The Weakening Of Layered Shale Immersion Mechanics And Its Influence On Wellbore Instability

Since the new century, due to the constant adjustment of energy structure and the increasing demand for clean energy such as natural gas. In order to optimize the energy structure of our country, it is necessary to increase the exploration and development of shale gas. This paper focus on the general wellbore collapse research of shale gas horizontal well which seriously restrict the development of shale gas exploration. The bedding shale, as the research object, which located in the Central South of Sichuan Changning shale gas blocks. Through the laboratory experiment, such as mineral group experiments, experiments of SEM (scanning electron microscopy), physical and chemical characteristics experiments, rock full water experiment and conventional triaxial compressive strength test. Combine with the Changning block geological and logging data, through a series of change rule research of rock mechanical parameters, including elastic modulus, Poisson’s ratio, cohesive force, internal friction angle. Taking well 201-HI as an example, analyzing the soaking mechanical weakening under angle of bedding and wellbore azimuth of wellbore instability. The main contents of this paper include:(1) Comprehensively analyses the target block of geology structure characteristics, stratigraphic section characteristics, the types and causes of weak structural plane of reservoir rocks. Changning structure main reservoir is located about 2300m, Longmaxi formation. The lithology is mainly dark grey, black shale and biologic limestone bottom. Longmaxi bedding shale is a layered rock mass structure, visual for anisotropic elastic body, its stability is poor, the instability of the structure may produce shear slip failure.(2) Through the analysis of rocks and minerals components experimental, the physicochemical characteristics tests and scanning electron microscopy (SEM) experimental results can be found Longmaxi bedding shale brittle mineral content is high, the clay minerals are dominated by illite and a small amount of illite smectite mixed layer, the development of a large number of weak bedding surface and micro fracture, pore is nano hole, after flooding patchy distribution of micro cracks, which belongs to the weak expansion of hard brittle shale.(3) The result of full water experiment and rock mechanics experiment showed that target core saturation water rate is low, after 24 hours full water process, the maximum value reaches around 1.1%, hydration expansion is weak; rock failure mode is brittle failure, easily destroyed along the bedding planes, elastic modulus and compressive strength showed obvious anisotropy, it shows lowest in the angle of bedding between 50 degrees to 70 degrees; bedding surface cementing strength is low and easily to be affected by hydration, along with the saturated water content increased, compressive strength decreased rapidly, maximum reduced by about 35%.(4) Combining with the Changning Well 201-H1 drilling data, analyses the wellbore instability influence of bedding angle and borehore azimuth under the weakening mechanical condition of water immersion study. In along the minimum horizontal principal stress direction drilling, the collapsed pressure is minimum; along with the saturated water content increased, the collapse pressure increases gradually, the bedding dip angle increases gradually as the destroy of weak plane, the weak plane destroy range of bedding dip angle in high water saturation rock samples expanded to 30 degrees to 85 degrees, it is causes the wellbore are more likely to collapse. It shows that the weakening mechanical condition of water immersion study affect the weak plane strength obviously, and the influence of collapse pressure is greater than the anisotropy, the collapse pressure increase to 73% under the mutual effects of mechanical weakening and dip.