A New Method of Forecasting the Thermal Breakthrough Time During Reinjection in Geothermal Reservoirs

Success of a reinjection process depends on the method used for estimating the thermal -ugh time. Estimating the thermal breakthrough time may be based solely on tracer data. Alternatively. it may be estimated solely by using thermal interference tests. While the tracer method usually yields ambiguous estimates, thermal interference tests are infeasible because durations of these tests are similax to thermal breakthrough times. Estimation of the thermal breakthrough time must be based on both tracer and thermal data. A new method of forecasting the thermal bnakthrough time during ninjection was developed by usmody practiced in many liquiddominated geothermal fields worldwide. Most of the time the objective of the reinjection is to dispose of the waste water[9.10], since it usually contains silica and toxic m i n d s such as arsenic, boron and mercury[3]. Reinjection of the waste water is also used to maintain the reservoir pressure[9,10] and to enhance the mergy recovery[l!5l. Regardless of the objective, however, the low temperature of the waste water is a serims constmint upon the reinjection. Many field experiences have shown that the reinjected water may 1 move through the fractuxw to the production zones in a very short time. The rapid migration of the reinjected water is undesirable, because it can produce ermal drawdown at the production wells. This theral drawdown has two detrimental effects. First, it reduces the discharge enthalpy causing the steam discharge rates to decline. Second, it decreases the total production because of the increasing hydrostatic of the fluid in the well[lO]. We ean avoid a of a fracture located in a porous matrix is used, the thermal breakthrough time is determined by two parametcrs, the water transit time t , and the parametool most commonly used to identify these fast 5ow e thermal front[lq. However, Pruess and pmposed thermal intcrfemce tests to make reliable estimates of the thermal characteristics of fast fiow paths. They reported that these tests had been car-