Results of Interference Tests From Two Geothermal Reservoirs

Results are presented from interference tests on two liquid-dominated geothermal reservoirs in the U.S. The collected pressure data show that interference effects can be masked by earth tides and other effects. Well known techniques of petroleum engineering and hydrogeology are used to estimate hydraulic characteristics and to infer the presence of barrier and leaky boundaries.

Introduction

An important facet in the problem of geothermal energy development is assessing the productivity and size of a geothermal reservoir. Modern techniques of well-test analysis (pressure drawdown, buildup, and interference tests) developed in petroleum engineering and hydrogeology have been applied successfully to two liquiddominated geothermal reservoirs in the U.S.: one in the Raft River Valley of Idaho and the other at East Mesa in the Imperial Valley of California. These tests gave reasonable estimates of the permeability and storage parameters for the two reservoirs. In addition, the tests also parameters for the two reservoirs. In addition, the tests also illustrate the type of instrumentation that can be used in testing geothermal wells as well as the nature of the data that can be collected.

A large body of literature is available on well testing in both petroleum engineering and hydrogeology. Ramey recently has summarized the practical aspects of modern well-test analysis.

This paper reviews the results of interference tests performed with a very sensitive pressure-measuring performed with a very sensitive pressure-measuring device on two geothermal reservoirs. As used in this paper, the term "interference test" denotes the fluid production from one well and measurement of pressure transients in a nearby observation well. The geologic setting for each reservoir is given and the instrumentation is presented. Some unusual features of the data are discussed, followed by the methods of interpretation that were used.

Description of the Geothermal Reservoirs

Raft River Valley Field, Idaho

The Raft River Valley geothermal field (Fig. 1) is located on a graben filled with Tertiary and Pleistocene sediments and volcanics, with an aggregate thickness of about 5,000 ft. The sediments rest on a basement of quartzites, schists, and quartz monzonites of Precambian age. Precambian age. Wells RRGE 1 and RRGE 2 were drilled in 1975 and successfully produced hot water at approximately 295 deg. F from a geothermal reservoir occurring at the base of the sediments at 3,500 to 5,000 ft below the land surface. Well RRGE 2 is northeast and 4,000 ft from Well RRGE 1. Both wells are artesian with wellhead pressures of about 150 psi when shut in. During construction, both wells indicated free flows of about 400 gal/min. The completion details for the two wells are summarized in Table 1.

Subsurface correlations of borehole data suggest that the sediments dip toward Well RRGE 2 with apparent northeasterly dips increasing from about 3 deg. in the upper portion to about 7 deg. toward the bottom. Also, it is known portion to about 7 deg. toward the bottom. Also, it is known that Well RRGE 1 pierced a fault zone between 3,800 and 4,500 ft. Apparently, Well RRGE 2 did not intercept any faults.

East Mesa Field, California

The East Mesa geothermal field (Fig. 2) is located on the eastern part of the Salton Trough in the Imperial Valley of California.

JPT

P. 10