Geothermal Exploitation From Depleted High-Temperature Gas Reservoirs By Recycling CO₂

The geothermal resource in oil&gas field is huge,which can be fully exploited by fully using existing wells and ground facilities to prolong the economic life of the oil&gas field in the late stage of development.The produced hot water in the oil field has been widely used,but the geothermal resource in the high-temperature gas field is seemed still not been discovered.Recently,the method of the enhanced geothermal system has been proposed,recycling the heat transmission fluid to exploit geothermal energy,which is similar to the technology of enhanced oil/gas recovery.Therefore,the geothermal energy in the high-temperature gas reservoir can be exploited using this method.Water is usually used as the heat transmission fluid to exploit geothermal energy in the conventional geothermal resource,but the low permeability in high-temperature may limit its application.However,CO₂ can be as an alternative to exploit geothermal energy from the high-temperature gas reservoir due to its unique thermal properties and excellent flow capacity.Nevertheless,the development mechanism and heat recovery capacity of CO₂ in the high-temperature are still unclear.To this end,a geothermal exploitation simulation model was established based on the condition in a typical depleted high-temperature gas reservoir in this paper.The heat mining mechanism using water and CO₂ and the heat potential of depleted high-temperature gas reservoirs were analyzed using this model,and the influence analyses of different factors on the heat mining rate of water and CO₂ was also conducted to reveal their adaptability for geothermal energy exploitation from high-temperature gas reservoirs.Theses influence factors include geological parameters,injection technology parameters and rock-fluid interactions induced by CO₂ injection.Meanwhile,combined with a real depleted high-temperature gas reservoir,optimization analysis of geothermal development plans and economic analysis of geothermal power generation were conducted,which can provide some guidance for high-efficiently geothermal exploitation from depleted high-temperature gas reservoirs by recycling CO₂ in theoretical basis and engineering design.The high-temperature gas reservoir(reservoir temperature is higher than 100oC)is mainly located in medium-high geothermal gradient basins,and its depth is generally deeper than 3000 m,so its porosity and permeability are low.All the geothermal reserves in medium-scale high-temperature gas reservoirs calculated by the volume method are higher millions of standard coal,showing the high-temperature gas reservoir has huge geothermal exploitation potential.Based on the characteristics of typical depleted high-temperature gas reservoirs,a geothermal exploitation simulation model of the depleted high-temperature gas reservoir was established,which can consider the change of density,viscosity and heat enthalpy of heat transmission fluid with the variation of temperature and pressure during geothermal exploitation.Using this model,the influence of reservoir structure,reservoir physical properties and the injection-production schemes on geothermal exploitation was conducted.Simulation results show that for a gas reservoir studied at 150oC and with a volume of 1000 m×500 m×50 m(permeability is 10×10^-3?m²),the heat mining rate of water is about 2 MW and total mined heat is 2.69×10¹⁵ J,while heat mining rate of CO₂ is about23.8 MW and total mined heat is 4.83×10¹⁵ J,when the injection-production pressure is 14MPa using a single injection-production well group.According to the analyses of all the influence factors,the CO₂ should be selected to exploit geothermal energy from the depleted high-temperature gas reservoir due to his high heat mining rate,and the water can be considered as the heat mining rate only when the edge-bottom water energy is strong,the water intrusion in the gas reservoir is serious,and the reservoir permeability and temperature are high.CO₂ will react with the formation water and the rock when CO₂ is used to exploit geothermal energy from the depleted high-temperature gas reservoir,which may affect the CO₂ heat mining rate.To analyze the influence of these rock-fluid interactions,laboratory experiments and numerical simulation are carried out for the salt precipitation and CO₂-formation water-rock geochemical reactions.For the water evaporation and salt precipitation,results indicated that the water evaporation would occur during CO₂ injection,but the salt precipitation has little effect on the CO₂ heat mining rate when the formation water saturation is low.However,a lot of Na Cl would precipitate in the area near the injection well and seriously reduce the CO₂ heat mining rate when the formation water saturation is high(the CO₂ heat mining rate decreases by about 24%in this paper).The method of low-salinity water injection prior to CO₂ injection was proposed and analyzed,which can effectively prevent salt precipitation and maintain the high CO₂ heat mining rate.For the CO₂-formation water-rock geochemical reactions,results showed that the influence of geochemical reactions on reservoir physical properties and CO₂ heat mining rate is little during geothermal exploitation from the typical sandstone gas reservoir whose rock is mainly consist of quarts and silicate minerals,while the geochemical reactions would increase the CO₂ heat mining rate by 6%due to the dissolution of calcite and dolomite during geothermal exploitation from the typical carbonate gas reservoir.In general,the geochemical reactions have little effect on the physical properties of sandstone reservoirs,but have a great impact on carbonate reservoirs.The geochemical reactions usually result in mineral dissolution in carbonate reservoir and increase its porosity and permeability,which is conducive to geothermal exploitation for CO₂.According to the reservoir characteristics and well pattern in a typical medium-low permeability depleted high-temperature gas reservoir in Zhongyuan Oilfield,a target block was selected and the geological and geothermal exploitation model was built.The adaptability of water and CO₂ was evaluated and CO₂ was selected for geothermal exploitation.The injection-production well pattern and injection-production parameters were optimized,and the technical and economic feasibilities of geothermal power generation using CO₂ were analyzed.Results show that for the target block with a temperature of 120oC and size of 1500m×1000m×50m,the CO₂ cyclic injection rate is 4500 t/d and the electricity generated by the geothermal energy is 6.2×10⁶ k W?h per year after 30 years of operation.The cost of geothermal power generation is about 0.664 yuan/(k W?h),which shows that the geothermal exploitation from depleted high-temperature gas reservoirs recycling CO₂ injection has certain economic competitiveness and excellent environmental protection compared with the cost of coal and wind power generation.