Detecting Fluid Movement and Isolation in Reservoir Core With Medical NMR Imaging Techniques

Abstract

Summary A medical nuclear magnetic resonance (NMR) imaging instrument has been modified to image water and oil in reservoir rocks by the construction of a new receiving coil. Both oil and water inside the core produced readily detectable hydrogen proton NMR signals, while the rock matrix produced no signal. Because of similar NMR half-lives, the water was doped with a paramagnetic ion, Mn+2, to reduce its relaxation time. This procedure enhanced the separation between the oil and water phases in the resulting images. Sequential measurements, as water imbibed into one end and oil was expelled from the other end of a core plug, produced a series of images that showed the location of the fluids as a function of flow time. For water-wet Berea sandstone, a flood front was readily observed, but some of the oil apparently was left behind in small, isolated pockets that were larger than individual pores. After several additional PV's of water flowed through the plug, the NMR image indicated a homogeneous distribution of oil. The amount of residual oil determined from the ratio of NMR intensities closely approximated the residual oil saturation of fully flooded Berea samples measured by the Dean-Stark method. A Berea sandstone core treated to make it partially oil-wet did not show a definitive flood front but appeared to channel the water around the perimeter of the core plug. The relative ease with which these images were made indicates that NMR imaging can be a useful technique to follow the flow of oil and water through a core plug for a variety of production processes.