Revisiting Log-Inject-Log NMR for Remaining Oil Determination: A Field Application of T₂-D NMR in the Permian Basin

As in many mature carbonate reservoirs, determination of remaining oil and oil-water contact in the Grayburg-San Andres dolomite formation at Vacuum Field (Permian Basin) is challenged by unknown water salinity (due to decades of water flood)and variable electrical rock properties (due to the presence of anhydrite nodules, vugs, and possibly variable wettability). To address these difficulties, a new protocol combining Log-Inject-Log (LIL) NMR and Relaxation-Diffusion (T₂-D) NMR was tested to improve accuracy in remaining oil estimation. In classic LIL-NMR, manganese mud doping is used to decrease the T₂ relaxation time of water between two successive NMR T₂ log passes. The difference between the two NMR T₂ logs is attributed to water, while the NMR free fluid signal remaining after doping is attributed to oil. However, it becomes very difficult to correctly differentiate oil from water if doping fails. Because oil signal develops a characteristic signature along the D dimension in T₂-D logging, which water does not follow, T₂-D NMR makes it possible to identify whether mud doping was successful. In turn, unflushed oil saturation and wettability can be interpreted with more reliability, which was verified against sponge core measurements. Using only one T₂-D NMR pass after manganese mud doping also alleviates the need for a pre-doping log pass. The new protocol therefore improves the quality control and simplifies the data acquisition of traditional LIL NMR method.