Pressure Transient Analysis of Injection/Fall-off Tests in the Geothermal Reservoirs with Free CO2 Phase

Abstract

Non-condensable gases consisting primarily of CO2 are commonly found in geothermal fluids. As the expansion and compression of CO2 can occur during the well test, those conditions further complicate the analysis of geothermal well test data. Early analytical methods for pressure transient analysis (PTA) commonly assume that the reservoir contains single-phase flow and slightly compressible fluids. However, in CO2-rich geothermal fields, the pressure transient testing in the geothermal reservoir involves a complex process of heat transfer, flow of multi-phase and multi-component conditions. This study examines the response of geothermal reservoirs that initially contains the free CO2 phase to injection/fall-off tests. Numerical modelling was carried out by using TOUGH2 simulator. Key parameters in the multi-phase geothermal reservoirs such as gas saturation and relative permeability were also investigated to identify their effect on the characteristic appearance in the derivative plots. The results illustrate that a pressure decline is observed in a log-log pressure derivative plot before reaching the infinite-acting radial flow due to the dissolution of CO2 in the reservoir during single-phase pure water injection. The pressure responses were then analysed using analytical models to estimate the reservoir properties. Furthermore, the estimated reservoir properties were compared with the actual values inputted to TOUGH2 to identify variables that may mislead the interpretation of the reservoir characteristics. Thus, the results provide insights into the interpretation of PTA data from CO2-rich fields.