Study On The Law Of Fracture Crack Extension In Tight Sandstone Reservoirs

In the Longdong region,tight oil is widely distributed in Chang-7 group with a preliminary evaluation of 20×10~8t.With the acceleration and deepening of energy resources exploration and development,oil resources in Chang-7 have become to the most important backup resources for the oil and gas increasing production.Tight sandstone reservoirs are usually characterized by low porosity and low permeability,so staged fracturing of horizontal wells technology is very important for the development of such unconventional resources.For the better performance,perforation will be formed during the process of field fracturing in the same well section to create will multiple hydraulic fractures as expectedare expected.However,the fild performance is usually not so good with,lower breakthrough in production capacity.On the one hand,because of the stress interference and reservoir heterogeneity between fractures,it is difficult for multiple fractures to extend synchronously and uniformly.This will be probably fails to form an effective fracture network communicating with natural fractures.On the other hand,large-scale and complex fracture networks are not formed during hydraulic fracturing,while the formation of fracture networks depends on the extension after the intersection of hydro-cracks and natural cracks.This paper studies the fracture propagation mechanism of multi-stage fracturing and the interaction between hydro-crack and natural crack in order to improve the optimisation effect of hydraulic fracturing.(1)Collecting data to carry out the acoustic emission Kaiser effect of stress test and triaxial compression test for measuring and analyzing rock mechanics parameters of the reservoir,rock mineral composition and natural fracture characteristics based on the long 7 region tight sandstone reservoir characters.The results show that the dominant orientation of Chang-7 fracture in Longdong area is NE-NEE direction,accounting for about70%,and the locally developed fracture orientation is NW-NWW direction.High and low Angle fractures are developed and come along with faults.(2)Calculate the brittleness index of acoustic anisotropy data from Chang-7 tight oil reservoir in well YC5 well of in Longdong area,and then solve out the shear wave time difference,rock density,the young’s modulus and poisson’s ratio through the above empirical formula,and predict the brittleness index according to the calculation formula of the brittleness index.The correlation between the brittleness indexs calculated by the well logging petrophysical method and the empirical formula prediction method respectively is good after comparison,the empirical formula is relatively accurate to obtain the brittleness characteristics of the rock,which can be used for quantitative prediction of the brittleness characteristics of tight oil reservoirs in Longdong region.(3)Combining with the geological characteristics,imaging logging fracture characteristics and rock mechanics experimental data of Chang-7tight reservoir in Longdong region,a numerical simulation model for fracture extension of Chang-7 tight reservoir in Longdong region was established by using the simulation software Abaqus.A cohesive element fluid flow model is introduced to simulate the cohesive damage mode during the process of artificial crack intersecting natural crack.The simulation results of different angles,different horizontal stresses and multi-stage fracturing spacing are in good agreement with those of large-scale physical simulation tests.The 3D static simulation model can well simulate the process of hydraulic fracture extension.(4)The formation mechanism and extension of complex fracture networks in the fracturing process of natural fractured reservoirs were studied through large-scale indoor physical simulation tests of hydraulic fracturing with tight sandstone samples from the outcrops of the reservoir.The results show that the fracture morphology is caused by the interference between fractures.Vertical extension is not affected by stratification,and fracture initiation does not strictly follow the orientation of the geostress field,which may be related to the lithological heterogeneity and borehole stress concentration effect.(5)Micro-seismic monitoring technology was used to study the fracture morphology,fracture height,fracturing reconstruction volume and hydraulic fracture extension law during the fracturing process of Chang-7section in Longdong.Field monitoring is consistent with numerical simulation and large-scale physical simulation test.When hydraulic fracturing extends to natural fracture,it is mainly affected by horizontal stress difference,the trend of natural fracture relative to the maximum horizontal principal stress,and the pressure inside the fracture.Complex fracture networks tend to form when hydraulic fracturing(fracking)fractures extend to natural fractures,stress zones,bedding,or other conditions that prevent fractures from extending along the maximum horizontal principal stress.