-OGY Exploration and Development of GEOTHERMAL POWER In California CA UNIVERSITY OF CALIFORNIA DAVIS NOV 1 1 1^3 SPECIAL REPORT 75 FORNIA DIVISION OF MINES AND GEOLOGY FERRY BUILDING, SAN FRANCISCO, 1963 L IODAE3(# Cover. Solfatara, southern Italy. The road to Naples once ran past the hot springs of Solfatara, where there was a perpetual bubbling and fuming of sulfurous waters. This illustration, which was made in the early part of the eighteenth century, shows the hot springs area. The sheds to the left were used for the extraction of alum. EXPLORATION AND DEVELOPMENT OF GEOTHERMAL POWER IN CALIFORNIA By JAMES R. McNITT, Mining Geologist California Division of Mines and Geology SPECIAL REPORT 75 CALIFORNIA DIVISION OF MINES AND GEOLOGY FERRY BUILDING, SAN FRANCISCO, 1963 STATE OF CALIFORNIA Edmund G. Brown, Governor THE RESOURCES AGENCY Hugo Fisher, Administrator DEPARTMENT OF CONSERVATION DeWitt Nelson, Director DIVISION OF MINES AND GEOLOGY Ian Campbell, Sfafe Geologist SPECIAL REPORT 75 Price $1.00 CONTENTS Page Abstract 7 Introduction 8 Parti. California steam fields 9 The Geysers 13 Geology 1 3 Thermal activity 14 History of development __ .__ 14 Production 1 4 Reservoir characteristics 16 Utilization _' 21 Economics 24 Casa Diablo 25 Geology 25 Thermal activity 27 History of development 28 Production 29 Reservoir characteristics __ 29 Salton Sea 31 Geology 32 Thermal activity 32 History of development _ 32 Production 33 Part II. Problems of natural steam exploration and development ___ 35 Geologic characteristics of steam fields __ 37 Preliminary evaluation of a thermal area 39 Determining location and depth of production wells 41 Estimation of steam reserve 43 References __ 44 (3) Geyser steam field seen through expansion bend in 2,000-foot steam line which transports steam from the field to the plant. Photo courtesy Pacific Gas and Electric Company. (4) Photographs Frontis. Photo 1. Photo 2. Photo 3. Photo 4. Photo 5. Photo 6. Photo 7. Photo 8. Photo 9. Photo 10. Photo 11. Page Geyser steam field seen through expansion loop in 2000-foot steam line 4 The Geysers steam field, Sonoma County.. 9 The Geysers power plant, aerial view 12 Drilling above the blowout in an attempt to seal permeable zone 14 Power generating station and condensing units at The Geysers 19 Turbine blades being removed for annual cleanup. The Geysers power plant 20 One of the two turbine generator units at The Geysers power plant 21 East end of Long Valley structural de- pression 25 Creek flowing through rhyolite near Casa Diablo, Mono County 25 Extent of thermal activity prior to drilling of steam wells at Casa Diablo Hot Springs 27 Geologist taking temperature of Casa Diablo Hot Pool 28 Early system of drilling wells in thermal areas at Larderello, Italy 35 Figures Figure 1. Location of thermal areas drilled in Cali- fornia 1 Figure 2. Geologic map of The Geysers thermal area 22-23 Figure 3. Mass flow — wellhead pressure curves for three Geysers wells 15 Figure 4. Temperature depth curves for The Geysers steam wells 16 (5) Figures— Continued Page Figure 5. Contours drawn on bottom of constant temperature zone at The Geysers 17 Figure 6. Contours drawn on top of groundwater zone at The Geysers 17 Figure 7. Surface contours of The Geysers thermal area 1 7 Figure 8. Section across A-A' of figures 5, 6, and 7 17 Figure 9. Flow diagram of steam cycle in The Geysers power plant _. 20 Figure 10. Distribution of units in The Geysers plant 21 Figure 11. Geologic and gravity anomaly map of Long Valley 26 Figure 12. Thermal gradient of Endogenous Nos. 1 and 2, Casa Diablo thermal area..__ .... 28 Figure 13. Mass flow — wellhead pressure curves for Casa Diablo steam wells 29 Figure 14. Geophysical data from Salton Sea thermal area 3 1 Figure 15. Thermal gradient of Sportsman No. 1 well, Salton Sea thermal area 33 LAGO, ITALY. 1850. (6) From 1955 to 1962, approximately 40 wells were drilled in 15 California thermal areas for the purpose of exploring and developing natural steam to utilize for electric power generation. Twenty-four of the wells were drilled in the three areas which at present seem to have the greatest potential for the production of natural steam: The Geysers, Sonoma County; Casa Diablo, Mono County,- and the Salton Sea area, Imperial County. Since June 1960, steam from The Geysers thermal area, produced at a rate of approximately 250,000 Ib/hr, has been utilized to operate a 12,500 kw generating unit. Completion of a second generating unit, now under construction, will increase the total capacity of this area to approximately 28,000 kw. Geologic mapping and inter- pretation of temperature and pressure data from the steam wells suggest that super- heated steam is confined in a steeply dipping fracture zone by an overlying body of ground water. The fracture zone is part of a complex system of normal faults which defines a graben structure at least 5'/2 miles long and about 1 mile wide. The density inversion represented by the steam phase underlying the water phase in the fracture zone is attributed to the thermodynamic equilibrium existing between the two phases in an open system. The Casa Diablo thermal area is located on the southwest side of a volcano-tectonic collapse structure which is approximately 23 miles long and 12 miles wide. Vertical displacement within the collapse structure may be as much as 5,000 feet on the west side and 18,000± 5,000 feet on the east side. Four of the tested wells, drilled to depths ranging from 570 to 1,063 feet, flow saturated steam at rates ranging between 19,000 and 69,300 Ib/hr at 7.5 to 39 psig wellhead pressure. The Salton Sea thermal area is located in the vicinity of five small volcanic domes on the southeast shore of the sea. The elevation of the sea, 240 feet below sea level, indicates that it occupies a tectonic depression. Positive gravit/ and magnetic anomalies suggest the presence of a large intrusive body beneath the volcanic domes. A 5,230 foot well, drilled over this anomaly in late Tertiary and Quaternary sediments, flowed 123,000 Ib/hr steam and 457,000 Ib/hr concentrated brine at 200 psig wellhead pressure. In light of the above data, and data now available from foreign projects, three fundamental problems of geothermal power development can be considered: a) pre- liminary evaluation of a thermal area; b) location of exploratory wells; and c) estima- tion of steam reserves. Preliminary evaluation of an area usually is based on natural surface heat flow. Experience to date, however, has shown that by drilling wells in a thermal area, heat flow has been increased 3 to 170 times the observed natural surface heat flow, depending on the permeability and structural characteristics of the thermal fluid reservoir, as well as the initial enthalpy of the thermal fluid. The efficiency of well location can be greatly increased by regional and local tectonic analyses based on geologic mapping and geophysical methods, including gravimetric, magnetic, resistivity, and thermal. Steam reserves and life expectancy of the field depend on rates of heat and fluid flow in an open system rather than on the more familiar condition of mechanical equilibrium associated with the more or less closed system of a petroleum reservoir. (7) Geothermal power is the electric energy generated by the utilization of natural steam. This power is harnessed by releasing steam from natural thermal areas through bore holes and conducting it through a system of pipe lines to a turbine-generator unit. The potential of geothermal energy was first recog- nized in Italy, where the first steam well was drilled at Larderello in 1904. By the late 1930s, the steam fields were producing electric power at a capacity of 100,000 kw. The generating plants were destroyed during World War II, but they have been rebuilt and ex- panded so that the present capacity is in excess of 300,000 kw. The second country to investigate the possibilities of geothermal power was New Zealand. The necessity for rapid development of power resources during the post-war period prompted the New Zealand gov- ernment to initiate a geothermal power project at Wairakei, North Island, in 1950. Development has progressed to the stage where plants having a total capacity of 192,000 kw have been authorized. These plants are being constructed in two stages. The first stage (69,000 kw) was completed in March 1960, and the second stage (123,000 kw) is scheduled for com- pletion in 1963. Tentative plans have been prepared for later expansion of the installation to a total capac- ity of 282,000 kw. Encouraged by the progress of geothermal power development in Italy and New Zealand, exploration and development programs recently were initiated in Iceland, Mexico, El Salvador, Japan, Russia and the