Surface relaxivity estimation of coals using the cutting grain packing method for coalbed methane reservoirs

Low-field nuclear magnetic resonance (NMR) has been widely used to express the pore-size distributions of conventional and unconventional reservoirs. Based on the equation 1T2 = & rho;Fs1r, the determination of the surface relaxivity p can help to quantitatively convert the transverse relaxation time T2 to pore radius (r). The existing calculation of p using NMR requires supplementary and expensive destructive or non-destructive methods, and consolidated cylinders are required as samples to be well tested. However, the operation of multiple drilling projects and the production of many drilling cuttings are barriers to making rapid measurement of the p value of the unconsolidated coal cuttings challenging. In this study, an NMR grain packing method is proposed using different coal grains. The surface-to-volume ratio was transformed into the ratio of the derivative of an arbitrary pore volume to its volume, and the governing NMR equation was established by considering only the NMR porosity and T2 distribution. Sphere and cubic-like models were also constructed. The results showed that the coal with different grain sizes had different relaxation responses, and the different assemblies with the same wgrain/wgrain,c also exhibited some relaxation differences, which might be affected by the diversity in grain packing patterns and grain morphology, resulting in the variation in porosity. In addition, the water film dis-tribution on the grain surface and the soluble minerals in the coal contribute to the variation in p values by influencing the relaxation properties of the voids between grains and the pores in the grains. The T2 spectrum indicated that the macropores contributed the most to the porosity, owing to the loose grain packing, and the larger grains might not strictly obey the fast diffusion limit. For the sphere-like model, the p values of the Ji15 and Ji17 coals were in the range of 19.47-44.04 & mu;m/s and 36.50-46.66 & mu;m/s, respectively; whereas for the cubic-like model, the corresponding p values of these two coals ranged from 24.16 & mu;m/s to 54.64 & mu;m/s and from 45.29 & mu;m/s to 57.90 & mu;m/s, respectively. Finally, different methods of p calculation are compared, and its po-tential field application is discussed.