Simulation and evaluation of enhanced coalbed methane recovery by CO₂ storage in coal reservoirs with different drainage and production levels

Under the background of “double carbon” strategy, it is of great practical significance to carry out the research on carbon dioxide sequestration and enhanced coalbed methane recovery(CO₂-ECBM) in coal reservoir. In terms of the research mentioned above, China has carried out some multi-faceted pilot tests and engineering demonstrations. The existing engineering experiments show that the drainage degree of coal reservoir has an important impact on the difficulty of carbon dioxide injection, and then there is a lack of quantitative data support and evaluation of the effect of carbon dioxide injection to enhance coalbed methane recovery. By means of numerical simulation, based on the coalbed methane geological and engineering conditions of SX018-5 H well in the Shizhuangbei Block of the Qinshui Basin, China, this paper establishes the CO₂-ECBM working area of No.3 coal seam with one horizontal well and two vertical wells. Six injection displacement schemes are designed for horizontal wells to continuously inject 10 000 t of carbon dioxide within 1 year after 0,1,2,3 and 4 years, and to continuously inject carbon dioxide after 4 years production. The gas well production and reservoir performance of each scheme without carbon dioxide injection and carbon dioxide injection during the 10-year simulation time are compared. The results show that under the geological conditions of No.3 coal seam in the Shizhuangbei block in the south of the Qinshui basin, the horizontal well has higher CO₂ gas injection efficiency, the No.3 coal seam has great storage potential, and the injection of 10 000 t of CO₂ is mainly stored in the range of 60 m near the horizontal wellbore(700 m in length). The drainage degree of coal reservoir, that is, the gas/water saturation of the reservoir before CO₂ injection, has an important impact on CO₂ injection, the more fully methane is discharged from the reservoir, the more favorable it is for CO₂ injection. The CO₂ injection has a “negative” effect of increasing reservoir pressure and a “positive” effect of methane displacement on the production of adjacent coalbed gas wells. In order to achieve a better displacement effect in the CO₂-ECBM project, on the one hand, the drainage degree of coal reservoir should be increased, on the other hand, the spacing between CO₂ injection wells and coalbed methane production wells should be appropriately reduced.