Research On Multiple Fractures Propagation In Horizontal Well With Multi-stage Hydraulic Fracturing

Unconventional oil and gas reserves are abundant,which has gradually become the backbone of oil and gas production in China.And horizontal well with multi-stage hydraulic fracturing is one of the most important technologies to develop such reservoirs economically and efficiently.In the horizontal well with multi-stage hydraulic fracturing,the multi-fractures in the process of simultaneous expansion are affected by the stress interference from each other,resulting in a significant change of the fracture morphology,which seriously affects the fracturing effect and the production after fracturing.Therefore,it is urgent to master the law of propagation of multi-crack in horizontal wells with multi-stage hydraulic fracturing,and then scientifically guide the fracturing construction design,providing technical support for the efficient development of unconventional oil and gas reservoirs in China.In this paper,we aimed at simultaneous initiation and propagation of the multiple fractures in horizontal wells.Firstly,considering integratedly the factors such as induced stress field of the fracture,induced stress field of the loss fluid,fluid flow in the fracture,fluid flow in the wellbore and the criterion of crack propagation,the mathematical model of multi-crack propagation in horizontal well was established.Secondly,based on the different characteristics of the induced stress field and the fluid pressure field,the displacement discontinuity method and the finite difference method were used to solve the fluid-solid coupling.The single crack propagation analysis model and the existing multi-crack propagation numerical model were compared respectively with our model,and the accuracy of the model was verified.Then,based on the model of this paper,the variation law of fracture morphology,induced stress field and nonplanar propagation were analyzed in single fracture,and the changes of multi-crack morphology,stress distribution and flow rate distribution during the process of multi-crack propagation are studied,and the influencing factors of multi-crack propagation are analyzed.Finally,the uniformity coefficient of fractures based on the fracture morphology quantitatively calculating was proposed to reflect the uniformity of multi-crack propagation.On the basis of this,the influence of the fonnation parameters on the uniformity of multi-crack propagation was analyzed,and the fracturing construction parameters of multi-crack propagation were optimized,and field application was carried out according to the established model.In this paper,the laws of multi-crack propagation in horizontal wells with multi-stage hydraulic fracturing are studied.The main achievements and understandings are following:(1)The plane propagation of the single fracture extends along the maximum horizontal principal stress direction.The influence range of the induced stress field is mainly controlled by the minimum of fracture length and fracture height.The extension direction of the non-planar propagation in the single fracture is gradually shifted to the maximum principal stress direction,when the smaller the initial fracture azimuth angle and the principal stress difference is,the fracture morphology is more curved.(2)The lateral cracks expand rapidly and deflect the outward in the multi-crack propagation,while the inner cracks grow slowly and almost don’t deflect.The stress interfe-rence between the fractures is larger.And the flow rate of the lateral cracks is larger than that of the inner cracks.The short and wide cracks are easy to be generated under the low elastic modulus,low fracturing fluid flow,high cluster spacing of the cracks and high fracturing liquid viscosity in the multi-crack propagation.(3)Aiming at the nonuniform expansion of multiple fractures,the crack uniformity coefficient is proposed to quantitatively describe the uniformity of multiple crack propagation based on the crack morphology.The larger its value is,the higher the degree of the uniformity of multiple crack propagation.In the multi-crack propagation the smaller elastic modulus,the smaller principal stress difference,the greater cluster spacing,the smaller viscosity of fracturing fluid,the greater fracturing fluid flow,the smaller perforation hole diameter,the smaller perforation hole numbers,the crack uniformity coefficient is higher,the more uniform the cracks will propagate.(4)The fracture construction parameters are optimized by using the crack uniformity coefficient as the objective function.By increasing the diameter of perforations and the number of holes in the middle crack,the uniformity of the crack propagation can be improved.The uniformity of the crack propagation of equal cluster spacing is higher than that of the non equal cluster spacing in the homogeneous reservoir.When the parameters of each cluster are equal in 3-cluster fractures,the optimal cluster spacing of is 30-40m,and when the perforation diameter is increased,the optimal cluster spacing can be reduced to 20～30m.In this paper,the law of the multi-crack propagation in horizontal well with multi-stage hydraulic fracturing is studied deeply.The mathematical model can be used to quantitatively analyze the morphological changes of multi-crack propagation.The uniformity coefficient of fractures can optimize the construction parameters of multi-crack propagation.The research results of this paper have some guiding significance for the optimal design of multi-stage hydraulic fracturing in horizontal well.