Application Of Two-Rate Flow Tests To The Determination Of Geothernal Reservoir Parameters
Authors: Rivera-R. Jesus , Ramey Henry 
Published: December 2007

Abstract
This paper deals with the application of well-established pressure transient analysis techniques to the determination of geothermal reservoir parameters. Among the pressure transient techniques available, those concerned with two-rate flow testing were chosen. A two-rate test may permit obtaining data while reducing interruption of power generation. Two-rate techniques have been applied successfully to both oil and gas reservoirs; however, no data have been published to date—to the authors' knowledge—on the application of this method to liquid-dominated geothermal reservoirs. Data from one test run on a well in the Cerro Prieto Geothermal Field are shown. The field data were interpreted by means of four different models. Three of the models produced results that agreed with each other, the fourth one produced data scatter.

Introduction
The Cerro Prieto Geothermal Field is located about 30 kilometers south of Mexicali, Baja California. As shown in Figure 1, this field is situated at the southern end of the Salton-Mexicali trough, which includes other thermal anomalies of great interest, such as Heber and East Mesa. The reservoir is a liquid-dominated system having a cap rock made of impervious plastic clays. This cap acts as a seal, keeping the hot water trapped and preventing the dissipation of heat to the surface. Figure 2 shows a schematic geological cross section of the reservoir drawn in an East-West direction. The permeable layers consist of alternating shale and permeable layers consist of alternating shale and sandstone layers resting on a highly fractured granitic basement. Basement rocks were encountered in one of the wells at a depth of approximately 2500 meters. The thickness of the cap rock varies, according to the location of the wells, from 700 up to almost 1000 meters in the portion of the field already drilled.
The first exploratory well in the area was drilled in 1961, and in 1964, four more exploratory wells were drilled. After an extensive field-test program, the Comision Federal de Electricidad program, the Comision Federal de Electricidad started the construction of a 75-MW geothermal power plant in 1968. The plant was located in the Mexicali plant in 1968. The plant was located in the Mexicali valley and named for the Cerro Prieto volcano. This plant started commercial operation in April 1973, and has been in operation since that date. The turbines operate on steam from 14 wells. The geothermal fluid is obtained as a water-vapor mixture, because flashing takes place at some depth inside the wellbore. The separated hot brine is disposed of in an evaporation pond; however, plans are being made to evaluate the feasibility of reinjecting at least part of the spent liquid.
Although well testing and pressure transient analysis techniques have been widely applied by petroleum engineers to gas and oil reservoirs (to petroleum engineers to gas and oil reservoirs (to find mean formation pressure, skin factor, and average permeability and porosity), the application of the permeability and porosity), the application of the same techniques to geothermal reservoirs encounters problems that are not commonly found in petroleum problems that are not commonly found in petroleum applications. In hot-water wells, two-phase flow and heat transfer influence the pressure response when the wells are shut-in. In addition to this, gathering of the data is difficult because of the high temperatures involved. Bottom-hole temperatures in the range of 300 to 320 degrees C are common in the Cerro Prieto Field. Until recently, there were no bottomhole pressure measuring devices that could withstand such temperatures for more than three hours. Another problem is mechanical damage due to extreme buffeting problem is mechanical damage due to extreme buffeting by the high production rates of boiling geothermal fluids. Rates in excess of 24,000 B/D are common.
Considering these facts, it was decided that the best choice for obtaining pressure-time data by means of standard bourdon-tube type pressure bombs would be short-time duration drawdown tests. These tests could be run by changing the flow rate to some predetermined value after the well was stabilized at a predetermined value after the well was stabilized at a given constant rate for some time. This type of test is known in petroleum technology as a "two-rate" flow test. It was first proposed by Russell in 1962. Further improvements in this technique were introduced by Selim and Odeh and Jones.