Staged Cracking Mechanism And Crack Propagation Law Of Supercritical CO₂ Fracturing Coal Mass

As an environmentally waterless fracturing technology,supercritical CO₂?SC-CO₂?fracturing technology has attracted considerable attention during the safe exploitation of coal resources.Fracturing coal mass with SC-CO₂ fluid can not only change the pore-crack structure of coal mass,but also achieve CO₂ geological storage.It has double effects at the same time,and has important practical significance and application prospects.At present,SC-CO₂ fracturing technology has been initially explored in the field of unconventional oil and gas resources exploitation,but the overall research still needs to be investigated further.The phase changes during SC-CO₂ fracturing,the crack propagation law,the formation mechanism of complex cracks,and the evaluation of fracturing effects have not been studied clearly.In this dissertation,the fracturing and reforming of coal mass was taken as the research theme.By using on-site investigation,laboratory tests,theoretical analysis,numerical simulation,artificial intelligence and other research methods,the staged fracturing process of SC-CO₂ fracturing and its crack formation mechanism were studied,the crack propagation law of SC-CO₂ fracturing coal mass was obtained,the crack initiation criterion considering the low pressurization rate and high diffusion ability of SC-CO₂ fluid was established,the staged fracturing numerical simulation method for SC-CO₂ fracturing was proposed,the hybrid artificial intelligence prediction model of SC-CO₂ fracturing effect was constructed,and the SC-CO₂ fracturing engineering design method based on this hybrid artificial intelligent model were given.The major innovative results are as follows:?1?The staged cracking mechanism of“jet impact+phase transition expansion”of SC-CO₂ fracturing coal mass was systematically studied,the formula for estimating the burst energy of CO₂ phase transition fracturing stage was derived,the pressure-time evolution law and the crack distribution pattern of SC-CO₂ fracturing coal mass under the coupling of temperature field,seepage field and stress field were analyzed,and the complex crack formation mechanism of SC-CO₂ fracturing coal mass was revealed.?2?The pressurization rate of SC-CO₂ fluid in fracturing boreholes and the pore pressure variation in the surrounding rock of the boreholes were analyzed,the crack initiation criteria considering the low pressurization rate and high diffusion ability of SC-CO₂ fluid was established,the calculation process of initiation pressure and initiation time for SC-CO₂ fracturing was given,and the quantitative analysis method of the shear slip distance before and after SC-CO₂ cracks intersecting with natural cracks was established.?3?The staged fracturing numerical simulation method based on fluid-solid coupling and phase transition equivalent for SC-CO₂ fracturing was proposed,the effects of in-situ stress,injection rate and other parameters on crack propagation of SC-CO₂ fracturing were revealed,the fracturing effect of SC-CO₂ fracturing stage and CO₂ phase transition fracturing stage was quantitatively analyzed,the crack propagation paths and the dynamic interactions between two opposing cracks originating from adjacent fracturing boreholes were studied.?4?A new hybrid artificial intelligence model that integrates support vector machine,gray wolf optimization algorithm,and differential evolution algorithm to predict SC-CO₂ fracturing effect was proposed,the accuracy of the DGWO-SVM hybrid artificial intelligence model was verified,the SC-CO₂ fracturing engineering design method and process based on this hybrid intelligent model were proposed,and the fracturing engineering design of the typical mine was given.There are 99 figures,14 tables and 203 references in this dissertation.