Diffusion coefficient measurements for moving samples under strong magnetic field gradients

Abstract

Among the numerous measurements carried out during a well-logging procedure, the Nuclear Magnetic Resonance (NMR) assessment is one of the fundamental analyses in determining the economic viability of a well for the oil industry. Nowadays, two reliable approaches, Wireline Logging (WL) and Logging While Drilling (LWD), stand out. WL comprises the acquisition of NMR data under static conditions. On the other hand, in LWD, the NMR measurements happen simultaneously with the drilling process, while the NMR tool experiences translation, rotation, and vibration motions relative to the rock formation. In order to comprehend better the NMR response acquired under LWD conditions, a setup emulating an LWD tool was developed, consisting of a single-sided magnet, rf probes, and a mechanical device that emulates a relative sinusoidal movement between the sample and the applied magnetic field. A bulk sample and three representative rocks, Fontainebleau Sandstone, Berea Sandstone, and Indiana Limestone, were investigated. Even though the diffusion coefficient measurements remain neglected for their intrinsic characteristics of data acquisition, the findings demonstrate that the diffusion coefficient parameter of a fluid in a bulk sample or confined in a porous rock can be precise and accurately predicted. In strong magnetic field gradients, the Hahn spin echo is predominantly weighted by the diffusion process, an effect used to measure diffusion coefficients. Under LWD conditions, the diffusion coefficient measurement is considerably affected by signal phase modulation due to sample movement in the presence of strong magnetic field gradients, making this measurement difficult. This article present solutions for correct diffusion coefficient measurements, synchronizing Hahn spin echo experiments with sample movement.