Bulk Saturation Measurement of Water and Oil in Porous Media Using 13C and 1H Magnetic Resonance

Knowledge of water saturation is essential to determine hydrocarbon-in-place during the exploration stage of petroleum reservoirs and to monitor the sweep efficiency during the production phase. Water saturation is usually measured using one or more of five independent techniques: resistivity logging, dielectric logging, capillary pressure saturation modeling, magnetic resonance (MR) logging and laboratory MR core plug analysis, and direct measurement of water in the core plug samples using Dean-Stark extraction. The latter requires the extraction of water using hot toluene vapor, which is collected in a graduated cylinder and measured by the water volume. Each of these steps is error prone. This conventional method is time consuming and involves hazardous organic solvents. This paper presents a radically different approach to measuring the water content in a porous rock sample; we exploit the presence of 13C in the hydrocarbon phase. 13C is an MR active nucleus that naturally exists in hydrocarbons but not, of course, in formation water. We show that water and hydrocarbon phase saturations can be derived using only two measurements: 13C and 1H MR measurements. While the 13C MR measurement gives hydrocarbon content in the rock sample, the 1H MR measurement yields oil and water content. The combination of these two measurements yields water and oil saturation in the rock sample. This method was tested and validated on Bentheimer and Berea core plugs saturated with brine and oil. Bulk saturation measurements based on 13C and 1H MR measurements were in close agreement with independent Dean-Stark saturation measurements. Results confirm that the calibrated MR signal of 13C was sufficient to obtain hydrocarbon content/saturation in the porous media, and it is applicable to realistic samples with complex hydrocarbon MR signals. The method requires sequential measurement of 1H and 13C in a core plug or similar sample. This may be undertaken with two MR instruments: one with a radio-frequency (RF) probe tuned for 1H and the other with an RF probe tuned for 13C. Alternatively, it may be undertaken using one instrument and a doubly resonant RF probe. These measurements are greatly facilitated by a variable field MR instrument, which permits sequential measurement of 1H and 13C in the same instrument with the same RF probe at the same frequency.