Nuclear magnetic resonance pore radius transformation method and fluid mobility characterization of shale oil reservoirs

The mobility of fluid is important for evaluating shale oil reservoirs, and it is closely related to the pore structure. Nuclear magnetic resonance (NMR) can effectively reveal the pore size distribution from nanometer to micron, providing a way to study the relationship between pore structure and mobility. However, the transverse relaxation time (T2) of NMR also contains bulk relaxation and diffusion relaxation, which will affect the accuracy of pore structure calculation. To characterize the mobility of shale oil by NMR, we proposed a pore radius transformation method with relaxation correction. The NMR simulations were carried out based on different digital core models, which were constructed according to X-CT results of shale cores. And the comparisons were performed to verified the accuracy and efficiency of the presented method. Then, the relation of geometric means of pore radius and mobility was studied through NMR experiments, and the NMR logging data of well J31 were processed by proposed method. The results show that the relative errors between corrected T2 distributions and surface relaxation distributions are smaller, and the geometric means of pore radius after relaxation correction are closer to those of digital core models, which proves the efficiency of the presented method. The logarithm of geometric mean of pore radius obtained from T2 distribution of shale core NMR experiment is positively correlated with S1/TOC, which can represent shale mobility. The processing results of NMR logging data in well J31 show two sections with large geometric means of pore radius, indicating that they have good mobility, which are consistent with the oil test results.