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HomeMy WebLinkAboutUnalaska Grothermal Executive Final Report 1985Pe EeBenLT teee TeON BTSeePOS ee ONA © Ol THE UNALASKA GEOTHERMAL EXPLORATION PROJECT EXECUTIVE FINAL REPORT Prepared By: Republic Geothermal,Inc. For: The Alaska Power Authority June,1985 1. THE UNALASKA GEOTHERMAL EXPLORATION PROJECT EXECUTIVE REPORT TABLE OF CONTENTS INTRODUCTION.0....ccc ccc ccc cece ecw cree cence cece eee eeeees SUMMARY....cece ccc ce cccrcccnceccece see eeesccenne eas cceaee A.Exploration SurveysS.....ccccccccccccccsscecccacsceesfee 1.Geologic Field Mapping......Cece cece cence cee eeees .2.Investigation of Thermal Manifestation Areas cece 3.Geochemical Survey......ccccccccccccccccccsccscces 4.Mercury Soil Survey.......cccccccccccscccccccscees 5.Self-Potential Survey......cc ccc cc cccccccccccecces B.Drilling Activities,Resource Discovery.......cceeeees 1.Temperature Gradient Hole Drilling to 1,500 Ft.... 2.Exploratory Well Drilling.......cccccccccccceccccs C.Resource Testing......ccccccnccccccsvccccsscccccvecece1.Test Facilities and Instrumentation cece ec eeececes 2.Flow Test Measurements.....ccccccccccccccsvccccces 3.Reservoir Fluid Composition.......cccccccccccccces 4.Interpretation of ST-1 Test Results............... D.Reservoir Delineation...ccccccccccssccsccncceveccess 1.Drilling of Temperature Gradient Hole ""A-1"....... 2.Electrical Resistivity Survey.......ccessccccccens E.The Makushin Valley Geothermal Reservoir Model........ CONCLUSIONS FOR COMMERCIALIZATION A.Estimated Electrical Generating Potential 1.Well Production Potential......ccc cece essen eces 2.Well Power Potential.....ccc ccc ccc cece severe eseees 3.Reserve Estimation.....ccccccccccccscvcessescceees B.Resource Commerctalization 1.Power Conversion Processes.......cece cece cence cece 2.Preliminary Economic Analyses........ccccccccccoes eo ke cat teaePgh.* MAKUSHIN'WELL ST#4:.10Qh rind Teer eee THE UNALASKA GEOTHERMAL EXPLORATION PROJECT 1.INTRODUCTION Geothermal Energy,in the form of naturally occurring underground steam and hot water,is now being used throughout much of the world to commercially produce electricity.Significant development of this viable alternative energy source is now taking place not only in the western United States but also in Japan,The Philippines,Italy,New Zealand,Mexico,and in eight other countries.Worldwide,the total installed electrical generating capacity that is "fueled"solely by geothermal energy 1s expanding at a 16% annual growth rate,having reached a level of 3,770 megawatts in June 1984. For comparison,the total amount of electricity now being produced in the State of Alaska is equivalent to about 690 megawatts of plant capacity. The Aleutian Archipelago,with its 46 active volcanoes,has for some time been recognized to have the same essential geologic characteristics that exist in those regions which are now producing geothermal power.One of these Aleutian volcanoes is Makushin Volcano,located on Unalaska Island at a distance of only 12 miles from the two communities of Dutch Harbor and Unalaska (Figure 1).Makushin Volcano is flanked by hot springs and fumaroles (steam seeps),and it clearly represents a potential natural energy source that could be tapped and utilized by the Unalaska Island residents. Because the State of Alaska is attempting to utilize indigenous energy sources colocated with population centers,a decision was made in 1980 to fund a geothermal exploration project on Unalaska Island.In 1981, approximately $5,000,000 was appropriated for this purpose and the Alaska Power Authority (APA)was given project responsibility.In 1982,Republic Geothermal,Inc.(Republic),was awarded APA Contract CC-08-2334 to conduct the geothermal exploration on Unalaska Island,together with its Alaska partner,Dames &Moore of Anchorage. From February 1982 through December 1984,the Unalaska Geothermal Exploration Project has advanced through three main phases.A final report of the Phase I project activities,which included data review,exploration surveys,and the drilling of three temperature gradient holes,was delivered to the APA in April 1983. Because encouraging results were obtained from the 1982 Phase I operations, Phase II activities were planned and undertaken during the summer field season of 1983.That work resulted in the successful discovery of a significant geothermal reservoir.Details of these results are contained in © the Phase II report delivered to the APA in March 1984.During this same time,a companion study and report titled,"The Unalaska Geothermal Exploration Project,Electrical Power Generation Analysis,"was completed by Republic Geothermal. Phase III project activities were subsequently designed to obtain during the summer of 1984 substantially more technical data on this reservoir's temperature and pressure,its apparent longevity for sustained production, and its apparent lateral extent (size).A summary of all the results of Unalaska Geothermal Exploration Project is provided within this Executive Report,including those results most recently obtained during the final Phase III activities. The report that follows is a compilation of numerous documents prepared by employees of Republic,Dames &Moore,and their several subcontractors. Considerable input has also been received from Alaska Division of Geologic and Geophysical Surveys (DGGS)employees,from University of Alaska staff, and from U.S.Geological Survey (USGS)scientists.Republic Geothermal acknowledges these contributions and wishes to express its appreciation for the cooperation and spirit of communication that has greatly facilitated the accomplishment of this project's objectives. 