ABSTRACT
Borehole stability means wellbore, drilled to get hydrocarbons from underneath, keeps its regular size and shape during drilling process. It’s a core technology guaranteeing drilling safety and high well quality(Jin & Chen 2012). According to statistics, cost on borehole instability worldwide is more than 6 billion dollars annually. Therefore, actively developing study on borehole stability is of significant economic consideration (Chen et al. 2008). Harold (1940) initialed borehole stability mechanics study on wellbore stress distribution. However, Santarelli et al. (1992), Franck et al. (2003), Helstrup et al. (2004), Germanovicha et al. (2000), Chen et al. (2005), Huang RZ et al. (1995), and Jin et al. (2004) have already assumed that surrounding rock and its stress distribution are isotropic, which is acceptable for shallow rock engineering. With the increase of drilling depth, encountered geological body presents various crossed structural planes (joint plane, fracture plane or bedding plane), and surrounding rock shows strong anisotropic characteristics. Generally coal seam gas reservoirs consist of sedimentary rocks with smooth laminar structures. Under normal conditions, the elastic modulus perpendicular to the bedding plane is slightly greater than that in the direction parallel to the bedding plane, and Poisson’s ratio as well as other mechanical parameters, also vary between these two orientations. This lamellar rock has mechani-
researchers prefer laboratory tests when determining these parameters. The original stress state of coal seam must be taken into account in laboratory tests. Researchers usually retrieve data such as well depth, confining pressure, pore pressure etc. from geological information before laboratory tests.
