According to the relationship of the percolation ability and the pore pressure of coal reservoir, an interpretation method of analyzing the pressure attenuation testing data is derived by use of the Duhamel principle. Consequently, the pore pressure of coal reservoir is obtained by the method. Depending on mechanics of porous elastic medium and rock mechanics, mechanical mechanism of borehole instability is analyzed for underbalanced drilling. Based on Hoek-Brown strength failure criteria, the limit drilling pressure difference mathematical model for keeping borehole stability is established. By calculating the annular pressure loss of drilling fluid in the laminar flow, the method for determining the proper drilling fluid density is proposed combined with the pore pressure of coal reservoir and drilling negative pressure difference. Comparing the measured values with the theoretical results of pressure difference and drilling fluid density of a coal-bed methane well in QinShui basin, the result shows the high degree of coincidence, verifies the correctness of the method and model in this paper, and supplies the theoretical basis for parameter design during underbalanced drilling.
Because of the low mechanical strength, the coal rock cannot brace the pressure of overlying strata, and is easily to collapse. As the microporosity, micro crack and joints are quite developed, and the formation pore pressure is low, it is easy to have the leakage. The coal liquid has the acid property, but the drilling fluid shows the alkalinity, so it is easy to cause acid-base neutralization producing metal salts and block coalbed methane channel. Therefore, in order to protect the coal reservoir effectively, reduce the pollution on coal reservoir, and improve the recovery efficiency, the underbalanced drilling technology is used in coal-bed methane mining more and more in recent years.