Brittle Shale Fabric And Its Influence On The Mechanical Characteristics

Borehole instability during the drilling of hard-brittle shale has always been the focus and difficulty of the wellbore stability studies, at the same time, with the accelerated development of shale gas exploration, hard-brittle shale formation as the reservoir of shale gas formation, the petrophysical characteristics and mechanical characteristics has not only received attention during the drilling process, but also get more and more attention during the reservoir exploitation. The studies on the petrophysical characteristics and mechanical characteristics of hard-brittle shale at this stage are not enough to solve the problems of shale gas exploitation. Be supported by National Natural Science Foundation of China Petrochemical joint fund key projects "Basic research of low-cost and efficient drilling and completion techniques of shale gas", in this paper, on the basis of the large number of domestic and foreign literature research, four typical brittle shale formation from Northeast Sichuan are carried out systematical experimental analysis, including basic physical properties, mineral composition, microstructure, physical-chemical characteristics and strength changes after steeping in drilling fluid, and compared reservoir characteristics with hydration swelling clay shale formations and major shale reservoir of US. Through reaching, this paper summarizes the common petrophysical properties (basic physical properties, mineral composition, microstructure, physical-chemical characteristics) and mechanical characteristics and quantitative research the factors which affect mechanical characteristics. The results showed that the hard-brittle shale from northeast Sichuan has the following characteristics:(1)The porosity is less than8%and the permeability is distribution in nD, so the hard-brittle shale is belonging to the low porosity and permeability formation.(2)The main minerals of hard-brittle shale are brittle minerals(quartz, feldspar, calcite), their content are more than50%, quartz is the major brittle mineral and its content is more than40%. The clay mineral content is less than45%, illite is the main clay mineral and its content more than40%. The content of andreattite is less than30%.(3) Most pore diameter of hard-brittle shale are distributed less than10nm, they are main distributed between3.5nm and4.5nm. The clay minerals are multilayer lamellar arrangement, the samples have some relative isolated, poor connectivity pore that the pore size can reach to μm size and micro-crack witch the fracture width is μm size.(4)The cation exchange capacity and surface to volume ratio of hard-brittle shale are less than the hydration swelling shale. (5)During the deforming process, the compaction stage of hard-brittle shale is not obvious and access to the elastic deformation stage quickly, the unstable failure stage is not obvious too, when the axial stress reach to the peak value, the cores will happen brittle failure quickly.(6) The deformation-failure characteristics have no significant change after interaction with different drilling fluid.(7)Due to the high content of siliceous minerals, the compressive strength and elastic modulus of hard-brittle shale is bigger than hydration swelling shale. Because of the heterogeneity, the elastic modulus and compressive strength are distributed of a wide range, drilling fluid have greater impact on mechanical properties of hard-brittle shale with high content of clay minerals.(8)The compressive strength of different confining pressures and the tensile strength are positively correlated to the content of quartz and brittle mineral and negatively correlated to the surface to volume ratio, the cation exchange capacity and the content of andreattite, kaolinit and chlorite. The elastic modulus is negatively correlated to the surface to volume ratio and the cation exchange capacity. Quantitative relationship between fabric parameter and mechanical parameter are established by multiple linear fitting.