Pressure Interference Data Analysis for Two-Prase Geotheryal Reservoirs

Abstract

Pressure interference tests are extremely valuable for establishing reservoir connectivity and for determining interwell transmissivity, but planning, executing and interpreting pressure interference tests in two-phase geothermal systems poses special difficulties. Since the effective "pressure diffusivity" for a two-phase system is much smaller than for a single phase system, it will take a long time to propagate pressure signals through a reservoir which is two-phase throughout. Consequently, a pressure interference test in a system which is initially two-phase may well be impractical. A more tractable situation occurs when a singlephase reservoir evolves into a two-phase system as a result of fluid production during the test. In this case, a boiling front propagates outward (during drawdown) from the producing wellbore(s); for practical purposes, the boiling front may be treated as a constant pressure boundary. If the initial reservoir pressure is high enough, and if the two-phase region created during the drawdown phase is not too extensive, the entire reservoir will return to single-phase conditions sometime after the cessation of fluid production (see Garg and Pritchett, 1984). In this paper, we consider a single phase reservoir which evolves into a two-phase system as a result of fluid production. Our goal is to examine the character of the pressure signaL to be expected at the observation well, and to develop practical methods for the analysis of this pressure signal to yield reservoir transmissivity and compressibility despite the two-phase effects.