A Reservoir Assessment of The Geysers Geothermal Field
Authors: Stockton A.D. , Thomas R.P. , Chapman R.H. 
Published: December 2007

Summary
Big Sulphur Creek fault zone may be part of a deep-seated, wrench-style fault system through which hydrothermal fluids from deeper horizons rise to relatively shallow depths. Upon cresting, hydrothermal fluids flow laterally and downward toward the reservoir's flanks where the fluids may drain back to the deep horizon. The areas where cresting occurs are shown as five distinct anomalies on the contour map of the top of the first reported steam entry. The flanks may be defined by the - 1500-m [-4,921-ft] contour and by several marginally producing geothermal wells. Field extensions are expected to be producing geothermal wells. Field extensions are expected to be on the southeast and northwest.
Electrical resistivity and audio-magnetotelluric anomalies evidently are caused by the local presence of hot water or a zone of hydrothermally altered rocks. Gravity, P-wave delays, and heat flow data may suggest the presence of an underlying heat source or magma body. presence of an underlying heat source or magma body. Microearthquake activity in the area of the reservoir may be caused by tectonic stress release resulting from fluid withdrawal and/or injection. The reservoir size and the possible recharge and production lifetime of the steam possible recharge and production lifetime of the steam reservoir may be estimated by judicious use of gravity, geodetic, and microearthquake surveys. Repeated precision gravity measurements taken over a 3-year period precision gravity measurements taken over a 3-year period while the reservoir was being produced indicate that large gravity changes are occurring. These changes imply a mass (fluid) loss equal to the mass produced and that no recharge of the reservoir is occurring. Also, maximum subsidence, gravity, and pore pressure decline overlap each other in the producing area. Seismic reflection data may be used in identifying fracture zones serving as conduits for steam.
Production wells drilled into the steam reservoir average one to seven steam entries per well. Wells produce slightly superheated steam at flowing wellhead pressures of 830 to 1030 kPa [120 to 150 psig]. The steam contains less than 1% noncondensable gases, mainly CO2.
Production decline curves are used mainly to determine the number of infill wells required to maintain a constant rate of steam production into power plants. Other studies in the older production areas of the field indicate that reservoir pressures decline over time.
Water injection is very important in sustaining and extending field life because little or no water influx or recharge is indicated. More than 20% of the produced steam is condensed and injected into the reservoirs. Typically, injection wells are completed below the main production interval to prevent liquid water breakthrough production interval to prevent liquid water breakthrough into the producing wells.
The approximate bulk-volume of the reservoir rock is calculated at 257.7 × 10 m3 [9.1 × 10 cu ft]. At the current generating capacity of 930 MW electricity, the estimated life of the reservoir is 129 years. The reservoir has an estimated life of 60 years at a maximum generating capacity of 2000 MW electricity (as of 1990).
Production and injection wells are drilled with conventional drilling fluid until the top of the steam reservoir is reached; then they are drilled with air. Mud, temperature, caliper, dual induction (surface only), and cement bond logs often are run on the wells. After the production casing is cemented to the top of the steam zone production casing is cemented to the top of the steam zone and the well is secured, a small amount of steam is allowed to vent from the well to prevent steam from condensing and quenching the well.

Introduction
The purpose of the study was to determine the nature and extent of the steam reservoir at The Geysers geothermal field. The study consists of four papers describing field subsurface geology, geophysics, reservoir assessment, and drilling technology.
The Geysers geothermal field, the world's largest dry steam, commercial geothermal resource, is about 65 km [40 miles] north of Santa Rosa, CA, and 19 km [ 12 miles] south of Clear Lake (Fig. 1). An early phase of development occurred in the early 1920's after which there was no further development. In 1955, Magma Power Co. drilled Magma 1 (Fig. 2). Then Magma Power Co. joined Thermal Power Co. to drill and test the geothermal reservoir potential. In Sept. 1960, Pacific Gas and Electric Co. installed the first power plant (Unit 1) with a generating capacity of 11 MW electricity, and, in March 1963, Unit 2 went on line and boosted the field capacity to 25 MW electricity (Fig. 3).
In 1966, Union Oil Co. of California drilled Ottoboni Federal 1 north of the Big Geysers area. The three companies pooled their leases in 1967, making Union Oil Co. the operator. By the end of 1968, there were 52 wells drilled and four power plants generating 80 MW electricity. As of Dec. 1982, there were 17 power plants constructed with a total generating capacity of 1128 MW electricity (Table 1).
JPT
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