Theoretical Mechanism and Application of Sphere-Cylinder Model in NMR for Oil-Water Porous Media

NMR is a unique logging tool that measures porosity, permeability, fluid components and wettability. It also shows different responses from rocks due to different pore-sizes in reservoirs; this gives opportunities to carry out a further study for pore structures and pore sharps in complicated reservoirs. The theoretical mechanism in NMR used for pore structure study currently is based on the Brownstein and Tarr theory (Phys Rev 19:2446-2453, 1979), but it shows that the pore structures are not sensitive to the connectivity of pores. In order to overcome this, we are proposing a theoretical approach called the Sphere-Cylinder Model to conduct NMR relaxation theories. In addition, a procedure for different pores has been discussed for porous media that is saturated by an oil-water phase. Consequently, considerations for the NMR relaxations for the water and oil phase have been taken into account in our model. The Sphere-Cylinder model has been used based on an NMR log in one of the gas fields in southwest China and shows satisfactory results.