Research Mechanical Effects Brittle Shale Pore Pressure Transmission

The brittle shale formations is not only the most common complex geological body encountered in drilling engineering, but also the mainly hosted geological body in shale gas. Different with the traditional hydration swelling shale formations,the brittle shale enjoys high illite、kaolinite、chlorite, and non-expansion or weak expansive clay mineral content, while the swelling clay mineral, such as montmorillonite, illite and smectite mixed-layer content is low, and micro-crack development, as opposed to the hydration swelling shale formations, such basic research of this type formation is still not systematic.Through literature research and collecting wild brittle shale outcrop, such research involves three aspects, that is, theoretical research, physical simultation expermients and numerical simulation expermients. Which aims at research the mechanism of pore pressure transimision of brittle shale, and the affect to crack propagation and collapse in brittle shale formation. Mainly achieved understandings are listed below.(1) When the brittle shale contact with an external fluid, the micro-cracks growth in the shale is the main channel for fluid flows into it, the chemical potential difference between hydraulic pressure, operating fluid power and hydration of shale which becomes an inner important motivation to drive the water from operating fluid into the mud shale formation.(2) The physical simulation results from the pore pressure transmission shows that the hydraulic fracturing is an important reason to cause instability and failure of brittle shale in pore pressure transmission.The effects of pore pressure transmission will result the micro-cracks in the inner part of rock, or it makes the expansion of native crack, run through it and then change the integrity of the rock, which reduces the strength of rock blocks, and eventually leading to the instability and failure of rock iteself. The characteristics of brittle shale refers to easily dispersed in water which makes the destruction process becomes more complex.(3) Based on the RFPA2D-Flow Software’s numerical simulation, the destruction process of rock mass in the transfer process of pore pressure is mainly divided into frive steps, that is, pressure aggregation, crack initiation, fracture closure, secondary cracks initiation, crack expansion and expand through. The first four steps is the stable crack expansion stage. When the cracks began to expand and through, the cracks accelerated expansion to rock mass destruction, which is the instability of crack expansion stage. Fracture geometry is also an important factor to affect the stability of brittle shale.