United States. By 1961, various development programs in these countries included the proposal of a 15,000 kw geothermal power station in Iceland, the operation of a 3,500 kw pilot plant to test wells of the Pathe field in Hidalgo, Mexico, and the operation of a 12,500 kw power station at The Geysers in California. The Geysers plant, located in northern Sonoma County, went on stream in June, 1960. In April 1962, construction began on a second generating unit which will raise the capacity of the plant to approximately 28,000 kw. The success of this power project has greatly stimulated exploration for natural steam throughout California and Nevada, as well as in some areas of Oregon, New Mexico, and Hawaii. The value of geothermal power exploitation is greatest in the California and Nevada desert regions, where neither fossil fuels nor hydroelectric power is available. But even in those regions where hydro- electric and steam generating plants are feasible, geo- thermal power offers considerable economic advan- tages: there is no fuel cost in comparison with the more conventional steam generating plant, or those utilizing atomic energy; and the capital investment needed to develop a steam field is, in most cases, smaller than that needed to construct either hydro- electric or conventional steam-generating facilities of the same capacity. In August of 1961, an international conference was held under the auspices of the United Nations for the purpose of collecting and exchanging data on "New Sources of Energy," including geothermal power. The ultimate objective of the conference was to make this widely dispersed, and mostly unpublished, information available for the use of underdeveloped countries, for whom geothermal power is of particular economic advantage. Thirty-nine papers on the geologic investi- gation of geothermal fields and 28 papers on the har- nessing of geothermal energy were presented. A con- siderable amount of the information discussed in the second part of this report was obtained from the papers presented at the United Nations conference. The writer also wishes to acknowledge the coopera- tion of the Magma Power Co. of Los Angeles and its affiliates, the Thermal Power Co. and the Natural Steam Corporation; the Pacific Gas and Electric Co. of San Francisco; O'Neill Geothermal Inc. of Midland, Texas, and the Rogers Engineering Co. of San Fran- cisco for making available much of the data presented in Part I of this report. (8) PART I. As of November 1962, 15 thermal areas have been drilled in California (figure 1). Table 1 lists the num- ber of wells drilled in each area as well as other perti- nent data. The temperatures listed in Table 1 were measured with maximum-recording thermometers, ex- cept at The Geysers, where test-metals of known melting points were used. The temperature listed is the maximum temperature recorded in the well or group of wells drilled in each area and does not cor- respond, in every case, to the temperature measured at the greatest depth reached by the drill. Because the successful development of power at The Geysers, and the well-test data from Casa Diablo and the Salton Sea have confirmed the economic potential of these three thermal areas, they will be described in detail. Although hot water or steam has been encoun- tered in most of the wells drilled in the other 12 thermal areas listed in Table 1, data from these areas are as yet insufficient for determining whether an ade- quate supply of steam will be available for power production. Photo l. The Geysers steam field, Sonoma County, California. Photo courtesy Pacific Gas and Electric Company. (9 ) THERMAL AREAS 1. Loke City 2. Cedorville 3. Terminal Geyser 4. Wendell 5. Amidee 6. Sulphur Bank 7. The Geysers 8. Calistoga 9. Foles Hot Springs 10. Bridgeport I I. Casa Diablo Hot Springs 12. Casa Diablo Hot Pool 13. Tecopa Hot Springs 14. Rondsburg 15. Salton Sea thermal area 75-1963 Geothermal Power in California 11 Table 1. Thermal areas drilled in California as of November 1962. Location Modoc Co., Sees. 23 & 24, T. 44 N., R. 15 E. Modoc Co., Sec. 6, T. 42 N., R. 17 E. Plumas Co., Sec. 36, T. 30 N., R. 5 E. Lassen Co., Sec. 23, T. 29 N., R. 15 E. Lassen Co., Sees. 8 & 5, T. 28 N., R. 16 E. • Lake Co., Sec. 5, T. 13 N., R. 7 W. Number of wells drilled Sonoma Co., Sees. 13 & T. 11 N., R. 9 W. Napa Co., Sec. 26 (projected), T. 9 N., R. 7 W. Mono Co., Sec. 24, T. 6 N., R. 23 E. Mono Co., Sec. 9, T. 4 N„ R. 25 E. Mono Co., Sec. 35, T. 3, S., R. 28 E. Inyo Co., Sec. 33, T. 21 N., R. 7 E. San Bernardino Co. T. 29 S„ R. 41 E. Sec. 25, Imperial Co., Sec. 23, T. 11 S., R. 13 E. and Sec. 10, T. 12 S., R. 13 E. Greatest depth reached, feet 2,150 734 1,270 630 1,116 1,391 2,100 2,000 413 982 Maximum tempera- ture measured, degrees C 1,063 805 422 772 5,232 160 54 129 79 107 186 300 (approx.) 137 51 178 134 116 340 Date when drilled 1959-1962 1962 1962 1962 1962 1961 1921-25, 1955-57 1959-61 1960-61 1962 1962 1959-62 1961 1962 1960 1927, 1957-58, 1961-62 Wells drilled by Magma Power Co. (and assocs.) Magma Power Co. (and assocs.) Geysers Steam Co. Magma Power Co. (and assocs.) Magma Power Co. (and assocs.) Magma Power Co. (and assocs.) Geyser Development Co 25) Magma and Thermal Cos. (1955-61) (1920- Power Calistoga Power Co. Magma Power Co. (and assocs.) Magma Power Co. (and assocs.) Magma and Natural Magma and Natural Steam Corp. Magma Power Co. (and assocs.) Magma Power Co. (and assocs.) Pioneer Development Co. (1927) Kent Imperial Oil Co. (1957-58) O'Neill Geothermal Inc. (1961- 62) Western Geothermal (1962) Figure 1. (Opposite page.) Map showing location of thermal areas drilled in California to November, 1962. Photo 2. The Geysers power plant, aerial view. Power plant under construction in lower right. Photo courtesy Pacific Gas and Electric Company. I 12 I THE GEYSERS The Geysers thermal area is located approximately 75 miles north of San Francisco on Big Sulphur Creek in the Mayacmas Mountains of northern Sonoma Count\\ This area is distinctive for two reasons: it is the only field outside of Italy which produces dry steam, and it is the only field in the United States developed to the point of actually producing electric power. Geology The Mayacmas Mountains of northern Sonoma County are underlain by the Jurassic-Cretaceous Fran- ciscan Formation which is a eugeosynclinal sequence of graywacke, shale, spilitic basalt, and serpentine. The oldest unit in this sequence is a massive graywacke with a very minor amount of shale. The graywacke is overlain by several hundred feet of spilitic basalt and associated chert beds. The basalt is in turn over- lain by a sequence of poorly bedded graywacke and shale (ratio approximately 1:1). Conformable bodies of serpentinized peridotite, in places over 200 feet thick, occur at the upper and lower contacts of the basalt. Bodies of hornblende and glaucophane schist are found near basalt-serpentine contacts and are thought to be the product of contact metamorphism of basalt by ultrabasic intrusions. Complex faulting, however, has obscured the original schist contacts. Geosynclinal deposition ceased in early Tertiary time when the Mesozoic rocks were uplifted and gently folded. There is no record of the Oligocene and Miocene epochs in this area, but by Pliocene time the uplifted rocks had been truncated by erosion. In the Pleistocene, volcanic rocks, including rhyolitic flows and tuffs, obsidian, basaltic lavas, and lavas of dacitic and andesitic composition, were erupted onto the eroded surface. These Pleistocene volcanic rocks —the Clear Lake volcanic series of Brice ( 195 3 ) — principally occupy the Clear Lake basin, which bor- ders the Mayacmas Mountains on the northeast. This basin is a northwest-trending structural depression 30 miles long by 15 miles wide. Approximately contemporaneous with the develop- ment of the Clear Lake basin, the Mesozoic rocks of the Mayacmas Mountains were uplifted and complexly faulted into a series of northwest-trending horsts and grabens. The individual horsts and grabens range be- tween 1 and 2 miles in width and retain their identity as structural units for lengths up to 10 miles. The grabens have not subsided on distinct major faults, but movement has occurred along numerous, inter-related normal faults, which dip between 60° and 80°. The fault traces are distinctly arcuate, having the downdropped block on the concave side of the fault. The lengths of the individual faults rarely ex- ceed 1 mile. The grabens are also complexly cross- faulted by arcuate faults which define small down- dropped units within the individual grabens. These units may be roughly circular, having diameters rang- ing from half a mile to a mile, or they may be oblong, having their long axes at various angles to