Pore-fluid characterizations and microscopic mechanisms of sedimentary rocks with three-dimensional NMR: Tight sandstone as an example

Abstract

Pore-fluid is directly related to the reservoir quality. Three-dimensional (3D) nuclear magnetic resonance (NMR) can be used for charactering pore-fluid due to its sensitivity to fluid components saturated in sedimentary rocks. In this paper, taken a tight sandstone as an example, investigating pore-fluid characterizations and microscopic mechanisms through 3D NMR. Firstly, a commonly used 3D NMR pulse sequence, tri-window pulse sequence, in actual oilfield was presented and its data processing workflow was exhibited briefly. Then, random-walk method was modified to simulate the pulse sequence measurements at different pore-fluid cases in tight sandstone, such as pure water case, oil-water case, and gas-water case. Finally, pore-fluid 3D NMR responses were exhibited and their microscopic mechanisms were explored. The results show that 3D NMR can identify different fluid components saturated in sedimentary rocks; wetting fluid saturation directly affect wetting fluid relaxation time, but the non-wetting fluid relaxation information is independent of fluid saturation; wetting fluid has an obvious restricted diffusion as a lower saturation; an obvious difference between gas signal and water or oil signal in two-dimensional relaxation spectra due to high longitudinal to transverse relaxation time ratio of pore gas. These simulations provide a theoretical basis for interpreting 3D NMR macroscopic responses, which should be helpful for pore-fluid identifying in tight oil and gas reservoirs.