Study On Mechanism Of Hydraulic Fracture Propagation In Coalbed Methane During Refracturing

Coalbed methane reservoirs have the characteristics of low permeability and low porosity,hydraulic fracturing must be carried out to obtain economic production.However,in the production process of CBM wells,the productivity decreases correspondingly due to the decrease of conductivity of initial fractures.Refracturing is an effective technology to restore productivity.With the mechanical characteristics of coal,the development of natural fractures,the strong anisotropy,the existence of initial fractures and the disturbance of early production activities,the morphologies of fractures created in refracturing(refractures)are very complex.At present,the field research is far beyond the theoretical study.The key problems such as the anisotropy of coal,the initiation and propagation mechanism of hydraulic fractures during initial fracturing and refracturing,and the main controlling factors in deflection angles of refractures have not been systematically studied.Therefore,it is necessary to study the mechanism of hydraulic fracture propagation in CBM wells according to the characteristics of coal and the strategy of refracturing,so as to increase the volume of reservoir re-stimulation and deepen the understanding of hydraulic fracture propagation in coal.In this paper,evaluation of the development and characteristics of cleats in the main coal seam in Shizhuang South of Qinshui Basin are carried out firstly.Then,the mechanical properties of coal under the influence of cleats and beddings,and the firmness coefficient of coal are studied,which will provide basic parameters for coal seam in refracturing.The complex fracture morphologies in coal after initial fracturing are revealed by the true triaxial fracturing simulation system and high energy CT scanning technology.The effects of bedding,cleat and in-situ stress on fracture morphology are investigated.Based on the understanding of hydraulic fracture morphology in initial fracturing,the experimental study on hydraulic fracture propagation during refracturing is carried out to verify the feasibility of refracturing.The influences of natural fracture such as cleat and in-situ stress inversion on refracture propagation are analyzed.The formation of refracture is confirmed according to different types of fracturing curves and breakdown pressure in the process of two fracturing,the favorable reservoir conditions for the formation of refractures are analyzed.The induced-stress field caused by initial fracture and production disturbance is calculated,and the distribution of in-situ stress near wellbore before refracturing is defined.Based on the extended finite element method,a fracture propagation model of refracturing near wellbore in porous media is established,considering the coupling effect of stress field and seepage field in porous media,the main factors of deflection angle are analyzed,and the effects of different horizontal stress difference,Young's modulus,tensile strength and initial fracture morphology and cleats on refracture are studied.Finally,the study of refracturing in field is carried out to deepen the understanding of hydraulic fracture propagation in coal seam.In this paper,the formation mechanism of refractures and the dominant factors of deflection angle are revealed through the combination of refracturing simulation experiment on natural coal and numerical simulation of deflection angle.The research results provide a theoretical basis for the formulation and optimization of refracturing strategy in coal seam.