Monitoring Dynamic Rock Fracture Behavior after Long-Term CO2 Injection in Tight Reservoir: Semi-Analytical Model and Case Study

Long-term CO2 injection will induce natural fractures to open and form an induced fracture network, which is dynamic during injection and shut-in because there is no proppant in the fracture. The high flow rate in the root and finger of the horizontal well causes the induced fracture network planar surface to be not rectangular but U-shaped. Induced fracture networks, dynamic rock fractures, and U-shaped planar surfaces hinder the tight reservoir parameter monitoring around gas injection wells, even causing gas breakthroughs and production well abandonment. Here, we develop a dynamic rock fracture monitoring (DRFM) model to describe the induced fracture network and dynamic rock fracture behavior. U-shaped flow, fracture closure, fracture network interporosity flow, and dynamic rock fracture effects are introduced into the mathematical model. The finite element method is used to characterize the dynamic rock fracture extension and closure. Rock mechanic functions are coupled with a seepage mathematical model to describe the CO2 seepage behavior within the tight reservoir and induced fracture network. The fracture storage effect, wellbore storage effect, and fluid pressure response are coupled using the Duhamel principle. Results demonstrate that the type curve has three innovative flow regimes including dynamic fracture linear flow with bipeak, fracture network flow with concave, and U-radial flow with horizontal lines. Five innovative parameters control these flow regimes' behaviors. Numerical simulation verified the DRFM model's accuracy. The field case shows that the DRFM model obtains a more reasonable induced fracture network and dynamic rock fracture parameters. In conclusion, the DRFM model can describe CO2 seepage behavior in induced fractures, dynamic rock fractures, and tight reservoirs. The field case shows the DRFM model's practicality. The obtained parameters can be used as a theoretical basis to guide engineers in designing appropriate gas injection programs to enhance tight oil recovery and prevent production well abandonment.