the sides of the major graben in which they occur. Because of the complexity of the faulting, the lack of clearly recognizable datum planes, and the variable thickness of the faulted units, it is difficult to measure accurately the amount of vertical displacement. The order of magnitude of this movement, however, may be estimated from one of the grabens in which Terti- ary gravels have been downdropped into the under- lying Mesozoic rocks. The minimum vertical displace- ment of this gravel unit is 2,200 feet, with the move- ment distributed among two or three normal faults. The Geysers thermal area is at the west end of a northwest-trending graben, 5'/ 2 miles long by 1 mile wide, which is 5 miles southeast of the Clear Lake basin. Numerous thermal areas, of which The Geysers is the largest, occur within the graben. The Geysers graben is flanked on the northeast by Cobb Mountain, a horst block capped by a rhyolite extrusion. Cobb Mountain is bordered on the southeast by a small graben containing two thermal areas, Castle Rock and Anderson Springs. The majority of thermal areas in the Mayacmas Mountains are located in the Geysers and Anderson Springs grabens. Cobb Mountain rep- resents the culminating uplift of the Mayacmas range. Because this uplift corresponds with a volcanic extru- sion center and is spatially associated with the thermal areas, it is possible that forceful magmatic intrusion is responsible for the uplift of Cobb Mountain, and that the flanking grabens were formed due to horizontal extension of the crust across the arched area. Figure 2 (p. 22-23) is a geologic map of part of The Geysers graben. In this area Franciscan gray- wacke is overlain by basalt, which in turn is overlain by a body of serpentine. Along the canyon of Big Sulphur Creek both the serpentine and basalt have been downfaulted into the underlying gravwacke, with the serpentine body marking the axis of the graben. The geometry of the faults and the relative stratigraphic position of the faulted units indicate that (13) 14 California Division of Mines and Geology [Special Report the thermal areas arc located on the fissures closest to the serpentine body. This body marks the fault block w liich has undergone the greatest amount of sub- sidence. The fact that thermal springs occur mainly on the southwest side of the serpentine unit rather than being equally distributed on both sides of this central block probably reflects the pattern of ground water flow. The fault traces on the southwest side of the serpen- tine are considerably lower in elevation and closer to the bed of Big Sulphur Creek than the fault traces on the northeast side of the serpentine. This difference in elevation produces a sloping ground water table which intersects the ground surface close to the ele- vation of Big Sulphur Creek. Thermal activity The Geysers thermal area, as defined by the effects of hvdrothermal rock alteration at the surface, is about 1 300 feet long by 600 feet wide. The longer dimension approximately parallels the northwest- trending fault block into which the steam wells are drilled (figure 2). .Most of the natural thermal activity is confined to Geyser Creek, which occupies a narrow canyon cross- ing the western part of the thermal area from north to south. Although this canyon contains numerous hot springs, whose temperatures range between 50 C. and the boiling point, there are only two rather feeble fumaroles. A third fumarole, also quite small, plus a few hot springs occur in the drilled area just to the east of Geyser Creek. Allen and Day (1927, p. 30) measured the discharge from hot springs in Geyser Creek and found the flow- to range between 2,770 gal/hr in the wet season and 1,775 gal/hr in the dry season. Because these authors estimate this discharge to be "at least half if not con- siderably more" than the total hot springs discharge from The Geysers area, an average year around flow- could be estimated at 5,000 gal/hr. The measurements of Allen and Day also show that the rate of ground water flow from the hot spring area varies with the season and the)' conclude, therefore, that part of the w ater from these springs is of local, near-surface origin. No significant chloride content has been found in the hot spring waters, and the springs have been clas- sified as the sodium bicarbonate type by White (1957, p. 1651). History of development The hot springs and steam vents of The Geysers were discovered in 1847, and the area became a na- tionally known spa in the latter half of the 19th cen- tury. Wells were first drilled for the purpose of gen- erating electric power in 1921, and by 1925 eight wells were completed. Although sufficient steam was pro- duced at that time to establish the feasibility of the project, there was no market for the steam and the project was abandoned. In 1955, Magma Power Company obtained a 99-year lease on the hot spring areas located along the north side of Big Sulphur Creek. Between 1955 and 1957 Magma Power Company and its partner, Thermal Power Company, drilled six wells. On the basis of flow tests taken in December 1957, Pacific Gas and Electric Company was approached with the proposal that it construct a steam-electric power plant at The Geysers. On October 30, 1958, a contract between the producing companies and Pacific Gas and Elec- tric Company was signed. Five more wells were drilled in the summer of 1959. In June 1960, a 12,500 kw generating plant went on stream utilizing approxi- mately 250,000 lb/hr of steam supplied by four wells. Construction began in 1962 to increase the capacity of the plant to approximately 28,000 kw. In 1960 and 1961 another well was drilled at The Geysers and two 2,000 foot wells were drilled in a thermal area located on Big Sulphur Creek a mile northwest of The Geysers. Although surface indica- tion of heat flow are meager at this latter area, which is on the northwest end of the thermal spring zone, the two wells resulted in potentially commercial pro- duction of steam and further development is planned. Production The steam wells at The Geysers are drilled to depths of 500 to 1200 feet and are spaced at an average of 150 feet apart. For the wells drilled in 1957, 11%-inch casing was set to depths ranging from 200 to 325 feet, cemented to the surface and then 8% -inch production casing was run to the bottom of the hole. The produc- Photo 3. Drilling above the blowout in an attempt to seal permeable zone. Photo by John Padan. 75-1963 Geothkrmal Powkr in California 15 tion casing was perforated at depths ranging be- tween 460 and 700 feet. In the wells drilled during 1959, 13%-inch diameter holes were drilled below the surface casing and left uncased to the bottom. Total depths, casing depths and perforation intervals for 8 wells are shown in figure 4. The wells were drilled with a modified diesel-pow- ered rotary drill rig with depth capability of 2500 feet. Drill pipe consisted of 40-foot stands of 4 '/i -inch A.P.I, pipe. The major problem encountered in drilling the wells was the loss of drilling fluid into the steam-bearing fissures and the consequent danger of the uncontrolled escape of steam. Precautions taken against blowouts due to lost circulation included the availability of an adequate and dependable water supply for "quench- ing" the well, and an adequate supply of mud and filler material for sealing the borehole against leakage. Because of the constant danger of blowouts, mul- tiple blowout prevention equipment was used. A 12- inch valve was installed on the 1 3-inch casing, which is the last string. This valve remains on the well as part of the wellhead equipment. A spool piece is mounted above the valve, a Shaffer blowout preventer above that, and above this a rotating blowout pre- venter. The Shaffer blowout preventer contains two sets of rams, one designed to close on the drill pipe, the other to close on the drill collars. The completed wellhead equipment includes a cy- clone separator mounted horizontally on the discharge line. This separator removes fine rock particles that are produced with the steam. In spite of these precautions, in 1957 a blowout occurred during the drilling of Thermal No. 4 well, and in the succeeding 5 years, all attempts to seal off the escape of steam have failed. This well was drilled on a flat bench about 100 feet back from a steep slope. While the well was being drilled, steam began to seep from the side of the bench below the wellhead. In a few days, the steam enlarged its own escape route until a crater was formed which measured approximately 5 or 6 feet in diameter. At first, an attempt was made to seal the blowout with filler materials, such as redwood bark, but the steam escaped with such a high velocity that the material was thrown from the