Mapping oil saturation distribution in a limestone plug with low-field magnetic resonance

Magnetic resonance imaging (MRI) is used to quantify in situ the recovery of crude oil from a strongly oil-wet microporous limestone core-plug. We demonstrate the capability of low-field MRI to continuously monitor oil saturation distribution by obtaining a series of spatially resolved transverse relaxation time (T2) distributions using the robust spin echo single point image (SESPI) profiling method to obtain T2 maps with a temporal resolution of 45 min. These T2 maps are shown to provide comparable data to nuclear magnetic resonance (NMR) well-logs. The low injection rate of 1.4×10−3cm3s−1 (equivalent to an interstitial velocity of 1 ft day−1 in the formation) allowed a large number of T2 maps to be acquired during the flood. Fluid-phase discrimination is achieved here in the T2 dimension; the brine relaxation time is reduced by addition of paramagnetic manganese. Some manganese is lost through adsorption on the limestone surface, but sufficient relaxation contrast is obtained to position an unambiguous oil/brine T2 cut-off. The spatial distributions of both the brine and oil are therefore determined simultaneously and independently. Capillary end effects are observed in the short core-plug due to the difference in wettability and permeability between the plug faces and the core-holder end-caps. The inclusion of the spatial dimension in the experiment allows a region of the plug, unaffected by end effects, to be considered representative of behavior in the reservoir. Overall, we highlight the importance of spatial resolution in laboratory-scale core analysis and demonstrate the capability of low-field MRI spectrometers to continuously monitor oil recovery experiments.