166°55° 54°00° POINT KADIN MAKUSHIN VOLCANO Mm Geologic Map and Geothermal Manifestations GEOTHERMAL MANIFESTATIONS fg}Fumarolic Area [@)Hot Spring Group 4 Warm Ground GEOLOGIC UNITS Quaternary All &Glacier Oep Makushin Volcanics Tertiary Piutonics Unalaska Formation MAKUSHIN BAY SCALE 6 1 2 : : FIGURE:1 e 3 wesbeen--ee KILOMETERS23@6ry ace crass 2.SUMMARY Project activities began with field work conducted during the summer of 1982intheMakushinVolcanoareaofUnalaskaIsland.These investigations resulted in the description of 23 thermal manifestation areas,some of which were previously unknown.Concurrently,a series of exploration surveys were conducted,including the drilling of three 1500 feet deep holes in order to determine the most promising area for the drilling of a deeper exploration well. Well ST-1 was drilled to 1949 feet during the following summer,successfullydiscoveringa317°F (158°C)shallow steam zone overlying a liquid-dominated geothermal reservoir with a temperature of 382°F (194°C).Preliminary tests Indicated that the reservoir is potentially commercial. Results from a long-term,34-day flow test of well ST-1 during the 1984 field season confirmed these results.Sustained flow of 63,000 Ib/hr was achieved through a three-inch diameter wellbore with less than two psi of pressure drawdown from the initial reservoir pressure of 497 psi.This indicates a very large permeability-thickness value for the reservoir.The well productivity index obtained during this test was approximately 30,000 lb/hr/psi. Wellbore flow modeling indicates.that 13-3/8 to 16 inch diameter commercial-size wells should be capable of flowing steam and hot water at maximum rates of between 1.3 and 2.0 million lb/hr.A single production well can be expected to produce up to about 10 megawatts of electrical power. Reservoir size calculations indicate that the reserve found is sufficient to meet the electricity needs of the island for several hundred years. Commercialization of this geothermal resource has been recently investigated in several reconnaissance studies undertaken to determine which available technology can most economically meet the present and future electrical energy needs of the community of Dutch Harbor and Unalaska.The most recent study,completed by the Alaska Power Authority itself,concludes that a geothermal power system plan,which uses diesel generators for only peaking and backup,appears to be the most economical.A detailed feasibility study has been recommended by the Authority. 3.PROJECT ACTIVITIES AND RESULTS A.EXPLORATION SURVEYS Prior to the start of this project several reconnaissance geologic surveys had been conducted on Unalaska Island.In 1919,Madden of the U.S. Geological Survey reported that sulfur deposits and fumaroles of superheated (310°F)steam are found in the summit crater of Makushin Volcano.In 1961, Drewes (and others)of the USGS published a geologic map of the island,which included the locations of two additional fumarole fields found on the vol- cano's southern flank. More recently,and in substantially more detail,Reeder and Motyka (both of the Alaska Department of Natural Resources,Division of Geological and Geo- physical Surveys)described in 198]eight fumarole fields and their accompanying hot springs located in the Makushin Volcano region.These investigations led to their preliminary conclusion that a large, high-temperature,vapor-dominated geothermal resource exists somewhere beneath Makushin Volcano. To actually confirm,delineate,and accurately characterize this suspected Unalaska geothermal resource,Republic designed an exploration program specifically appropriate for the Makushin area,taking into consideration the limited available geological data,the rugged terrain,the severe logistics, and the short field season.The final program selected consisted of 1) geologic field mapping,2)a detailed investigation of all thermal manifestation areas,3)a geochemical survey,4)a mercury soil survey,and 5)a self-potential survey. 1.Geologic Field Mapping From April 26,1982 through September 8,1982,the geology of the Makushin Volcano geothermal area was studied in the field by Republic geologists. Work accomplished during the first field season included remapping and sampling of the major lithologic units,an examination of the surface traces of fractures and faults,and,most significantly,the locating of all geothermal manifestations in the area for purposes of more thorough investigation. The results of the geologic mapping studies indicate the area is underlain by the early Miocene Unalaska Formation that consists predominantly of interbedded volcanic rocks and volcaniclastic rocks.Plutonic rocks of a gabbronorite composition intrude this unit.The overlying Pleistocene toRecentvolcanicsconsistofandesiticlavas,pyroclastics,and cinders,with the most recent volcanics being of post-glacial age.The recent volcanism on Unalaska Island is the surface expression of an underlying magma source, which is available as a heat source for a Makushin geothermal resource. 