Modeling on Enhanced Gas Recovery and Evaluation CO₂ Sequestration Capacity Under Different Mechanisms in Shale Gas Reservoirs

Carbon capture and storage in depleted shale gas reservoirs offer an opportunity to utilize CO₂ for enhanced gas recovery while providing access to fossil fuels.

To evaluate CO₂ sequestration coupled with enhanced gas recovery (CO₂-EGR-CCUS), we have developed a model that takes into account all the major contributing mechanisms of enhanced gas recovery and CO₂ sequestration in shale gas. The CO₂-EGR-CCUS process is divided into pe-riods of primary production, CO₂ huff and puff, and CO₂ sequestration. Firstly, a dual-porosity, dual-porosity-dual-permeability model was built for a shale gas well in Weiyuan Field, the production history was matched, and the depletion production was predicted. Then, the CO₂ huff and puff scheme was optimized to maximize gas recovery, including CO₂ injection timing, injec-tion rate, injection time, soaking time, and production times. After that CO₂ sequestration mass at different times under different mechanisms was calculated, including adsorption sequestra-tion, structural sequestration, residual sequestration, dissolution sequestration, and mineral se-questration.

This investigation shows that 17.11% percent of the injected CO₂ can be sequestered in shale while providing 4-13% incremental gas recovery. The main CO₂ storage mechanisms in this shale gas reservoir for the long term is adsorption sequestration(73.73%), followed by structural sequestration(22.68%), dissolution sequestration (0.98%), mineral sequestration(0.62%), and re-sidual sequestration(1.99%), while the dissolution sequestration and mineral sequestration mass will increase with time.

This study has made made a number of significant contributions to the field of evalution CO₂ sequestration capacity for unconventional resources, including evaluate CO₂ sequestration ca-pacity under different mechanisms, and identify the main CO₂ storage mechanisms during CO₂ enhanced gas recovery in shale gas reservoirs.

To address the shortage of conventional energy and meet international commitments to "carbon neutrality", the development of unconventional energy sources is of great stra-tegic significance for adjusting the domestic energy structure, ensuring energy security, and reducing carbon emissions ^[1]. Unconventional energy mainly refers to oil and gas re-sources including tight oil, shale oil, tight gas, shale gas, and coal bed methane. Shale gas, as a clean energy source, has good development potential and value ^[2], and China has abundant shale gas resources ^[3], mainly concentrated in the Sichuan Basin and the Ordos Basin ^[4]. The Sichuan Basin, as the main area of shale gas resources in China and one of the most successful development areas, mainly distributes in the east and southwest. By adopting the concept of "pressure-controlled production" for Silurian-Ordovician shale gas, the average ultimate recoverable reserves (EUR) per well has been increased from 0.9×10⁸ m³ to 1.2×10⁸ m^{3 [5]}.