Study Of Shale Gas Enrichment And Pressure Increase Processes In Horizontal Wells

With the increasing global demand for energy and the gradual depletion of conventional oil and gas resources,shale gas,as an emerging unconventional natural gas resource,has attracted widespread attention.Due to its low porosity and permeability,shale gas requires horizontal drilling and reservoir fracturing for extraction.In-situ combustion fracturing technology for shale gas is a disruptive technology that has advantages such as safety and environmental protection compared to hydraulic fracturing.The process of enrichment and pressurization of shale gas in horizontal wells is a crucial step in combustion fracturing,which requires further research.Firstly,the mechanisms of shale gas accumulation,the characteristics of shale gas reservoir structure,the occurrence forms of shale gas,the flow mechanisms of shale gas,and the production mechanisms of shale gas were analyzed.Based on this,a fully coupled mathematical model for the deformation of shale matrix and gas flow was established.Meanwhile,based on the dual medium model theory,a discrete fracture model was introduced,and a physical model of shale matrix-artificial fractures-natural fractures was established.The random distribution of natural fractures was implemented using MATLAB.Then,using COMSOL Multiphysics,numerical simulations were conducted to study the process of shale gas enrichment and pressurization in horizontal wells,as well as the re-enrichment process after combustion fracturing,considering parameters such as pre-fracturing operations,initial reservoir pressure,quantities,apertures,and lengths of natural fractures and artificial fractures,and compressibility conditions.The results show that the time required to reach pressures of 2 MPa and 10 MPa in the horizontal well without pre-fracturing operations is 2 days and 38 days,respectively,while with pre-fracturing operations,it only takes 5 hours and 4 days.Lowering the initial reservoir pressure significantly reduces the rate of pressurization in the horizontal well,and the lower the initial reservoir pressure,the more severe the reduction in pressurization rate.The quantity and aperture of natural fractures have a significant impact on the rate of pressurization in the horizontal well,with more fractures and larger apertures resulting in faster pressurization.However,the compressibility of natural fractures has a negligible effect on the pressurization process in the horizontal well,but still follows the trend that higher compressibility leads to slower pressurization.Artificial fractures have a significant influence on the pressurization process in the horizontal well,with more fractures,longer lengths,larger apertures,and lower compressibility resulting in faster pressurization.However,the quantity,length,and aperture of artificial fractures are not necessarily better when larger values are used,as beyond certain values,increasing these parameters has minimal impact on the improvement of pressurization effects.The rate of re-enrichment and pressurization of shale gas in the horizontal well after combustion fracturing is slower than the initial enrichment and pressurization process.Finally,an experimental platform was designed and constructed to investigate the desorption and flow-induced pressure buildup of shale gas under different samples and confining pressures.The experimental results showed that the desorption and flowinduced pressure buildup process of shale gas can be divided into three stages: rapid pressure buildup,slow pressure buildup,and extremely slow pressure buildup.In the rapid pressure buildup stage,shale samples with natural fractures exhibited much faster pressure buildup rates compared to samples without fractures,and the pressure buildup rate of fractured shale initially increased rapidly and then slowed down,while the pressure buildup rate of non-fractured shale remained relatively constant.This pressure buildup pattern in the rapid stage agreed well with the simulated results.Moreover,as the confining pressure increased,the permeability of shale,the desorption and flowinduced pressure buildup rate of shale gas,and the maximum pressure all decreased.