2.Investigation of Thermal Manifestation Areas The geothermal manifestations that occur at the surface in the Makushin Volcano region consist of fumaroles,hot springs,warm ground,mud pots,mud volcanoes,and hydrothermal alteration zones.Temperatures of the manifestations range from 310°F (154°C)to slightly above ambient air temperature.Of the 23 manifestation areas currently identified in the region (Figure 1),a number of these were discovered during the 1982 field work. 3.Geochemical Survey Geochemical field work was conducted by Republic during the summer of 1982 in cooperation with the Alaska DGSS.Several thermal and nonthermal waters were sampled and analyzed,and numerous samples of fumarole gases were collected and analyzed. The chemical analyses of Makushin thermal waters indicates two water types exist.The predominant type is a chloride-poor (<10 mg/1),near-neutral to acidic water that contains significant amounts of sulfate and carbonate with varying concentrations of calcium,sodium,and magnesium.The second type is a chloride-rich,nearly neutral water that is enriched in magnesium, potassium,and boron. The chloride-poor thermal waters,which are classified as acid sulfate, obtain their thermal energy from steam and conductive heat flow.The low chloride values indicate that they originate from rising steam accompanied by hot gases,and that a reservoir water is lacking.The steam has migrated from a steam cap that also supplied the fumaroles. The chloride-rich thermal waters occurring in both Glacier Valley and Driftwood Bay valley suggest that a liquid-dominated geothermal resource exists beneath the steam cap.The magnesium concentrations,stable isotope values,and surface temperature suggest the reservoir water has mixed with large amounts of groundwaters to form these hot springs.Stable isotopes of Makushin waters plot near the meteoric water line,except for the high temperature steam condensate samples.The chloride-rich water has a slight 8180 shift that confirms the mixing of groundwaters with the reservoir water. 4,Mercury Soil Survey Minor amounts of mercury (Hg)are typically contained in volcanic and plutonic rocks.The mercury is easily vaporized at moderate temperatures,migrating toward the surface where 1t is absorbed on clay minerals in thesoil.Consequently,areas of near-surface thermal activity are usually overlain by soil with anomalously high mercury concentrations. 166°55° 54°00° POINT KADIN SUGMALOAF Fess {\ AAKUSHIN VALLEY Mercury Soil Survey Results -=ppb Mascury In SoilOQ)108-252 ppb >252 ppb FIGURE:2 53°45' MAKUSHIN BAY The mercury soil survey conducted by Republic on Unalaska Island covered a 175 square mile area centered on Makushin Volcano.A total of 230 soil samples were collected and analyzed to determine the mercury concentrations. The concentration range is between 8 and 31,450 ppb Hg,with an average (mean)value of 454 ppb Hg.Contours of the data in Figure 2 are drawn at three times background (108 ppb),and at seven times background (252 ppb). The 108 ppb contour outlines six separate mercury anomalies in the Makushin Volcano region,with each anomaly exceeding 252 ppb Hg. The largest of the six mercury anomalies is situated on the top and flanks of Makushin Volcano and covers approximately 20 square miles.The five other mercury anomalies present in the Makushin Volcano region are of substantially smaller size and are of relatively minor significance.The mercury soil survey results suggest that much of the eastern half of Makushin Volcano is an excellent geothermal drilling target. 5.Self-Potential Survey Consideration of the applicability of many different geophysical techniques to the exploration of the Makushin Volcano geothermal area lead Republic to chose the Self-Potential (S-P)method as being most cost effective. The S-P survey covered approximately 48 line-miles on the flanks of Makushin Volcano.Field data was collected along traverse lines,with a normal station spacing on the lines of 200 yards.The survey results outlined two significant anomalies:a 600 mV negative anomaly centered northeast of Sugarloaf Cone;and a 500 mV negative anomaly occurring near Fox Canyon about 3 km southwest of Sugarloaf Cone. Computer modeling indicates that the most probable source of the two large S-P anomalies appears to be geothermal activity,with the flow of heat and/or fluid in the source regions generating the anomalies.Modeling of the maximum depth to the top of the geothermal source region is approximately 0.4 to 0.5 km (1300 to 1650 ft.)for the southwestern anomaly. In addition,a careful examination of the data reveals that a minor low S-P trough extends from upper Glacier Valley to Fumarole #1.This trough mayTackgreateramplitudebecauseofthepresenceoftheMakushinplutonic stock,so the lack of a major S-P signature over the major Fumarole Fields #1,#2 and #3 does not indicate the absence of a geothermal system in this area. B.DRILLING ACTIVITIES,RESOURCE DISCOVERY Analysis of all the acquired geological,geophysical,and geochemical exploration survey data resulted in the initial selection of several target areas judged promising for drilling. 1.Temperature Gradient Hole Drilling to 1,500 ft. In May 1982 drill rig equipment and related supplies were barged from Anchorage to Dutch Harbor,partly disassembled,and then airlifted in 1,400 lb.weight-limited loads up to the Makushin project site by helicopter. Temporary camp facilities had concurrently been erected,and 24-hour full-scale drilling operations were begun in early June. Three temperature gradient holes (TGH)were drilled that first summer using a Longyear 38 continuous wireline coring rig.TGH "D-1"was drilled in the Fox Canyon area about one mile northwest of the base camp,near the center of superimposed mercury soil and S-P anomalies.The gabbronorite intrusive was reached after drilling through 1,222 feet of fresh volcanics,and core drilling was continued several hundred feet into highly altered and fractured gabbronorite,down to a total depth of 1,425 feet.Temperature measurements made in TGH "D-1"after equilibration indicate that there is almost no temperature increase from the surface down to a depth of 900 feet (see Figure 3).Below that,temperatures increase at a high rate of 25 to 30°F per 100 feet,reaching a maximum of 212°F (100°C)at bottom.The results indicate that higher temperatures extst at depth,and that a relatively shallow (but deeper)hydrothermal system exists. TGH "E-1"was then drilled,near the base camp,on the NE-SW geologic fracture trend that is suggested by our lineament studies and the surface locations of Fumarole Fields #1,#2,#3 and #8.The gabbronorite was found, as anticipated,less than 100 feet below the surface,and E-1 was drilled to a total depth of 1,501 feet,still in the gabbroic pluton.The temperature profile in TGH E-1 was initially determined to be essentially conductive from the surface to total depth.Gradients in the first 1,000 feet were quite consistent and high,averaging 26°F/100 feet.At 1,000 feet,the gradient gradually decreases to 5°F/100 feet in the last 100 feet.The hightemperatureof383°F (195°C)at 1,485 feet in TGH E-1 is well within the temperature range for commercial electrical power generation. TGH "I-1"was the third and last drilled in 1982,at a location in upper Glacier Valley three miles southwest of the base camp.This drill site wasselectedtodetermineifthesuperheated(306°F)steam escaping at Fumarole #3,located 4,000 feet away (upslope),was coming from a fracture zone thatcontinuedatdepthdownintoGlacierValley,towards a developable site.TGHI-1 was drilled to 1,500 feet,and although it found evidence of an older geothermal system,the observed maximum temperature of only 186°F (80°C)indicates that it is located south of the presently active thermal region of Makushin Volcano. DEPTH(FEET)FIGURE:3 MAKUSHIN STATIC TEMPERATURE PROFILES TEMPERATURE °F 50 100 150 200 250 300 350 400 200 VAN \ IN e-1 1200 . .\ \1600 ||2000 |a AGI E 1683 10 2.Exploratory Well Oritlling The primary objective of the second (1983)project field season on Unalaska Island was the drilling of an exploratory well capable of producing geothermal fluids.As originally conceived,the project was to drill a full-size geothermal production well itn the second field season.This plan was determined to be too expensive to undertake with the funds available, therefore,a "slim"well (Makushin ST-1)was programmed and designed to be drilled to a maximum depth of 4,000 feet. Drilling of Makushin ST-1 began in June 1983 and proceeded through assorted volcanic rock types down to a depth of 136 feet,at which point the gabbro- norite pluton was first penetrated.Coring proceeded into the pluton,7-inch casing was cemented in the well at the depth of 160 feet,and the coring of a smaller diameter hole was begun.On July 18,while coring at 670 feet,the bit dropped free for one foot and all circulation was lost.Because these conditions are characteristic of a possible resource-producing interval,the zone was tested. Flow-testing this zone indicated it to be a 325°F low-pressure steam zone of apparently noncommercial magnitude.The decision was made to plug the interval with cement and to deepen the hole in search of a more productive horizon.Coring continued with only minor lost circulation noted until a depth of 1,916 feet where all circulation was lost for a brief period. Coring was continued to 1,924 feet where returns were again lost.An additional two feet were cored "blind"to 1,926 feet with the well on vacuum.Republic then decided to flow test the 1,916 to 1,926-foot interval. The results of this test were that only intermittent flows of muddy water and steam were achieved.Coring operations were continued,al?returns were again lost,and the well was cored "blind”from 1,926 feet to 1,946 feet.At 1,946 feet the core bit dropped three feet to a depth of 1,949 feet.This event prompted the decision to conduct a third flow test. At 7:45 a.m.on August 26,the wellhead valves were opened and the pressure immediately bled to 0 psig as the well unloaded muddy water and steam intermittently for approximately one hour.The well then ceased to flow and was shut in.When the wellhead pressure had risen to 25 psig,the master valves were again opened and the well flowed for one hour in a fashion similar to that exhibited during the preceding period.Maximum reading thermometers were run down the hole and:315°F was measured at a depth of 680 feet.Then two successive temperature instrument runs to 1,949 feet recorded 388°F and 395°F.The well was shut in at 4:00 p.m. On August 27 at 7:40 a.m.,the wellhead pressure had increased to 33.5 psig.The well was opened for a third time and after about one minute of modest flow,a very strong flow of steam and water continued at a visually estimatedrateof30,000 lb/hr until the well was shut in at 10:40 a.m.Following this very successful preliminary flow test,the decision was made to instal] flow-line,metering,sampling,and survey equipment for a production test. -11- C.RESOURCE TESTING Makuskin Well ST-1 now has been production flow tested two times;the first period (during Phase II)was for 4 days in early September,1983,the second period was for 34 days from July 5 through August 8,1984 during Phase III project operations. 