Influence Of The Geothermal Reservoir In Power Production Rates
Authors: Mann Lloyd D. 
Published: December 2007

The Heber geothermal anomaly is located in the Imperial Valley of southern California. This paper gives a description of the geologic aspects of the Heber reservoir. Details of the production plan visualized for the reservoir are presented. Particular emphasis is given to the limitation and Particular emphasis is given to the limitation and performance of the reservoir at different production performance of the reservoir at different production rates. These limitations are both technical and economic. This is demonstrated by an example comparing flowing well performance to pumping well performance. The Heber reservoir appears capable of performance. The Heber reservoir appears capable of flowing sufficient fluid to support a 200-MW power development. This can be increased to near 500 MW if wells are pumped mechanically. It is concluded the methods developed for prediction of oil and gas reservoir performance can be used to examine geothermal reservoir behavior. As in oil and gas, however, each reservoir must be analyzed for its unique characteristics.

Introduction

Through the history of oil and gas production, many reservoir case histories have been published. Due to the scarcity of development projects, has not been true for the geothermal industry. The Heber anomaly in the Imperial Valley may be the first commercial hot-water project in the United States and its development may be of particular interest. In many respects, production from a geothermal reservoir is similar to production from oil and gas reservoirs. The similarities are in the areas of well drilling, well completion, and general reservoir engineering. There are, however, great differences in fluid handling techniques, required fluid volumes, and ability to use readily the produced fluid. These differences generally translate into more difficulty in achieving economic development.

The large equipment required is an area that introduces a major economic burden on geothermal development. Pipeline sizes will commonly be 30 to 40 in. and pressure separation vessels may have to accommodate flow rates up to 10 million lb/hour. This type of equipment requires large investment capital. It is generally believed that most geothermal systems will require reinjection for environmental reasons or to maintain reservoir production capacity. This requirement weakens the geothermal economic position. Lastly, whereas in the early development of an oil field, some revenue is generated from the sale of production that may be marketed in many places; production that may be marketed in many places; revenue from geothermal production must wait for the construction of a large electrical generating plant.

It is a generally accepted fact that economics influence resource size. After all economic parameters are determined, it is possible to parameters are determined, it is possible to determine the reserve contained in a geothermal reservoir. Therefore, all reserve numbers should be based on today's economic environment. A geologic study of a reservoir or area can be used to determine heat-in-place satisfactorily. It is somewhat more difficult to translate this number into a satisfactory recoverable reserve. In addition to the economics, this requires detailed long-term reservoir performance predictions. Even after reserves are known, performance predictions. Even after reserves are known, power-plant output is not known until studies are power-plant output is not known until studies are made regarding conversion efficiencies. It would be advantageous for the industry to adopt standards for reporting heat-in-place, reserves and electrical generating potential. The significance of these parameters can be emphasized by studying an example parameters can be emphasized by studying an example reservoir. Heber has been selected for this purpose.

RESERVOIR DESCRIPTION

Temperatures and Development Plan

Fig. 1 shows the Heber temperature distribution at 3,000 ft based on early well drilling. Fig. 2 is a typical temperature cross-section through three wells in the field traversing from the center of the reservoir on the left to the periphery of the field on the right. Since there is a large thermodynamic penalty for using cool fluid, a logical development penalty for using cool fluid, a logical development concept for this field would situate producers near the temperature high. At Heber, producers will be spaced equally around a 4,000-ft diameter producing circle near the center.