Application and Study of Permeability Testing Method of High Rank Coal Bed Methane Available to Purchase

Because of the four characteristics: low gas saturation, weak permeability, low reservoir pressure, and scarce resource abundance of CBM(coalbed methane)in China, the mining of CBM still has not good progress. The permeability of coal is usually less than 1mD (1mD=1×10⁻³μm²), and is general 0.001~0.1mD in high rank coal reservoirs which are existed widely in China. Under super low permeability conditions, production practice of coal bed adsorption methane is obviously different from conventional gas. Different permeability of coal bed generates a considerable variance in the optimum condition for drilling and well completion and development technology. Therefore, permeability determination plays a very important role in CBM development. When the coal core permeability is lower than 0.01mD, conventional permeability test method is difficult to obtain accurate permeability measurement values. A permeability measuring instrument basing on the principle of pressure oscillation is designed in view of this difficulty. The coal sample is first stabilized at certain pore pressure, then a specific sinusoidal pressure wave is applied to the upstream side of the sample, and the pressure response at the downstream side is recorded. Permeability is deduced from the attenuation and phase variation of the downstream signal as it pass through a sample under test. The diameter range of sample is 50±0.5mm, which can better react the heterogeneous reservoir characteristics. The instrument which is equipped with drilling fluid pollution system can simulate downhole conditions and changes in permeability by drilling fluid pollution. The lowest permeability could reach 1×10⁻⁷mD. The permeability of coal samples which are from Sihe /Shanxi Province, Wu'an /Hebei province and Gerick coal mine is measured with this instrument. Multiple experiment results show that the instrument can be in accordance with repeatability accuracy requirement of permeability test in super low permeability coal reservoir. The study provides the basis for the reserves evaluation and drilling and well completion technological design.