1.Test Facilities and Instrumentation A relatively simple two-phase orifice meter and James tube were installed at the end of a flow line to measure the flow rate.Upstream and downstream orifice pressures were recorded continuously with a differential pressure flow meter.James tube lip pressure was monitored continuously and recorded at frequent intervals using a carefully pre-calibrated test quality gauge. Pressure and temperature were also recorded frequently at the wellhead and elsewhere in the system.The fluid enthalpy was determined by both the two-phase orifice pressure drop data and by estimation from the measured pre-flash flowing wellbore temperature adjusted for estimated uphole heat losses. During the first (1983)flow period downhole pressure and temperature were measured using conventional Amerada instruments modified for high temperature service.In 1984 significantly improved instrumentation was acquired and utilized.The pressure monitoring equipment was a capillary tube system which utilizes a helium-filled,volumetric chamber downhole connected by a very small diameter capillary tube with a surface recording pressure transducer.This equipment has an accuracy of approximately 0.3 psi,with a sensitivity of 0.1 psi on the transducer.The temperature measurements were obtained using a thermocouple cable system with an accuracy of +3°F and a sensitivity of +3/4°F. 2.Flow Test Measurements The first production test of ST-1 was performed in five steps consisting of: 1)initial static pressure/temperature (P/T)profile and bottomhole surveys; 2)flow until stable at the highest practical rate with bottomhole P/T measurements during the rate change;3)flow at a reduced rate until stable with bottomhole P/T measurements during the rate change;4)shut in and buildup with two pressure instruments on bottom;and 5)final static P/T surveys. The initial static P/T surveys preliminarily indicated that there is a shallow zone of underpressured (77 psig)steam at a temperature of about 325°F (163°C)extending from a depth of 300 ft.down to the reservoir's water level at 700 ft.Below that level static temperatures gradually increased to a maximum of 378°F (192°C)at 1700 ft.,with a slightly lower temperature of 376°F (191°C)and a pressure of 482 psig recorded on bottom (1949 ft.).The maximum stabilized flow rate measured was 47,000 1b/hr of steam and hot water,at a wellhead pressure of 36 psig.The flow rate was then intentionally reduced to 34,700 lb/hr by partly closing the surface valve, -12- which increased the wellhead pressure to 52 psig.Downhole P/T surveysrecordedduringandafterflowindicatedthatthefluidsproducedentered the well within the 1900-1949 ft.depth interval at a temperature of 380°F.The available Amerada instruments could not accurately measure the much smaller than anticipated bottomhole pressure changes,which were indicated to be only about 10 psi. The second (1984 long-term)production test of ST-1 was similar in basic plan to the first.The important differences were that it was conducted with extremely sensitive instrumentation and was performed over a substantially longer flow period.Initial static pressure/temperature surveys reconfirmed the location of the shallow steam zone and more precisely determined its temperature to be at 316.9°F (158.3°C)down to the steam/water interface at 810 feet (see Figure 4).Static temperatures gradually increased below that level,reaching a maximum of 390.2°F (199.0°C)at 1500 feet,with a slightly lower temperature of 385.1°F (196.2°C)and a pressure of 496.8 psig recorded on bottom at 1946 ft.The well was then allowed to flow at a 33,000 lb/hr (throttled)rate for 15 days,and then fully opened to achieve maximum stabilized flow rate of 63,000 lb/hr at a wellhead pressure of 27 psig for an additional 19 days.Temperature surveys conducted during flow periods indicated that the deep reservoir water entering the well is 382°F (194.4°C).The observed pressure drawdown during the first stage (low-rate) flow period was on the order of one psi,where,during the second (maximum rate)flow period,it was about two psi.After the well was shut in,the pressure tool recorded the buildup data for the next 17 days and showed a very small increase in bottomhole pressure. 3.Reservoir Fluid Composition The chemical analyses of the flashed samples collected tn 1984 have been combined with the physical measurements to calculate the composition of the reservoir fluid.The physical measurements indicate that a 22.4 percent flash to atmosphere was occurring during the 63,000 lb/hr rate test.The back calculated reservoir fluid's composition is: CALCULATED RESERVOIR FLUID COMPOSITION $109 277.8 mg/l Mg <0.49 =mg/] Ca 125 =mg/l Fe <0.024 mg/1 Na 1,836 mg/1 Hg <0.0002 mg/1 K 232 =mg/1 TOS 5823 mg/1 HC03 10 =mg/} $04 80.8 mg/1 C02 173 mg/1 Cl 3,180 mg/l No 11.9 mg/1 F 0.1 mg/1 H2S 1.7 mg/1 As 11.2 mg/1 Ar -158 mg/l Li 8.6 mg/1]Ho 024 =mg/1 B 44.7 mg/]CH4 .008 mg/) Br 14.0 mg/1 _He 001 mg/1 Sr 2.7 mg/1 -13- 121333NIHld30FIGURE:4 MAKUSHIN WELL ST 1 Temperature Survey Results TEMPERATURE°F 310 320 -330 340 350 360 370 380 390 400 400 SS ,LOWING TEMPERATURE (July 20,1984) 600 N N STATIC TEMPERAT URE (July 3,1984) =y L-in -_e-oeoa. \q t t f 362.1 06 385.1 AGI E 1789 IQeuicad £/18/86) The stable isotope analyses suggest that ground waters originating on theflanksofMakushinVolcanopercolatedownwardtobecometheliquidwithin the geothermal reservoir.The isotope values clearly indicate that seawater isnotacomponentinthereservoir.The 5180 shift towards heavier values shows that hydrothermal alteration of the reservoir rock 1s occurring.This rock-water interaction allows exchange of oxygen isotopes between the water and the silicate minerals. 