NMR Response of Brine, Oil, and Methane in Organic Rich Shales

Abstract

Abstract The application of NMR methods to evaluate the fluid content in hydrocarbon reservoirs requires the understanding of the NMR response of the fluids present in the rock. The presence of multiple fluids which can be in liquid, gaseous or adsorbed phase in nanometer size pores (associated with various minerals and organic matter) add another degree of complexity to the interpretation of NMR data in shales. This report presents the findings of a laboratory study on the NMR responses of brine, oil and methane in shales. The acquisition of the NMR data was made at 2MHz. NMR T2 distributions were acquired at TE = 0.2ms on companion core plugs from the Haynesville shale and at TE = 0.3ms for Barnett and Woodford shale samples. The NMR T2 distributions acquisitions were conducted on samples saturated with brine (2.5% KCl) and oil (dodecane). The NMR T2 data acquired on the native state samples (as received) showed a bimodal distribution with the dominant peak at T2 less or equal to 1ms. After saturation in parallel with oil and brine with increasing saturation pressures we observed in the case of brine an increase in amplitude of the NMR peak located at T2 less or equal to 1ms. At higher saturation pressures we observed a shift in the T2 time of this peak toward longer T2 times. When the samples were saturated with oil and the saturation pressure increased we observed the development of second NMR T2 peaks around 6-20 ms with no substantial increase of the peaks at T2 less than 1ms; the observed distributions are bimodal. The measurements with methane were conducted at TE = 0.3ms on 2 Haynesville core plugs at a methane pressure of 4000 psi and a confining pressure of 5000psi. The NMR T2 distributions that were essentially unimodal in the as received state with a peak at 0.35-0.4ms, evolved into a bimodal distribution in response to the injection of methane. The first peak was observed at the same T2 time as in the as received state with no substantial change in amplitude and second peak was located at 10-20ms ms. This results show that the NMR response of methane and oil is similar. Monitoring the saturations increase with NMR show that brine can enter the entire pores spectrum while oil and methane have access only to a fraction of the pore space.