crater. Next, surface rock material was bulldozed over the crater, but eventually this too was blown out and a new crater formed. In October 1959, a 550-foot well was directionally drilled to intersect Thermal No. 4 well below its connection with the blowout. This new well succeeded in diverting much of the escaping steam, but attempts to quench the blowout by pumping water down the directional well failed to stop the uncon- trolled flow. In August 1962, a well was drilled di- rectly adjacent to Thermal No. 4 in an attempt to seal the fracture zone with gravel and cement at a depth of about 300 feet (photo 3). Although this attempt was not successful in sealing off the blow out, it re- 100 o o o * 60 40 N 1 Mogmo No. 1 Thermol No. 5 *\ Thermol No.2^ \ \ 3 80 100 120 140 160 180 Pressure, psig ■■■HnnnnHn Figure 3. Mass flow-wellhead pressure curves for three Geysers wells. (After Bruce and Albritton, 1959.) suited in a slight increase of steam pressure in the adjacent wells. The quantity and pressure of steam flowing from the wells are determined by the size of the wellhead orifice which is controlled by a manually operated valve. Figure 3 shows the wellhead pressure-steam flow relationship for three representative wells. The wellhead temperature is in turn dependent upon the steam pressure. The following table shows representa- tive wellhead pressures (in pounds per square inch, absolute) and corresponding temperatures of the ef- fluent from Magma No. 1 and Thermal Nos. 2 and 5 measured in September 1958. Pressure psia Temperature °C. 59.7 _ 169 62.4 171 75.4 181 94.2 ..... 184 104.1 ... 181 132.1 188 Enthalpies calculated from these temperature and pressure data for the three wells are: Enthalpy litu per lb Well (referred to S2"F.) Magma No. 1 1207 Thermal No. 5 .. .. 1199 Thermal No. 2 .. 1204 Analyses of the steam from these three wells indicate that the percentage of non-condensable gases ranges 16 California Division of Mines and Geology [Special Report from 0.68 percent to 0.83 percent by weight. A weighted average composition of the gases in the three wells is shown below (Bruce, 1961, p. 12). Gas CO, CH. H* Ns + A H,>S Volume percent at 60° F and 30" Hg 69.32 11.81 12.70 1.59 2.99 1.59 Weight percent 88.73 5.49 .74 1.29 2.96 .79 Reservoir characteristics On April 21 and 22, 1960, temperature logs were run by the Thermal Power Co. on 8 steam wells. These temperatures were taken with an iron-constan- tan thermocouple after the wells had been shut in for periods ranging between 1 hour and 2 months. The resulting temperature-depth curves for these wells are shown in figure 4. The static wellhead pres- sure recorded during the time of temperature meas- TEMPERATURE *C 200 210 "X V 3 > Per. \ 6or.dC Sot. T-179*C O ISO 190 200 ZK V C — B P-IOlp.s.i.o. Sot. T.-165'C i- Stotic (or 4 doyt* I7D ISO 190 200 2K a; V >, — B ortd C P-M4 p.j.i.a. Sol.T.-i?0*C 1- Stotic for IB hours 200 210 170 180 190 ZOO 210 170 ISO 190 200 ZtO *- ■v Per. ^ \ - — Bond C P-l24p.Uo. Sol. T.-I75-C 1- StOtiC tO' Ihour" \ I — c Sol. T.-180'C t- Static lor 2 months - B =S: B \ C ~ i r ^ J — 8 A P-109 p s,io. sot T.-ierc I- Static lor l.Shours B P-H2p.s.i.c. Sot T.-169'C 1- Stotic tor IT hours" X > >,. ^i. - 8 Aid C p- ti6 p. s.t- at Sot. T.-I70*C t- Stoltc lor 16 hours' WELL T2 170 (80 190 200 210 \ — C P-I24p.i.i,'a Sat. T.-"73*C 1- Stalk (or Ihour — B B-80TT0M OF WELL C-BOTTOM OF CASINO Ptr -PERFORATION 20NE P- STATIC WELL HEAD PRESSURE Sat. T -SATURATION TEMPERATURE AT P. 'TIME ELAPSED BETWEEN CLOSING OF WELL AND MEASUREMENT OF TEMPERATURE TEMPERATURE- DEPTH CURVES FOR THE GEYSERS STEAM WELLS LsssssssssHHLBHi 75-1963] Gfothfrmal Powkr in California 17 Figure 5. Contours drown on bottom ot constant temperature zone ot The Geysers Figure 6 Contours drawn on top of ground water zone as defined by calculations described in text Figure 7 Surface contours of The Geysers thermol area urement and the elapsed time between well shut-down and temperature measurement are indicated for each well. A striking characteristic of these logs is the interval of constant temperature encountered in the upper part of the wells. In only one of the wells, T 7, does this constant temperature interval represent saturation conditions at the measured pressure. In all the other Elevation in feet above sea level 1800, ^Top of ground water zone -Steam-water interface Figure 8. Section across A-A of figures 5-7 Scole for figures 5-8 wells, the constant temperature shows various degrees of superheat, ranging from 8° C to 19° C, indicating that a liquid phase does not exist over the interval measured. There is a distinct temperature increase at the bottom of the constant temperature zone in six of the wells (figure 4). The maximum temperature below this temperature "break" is 207.5° C which was re- corded in well No. T 7. It should be noted that the 18 California Division of Minks and Geology [Special Report highest temperature measured in The Geysers steam held is approximately 300° C. This temperature was measured in 1957 at 600 feet in Magma No. 1 by the use of test-metals which melt at different tempera- tures. The surface representing the bottom of this con- stant temperature zone is shown by contours in figure 5. Because geologic mapping indicates that steam is conducted through the dense, indurated gravwacke of the Franciscan Formation by dipping fracture sys- tem (figure 2), it is not probable that the surface shown in figure 5, which dips only 25°, corresponds to one of these steam conducting fractures. Further- more, this surface cannot be correlated with lithologic discontinuities disclosed by well cores and cuttings. From the observations of Allen and Day (1927, p. 26-31) it is known that a body of ground water over- lies the superheated steam zone. Therefore, it is sug- gested that the base of the constant temperature zone corresponds to a steam-water interface at the bottom of the body of ground water. Immediately after closing a flowing well, a film of water should condense on the wall of the well where superheated vapor comes in contact with ground water or with the well casing, which is in contact with ground water. As heat is absorbed by the sur- rounding ground water, the temperature of the super- heated vapor will decrease until saturation conditions for the pressure in the well pertain. At any time be- tween the shut-down of the well and the final attain- ment of saturation conditions, the temperature in the part of the well that is in contact with ground water should not vary with depth. This is due to the fact that the temperature at which steam will condense on the wall of well is dependent on the steam pressure, which, under static conditions, would be essentially constant with depth. The validity of this proposed explanation for the constant temperature zone is sup- ported by the fact that the only well in which satura- tion conditions exist in the constant temperature in- terval is well No. T 7. This well had been closed for the longest period of time before the temperatures were measured, suggesting that all the wells would eventually reach saturation conditions within the depth interval of constant temperature. If the steam is not confined in the reservoir by over- lying impermeable beds, then a hydrostatic equilibrium must exist between the nearsurface ground water bodv and the steam reservoir, i.e., the expansive pressure of the steam must equal the hydrostatic pressure of the water body which confines it. Therefore, from the static wellhead pressure it should be possible to com- pute the height of the overlying water body at the point at which the well is drilled. This hypothesis has been tested by calculating the height of a column of water which would produce the static wellhead pres- sure given in figure 4, and adding its height to the elevation of the temperature "break" at the bottom of the constant temperature interval. The surface thus defined, contoured in figure 6, should represent the top of the ground water bodv overlying the steam. Two features of the surface defined in this manner support the validity of these calculations: a) there is a general similarity in configuration between the topog- raphy of The Geysers area (figure 7) and the config- uration of the upper boundary of the proposed ground water body (figure 6), thus indicating a relationship which would be expected between the two surfaces; b) the elevation of the surface shown in figure 6 ranges between 1500 and 1600 feet, which corresponds to the range in elevation of the principal natural springs in Geyser Canyon. Figure 8 is a cross-section through The Geysers area illustrating the spatial relationship between the ground surface, the top of the ground water body, and the steam-water interface, as defined