Subsurface Fluid Characterization Using Downhole and Core NMR T1-T2 Maps Combined with Pore-Scale Imaging Techniques

Characterization of the subsurface fluid types, porosity, saturations, and wettability are critical for understanding the type and volumes of fluids that will be produced during primary completions and secondary waterflood recovery. The oil reserves within the Green River Formation of the Uinta Basin (Utah, USA) in the Greater Monument Butte Unit (GMBU) have variable fluid volumes, saturations, and wettability. Within a potential pay section of over 2,000 feet are over 20 defined producing sandstone reservoir intervals within the Green River Formation with variable depositional environments, mineralogy, and rock quality. Traditional core analyses for saturations and wettability are time-consuming and expensive because of variable reservoir properties within discontinuous sands and high-paraffinic oil containing asphaltenes and resins. Similarly, the variable wettability complicates standard analyses of NMR (nuclear magnetic resonance) data for fluid type and volume estimations. Newfield has approximately a dozen wells with NMR T-1, T-2, and diffusion data in the producing section of Monument Butte Field. We have identified patterns of NMR, dielectric, and standard triple-combo log data that are associated with differences in estimation of clay-bound water volumes from NMR and XRD data. We present results from pore-scale imaging, NMR coreflood experiments, USBM (United States Bureau of Mines) measurements of wettability, and 2.5D inversion of NMR data used to characterize the variable wettability of GMBU sandstone reservoirs. We found that the sandstone reservoirs are mixed-wet at the micro-and macropore scales, including presence of oil-wet clays. Mixed-wettability complicates estimation of fluid types and volumes from NMR data using standard interpretation techniques. An analysis protocol involving pore-scale imaging, coreflood NMR experiments, and 2.5D NMR processing and analysis permit reduction of interpretation ambiguity of the NMR data.