Research On Evolution Of Borehole Breakout In Heterogeneous Rock:Insights From Numerical Simulations

China is a major producer and consumer of oil and gas resources.Drilling technology is widely used in the exploration and production of oil and gas resources.Drilling will redistribute the original stress state of the gas reservoir,damaging the surrounding rock and borehole breakout even borehole instability,which lead to huge economic losses.A consideration of the mechanism of borehole breakout is necessary to optimize the drilling design and to ensure the engineering safety.Previous researches have made tremendous progress to study the mechanism of borehole breakout,however,no unified consensus has made due to the complicated mechanism of borehole breakout.Moreover,the rock mass was assumed as a homogeneous material in most in previous studies,in which the rock heterogeneity was ignored.In fact,the rock heterogeneity has an important impact on the mechanical properties of the rock damage and borehole breakout.In this paper,laboratory tests and numerical simulations was combined to clarify the mechanism of borehole breakout in heterogeneous rocks.The specific work is summarized as follows:(1)According to the suggestion of International Society of Rock Mechanics,the red sandstone of a mine in Hunan Province is adopted to make standard cylinder and disk samples.Its basic mechanical properties were investigated though the uniaxial triaxial compression test,triaxial compression test,and Brazilian fracture test.Meanwhile,the borehole breakout test under hydrostatic pressure was carried out by cylinder samples with a frilled hole at different diameters.The process of borehole collapse was observed via the CT scanning technology,as well as the final V-shaped breakout.(2)Using the Voronoi program of Universal Distinct Element Code(UDEC),the UDEC-GBM model is established,in which the rock is regarded as a polygonal particle polymer with a combined boundary.Based on sensitivity analysis of contact parameters the standard calibration programs are established,though which the contact parameters were obtained.The numerical results are in good agreement with that of laboratory test.(3)Considered the heterogeneity of rocks,the contact parameters of UDEC-GBM model is assumed to conform the Weibull distribution.The effects of contact stiffness,contact cohesion,and contact internal friction angle on the crack generation and macroscopicresponse were studied under different homogeneity coefficients.After the second calibration,the UDEC heterogeneity model is established assuming that the contact parameters satisfy the same homogeneity coefficient;then,the effects of the far-field stress,homogeneity coefficient,and borehole diameter on borehole breakout are discussed via a two-dimensional sandstone borehole model.(4)The RFPA software a reasonable to fully consider the heterogeneity of rock and simulate the spontaneous generation of cracks,though which a two-dimensional sandstone drilling model is established.The numerical simulation study is carried out on the influence of the borehole diameter,far-field stress,and homogeneity coefficient on the process of the stress-induced borehole breakout in heterogeneous rocks.In brief,borehole diameter reduction remarkably increases the critical hydrostatic pressure,and shifts the crack failure form from tensile to shear.Far-field stress anisotropy significantly affects the shape of the borehole breakout.Rock heterogeneity dictates the location of the preferred crack under the hydrostatic stress,which leads to local stress concentration,and determines the types of breakout around the borehole.The findings in this paper can facilitate in-depth understanding of the classic borehole stability problems in heterogeneous rocks.