by the method described above. Although the static pressure of the steam is depend- ent upon the hydrostatic head above the steam, the height of this water column is principally determined by the rate of heat flow into and out of the overlying water body. This water body is a mixture of mete- oric water, originating in the immediate vicinity of The Geysers, and condensed steam from below. Under natural conditions, heat is released from this ground- water body by flow of hot water from the springs in Geyser Canyon and the rate of this water flow is determined by the permeability of the fracture zone. Evaporation of water must also be an important cause of heat loss because water in the ground water body is at, or very near, its boiling point throughout its pressure range. The rate of heat loss from the ground water body must be equal to the rate at which heat is supplied at the steam-water interface. If the rate of heat flow into the water body were greater than the rate of heat loss, the water would "boil away" and superheated steam would escape directly to the sur- face. If the rate of flow of cold meteoric water into the ground water body increased, the rate of heat flow into the water body by steam condensation would also increase because of the necessity of raising a greater volume of water to its boiling point. Eventually the rate of heat flow into the area would be limited by the physical dimensions of the feeding fractures, and, on continued inflow of cold meteoric water, a super- heated steam phase could no longer exist in the system. The balance of heat flow into and out of the ground water body, as described above, is responsible for maintaining the density inversion of the two phases in the system. Under static conditions, it would not be possible for a steam phase to be in mechanical equi- librium with an overlying liquid phase due to their difference in density. Because the movement of both phases through the system is determined not only by density differences, but also by the thermodynamic equilibrium existing between the two phases in an open and flowing system, such a density inversion is made possible. W i ■ >*. .. Photo 4. Power generating station and condensing units at The Geysers. Photo courtesy Pacific Gas and Electric Company. 20 California Division of Mines and Gkology [Special Report The rate at which steam is condensed in the ground water body under natural conditions can be estimated in the following manner. The natural thermal spring water, which is derived from the ground water body, is a mixture of meteoric water and steam condensate. The enthalpy of the meteoric water is approximately 20 cal/gm and the enthalpy of the steam is 667 cal/gm. Mixing of these two waters results in a water body having an enthalpy of approximately 100 cal/gm. Water is released from this body at the approximate rate of 5,000 gal/hr. If x is the percentage of con- densed steam in the natural spring flow, then the fol- lowing equation expresses the above conditions: 667x + 20 (1 — x) = 100, and x = 11.6% Therefore, under natural cond 1.3 lb. of steam is condensed in average enthalpy of 667 cal/gm, flow gives an estimated natural Geysers thermal area of 4.1 X 10 however, does not include heat from the ground water body or the atmosphere. itions, approximately one second. At the this estimated steam heat flow from The "' cal/sec. This figure, loss by evaporation radiation of heat to Photo 5. Turbine blades being removed for annual cleanup, The Geysers Power Plant. Photo courtesy Pacific Gas and Electric Company. An estimated limit for the maximum rate of mass flow through the steam wells at The Geysers is 600,000 lb/hr. Water loss from natural spring flow (approxi- STEAM FROM WELLS 250.2M GENERATOR 240 M I OOP 348 F 1196 h_ TURBINE I2.5MW OUTPUT P = LBS/SQ. IN. GAGE M= 1000 LBS/HR F = DEGREE F h = BTU/LB WATER STORAGE TANK t/S INDUCED 2I6.5M f- DRAFT FANS I20F I0.2M CIRC. PUMP o- 5.800M COOLING TOWER 65F WET BULB AIR 5.500 M,, 80 F T 5.3 M OIL COOLER n MAKE-UP PUMP 240 M n-i 1011 h U- I55M 4.9M I00M GAS OUT ' H 9 45M 1ST. STAGE GAS EJECTOR & INTER- CONDENSER TURBINE BAROMETRIC CONDENSER 5.740 M 2ND. STAGE GAS EJECTOR & AFTER- CONDENSER 104. 8M 160.3 M 6.050.1 M OVER- FLOW 33.5M CIRC. PUMP Lj HOTWELL Bl G SULPHURJE!!H^- Figure 9. Flow diagram of steam cycle in The Geysers power plant. (After Bruce and Albritton, 1959.) 75-1963] Gf.othf.rmai. Power in California 21 Figure 10. Distribution of units in The Geysers power plant. (After Bruce and Albritton, 1959.) mately 40,000 lb/hr), from evaporation, and from the uncontrolled "blowout", however, are not included in this figure. If these other factors are considered, a conservative estimate for maximum possible total steam flow from the area would be about 800,000 lb/hr. At an average enthalpy of 667 cal/gm, this estimated steam flow gives a maximum estimated heat flow of 6.7 X 10" cal/sec. Comparing this figure with the cal- culated natural heat flow shows that drilling the wells made it possible to increase the rate of heat flow from the thermal area by approximately 170 times the rate of natural heat flow. Utilization Photo 6. One of the two turbine generator units at The Geysers power plant. Photo courtesy Pacific Gas and E/ecfric Company. The steam produced at The Geysers has a com- paratively low content of non-condensable gases, and it is superheated. Because of the low non-condensable gas content, elaborate gas removal equipment is not necessary in order to achieve a low back pressure behind the turbine; and because the steam is dry, it can be fed directly into the turbine without first pass- ing through water separating tanks (as is necessary in most other steam producing areas). The flow diagram for the steam cycle is shown in figure 9, and an elevation of the plant, illustrating the distribution of the units, is shown in figure 10. The turbine is designed for 100 psig (pounds per square Base from U. S. Geological Survey 4000 3000'- The Geysers Q s lL ■ thermal area JKs \sch^p^i Figure 2. Geologic map of The Geysers thermal area. I22°45' EXPLANATION Landslide Rhyolite flows and tuffs — JKms— _ Micaceous graywacke-, shale Amphibole schist Serpentine Chert Greenstone Graywacke and shale Contact Dashed where approximately located Fault (down on concave side) Dashed where approximately located n i Concealed fault Strike and dip of beds Horizontal beds Hot spring Steam well California Division of Mines and Geology [Special Report inch, gauge) and 348° F inlet steam conditions and a back pressure of 4 inches Hg. Because there is no boi-ler, it is not necessary to re- cycle the steam condensate through the turbine and a barometric condenser is used in order to achieve the low back pressure. This type of condenser is one in which the cooling water is sprayed directly into the exhaust steam. Because less water is lost by evapora- tion than is supplied by the condensed steam, no ex- ternal make-up water is required except to fill the cooling system initially. The amount that overflows from the cooling tower is 12 percent to 40 percent of the steam condensate depending on the atmospheric conditions. The use of this condensing system pro- vides about twice the electrical output per pound of steam compared with one which exhausts to the at- mosphere. Approximately 240,000 lbs of steam per 'hour is sup- plied from four of the wells to the turbine through a 2,000-foot line, 20 inches in diameter. An 84-inch diameter duct carries the exhaust steam from the tur- bine to the barometric condenser. The non-condens- able gases are removed at the top of the condenser by a two-stage steam jet gas ejector. The condenser vessel is mounted vertically 28 feet above ground level, in order to allow room for a 34- foot long, 30-inch diameter barometric leg which is connected to the bottom of the condenser. The mix- ture of cooling water and condensate falls down to the hotwell in which the lower end of the barometric leg is immersed, thus maintaining a seal for the vac- uum. The flow of circulating water through the hot- well is about 12,000 gallons per minute. From the hotwell, the condensate is cycled to the cooling tower, and then back to the condenser. The cooling tower is a three cell induced draft type, de- signed to cool 1 2,000 gallons per minute of water from 120°F to 80°F with a wet bulb air temperature of 65°F. The alternating current generator is rated 12,500 kva, 1,000 rpm, 3 phase, 60 cycle and 11.5 kv. The current is stepped up to 60 kv for transmission over a new 10-mile line to the existing Fulton-Hopland transmission circuits. Because of the sulfur content of the thermal fluid, intensive tests were made to study the corrosiveness of the steam condensate and circulating water. These tests showed