Geothermal exploitation from depleted high temperature gas reservoirs via recycling supercritical CO2: Heat mining rate and salt precipitation effects

Abstract

The geothermal energy in depleted high temperature gas reservoirs can be developed using existing wells and surface facilities via recycling water or supercritical CO2 after natural gas production. For a typical medium-size gas field, the recoverable geothermal energy can be equivalent to over 10milliontons of standard coal. The injection of CO2 can improve the heat mining rate, and it can also enhance gas recovery at the early stage of the process and achieve geological storage of CO2. However, a big concern in the injection of CO2 is the salt precipitation induced by the interactions between the injected CO2 and the formation water, which might cause reservoir damage and subsequently affect the flow behavior and the heat mining rate. In this paper, a comprehensive model of geothermal exploitation from gas reservoirs via CO2 injection was established, in which the processes of formation water evaporation, salt dissolution and precipitation, and their effects on formation porosity and permeability were incorporated. The influences of various parameters on geothermal recovery and salt precipitation were investigated by using this model, including the saturation and salinity of formation waters, injection-production pressure difference, and the permeability and porosity of the gas reservoirs. The results show that, for the gas reservoir studied at a temperature of 130°C (i.e. with a volume of 1000m×500m×50m), the heat mining rate of one injector-producer pair can be maintained at about 5MW with a CO2 recycling rate of 3000t/day for 30years. The effect of salt precipitation is moderate, and it is dependent on the reservoir conditions. Especially, salt precipitation occurs severely in the near well region when the remaining water saturation is higher than the irreducible water saturation. Meanwhile, water evaporation induced by CO2 injection may cause a back flow of formation water due to the effects of gravity and capillarity, which can intensify the evaporation and increase the salt precipitation and enrichment in the region. This can cause a reduction of permeability which therefore decreases the heat mining rate. Different methods for reducing salt precipitation was proposed and evaluated accordingly, including injection of low salinity water prior to CO2 injection and co-injection of CO2 and fresh water.