Study On Initiation And Propagation Of Multi Cluster Fracturing In Horizontal Wells Of Tight Reservoir

The permeability of tight reservoirs are extremely low,multi-cluster hydraulic fracturing in combination with horizontal drilling is necessary to improve production.However,production data and fracturing field experiments both indicated that multiple-fracture in horizontal well are unevenly growth,and even some fractures are invalid.Based on the existing research achievements,this thesis takes advantage of rock mechanics,mechanics of elasticity,mechanics of composites,displacement discontinuity method,fracture mechanics,hydromechanics,and hydraulic fracturing mechanics to study the fracture initiation and propagation of horizontal well.The achievements and cognitions of this thesis are as follows:(1)Since the existing hydraulic fracturing initiation pressure prediction model not consider the influence of casing and cement sheath,this research combine the rock mechanics and elastic mechanics to establish a new hydraulic fracturing initiation pressure prediction model that consider the influence of casing and cement sheath.The research show that the properties of cement sheath,nature fracture and perforation scheme have influence on hydraulic fracturing initiation pressure,therefore,the perforation scheme(perforation orientation and perforation depth)should be optimized before the fracturing operation,so that the initiation pressure of each perforating cluster is not different and all the perforating clusters can initiation effectively.(2)A coupling model for simultaneous multiple-fracture propagation within a stage in horizontal well is established.In this model,stress interference among fractures and fracture width are calculated by boundary element theory,the control equation of fracturing fluid flowing in wellbore,perforation and hydraulic fracture are established based on hydromechanics and hydraulic fracturing mechanics,fracture propagation conditions and propagation direction are determined through fracture mechanics,and the multiple-fracture expansion length are calculated according to their energy release rate.(3)Based on our numerical model,we investigate the influence factors of multiple-fracture even propagation within a stage.The results show that the multiple fractures even propagation are influenced by factors such as the number of clusters,the cluster spacing,the perforation diameter and the perforation number of each cluster.The more perforation clusters or the smaller the cluster spacing,the more serious fractures non-uniform propagation.Under the condition of four clusters fracturing,it is difficult to keep multiple-fracture even propagation if only adjust perforation diameter or perforation number,it is necessary to simultaneously adjust the clusters spacing,perforation diameter,and perforation number to promote multiple-fracture uniform propagation.From the study of this paper,we can see that designer can adjust the construction parameters to make the hydraulic fracture initiation and extension effectively.Therefore,designer can adjust the perforation orientation,perforation depth,perforation number,inject rate,and fracturing fluid volume to meet the requirements of development program.