4.Interpretation of ST-1 Test Results In ST-1 produced fluid enters the wellbore at the bottom of the well,near 1,950 feet,at a temperature of 382°F,which is slightly lower than the 385°F static temperature in the wellbore at that level,and 8°F less than the maximum of 390°F observed at a depth of 1500 feet.This indicates that the slightly cooler water is entering the well through a fracture zone that connects the well with the reservoir.During shutin,the reservoir water is restricted from flowing up the wellbore,allowing the well to heat up to its observed static condition.This means that the fluid density within the wellbore column lightens over a period of time as it returns to its static temperature level,and thus reduces the measured pressure buildup. The maximum stabilized flow rate of 63,000 Ibs/hr by a resource at 382°F is the maximum physically attainable from a well of the small (3 inch) dimensions of Makushin ST-1.The calculated productivity index of 30,000 lb/hr/psi indicates that the Makushin reservoir is extremely productive. This point is discussed further in the "Estimated Electrical Generating Potential"section of this report. -15- D.RESERVOIR DELINEATION The 1983 discovery of a significant geothermal resource by well ST-1 dictatedthatconsiderableeffortbemadeduring1984tofurtherdelineatethe boundaries of that reservoir,in particular its northeastern boundary,inhopesthatpotentialdevelopmentsitescouldbeidentifiedclosertoDutch Harbor.Two operations were conducted;a fourth temperature gradient hole was drilled,located 1.2 miles north-northeast of ST-1,and a ground-surface resistivity survey of 1]square mile size was made.This survey covers the vicinity of ST-1 and the area extending 4 miles northeast of this well,down into lower Makushin Valley. 1.Drilling of Temperature Gradient Hole "A-1" The location of TGH "A-1"was controlled by the objective to drill the most promising resource site to be found on the relatively easily accessible plateau region near Sugarloaf Cone.This plateau is separated from ST-1 by two deep tributary canyons of the Makushin River,the significance being thatbyavoidingtheircrossing,the costs of road construction to resource development site could be materially reduced.Consequently,TGH "A-1"was sited at the plateau location closest to ST-1 and still within the same NE-SE structural-resource trend that includes most of the Makushin volcano thermal manifestations. Drilling rig equipment stored in Dutch Harbor over the winter was mobilized by helicopter to the "A-1"drill site and drilling began on June 22,1984. About 100 feet of young surface volcanics were cored through,below which are 1500 feet of Unalaska formation Miocene flows and volcanic agglomerate units above the Makushin gabbronorite.At the final drilled depth of 1867 feet the hole was still in gabbronorite. A temperature survey acquired on October 8,40 days after completion, indicated that the bottomhole temperature was 368°F.This is almost equal to the equilibrated temperature results obtained in both ST-1 and TGH "E-1". However,no evidence of effective fracturing exists within the 265 feet of gabbronorite in the bottom of "A-1",meaning that if a significant geothermal reservoir is present at this site,it must lie still deeper. 2.Electrical Resistivity Survey An "E-SCAN"electrical resistivity survey was carried out in the Makushin Geothermal project area during the summer of 1984.Approximately 11 square miles were surveyed,covering all of the potentially developable area within the upper Makushin Valley,and extending down into lower Makushin Valley. The survey examined in detail the area known to be underlain by the geothermal reservoir discovered by ST-1,and was used to obtain evidence as to the location of that reservoir's boundaries.In addition,a search was made for any evidence that a geothermal reservoir is present within the physically more accessible areas located northeast of ST-1,toward Dutch Harbor. -16- The survey results reveal that the gabbronorite geothermal reservoir rock inST-1 has an apparent resistivity of about 50 ohm-meters.In lower Makushin Valley,4 miles to the northeast,the same intrusive rocks are ten times more resistive,indicating significantly less hydrothermal alteration,lower temperatures,and the probable absence of a geothermal reservoir at reasonably exploitable depths. An examination and interpretation of the resistivity data for all of the area in between,and in all directions around ST-1,clearly shows that the southern and western limits of the known geothermal reservoir extend for at least a mile from ST-1 in those directions,beyond the surveyed area.To the southwest of ST-1 resistivity values for the 200-1000 meter (650-3300 ft.) depth interval are in the range of 30 to 100 ohm-meters. There is now considerable evidence that the ST-1 reservoir extends only a few hundred feet to the north of the well,where resistivity values abruptly increase to 300 ohm-meters (see Figure 5).This increase is attributed to the presence of a south-dipping fault boundary separating the two areas ("A" and "C"),with extensive hydrothermal alteration and the shallow geothermal reservoir limited to only the south (low resistivity)side.The reservoir's eastern limit is positioned only 1000 feet east of ST-1 and is represented by a near vertical fault boundary,separating Area A from Area B (Figure 5). Based on the interpreted results of both TGH "A-1"and the resistivity survey,it is now recommended that development plans anticipate future production and injection wells will be drilled in relatively close proximity to ST-1,and not (unfortunately)significantly closer to Dutch Harbor.This recommendation could ultimately be proven to have been overly conservative, in unnecessarily restricting the area now considered potentially available for resource development.It 1s,however,judged to be the most responsible interpretation that can presently be made,in light of all the data available. -I7- *REPUBLIC GEOTHERMAL INC. UNALASKA=\GEOTHERMAL EXPLORATIONPROJECT UNALASKA ISLAND,ALASKA _p.E-SCAN RESISTIVITY SURVEY ":MAKUSHIN VOLCANO AREA JULY,AUGUST,1964 me SUMMARY OF SUAVEY FINDINGS, PRELIMINAAY INTERPRETATION COMPLETED. KEY TO REFERENCES IN REPOAT RESISTIVITY CONTACT OA FAULT IMPLICATION DEAIVED FROM SHALLOW (<700 METERS) DATA