that the main steam line could be made of carbon steel pipe, but it was necessary to use type 304 (18 percent chrome, 8 percent nickel) stainless steel pipe for the condensate lines. Type 304 stainless steel plate is used for the exhaust pipe from the tur- bine, and Type 316 (18 percent chrome, 14 percent nickel, 2'/ 2 percent molybdenum) stainless steel is used in the barometric condenser. The structural parts of the cooling tower are made of redwood, and the fill made of styrene plastic. All nails, fasteners and bolts are of Type 303 or 204 stain- less steel and all cast iron fittings are coated with a baked phenolic coating. Cement asbestos board is used to enclose the tower. Economics On the basis of feasibility and economic studies, Pacific Gas and Electric Co. signed a contract on October 30, 1958 which provided that the first unit constructed by P.G. & E. would be 12,500 kw and a second unit would be at least equal in size if warranted by the supply of steam. P.G. & E. would pay for the steam at a rate of 2.5 mills per net kwh of electric energy delivered to the transmission line for the first two units and an escalated price for energy from any- additional units (Bruce, 1961, p. 5). During 1961, the gross income to Magma and Thermal Power Com- panies for the sale of steam was approximated $250,- 000. A capital cost of $40 to $60 per foot of completed steam well has been estimated (English, 1961, p. 12). This cost includes such items as roadways, cellars, warehousing, geologic studies, well testing, wellhead equipment, casing and administration. The cost of the first 1 2,500 kw generating plant plus the 11.5/60 kv step-up switchyard was approximately $1,900,000. A second-hand turbine generator was used in this unit, and if it had been necessary to supply a new unit about $500,000 would have been added to the plant cost. The 10-mile 60 kv transmission line cost is about $220,000 (Bruce, 1961, p. 5). The technic.il and economic success of The Geysers geothermal power plant is well established. In April 1962, construction began on a second generating unit which will raise the plant's capacity to approximately 28,000 kw. The new design pressure will be 80 psig for the first (original) unit and 65 psig for the second unit. Steam from the thermal field will be supplied to the two units at the rate of approximated 550,000 lb/hr. CASA DIABLO Casa Diablo Hot Springs are located in Mono County, about 40 miles northwest of Bishop on High- way 395. These springs are one of several hot spring groups on the west side of Long Valley, near the headwaters of the Owens River. Long Valley occupies the east side of a large topographic basin which is oval in shape and oriented with its long axis east-west. This basin is known as the Mammoth Embayment because of the east-west offset which it produces in the steep, northeast front of the Sierra Nevada. Mono basin, a similar topographic depression, is located about 10 miles north of the Long Valley area. Photo 7. East end of Long Valley structural depression. Photo by Mary Hill. Geology The Sierra Nevada on the west and south of Long Valley, and the Benton Range and the Black Moun- tains on the east of the basin, are composed of Late Paleozoic metasedimentary rock which is intruded by Cretaceous rocks ranging in composition from gabbro to granite (Rinehart and Ross, 1956, p. 5-7). Cenozoic volcanic rocks, as well as alluvial and glacial deposits, fill the Long Vallev basin and cover much of the pre- Tertiary rocks to the north. The gravity data shown in figure 1 1 indicate that Long Valley is a structural depression as well as a physiographic basin, and is bounded on all sides by steep faults. The structural depression is also elliptical in shape, 23 miles long and 12 miles wide. The Ceno- zoic deposits in the depression increase gradually from a thickness of less than 5,000 feet on the west to 18,000 ±5,000 feet on the east (Pakiser, 1961, p. 253). Pakiser has interpreted Long Valley to be "a volcano- tectonic depression caused by subsidence along faults, following extrusion of magma from a chamber at depth". Gilbert (1938, p. 1860) believes that the Long Val- ley depression was the major locus for vents which erupted the Pleistocene Bishop Tuff, a pyroclastic de- posit of the nuee ardente type. The visible extent of the Bishop Tuff is approximately 350 square miles, but because of its probable continuation beneath the allu- vium of Owens, Long and Adobe Valleys, the total areal extent of the tuff should be about 400 to 450 square miles. The thickness of the tuff exposed in stream gorges ranges between 400 and 500 feet; con- sequently, the total volume of the Bishop Tuff ap- proximates 35 cubic miles (Gilbert, 1938, p. 1833). Approximately 200 feet of Pleistocene lacustrine de- posits (Cleveland, 1961) overlying rhyolite flows in -**.. riysyiH^?** %*'■'• £«iL- '*«> Photo 8. Creek flowing through rhyolite near Casa Diablo, Mono County. Photo by Mary Hill. (25) z o h < z < _l Q. X UJ 3 .2 BQ .2 ■a c lJ © >> 3 J2 1 8 D| Ds Dl 1 1« c •5.1 E JS to 3 8 1 1 r 1 1 = £ 2 (p8i|Si|qndun) J»qnH pue ueyauiy O uj X o at O >- 8 = 8 o K O - UJ ~ X O UJ 3 z * z in— i to ■o c o i? - z 75-1963] Gkothermai. Power in California 27 Long Valley is evidence that the depression has ex- isted at least from mid-Pleistocene. Volcanic activity in this region has continued to a very recent time, as indicated by the presence of a basalt flow in the Mam- moth Creek area which overlies lacustrine sediments thought to be deposited during the Tahoe glaciation (Cleveland, in press). An aeromagnetic survey of the region disclosed a sharp magnetic high, having a relief of from 2500 to 5000 gammas, located over the center of Long Valley (Pakiser, 1961, p. 252). This magnetic anomaly roughly corresponds to a local positive gravity ano- maly which has a relief of about 10 milligals, as com- pared with a 60 milligal negative anomaly over the major part of the Long Valley depression. The calcu- lated depth to the upper surface of the magnetic body is about 3000 feet below the valley floor, which places the top of the body in the upper part of the Cenozoic section as determined by gravity methods. This mass of dense and magnetic material may represent the buried volcanic or intrusive rock which is the heat source for the various thermal springs in Long Valley. The principal drilling in this area for geothermal power has been done at Casa Diablo Hot Springs. The springs are located near two structural features: a) a fault trending north-northwest, and b) a west-trending contact between a Quaternary basalt flow and a late Tertiary rhyolite (Rinehart and Ross, in press). The rhvolite is the principal flow covering the west half of the Long Valley depression. Because of poor out- crops in the Casa Diablo Hot Springs area, the nature of the basalt-rhyolite contact is not known with cer- tainty. Rinehart and Ross (written communication, 1962) are of the opinion that the thermal activity at Casa Diablo Hot Springs, as well as in most of the Long Valley area, appears to be localized along steeply dipping to vertical faults that trend north to north- west. The writer believes that arcuate faults, trending to the northwest from Casa Diablo Hot Springs are suggested by the configuration of the contact between the Tertiary rhyolite and younger Pleistocene units. Moreover, inspection of aerial photographs strongly suggests that collapse structures enclosed by arcuate faults are common in the Long Valley depression. If this is correct, then there is a striking similiarity be- tween the geologic structure of the Casa Diablo ther- mal area and the graben structure at The Geysers in Sonoma Countv. Thermal activity The surface temperature and approximate discharge of seven spring groups in Long Valley are given in table 2 (Stearns, Stearns, and Waring 1935, p. 126- 127). The number given to each spring group corre- sponds to the number in figure 11. In addition to these thermal areas, there are many localities, mainly northeast of Casa Diablo, where the rhyolite has been altered to clay and opal by hydro- thermal solutions. Although there is evidence for very recent thermal activity at a few of these clay deposits, it is believed that most of them formed in the mid- Pleistocene and are not closely related to present ther- mal activity (Cleveland, in press). Table 2. Temperature and discharge of Long Valley thermal springs (after Stearns, Stearns and Waring, 1935, p. 126-127). Photo 9. Extent of thermal activity prior to drilling of steam wells at Casa Diablo Hot Springs. Photo by Mary Hill. Map no. Location Name Temp. °C Total dis- charge, gal/min No. of springs 1 NWX, sec 32, T.3S., R.28E. Casa Diablo Hot Springs 46-90 35 20 2 NWK\ sec. 35, T.3S., R.28E. Casa Diablo