The Evaluation Technology Of Rock Mechanics And Brittleness Characteristics For Marine High Mature Shale Reservoir

Rock mechanics and brittleness of shale gas reservoirs are important foundations for the evaluation of shale gas reservoir development potential.The high mature marine shale in southern China has large differences in organic matter content and mineral composition,complex structure and bedding development.At present,there is no systematic understanding of its reservoir rock mechanics and brittle characteristics.This paper takes the organic-rich shale of the Wufeng-Longmaxi Formation in the Sichuan Basin as an example,according to field profile and sheet observation,organic geochemical analysis,X-ray diffraction analysis,scanning electron microscopy and energy spectrum analysis,axial compression test,Brazil split test.These tests are used to analyze the material composition,microstructure and structure,rock mechanics and brittleness of high mature marine shale,combined with logging rock mechanics and brittle parameter response characteristics.The evaluation method was studied and the following results and understanding were obtained:1)From the bottom to the top,organic matter and quartz content gradually decreases,clay mineral content gradually increases,and the particle size of the quartz particles becomes coarse.In the horizontal direction,compared with the eastern region of Sichuan,the content of carbonate minerals in the southern part of Sichuan is higher,while the content of clay minerals is lower.2)When the TOC is less than 2%,the organic matter is mainly distributed in abundance between the mineral particles.When the TOC is higher than 2%,the organic matter is distributed in a living body such as a graptolite or a radiolaria,or mixed with floc and particles.With the increase of organic matter content,the quartz content gradually increases,and the particle size of quartz particles gradually decreases.When the carbonate mineral content is high,it is often distributed in a strip-like layer.In shale with TOC higher than 2%,clay minerals are arranged in layers.The distribution of crumb particles,organic matter,carbonate minerals,clay minerals,etc.in the vertical bedding direction is formed.3)In the pre-peak stage of the stress-strain curve,linear elastic deformation and elastoplastic deformation are dominant,and brittle fracture and strain softening are dominant in the post-peak stage.Under the uniaxial condition,the linear elastic deformation is dominant,and the plasticity is strong when the confining pressure is applied.Among them,high TOC and high clay mineral shale have the strongest plasticity.The higher the confining pressure,the higher the compressive strength,the greater the peak strain and the plastic strain before the peak,but the confining pressure has a lower effect on Young's modulus and Poisson's ratio.Compared with the saturated sample,Young's modulus after water saturation is significantly reduced,and the Poisson's ratio increased.4)The influence of bedding on shale mechanical anisotropy is mainly reflected in compressive strength,tensile strength and dynamic mechanical parameters.In axial compression test,the shale compressive strength anisotropy of high organic matter,high quartz,medium or low clay mineral content is obvious,and the compressive strength first increases with the angle ? between bedding plane and sample axis.After the decrease,the compressive strength has the lowest value when ? between 30°?45°;the compressive strength of other types of shale is not obvious with the bedding.In the experimental test,the textured surface often exhibits a weak surface feature of the mechanical structure,resulting in cracking along the bedding plane.In the Brazilian split test,the tensile strength along the bedding plane is significantly lower than the tensile strength at other loading angles.As the angle ? increases,the acoustic velocity decreases and the dynamic Young's modulus decreases.5)Brittleness is a comprehensive feature of elastoplastic deformation and fracture strength of rock.High brittle rock generally has the characteristics of low strain,high Young's modulus,low elasticity,low plasticity,low tensile strength,high internal friction angle and low Poisson's ratio.It is prone to rupture under external force and the rupture process is accompanied by obvious stress drop and acoustic emission events.In this paper,peak strain,Young's modulus and tensile strength are used to describe the brittleness of rock,and brittleness index B_S is established,based on deformation and strength: the smaller the peak strain,the higher the Young's modulus.The stronger the rock elasticity,the weaker the plasticity and the higher the brittleness;the lower the tensile strength,the more likely the tensile failure occurs and the higher the brittleness.6)Consider the difference of brittleness of different minerals,the influence of organic matter distribution on brittleness,the influence of differences in mineral distribution patterns of different sedimentary environments on brittleness,and the relationship between peak strain,Young's modulus,tensile strength and material composition of shale.Based on the established brittleness index B_mine6 of the material composition.The brittleness index B_mine6 has a good correspondence with the brittleness index based on rock mechanics.7)A method for logging interpretation of rock plasticity parameters is proposed for the first time,which enriched the rock mechanics logging interpretation system.According to this method,the peak deformation modulus,peak strain,pre-peak plastic strain and log interpretation of the brittleness index based on deformation and strength can be realized.The brittleness index B_S and B_mine6 proposed in this paper have effect in Wufeng-Longmaxi Formation in Sichuan Basin.The introduction of plastic parameters in logging interpretation can more accurately describe the brittle-plastic characteristics of rock.This method also has effect in the Wufeng-Longmaxi Formation in Weiyuan-changning Area,Sichuan Basin.