Direct Hydrocarbon Saturation Imaging in Porous Media With 13C Magnetic Resonance

Executing a successful waterflood recovery project in the field requires prior laboratory coreflooding studies to understand fluid displacement and predict recovery efficiency. When dealing with reservoir rocks that display complex fluid distribution, the use of an imaging technique is crucial. Traditionally, fluid saturation changes during corefloods are monitored using X-rays. However, dopants are usually required to improve the density contrast between fluids. These dopants may alter the wettability of the rock or could chemically inhibit some enhanced oil recovery (EOR) operations, such as low salinity injection. In this work, we introduce a simple magnetic resonance imaging (MRI) measurement to monitor hydrocarbon saturation profiles. This measurement, selective for hydrocarbons, requires no contrast agents or unusual fluids. We employ 13C measurements of carbon in the hydrocarbon phase to directly detect oil with natural abundance 13C. This allows for spatial quantification of oil saturation in core plug samples. The core plugs tested were water-wet Berea and Bentheimer sandstones and oil-wet (treated) Bentheimer sandstones. Viscosity standard oils (S6 and S20), which are synthetic oils with precisely known viscosity values, were used as the oil phase, and brine (2.1 wt% NaCl) was used as the water phase. Imaging was performed using the one-dimensional (1D) hybrid spin-echo single-point imaging (hybrid SE-SPI) MRI method. The results showed that the 13C 1D images provided high-quality saturation information. Dean-Stark analysis on the final state of the core plugs confirmed the saturations. The saturation difference between the proposed method and the Dean-Stark analysis was less than 1 s.u. The 13C bearing phase was directly observed, and there was no need for further processing to separate the water and oil signals. Flooding in different combinations of fluids, rocks, and wettability conditions was examined by 1D imaging. The 13C profile method clearly reveals hydrocarbon capillary end effects in oil-wet samples. The ability to generate direct 13C profiles is a general capability that is applicable to a wide range of flooding experiments and core analysis measurements. These studies are greatly facilitated by a variable field magnet, which permits sequential measurement of 1H and 13C in the same magnet with the same radio-frequency (RF) probe. In this work, 13C MRI measurements were undertaken at a magnetic field of 3.1 T. Despite the lower sensitivity of 13C compared to 1H, we have demonstrated that accurate results can still be obtained.