Hot Pool 82 Inter- mittent -- 3 NWX, sec. 6, T.4S., R.29E. Whitnjore Hot Springs 36 306 2 4 NEX, sec. 31 T.3S., R.29E. -- 23-38 450 4 5 NEK, sec. 30, T.3S., R.29E. "The Geysers" 49-94 500 5 6 NWX, sec. 13, T.3S., R.28E. -- 11 5 1 7 NEX, sec. 7, T.3S., R.29E. -- -- -- -- Photo 10. Geologist taking temperature of Casa Diablo Hot Pool. Photo by Mary Hill. Figure 12. Thermal gradient of Endogenous Nos. 1 and 2, Casa Diablo thermal area. History of development In 1959, two wells were drilled at Casa Diablo Hot Springs by the Magma Power Company. The first well, Mammoth No. 1, was drilled about 1200 feet northeast of Highway 395 and the second just west of the highway near an active fumarole. Mammoth No. 1 was drilled to 1,063 feet, but the second well was aban- doned at a shallow depth because extensive steam seeping around the well area indicated very pervious ground. In 1960 Magma Power Co. entered into a partnership agreement with Endogenous Power Co. (now known as Natural Steam Corporation). This new company drilled the third, fourth and fifth wells on the west side of the highway, approximately 100 yards apart in a northwest-southeast line. From south to north these wells are named Endogenous No. 1 through 3, and were drilled to 630, 810 and 570 feet, respectively. In 1961 Endogenous No. 4 was drilled adjacent to the other Endogenous wells, but east of the highway. Also in 1961, Endogenous Power Co. drilled Chance No. 1 at Casa Diablo Hot Pool, a thermal spring 3 miles east of Casa Diablo. In 1962, Endogenous Nos. 5, 6, and 7 were drilled at Casa Di- ablo on the east side of Highway 395 to 405, 756 and 670 feet, respectively. TEMPERATURE, °C 100 115 130 145 160 175 190 100 200 N UI300 UJ u. \ t- 0. UJ400 o 500 600 Endogenous No. 1 Endogenous No. 2 r J 75-1963 Gfothkrmai. Power in California 29 •s. o g300 200 100 / t 1. Endogenous No, 1 2. Endogenous No.2 1 3. Endogenous No.3 1 4. Mommoth No. 1 t .X woter / \ o O / \ i \ \ c > v" \ ^A Y \ 3- \ s ^ \ © ^ ** 1 O ^ I ^ 2- «-* 2^ 1- 4 3__ 10 20 30 40 50 60 Pressure, psig 70 Figure 13. Mass flow-wellhead pressure curves for Casa Diablo steam wells. Production The four wells tested to date are Endogenous No. 1 through No. 3 and Mammoth No. 1. The Endog- enous wells were completed with two strings of cas- ing, leaving an open hole below. The outer casing is 1 3 % inches in outside diameter and was set to depths ranging from 140 to 220 feet. The inside casing is 9% inches in outside diameter and was installed to depths ranging between 350 to 400 feet. Both sets of casings are hung from the surface and cemented from bottom to top. Mammoth No. 1 has one string of casing 165 feet long and 9% inches in outside diameter. Temperatures in the wells were taken with a maxi- mum recording thermometer after the wells had been static for several weeks. Under these conditions, the maximum temperatures recorded in Endogenous No. 1 through 3 were 178° C, 174° C and 172° C, respec- tively, and 148° C in Mammoth No. 1. Figure 12 shows the thermal gradients measured in Endogenous Nos. 1 and 2 after the wells had been static for a period of six months. The temperatures shown in fig- ure 12 were measured with a mechanical temperature logging device in the latter part of 1962. This device was developed in New Zealand and is considered more reliable than electrical instruments for recording well temperatures under these extreme conditions. The four wells produce a mixture of saturated steam and hot water. Well performance was tested by W. M. Middleton in October 1960. The following table sum- marizes the well characteristics under producing con- ditions. Tempera- ture Pressure Steam Water Well no. °C psig Ib/hr Ib/hr Kndogenous No. 1 148 39 69,300 473,000 Endogenous No. 2 181 38.5 45,000 233,500 Endogenous No. 3 . 157 30 19,000 330,000 Mammoth No. 1 132 7.5 25,000 471,000 The chemical constituents in water and condensate from the Casa Diablo wells are given in table 3. The steam produced from Endogenous No. 4 con- tains 0.36 percent by volume and 0.87 percent by weight of non-condensable gases. This gas is composed of 98.25 percent by volume or 98.64 percent by weight of C0 2 and 1.75 percent by volume or 1.36 percent by weight of ETS. An infrared spectrum of the samples indicated that there were no other gases present. Reservoir characteristics The variation of steam and water flow with well- head pressure for the four wells is shown in figure 13. As would be expected, steam flow increases with decreasing pressure in all the wells. In Endogenous No. 1, water flow increases with decreasing pressure, but in the other three wells, water flow decreases with decreasing pressure. The anomalous relationship of Table 3. Chemica constituents of fl jids from Casa Dia 1I0 wells in ppm. Endog- Endog- Endog- Mam- Endog- Endog- enous enous enous moth enous enous No. I 2 No. I 1 No. 2 2 No. I 1 No. 4 3 No. ¥ SiO-2 250 278 256 292 200 0.8 Ca 2 30 4 Mg tr. tr. Na 380 236 375 247 308 5 K 47 62 45 71 32 Li 4 3 0.3 Fe 5 4 Al 2 1 B 60 49 11 0.3 CI 276 266 276 301 227 5 S0 4 61 108 62 124 96 2 H 2 S 14 11 F 20 NH, 0.1 0.5 CO, 180 205 As 0.2 P H 8.86 7.5 8.61 8.0 6.5 4.9 Analyst: Abbot A. Hanks, Inc., San Francisco. 1 Sample taken from wellhead immediately after flowing. Some water flashed to steam. ! Sample taken from wellhead after cooling. No flashing to steam. 3 Water sample taken during flow test. 4 Condensate of steam sample taken during flow test. 30 California Division of Minis and Gf.ology [Special Report decreasing water flow with decreasing wellhead pres- sure is probably the effect of relative permeability. When both a vapor and liquid phase are being pro- duced, a reservoir is considerably more permeable to the vapor than to the liquid. At low wellhead pres- sures, vapor expansion in a reservoir could be so great as to block the passages to liquid flow. The effect of relative permeability becomes more pronounced as the absolute permeability of the rock decreases. The relative permeability effect should not be no- ticeable where water is flashing to steam within the well bore, provided that the bore has not been re- stricted by mineral deposition. If on the other hand, some water flashes within the reservoir rock, the rela- tive permeability effect should become more pro- nounced. Where flashing occurs within the reservoir rock, the relative permeability effect offers a method for comparing reservoir permeability from one bore to another, irrespective of the area of the hole open to production: the greater the tendency of a water pro- duction curve to flatten or even to slope in a positive direction (figure 13) with decreasing wellhead pres- sure, the less permeable the reservoir in the vicinity of the well displaying this type of water production curve. In regard to the four Casa Diablo wells, the water production curve of Endogenous No. 1 reflects the greatest reservoir permeability, Mammoth No. 1 the least permeability, and Endogenous No. 1 and 2 inter- mediate permeability. The comparatively large total flow of Mammoth No. 1 is most probably due to the fact that its "open hole area" is approximately three times greater than that of any of the other wells. The comparatively low permeability reflected by the pro- duction curves of Endogenous No. 1 and 2 and Mam- moth No. 1 may be due to either or both of two fac- tors: (a) the well flow is restricted by the deposition of calcite and silica in the region where hot water is flashing to steam, or (b) these three wells did not intersect the main steam bearing fissure, but are pro- ducing from relatively small subsidiary fractures. In order to explain the differences of steam production as well as the slope of the water production curve be- tween Endogenous No. 1 and Mammoth No. 1, the latter interpretation is preferred, because both bores were cleaned of calcite deposits before the tests were made. On the other hand, Endogenous No. 1 and 2 seemed to sustain equal mass flows directly after com- pletion of the wells, so that calcite deposition in En- dogenous No. 2 is the more probable explanation of its relatively low rate of flow. SASSO. ITALY. 