The synergistic mechanism of CO2 fracturing stimulation and storage of tight oil reservoirs using a cohesive zone model

Supercritical CO2 (SC-CO2) fracturing technology is a key approach for enhancing production and achieving carbon neutralization in tight oil reservoirs. Current SC-CO2 fracturing models primarily focus on the effect of CO2 fracturing fluids on fracture propagation, often overlooking the mechanisms by which CO2 impacts tight oil recovery during shut-in, production, and storage stages. To address this gap, this study develops an integrated model for SC-CO2 fracturing, shut-in, and storage in tight oil reservoirs based on a stress-seepage fluid–solid coupling finite element method. The model systematically determines the mechanisms underlying CO2 fracturing stimulation and storage, as well as their contributions to production, quantified through weight coefficients. This forms the basis of an integrated technology for CO2 fracturing and enhanced oil recovery in tight oil reservoirs. Key factors such as horizontal stress difference, natural fracture distribution, CO2 injection volume and rate, and shut-in duration were evaluated through numerical simulation to analyze their effects on crude oil production and CO2 storage. The results are as follows: (1) During CO2 fracturing, horizontal stress difference is the primary factor influencing the complexity and geometry of the fracture network. A smaller horizontal stress difference promotes the formation of a complex fracture network. Additionally, a critical reservoir Young's modulus exists, which optimizes the fracture network morphology for CO2 fracturing. (2) During shut-in, higher permeability and porosity enhance CO2 imbibition into the formation. Shut-in also allows sufficient time for ion exchange between the reservoir and CO2, which expands the CO2 huff-and-puff area. As crude oil comes into contact with CO2, it undergoes partial expansion and extraction, facilitating production with free CO2. (3) In the production phase, CO2 stimulates recovery through two key processes. First, CO2 dissolves into crude oil, causing expansion and extraction, which enhances recovery as crude oil is produced alongside free CO2. Second, dissolved CO2 separates from crude oil as pressure decreases during production, forming a dissolved gas drive mechanism. (4) Reservoir permeability, porosity, CO2 injection volume, and injection rate are positively correlated with both crude oil production and CO2 storage. Moreover, there exists an optimal shut-in duration that maximizes crude oil recovery and CO2 storage. The findings of this study provide critical technical and theoretical support for significantly improving recovery rates in tight oil reservoirs in China.