Adaptive finite element–discrete element analysis for the multistage supercritical CO2 fracturing and microseismic modelling of horizontal wells in tight reservoirs considering pre-existing fractures and thermal-hydro-mechanical coupling

Abstract

Compared with conventional water-based fracturing, the supercritical CO2 (SC-CO2) fracturing technology can potentially improve the fracturing effect and gas production in unconventional tight reservoirs. To comprehend the key mechanical mechanism of this technology, some governing issues, such as the heat transfer between the injected SC-CO2 and rock matrix, multistage fracturing, pre-existing fractures, and fracturing-induced damaged, and contact slip events, need to be properly simulated via numerical approaches. However, the challenge of characterizing the complex structure of natural fractures and the physical properties of SC-CO2 that significantly affect fracturing and heat transfer in porous rock matrix have not been satisfactorily solved. To overcome the shortcomings of the conventional finite element methods that impede the automatic remeshing to fit the simulation of fracture propagation, in this study, we introduce an adaptive finite element–discrete element method and local remeshing strategy to simulate the propagation of fracturing fractures. The proposed numerical model involves the crucial governing issues of a multistage SC-CO2 fracturing, such as heat transfer, thermal-hydro-mechanical coupling, the interaction between the fracturing fractures and the embedded pre-existing fractures, leak-off of fracturing fluid, proppant transport, and gas production prediction. Based on the changes of the computed stresses, the distribution and magnitudes of microseismic damaged and contact slip events can be identified, allowing us to predict the microseism caused by fracturing. The fracture network and consequent heat transfer and fluid flow induced by slick water and SC-CO2 fracturing in engineering-scale unfractured and naturally fractured models are compared in the same manner to evaluate the influence of SC-CO2 on multistage fracturing behaviour, thermal effects, gas production, and microseismic effects. Numerical results show that SC-CO2 fracturing can improve the fracturing effect as well as increase the production rates but may not simultaneously induce additional microseismic events.