Numerical investigation on heat extraction performance of supercritical CO2 in depleted oil and gas reservoirs

Using supercritical CO₂ (sCO₂) for geothermal exploitation not only improves the heat extraction rate and saves injection-production energy consumption, but also gains environmental benefit of carbon sequestration. Compared to hot dry rock, depleted oil and gas reservoirs are nature porous reservoirs harboring abundant geothermal resources, in which artificial reservoir fracturing is unnecessary prior to the geothermal extraction. Besides, a lot of pre-existing oil-gas well networks along with ground facilities can be reutilized, significantly reducing geothermal drilling costs. This paper conducts a numerical investigation on using sCO₂ for heat extraction in depleted oil and gas reservoirs. Firstly, A coupled wellbore-reservoir model is established to analyze the flow and heat transfer characteristics of sCO₂ in the reservoir and injection-production well. Secondly, the variation of sCO₂ gas saturation, production temperature, production rate and heat extraction rate are studied. Finally, the influence of different factors on sCO₂ heat extraction performance is examined. The results indicate that the maximum sCO₂ heat extraction rate is 18.45 MW, and the temperature rise of sCO₂ within the reservoir is still higher than 24 °C after 30 years. To enhance sCO₂ heat extraction performance, reducing injection temperature, increasing production pressure and well spacing are advisable. In the case of multi-well scheme, a higher ratio of production wells to injection wells and a decentralized arrangement pattern are encouraged. The findings of this paper are anticipated to provide theoretical basis and technical support for efficient geothermal harvest in depleted oil and gas reservoirs and potential appropriate option in building heating.