Optimization of CH ₄ Recovery and CO ₂ Sequestration in Yanchang Shale Gas Reservoir through Hydraulic Fracturing Design and CO ₂ Injection Strategies: A Numerical Simulation Study

Unconventional shale gas is vital for clean energy and energy security. However, its ultralow permeability leads to low primary recovery. This study investigates CO2-enhanced shale gas recovery (ESGR) in the Yanchang shale reservoir by using a CMG-GEM compositional simulator with a dual porosity/permeability model, evaluating well placements and both continuous and huff-and-puff injection methods. Findings show that continuous CO2 injection increases CH4 recovery by 7.59% over no injection. For huff-and-puff, a shorter 1 year injection period yielded the highest CH4 recovery; extending injection to 2 and 3 years caused declines of 1.93 and 3.89%, respectively. Conversely, using five injection cycles resulted in the highest cumulative CH4 recovery. Starting CO2 injection later in the production lifecycle also optimized recovery, with a 1.15% increase observed after 10 years of initial CH4 production, as it utilizes more favorable reservoir pressure conditions. Moreover, for CO2 storage, the reservoir exhibited 99.565% efficiency during continuous injection. In huff-and-puff, longer injection durations improved storage, with a 3 year period achieving 98.35% efficiency. Similarly, five injection cycles yielded the highest storage efficiency, at 99.12%. Delaying the injection start time also significantly enhanced CO2 storage, with efficiency improving from 96.38% after 1 year to 98.35% after 10 years, leveraging improved pressure dynamics over time. In addition, a sensitivity analysis confirmed that key reservoir parameters, such as matrix porosity, permeability, and pressure, significantly influence both gas recovery and storage capacity. Critical hydraulic fracture parameters, including half-length, spacing, conductivity, and bottom-hole pressure, are essential for optimizing gas flow and CO2 injection efficiency. This study applies to tight shale gas formations worldwide, offering insights into optimizing hydraulic design and injection strategies to enhance shale gas production and CO2 sequestration, supporting global carbon management and climate change mitigation.