Influence of the abandoned oil layer on heat mining in U-shape well geothermal system with ScCO2 storage considering the natural convection heat transfer

Abstract

In view of the current situation that there are large numbers of high water-cur oil and gas reservoirs, the switching of geothermal development using the available well pattern has become the effective way to reduce carbon emission in oil field. However, the influence of oil layer features (including the inclination, permeability and quantity of the oil layer) on geothermal development and the optimization measures for heat mining are still unclear. This study proposes to use the oil-extracted layers for CO2 geological storage, and construct the U-shape well geothermal system with ScCO2 storage based on the existing well pattern. Considering the fluid natural convection in porous medium with temperature gradient, the heat and mass transfer in wellbore and thermal reservoir as well as the heat transfer between pipes and reservoir are coupled, then the effect of oil layer features, wellbore insulation conditions and injection parameters on heat mining of the U-shape well geothermal system are discussed. The results show that in CO2 buried oil layer due to the low viscosity of ScCO2 and the sensitivity temperature-independent density, the natural convection of ScCO2 drives the vertical migration of high temperature heat flow, which raise the temperature gradient around the oil layer and then provide more sufficient heating for heat-carrying fluid in wellbore. Comparing the water-flood abandoned layer with the ScCO2-flood abandoned layer, the outlet temperature of geothermal system with ScCO2 storage increased by 4.63% and the heat mining rate increased by 9.49% after 10-year exploitation, and the increment increases with the exploitation duration. The increase of the inclination, permeability and quantity of the oil layers can enhance the natural convection, and raise the outlet temperature and heat mining rate. After 10-year exploitation, the outlet temperature of oil layer permeability of 90×10-12m2 increased by 13.25% compared with that of 10×10-12m2, and the heat mining rate raised by 20.23%. The outlet temperature of oil layer inclination of 15° increased by 7.93% compared with that of 5°, and the heat mining rate raised by 13.4% after 10-year exploitation. The outlet temperature of three parallel oil layers increased by 5.6% compared with that of single oil layer, and the heat mining rate raised by 7.1% after 10-year exploitation. With the increase of circulation flow rate, the outlet temperature decreases due to the time shortening of heat transfer, while the heat mining rate increases due to higher mass flow rate. For the insulation of production well, there are optimal location and length of insulation section according to the temperature gradient of thermal reservoir. And in this case, the optimum insulation length is 1500m. This research will provide technical support for the site selection and exploitation scheme design for the geothermal development in abandoned oil reservoirs with CO2 geological storage, and contribute to the carbon reduction in oil field.