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.

中文翻译：

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考虑预先存在的裂缝和热-水-机械耦合的致密油藏水平井多级超临界 CO2 压裂和微地震建模的自适应有限元-离散元分析

摘要 与常规水基压裂相比，超临界CO2（SC-CO2）压裂技术具有提高非常规致密油藏压裂效果和产气量的潜力。为了理解该技术的关键力学机制，一些控制问题，例如注入的 SC-CO2 与岩石基质之间的传热、多级压裂、预先存在的裂缝以及压裂引起的损坏和接触滑移事件，需要通过数值方法进行适当的模拟。然而，表征天然裂缝的复杂结构和显着影响多孔岩石基质中的压裂和传热的 SC-CO2 物理性质的挑战尚未得到令人满意的解决。为了克服传统有限元方法阻碍自动重新划分以适应裂缝扩展模拟的缺点，在本研究中，我们引入了自适应有限元-离散元方法和局部重新划分策略来模拟压裂裂缝的扩展。所提出的数值模型涉及多级 SC-CO2 压裂的关键控制问题，例如传热、热-水-机械耦合、压裂裂缝与嵌入的预先存在的裂缝之间的相互作用、压裂液的泄漏、支撑剂输送和产气预测。根据计算应力的变化，可以识别微震损坏和接触滑动事件的分布和大小，使我们能够预测由压裂引起的微震。以相同的方式比较工程规模未压裂和天然压裂模型中滑溜水和 SC-CO2 压裂引起的裂缝网络和随之而来的传热和流体流动，以评估 SC-CO2 对多级压裂行为、热效应、产气量和微震效应。数值结果表明，SC-CO2 压裂可以改善压裂效果并提高产量，但可能不会同时诱发额外的微震事件。