Summary
The application of nuclear-magnetic-resonance (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 such as liquid, gaseous, or adsorbed phases in nanometer-sized pores (associated with various minerals and organic matter) adds another degree of complexity to the interpretation of NMR data in shales. We present a laboratory study on the NMR responses of brine, oil, and methane in shales at 2 MHz.
NMR transverse relaxation time (T2) distributions were acquired on core plugs from the Haynesville, Barnett, and Woodford shale formations. The NMR T2 distributions were acquired after brine (2.5% potassium chloride) and oil (dodecane) imbibition and saturation. After brine imbibition, we observed an increase in porosity at T2 ≤ 1 ms. However, after saturation at increasing pressures we observe a porosity increase at T2 ≈ 6–20 ms. Dodecane imbibition and saturation induced a porosity increase at T2 ≈ 10 ms.
The measurements with methane were conducted on Haynesville core plugs at a methane pressure of 4,000 psi. The NMR T2 signal of methane in shales appears to be at approximately 10 ms.
These results show that the NMR response of methane and oil is very similar in shales. Monitoring the saturation increase with NMR shows that brine can enter the entire pore spectrum, whereas oil and methane have access only to a fraction of the pore space.