ONLY.NO STRUCTURAL INFERENCES FROM DEEPEA DATA ARE SHOWN. +SUAVEY ELECTAOOE SITE :\ e DAItL HOLE SITE rrvity byenehe:KILOMETERS ,0 0.8 '15L THOUSAND FEET 2 3 #4 1 i ||}| PREMIER GEOPHYSICS INC. VANCOUVER,CANADA FIGURE:5 E.THE MAKUSHIN VALLEY GEOTHERMAL RESERVOIR MODEL The heat source available for any Makushin geothermal system is the buried, partly molten,magma body that is genetically related to the Makushin volcanic suite.This heat source was still molten after the last glacial period,approximately 3,000-4,000 years ago,and the 14 historic eruptions of Makushin Volcano suggest that molten or semi-molten rock is likely to still exist beneath the mountain.. The mechanism for enthalpy transfer from this heat source to a Makushin geothermal system 1s dominantly conductive heat flow.This is indicated by the stable tsotopes of the thermal waters,the isotopic composition of steam from Makushin's summit,and the carbon isotopes of fumarolic methane. The Makushin geothermal system discovered in upper Makushin Valley is a 382°F liquid-dominated resource,consisting of a simple NaCl type water of relatively low salinity having approximately 5820 ppm of dissolved solids. The reservoir waters rise upward (convect)and boil at an elevation of about 500 feet above sea level in localized open fractures to form a steam cap thatislimitedinsizeandextent.Leakage of steam from this cap feeds fumaroles and mixes with ground waters to form chloride-poor thermal waters. Reservoir waters also mix with ground waters before surfacing as chloride-rich hot springs in Glacier Valley and in Driftwood Bay valley. This Makushin geothermal reservoir is situated primarily within the Makushin gabbronorite stock at commercially exploitable depths.However,it is possible that beneath and to the west of Makushin Volcano's summit crater another reservoir or reservoirs may exist within the Makushin Volcanics or the Unalaska Formation rocks.An impermeable seal for the reservoir exists, comprised of clayey,altered volcanic rocks,and chemical precipitates which have "self-sealed"the gabbronorite,as seen in temperature gradient hole E-1 and Makushin ST-1. The specific location of the discovered Makushin geothermal reservoir within the gabbronorite stock is structurally controlled by a major northerly striking fracture zone.Contemporary seismicity and recent movement along this north-trending zone maintains the permeability of the fractures in the present-day geothermal reservoir,and in the impermeable cap along which the majority of the surface geothermal manifestations occur. There are two subparallel structures that intersect the north-trending zone and which expand the area of the Makushin geothermal reservoir.These are an inconspicuous east-west striking fracture zone that expands the reservoir beneath Fumarole Fields #3,#4,#5,and #23,and a northwest-trending fracture on the south edge of Fox Canyon that may extend the reservoir towards Fumarole Field #7.Assuming that these three fracture systems control the extent of the resource,the commercially exploitable reservoir in the Makushin geothermal area might well cover approximately 5 square miles. -19- 4.CONCLUSIONS FOR COMMERCIALIZATION A.ESTIMATED ELECTRICAL GENERATING POTENTIAL 1.Well Production Potential Well ST-1 has been demonstrated to produce 63,000 lb/hr steam and hot water at a wellhead pressure of 42 psia.That production could be used to operate a 400 kilowatt electricity generator.However,as pointed out previously, the flow rate of ST-1 is severely limited by the small (3 inch)diameter of the well.Significantly larger production rates and greater electricity generating potential could be achieved from a larger diameter commercial size well.Fortunately,computer programs are available to make such production rate predictions,based on factors determined initially by small diameter well performance. A wellbore flow model yielding wellhead pressure vs flow rate must first be validated against the measured slim hole conditions.Once a match is achieved,then wellhead pressure vs flow rate curves for various commercial-size wellbore configurations may be generated and related to appropriate power cycles with some degree of confidence. The flow simulator used in this study is a vertical,multiphase flow simulator which incorporates variable well diameter with depth,heat losses, and noncondensable gases.The "nominal”commercial well conditions arrived at were as follows: Initial Pressure Inflow Temperature Salinity CO.Content Productivity Index Wellbore Size 497 psig at 1,946 feet 382°F at 1,946 feet 5,800 ppm TDS 170 ppm 30,000 Ib/hr/psi 13-3/8 or 16 inch Using these conditions,simulator-generated curves for wellhead pressure vs flow rate were constructed for the two different wellbore sizes.Ata reasonably optimum wellhead pressure (WHP)of 60 psig for power generation from this resource,flow rates of 1,170,000 and 1,860,000 lb/hr are predicted for the wellbore sizes of 13-3/8"and 16"respectively. 2.Well Power Potential Using the above predicted flow rate data for commercial size wells,the electrical power generation potential of a single well can be estimated. Based on figures presented later in the commercialization section of this report,a commercial size well at Makushin (with,for example,a WHP of -20- 60 psig)can have an electrical generating potential of between 5,700 and14,100 kilowatts (5.7 to 14.1 megawatts),depending on its diameter and the type of power cycle selected. 