Study Of Breakdown Pressure Prediction In Fractured Tight Gas Reservoirs In The Second Section Of Xujiahe Formation

The reservoir in the second section of the northeastern Sichuan Basin is the tight gas reservoir and natural fractures are very developed in the sandstone and mudstone.Hydraulic fracturing is the necessary means of production,and the characteristics of natural fracture development brings the stimulation more challenges.Therefore,it is necessary to study the relevant reservoir characteristics of fractured reservoirs and deepen the understanding of reservoir rock breakdown.In this paper,the geological conditions of the second section of the northeastern Sichuan area were firstly summarized,and the corresponding experiments of the rock mechanics were carried out on the cores from the target reservoir.In view of the characteristics of the natural fractures,the influence of the natural weak planes on the rock mechanics was studied,and the various failure modes of rock samples in indoor compression experiment were analyzed and summarized from multiple angles.The stress characteristics of the reservoir were determined by laboratory experiments,and the development of natural fractures was evaluated by core observation,thus the basic characteristic parameters of the reservoir were obtained.Then,considering the stress concentration caused by drilling and perforation,the stress caused by the internal pressure of the liquid and the additional stress caused by the infiltration of the fracturing fluid from the perforation,and especially considering the additional stress caused by the infiltration of the fracturing fluid from the natural fracture surface,the calculation models of the stress field distribution of the bullet hole orbital surrounding rock were established,and the stress field was calculated and the corresponding influencing factors were analyzed by using the basic characteristic parameters of the reservoir.According to the established stress field distribution models,different fracture criteria are selected for different fracture modes,and the calculation models of breakdown pressure for fractured reservoir were established,and the calculation of fracture pressure and the analysis of relevant influencing factors were carried out.Finally,the breakdown pressure prediction model proposed in this paper was applied to the field fracturing well from the target block and compared with the site construction data.The following points can be concluded from the work done in this paper:(1)The porosity and permeability of the sandstone in the second section of the northeastern Sichuan area are low,and the reservoir is tight,with abnonnal high pressure.Natural fractures of high dip angles are developed in the reservoir,and the azimuth angle is basically the same with the maximum principal stress direction.In the triaxial compression experiment,the existence of weak surface makes the rock failure modes more complicated,so a variety of failure modes could possibly occur.(2)The variation of the stress is mainly concentrated in the range of 1～3 times of the borehole/bullet hole radius.With the distance from the borehole/bullet hole increasing,the three-way normal stress of the rock gradually stabilizes.When the perforation depth is different,with the depth increasing,the changing trend of the stress of the orbital rack is basically unchanged,but the variation range of the numerical value is reduced.As the fracturing fluid is leaked from the fracture,the pore pressure of the rock around the fracture increases correspondingly,resulting in an additional stress field,and the elliptical distribution of the stess decreases from the frature center.(3)There are three kinds of failure modes in the fractured reservoir:breakdown from the rock body,shear breakdown along the natural fracture and tensile breakdown from the natural fracture.For the case of breakdown from the rock body,the optimal perforation orientation is the maximum horizontal principal stress direction,the hydraulic fracture initiates from the root of the hole,and the position is the vertical direction.According to the basic situation of the reservoirs in the second section of the northeastern Sichuan Basin,if the bullet hole intersect with the natural fracture,the calculation results show that the maximum possibility of the hydraulic fracture initiation is the shear breakdown along the natural fracture.(4)Compared with the field construction data,the model proposed in this paper can effectively predict the failure mode and breakdown pressure in the second section of northeastern Sichuan,and it is of propagation value for field application and the follow-up research.