Measurement of Pore Distribution and Compression Anisotropy by Nuclear Magnetic Resonance

Coal pores and fractures vary in geometric structure, size distribution, and compressibility in different directions, referred to as the anisotropy of coal pores and fractures (APF). For high-dip coal seams, this anisotropy impacts coal reservoirs more severely owing to stress variations. A low-field nuclear magnetic resonance (NMR) test can detect the pore size distribution under different confining pressures. By changing the angle between the axis of the columnar sample and the coal bedding plane, the NMR test could express the evolution of the APF with stress. Pressurized NMR tests were performed to extrapolate the variation in the pore size distribution and compressibility with the dip angle. T 2 spectra of each columnar sample were divided into two peaks (P1 and P2) with T 2 = 2.5 ms as the boundary. By establishing a linear elastic model, the pore size distribution of P1 can be regarded as a superposition of the micropore deformation controlled by APF. Based on the predominant orientation of the pore structure, the anisotropy of the micropores was created before the formation was tilted. Most (80.1%, volume percentage) of the micropores had geometric spindles parallel to the bedding plane, and most were ellipsoids. Through stress and spectral analyses, we found that the seepage fractures were mostly (80.45%, volume percentage) occupied by the cleat system perpendicular to the bedding plane. The fracture compressibility was related to the normal stress received. The dip angle decomposed the confining pressure in the coalbed and changed its compressibility. As the pressure increased, the multi-scale effect narrowed the main fracture, but did not change other fractures, which reduced the anisotropy of fracture.