Basic Research Of Coal And Rock Mechanics Borehole Stability

With the accelerative exploitation of coalbed methane resources, coal rock borehole wall stability now is paid more attention. The collapse and falling pieces at the coal rock stratigraphic position and tight pull when tripping during the drilling process not only affect the drilling progress and the well depth, but also bring unpredictable danger to the following production of coalbed methane. At present, coal rock borehole wall stability study is still in the initial stages:first of all. Coal rock has complex composition, almost orthogonal cleat and bad homogeneity, failure mechanism is not clear. Secondly, there are not relatively accurate theoretical models. Unable to quantitatively determine the pore pressure, crack pressure, collapsed pressure and safety density window of drilling mud. In addition, in the on-site construction process, have no effective anticollapsed measures. Therefore, the study of coal rock borehole wall stability should be strengthened.Using the laboratory test and numerical simulation to study coal rock borehole wall stability, laboratory test mainly analyzed the Qinshui Basin and the Chongqing coal mine, the mechanical properties and failure mechanism by comparative studied. The reasons of mechanical parametersand discreted are explained from meso-structure characteristics. Numerical simulation mainly use RFPA-Flow fluid-solid coupling role analysis system to establish a coal rock borehole wall stability numerical model, based on the model to analysis the effect of the density of cleat.downhole pressure, the pore pressure to coal rock borehole wall stability.Main results and knowledge attained are as follow:(1) There are a large number of micro-pore,tiny cracks within the coal rock observed by scanning electron microscopy. CT image test, that make the coal rock to precent highly heterogeneous and anisotropic in mechanical properties. Native damage variable and Crack development direction have a greater impact on the strength of coal rock and the mechanical properties.(2) By coal rock uniaxial and triaxial compression tests found that, compared with other rocks, coal rock strength is low7, less elastic modulus. Poisson’s ratio discrete nature. Uniaxial destruction or high-angle-splitting damage; the triaxial failure mode is more complex.(3) Under normal temperature and humidity conditions, coal rock strength is mainly associated with the organic component type, it does not exist a clear relationship between the inorganic components of coalrock and strength, but the coal rock soaked in different drilling fluid will swell, also causing strength reduction, and the degree of expansion is directly affected by the clay mineral content of the coalrock. (4) It can be found by numerical simulation test, the more density of cleats in the coalrock, the borehole instability phenomenon gradually intensified, the higher collapse pressure around the borehole, the more obvious deformation of the borehole; the bigger caliper, the more number of cleats through the borehole surface, the seepage effect are more obvious, resulting in borehole instability phenomenon is more serious:smaller under the conditions of the borehole fluid injection pressure, borehole obvious necking phenomenon, with the increasing pressure of the well fluid injection, wellbore can obtain effective support, stability is getting better, wells circumferential stress concentration range also smaller; lower formation pore pressure, rock subject to greater stress, the better the stability.