3.Reserve Estimation The last step is to estimate the size of the Makushin reservoir,in terms of the amount of hot water available.That reserve estimate can then be used to predict for how many years a commercial size well could continuously produce and generate electricity. Material balance calculations for reserve estimation of systems such as the Makushin geothermal reservoir have been developed and used by a number of investigators.The initiating step is an expression providing the total isothermal compressibility (c¢). 1 (dV)T;where V=reservoir volume,p=pressure,T=time (1)c=-1 ( t V (dp) Assuming that the total compressibility of the system ts constant,Equation 1 may be integrated: V7 =ect 4P ;where Vy zinitial volume,Vo=vo lume at time T (2) ] and because the recovery in terms of reservoir volumes is defined as: r=Vo-Vq (3) Vy then a combination of Equations 2 and 3 results in: Vo-V1 =eftAp -](4) Vy The cumulative production in terms of reservoir volumes is,of course, Vo-V]and,because the fluid 1s considered incompressible,the ratio Vo-Vq Vy may be taken as: Wp W which is the ratio of the cumulative mass produced to the initial mass-in-place.Hence,Equation 4 becomes: -21- Wwe =e"t Using Equation 5,the initial-fluid-in-place may then be calculated: -] 4.06 x 107=e(6 x 10-8 x 1)_y W yielding W =6.8 x 1012 Ibs.Given the uncertainties inherent in this calculation,the value of "W"can be considered order of magnitude only. Nonetheless,assuming a single 16 inch diameter full-size production well drilled on the site of ST-1 yielding (for example)1,500,000 lb/hr,and (depending on the power cycle used)generating between 7-13 MWe,the longevity of this reservoir is extremely large.The calculated initial-mass-in-place indicates that the Makushin reservoir could deliver this flow rate for over 500 years. -22- (5) B.RESOURCE COMMERCIALIZATION 1.Power Conversion Processes The conversion of hydrothermal energy from a Makushin-type liquid-dominated geothermal resource into electric power can be accomplished by any one of the following technologically proven processes: Flash Steam In the flash steam process,steam is produced from the geothermal fluid by reducing the pressure of the fluid below the saturated liquid pressure.The steam is then used to directly power a turbine,which in turn drives an electric generator.Both single and double flash systems are available. Binary In the binary process,a low boiling point fluid,such as freon or isobutane,is passed through a heat exchanger where it is vaporized by proximity to the geothermal brine.The superheated vapor is then used to power a turbine,which in turn drives an electric generator. Hybrid In the hybrid process,part of the geothermal fluid is flashed into steam which is used to drive a steam turbine-generator.The residual fluid is then used to vaporize a low boiling point fluid through a heat exchanger.The superheated vapor produced is then used to power a second turbine-generator. Total Flow In the total flow process,all of the geothermal fluid is expanded through a mechanical device which converts both thermal and kinetic energy of the well fluid into shaft work (torque).This shaft work is then used to drive an electric generator. Figures 6 and 7 t1lustrate the relative efficiencies of the five availablepowerconversionsystems.For example,in the case of a 16"diameter well (Figure 7)at a wellhead pressure of 75 psia,the same well flow of 1,860,000 lb/hr can be used to generate 9.0 MW by using the single flash steam process, or 11.4 MW with the double flash process,or 12.0 MW with the total flow process,or a maximum of 14.0 to 14.1 MW by using either the binary or hybrid cycle process. -23- FIGURE:6 POTENTIAL NET POWER GENERATION OF A MAKUSHIN PRODUCTION WELL (With 13 3/8”diameter casing) oT120 110 =AVERAGEoman,AGE 100 pci SER 4 < SPieg%90 _ ES a ya "2” a 7 TAS WE9cofapeeLFecMATEY 7 >OAD Lh 3 <isolIFeka 20 SEE10EZAt 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 WELL FLOW (In 1000 Ibs/hr) RG!E 1684 -24-POTENTIALNETPOWERGENERATION-MW WELLHEADPRESSURE-,PSIAFIGURE:7 POTENTIAL NET POWER GENERATION OF A MAKUSHIN PRODUCTION WELL (With 16”diameter casing) yi .] LOA EZLY AVERAGE WELL.V4 I MAXIMUMFLOWRATEae2eeeeeeeeee200 400 600 800 1000 1200 1400 1600 1800 2000 WELLFLOW (In 1000 !bs/hr) -25- 2200 15 14 POTENTIALNETPOWERGENERATION-MW 2.Preliminary Economic Analyses The geothermal power process data have already been incorporated into several recently completed reconnaissance studies that investigate the apparent economic effect of developing the Makushin geothermal reservoir to supply thepresentandfutureelectricalenergyneedsofthecommunitiesofDutchHarbor and Unalaska. These studies,starting with a determination of the present load requirements and the selection of several alternative projections of future demand growth, analyze the costs of using each of the various power generation technologies available to individually meet at least part if not all of the community energy requirements.The available technologies include (presently used) diesel-powered generators,hydroelectric systems,and technically-proven geothermal systems. The most recent of these preliminary studies,completed in April 1985 by the Alaska Power Authority itself,concludes that either of two geothermal power system plans,both of which use diesel generators for only peaking and backup,appear to be more economic than the use of diesel generators alone as the source of electric power for Unalaska/Dutch Harbor.That conclusion is supported by other studies,and the report's recommendation to now undertake a detailed feasibility study is certainly justified and appropriate. -26- Main Office:11823 E.Slauson Ave.,Suite One,Santa Fe Springs,CA.90670 (213)945-3661 Land Office:1101 College Ave.,Suite 220,Santa Rosa,CA.95404 (707)527-7755 Field Office:P.O.Box 1921,El Centro,CA.92243 (714)353-4364