煤层气水平井扇形洞穴完井应力—渗透率演化机理OA北大核心CSTPCDEI

In China,coal seams are characterized by low permeability and complex geological conditions,making conventional reservoir stimulation techniques challenging and inadaptable to apply.A novel coal seam stimulation technique based on stress relief and hydraulic jet cavitation in coalbed methane horizontal wells has effectively addressed these issues.However,the mechanism and principal controlling geological factors of stress relief and permeability enhancement during cavitation remain poorly understood.In this paper,a numerical model of stress-permeability evolution in fan-shaped cavity completion of coalbed methane horizontal wells is established using the finite-discrete element method(FDEM)and considering the influences of beddings and natural fractures in coal seams.With the numerical model,the stress evolution process of rock mass and the mechanism of stress relief and permeability enhancement of reservoir during cavitation are investigated.Furthermore,the effects of different reservoir parameters(e.g.porosity compressibility coefficient,reservoir strength,weak plane strength,and in-situ stress field)on stress relief patterns are compared.The results indicate that the evolution process of the surrounding rocks involves contraction of the rock mass after cavitation,leading to stress redistribution in the reservoir,a gradual reduction in the strength of the surrounding rocks,the initiation of new fractures,and the expansion of pre-existing fractures,culminating in the formation of an excavation damaged zone and a stress relief zone.A sensitivity analysis on parameters reveals that the porosity compressibility coefficient is critical in determining the adaptability of the reservoir to cavitation,while the strength of weak planes,reservoir strength,and distribution of in-situ stress field dictate the stress evolution pattern and fracture expansion morphology of the surrounding rocks.The mechanism of permeability enhancement in the reservoir after cavitation involves the enhancement of fracture conductivity due to the expansion of fractures and the increase in matrix permeability as a result of stress relief in the reservoir.The conclusion suggests that the established numerical model takes into account the effects of formation characteristics,cavitation process and coal fractures,for the first time.The study results show the stress evolution process and the mechanism of stress release and permeability enhancement during cavitation in coal seams,providing further insights on the mechanism of stimulation through completion with cavitation in coalbed methane horizontal wells and also valuable engineering reference for the stimulation of coal reservoirs in China.