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Lots of cleats, fractures and other weak structures have developed in coal reservoirs, and as a result, there are relatively big difference between its mechanical properties and coal matrix. During the drilling process, the cleats and fractures near the sidewall are more prone to surfer damage than coal matrix under the ground stress, crack stress, fluid pressure and borehole fluid additional stress, and thus destabilizing phenomena such as borehole collapse and circulation loss would take place. This paper used elasticity and fracture mechanics theories, and took the time effect of sidewall rock drilling fluid seepage into consideration to deduct stress intensity factorIandIIat the cleat tip under the action of the fluid-solid coupling, to establish criterion for the failure condition of coal matrix and cleat fractures, and to establish the wellbore stability calculation model for the cleat-featured coal body. Applying the model can calculate the safe drilling fluid density during the drilling process in cleat-featured coal seam, get the influence law of the factors which affect the wellbore stability such as the length of cleats, horizontal inclination of cleats and opening time of borehole, etc, and it also can analyze the time-delay effect of wellbore destabilization during the drilling process in cleat-featured coal seam. The study achievement was the supplement and perfection of the existed estimation model for the wellbore stability of coal seam gas well.

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Taking displacing pressure gradient and threshold pressure gradient into account, we have carried on a research of effective displacement theory in low permeability formation. Based on displacing pressure gradient expression of different production diverging/converging, we get displacing pressure gradient engraving when formational pressure, injection pressure, bottom hole producing pressure, and inter well distance is changing. And analyze its regular as well. On the basis of experimental result, establishing the relationship between threshold pressure gradient and displacing pressure gradient, and analyze the critical value of different pay zones for effective displacement. Study has shown that if we want to drive low permeability formation it is necessary that the displacing pressure gradient is larger than the threshold pressure gradient. It is the optimal choice for low permeability formations effective displacement that the well spacing should be smaller. It can achieve a goal that effective driving low permeability reservoir by the adjustment of taking measures and the injection-production pressure systems modulation. The study has important guiding significance for the development of low permeability formation, and it has got effective verification in oil field test.