1850. SALTON SEA The Salton Sea thermal area is located in Imperial County approximately 3 miles southwest of Niland and at the southeast end of the Salton Sea. Although little development work has been done on this area as compared to the Geysers steam field, the future of the field is very promising because of the high heat flows measured in exploratory wells and because of the large implied area of the field. TllS RI2E TI2S TI3S TI4S R 14 E Figure 14. Salton Sea thermal area. Magnetic data after Kelley and Soske, 1936; gravity data after Kovach, 1962. R 15 E 32 California Division of A4inf.s and Geology [Special Report Geology The Salton Sea is approximately 30 miles long and 10 miles wide and stands at an elevation of 240 feet below sea level. The lake occupies the lowest part of a large topographic and structural depression which extends 175 miles from the head of the Gulf of Cali- fornia northwestward through the Colorado River delta, the Imperial Valley, and Salton Sea, to the Coa- chella Valley. The Colorado River delta protects the Salton basin from inundation by water from the Gulf of California. In the vicinity of the Salton Sea this structural trough is about 75 miles wide. It is bordered on the west by the Peninsular Ranges, which are primarily granitic rocks of probable Cretaceous age (Dibblee, 1954, p. 21), and on the east by the Chocolate Moun- tains, a complex of Precambrian igneous and meta- morphic rocks capped by Tertiary volcanic rocks. The clastic sediments filling the Imperial depression represent essentially continuous deposition since Mio- cene time. The sediments attain a maximum exposed thickness of 16,500 feet in southwestern Imperial Val- ley, 18,700 feet in northwestern Imperial Valley and 8,600 feet in northeastern Coachella Valley (Dibblee, 1954, p. 21-22). Seismic data suggest that the fill is more than 20,000 feet thick in the central part of the depression. The sediments are primarily continental deposits consisting of fanglomerate, conglomerate and lacustrine sandstone and claystone. The Miocene Im- perial Formation, however, is a shallow water marine claystone interbedded with oyster-shell reefs. Three major right-lateral fault zones can be traced into Imperial Valley. The San Andreas zone can be followed along the northeast side of Coachella Valley and the San Jacinto and Elsinor fault zones have been mapped on the northwest side of Imperial Valley. There are only a few traces, however, which indicate the location of these faults on the flat, alluviated sur- face of the basin. Gravity surveys across the depres- sion (.Kovach, 1962, p. 2869-2870) have not been par- ticularly successful in demonstrating the continuity of major fault zones within the depression, although grav- ity and seismic data confirm the presence of large ver- tical displacements beneath the valley alluvium. The most striking surface feature of the Salton thermal area is the presence of five volcanic domes forming a five-mile-long north-northeast-trending arc which parallels the shore of the Salton Sea (figure 14). These domes, Mullet Island, Pumice Buttes (two coa- lesced domes), Salton Dome and Obsidian Buttes, are spaced at equal distances along the arc and rise about 100 feet above the surrounding alluviated surface. The domes are composed primarily of rhyolitic lavas, pum- ice, and obsidian. Both a positive magnetic anomaly (Kelley and Soske, 1936) and a positive gravity anomaly (Kovach, 1962, figure 4, p. 2850) have been found distributed symmetrically about the southeast side of the volcanic arc (figure 14.) The fact that these geophysical anom- alies coincide with a thermal anomaly, as well as with the position of the volcanic domes themselves, suggests that the dense magnetic mass underlying the area may be a cooling intrusive body from which the domes have been extruded. There is also evidence suggesting that buried intru- sive bodies may exist to the southeast of the Salton domes. A small positive gravity anomaly is located ap- proximately 12 miles southeast of the volcanic domes in an area just north of Brawlev (figure 14). In 1945, Amerada Petroleum Company drilled their Vevsey No. 1 well in the SE'/ 4 , sec. 9, T.13S., R.14E., which is on the northwest flank of the gravity high. The well was drilled to 8,350 feet and abandoned because of the lack of oil indications. Although no steam was produced from this well, a temperature log indicated 138°C at 5,500 feet. The coexistence of a thermal and gravity anomaly in this area may indicate a cooling magma body. It is unlikely that the steam field ex- tends between the volcanic domes and this small anom- aly, because the Sardi Oil Co. well, drilled in sec. 24, T.12S., R.13E., (figure 14), did not encounter particu- larly high temperatures, although the thermal gradient here is also above normal. Thermal activity Surface thermal activity is confined to several groups of mud volcanoes extending southeast from Mullet Island on a line approximately 1 mile long, and to a group of mud pots just northwest of Mullet Island on an extension of the same line. Some of these vents emit only steam, water and carbon dioxide, while others erupt a viscous mud which forms a small cone around the orifice. Water temperatures around some of the vents have been measured as high as 79°C (Rook and Williams, 1942, p. 26), but generally the temperatures are considerably lower. Most of these mud volcanoes have been covered by a recent rise in the water level of the Salton Sea. History of development The first attempt to find natural steam in this area was in 1927 when three wells were drilled about one- half mile east of Mullet Island. The deepest of these wells was 1,473 feet and, although the circulating drilling mud was heated to a temperature of 118°C, the pressure and volume of steam obtained were in- sufficient for commercial purposes, and the project was abandoned (Rook and Williams, 1942, p. 19). Although this first attempt to develop natural steam was unsuccessful, it demonstrated the probable exist- ence of commercial accumulations of carbon dioxide 75-1963] Geothf.rmal Power in California 33 gas. In 1932, a well was drilled on the southeast side of Salton Dome for the purpose of exploring for car- bon dioxide. At a depth of 1,054 feet drilling was dis- continued because the high temperature in the well made it impossible to handle the drill pipes when "coming out of the hole"; however, carbon dioxide was found at 310 feet. A second well was drilled about 2 miles east of Mullet Island to a depth of 750 feet where 99.1 percent carbon dioxide gas was found in considerable quantity (Rook and Williams, 1942). This was the discovery well of a field which was de- veloped continuously from 1932 to 1954 and which produced over 2 x /i billion cubic feet of carbon di- oxide gas. Gas was obtained from wells drilled over an area approximately 3 miles long and 2 miles wide (figure 14), and principally from depths ranging between 200 and 700 feet. Addition of irrigation water to the Salton Sea in the early 1950s resulted in a rise of the lake level and was an important factor in forcing the abandonment of the field in 1954. In late 1957 and early 1958, Kent Imperial Oil Co. drilled a wildcat well, the Sinclair No. 1, to a depth of 4,720 feet in NE % sec. 10, T. 12 S., R. 13 E. (figure 14). Production casing of 3 x /i -inch tubing was set to a depth of 4,692 feet and perforated with 4 holes at 3,310 feet. Instead of oil, however, steam and hot water were produced. A temperature log, taken several months after the well has been shut-in, re- corded a temperature of 294° C at 4,600 feet. In 1960 the Imperial Irrigation District granted a long term lease to O'Neill Geothermal, Inc. of Mid- land, Texas, for the purpose of developing natural steam resources in the Salton Sea thermal area. Production O'Neill's first well, Sportsman No. 1, was drilled in Januarv and Februarv 1961 near the center of sec. 23, T.11S,, R.13E. (figure 14), to a depth of 4,729 feet. The formations encountered in the well are shown in the following table: Depth (in feet) Formation to 1,685 Borrego-Brawley, Pleistocene to Pliocene, nonmarine 1,685 to 3,485 Imperial, upper Miocene, marine 3,485 to 3,805 Alverson Andesite, upper Miocene, volcanic 3,805 to 4,729 Split Mountain, middle Miocene, nonmarine TEMPERATURE, °C 100 200 300 The 5!/ 2 -inch OD production casing was perforated in the Split Mountain Formation with two shots per foot from 3,980' to 4,100 / ; from 4,140' to 4,250'; and from 4,560' to 4,720'. During a three day testing period 56,000 lb/hr steam and 258,000 lb/hr concentrated brine were produced from the well at 200 psig and 199° C. (tested by C. F. Braun & Co., Alhambra, Calif.). Figure 15 shows the thermal gradient measured in Sportsman No. 1. Temperatures were recorded with 5000 Figure 15. Thermal gradient of Sportsman No. 1 well, Salton Sea thermal area. a maximum-reading thermometer after the well had been static for approximately 30 days. Although the thermometer was graduated only up to 500° F, extra- polation of the smooth curve indicates a bottom hole temperature of 340° C (643° F), the highest tempera- ture yet reported from a well drilled for natural steam. A representative analysis of the brine from Sports- man No. 1 is given in the following table. Constituent ppm S1O2 5 .0 Fe ._ 4200.0 Ca 3 4470.0 Mg 18.0 Na 70000.0 K 24000.0 Li 1 49.9 CI - _ 201756.7 B