Comparison between steady state method and pulse transient method for coal permeability measurement

Coal permeability measurement is significant for research on reservoir characteristics and development of coalbed methane. In order to find a favourable method to obtain exact data of coal permeability, two different methods are operated to investigate coal permeability evolution. Steady state method is applied with constant upstream gas pressure and downstream pressure to keep constant pressure difference. Permeability is measured when velocity of gas flow is constant. Pulse transient method is applied with a pulse upstream pressure increment to discharge gas through coal sample to downstream gas reservoir. Permeability is calculated when the upstream reservoir and downstream reservoir reach a new equilibrium pressure. Experiments by different methods are both conducted on 2.5 cm diameter, 2.5–5 cm long cylindrical samples at same constant confining pressure and axial pressure, using both non-adsorbing (helium (He)) and strongly adsorbing (carbon dioxide (CO₂)) gases. Every single coal sample is measured by both methods, while all samples are collected from Southeast Ordos Basin in China. The results are compared to investigate the advantages and disadvantages of the two methods. Permeability measured by steady state method is slightly larger than that measured by pulse transient method, which indicates that pulse transient method is preferable for measuring low permeability measurement. Factors that may influence the results of measurement are analyzed. Steady state method may destroy the structure of coal and slightly enlarge the pore fracture, with constant pore pressure, which results in higher permeability. On the contrary, pulse transient method keeps samples close to in situ condition, which obtains more exact data. However, several improvements are needed to minimize the error of the pulse transient methods. Temperature has important influence on permeability measurement. (paper)