Interpretation of Flowing Pressure Transients to Monitor Mobility Front Movement around Geothermal Injectors

Non-isothermal injection in wells (e.g. cold water into hot formation) brings challenges to Pressure Transient Analysis (PTA). The fluid properties may show large changes with temperature, which makes interpretation of pressure transients for injection periods a moving-front problem as opposed to classical well shut-in interpretations, with close-to-stable temperature front. Therefore, the classical PTA works well for fall-off interpretations of geothermal injectors, although requiring well shut-in and additional operational cost. Interpretation of flowing well data doesn’t bring additional costs and provides information about well-reservoir performance after injection started. In this paper, we study pressure transients at (1) non-isothermal single-phase injection to demonstrate similarities and differences between injection and fall-off periods; (2) multi-rate injection to estimate mobility front movement using PTA. Numerical simulations were performed and interpreted with analytical models using an industry-standard software. The study has shown that pressure transients for flowing periods are different from shut-ins, where the moving mobility front at injection is reflected with two transitions between low- and high-mobility derivative levels, as opposed to one transition observed for shut-ins. The interpretation also provides the mobility front position, helping in assessing geothermal injection performance. These results facilitate use of real-time pressure measurements for well-reservoir monitoring during injection operations.