INVESTIGATION OF PRESSURE TRANSIENT ANALYSIS METHODS FOR CO2-RICH GEOTHERMAL RESERVOIRS

Abstract

Many geothermal reservoirs contain considerable amounts of non-condensable gases (NCG), particularly CO2. Thus, the analysis of geothermal well test is often complicated by multi-phase and multi-component effects as pressure transient analysis methods are mainly developed for single-phase flow and slightly compressible fluids.
This work aims to investigate the suitability of commonly used pressure transient analysis methods under various phase and component conditions. A series of numerical models were constructed for a geothermal reservoir containing a mixture of water and CO2 incorporating the production/injection record and the corresponding pressure response. The pressure response of the numerical models was analysed using commercial software to generate the pressure derivative plot and to estimate the reservoir properties, and explore the accuracy of the interpretations. The parameters used to analyse the pressure transient tests were also subjected to manual sensitivity analysis to identify their effect on the characteristic appearance of the pressure derivative plot shapes for realistic interpretations of the reservoir characteristics.
The results obtained show that conventional analytical methods using the pressure derivative can be reliably used to some extent in CO2-containing reservoirs. However under different multiphase conditions, well test derived properties should be interpreted with great caution.