Geological Sequestration in Tight Reservoirs with Different Fracturing Schemes: The Weiyuan Shale Gas Field, Sichuan Basin, China

Abstract

Research on caprock safety during CO₂ geological sequestration is essential for preventing gas leakage and safeguarding the environment. This study examines the impact mechanisms of different fracturing schemes on caprock safety during CO₂ sequestration in tight reservoirs to optimize sequestration strategies and provide theoretical support. Using a developed CO₂ sequestration model, we systematically evaluate the leakage characteristics of caprock under various fracturing schemes. For the first time, this study introduces the use of the Analytically Modified Embedded Discrete Fracture Model (AEDFM) to describe the flow interactions between fractures and the matrix during long-term CO₂ sequestration. Through an in-depth analysis of critical factors such as the different mechanisms of fracture parameters, sand concentration, flow rate, and fracturing fluid viscosity, we comprehensively evaluate how these fracturing schemes influence caprock safety for a case study. Validation results confirm that the AEDFM method successfully overcomes the low accuracy exhibited by the Embedded Discrete Fracture Model (EDFM) in simulating early-time transient flow. The proposed CO₂ sequestration model demonstrates strong agreement with production history matching performed using commercial software, affirming its reliability and practical applicability. The results from a case study show that increasing the number, half-length, and height of hydraulic fractures in fracturing designs will increase the risk of CO₂ leakage, thereby compromising caprock safety. If hydraulic fractures penetrate the caprock, the risk of CO₂ leakage through the top caprock markedly increases. In scenarios where fractures do not penetrate the caprock, the volume of leaked CO₂ over a 100-year sequestration period remains below 5% of the injected gas. However, leakage can rise to as much as 12% if caprock penetration occurs. Furthermore, increasing the flow rate and sand concentration and decreasing the viscosity lead to larger fracture dimensions, further increasing the risk of CO₂ leakage and diminishing caprock safety. An investigation of various mechanisms affecting caprock safety reveals that heterogeneity-controlled preferential flow and hydrochemical reactions tend to compromise caprock safety, while capillary forces and stress-sensitive fracture behavior increase sealing capacity. This study offers theoretical insights and practical guidance for CO₂ sequestration in tight reservoirs.