A THEORETICAL STUDY ON NUCLEAR MAGNETIC RESONANCE OF PORE SPACE SATURATED WITH OIL AND WATER
油水双相饱和孔隙模型核磁特性理论研究

We study the basic characteristics of the model spin-spin relaxation. By the series expansion method, the Bloch equations controlled by diffusion effect are solved for double-phase pore space of slab and sphere saturated with oil and water. The result shows that the Nuclear Magnetic Resonance (NMR) relaxation is controlled by multi relaxation modes and associated with pore size, surface relaxation ability, diffusion coefficient of pore fluid and oil saturation. Numerical results indicate that for water saturated pore space, relaxation is mainly related to pore size. For small water saturated pore, the relaxation is dominated by the smallest relaxation mode and the corresponding relaxation time is proportional to pore size. But for large pore space, multi-mode relaxation composes the pore water relaxation process, even when the pore size is uniform. And under this situation, it is no longer a linear function between relaxation time and pore size. For water-oil double-phase saturated pore space, when pore size is small and as oil saturation increases, the water relaxation time decreases linearly. However, when pore size is relative large, the multi relaxation mode turns to be single mode with the increasing of oil saturation. And at the same time, the variation of relaxation time with oil saturation is non-linearity. Using the formulae in the paper, the responses for the pore space model obtained from real rock core have been calculated. There is a fine agreement between the theoretical and experimental results.