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 CO 2 -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 CO 2 injection increases CH 4 recovery by 7.59% over no injection. For huff-and-puff, a shorter 1 year injection period yielded the highest CH 4 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 CH 4 recovery. Starting CO 2 injection later in the production lifecycle also optimized recovery, with a 1.15% increase observed after 10 years of initial CH 4 production, as it utilizes more favorable reservoir pressure conditions. Moreover, for CO 2 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 CO 2 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 CO 2 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 CO 2 sequestration, supporting global carbon management and climate change mitigation.