Derivation of Residual Oil Profile for Enhanced Recovery

Abstract

Petrophysical evaluation of a given formation is not complete without looking into all means of exploitation of resident hydrocarbons. With this intent, we studied the hydrocarbon potential of the subject formation via conventional Petrophysics and identified only the possibility of recovering residual-oil(Sro). The remaining question and the objective of this study was the quantification of Sro that can make EOR sweeping of residual-oil economically viable or cost-prohibitive. It is a well-known phenomenon that additional residual oil (Sro) may be present below the conventionally defined oil-water-contacts as a function of geologic and hydrodynamic conditions. In addition, the oil-wet formations force the contact to be below Free-Water-Level leaving a sizable Sro "Stranded Oil" in or below the transition zone. The zone of "Stranded Oil" can be quite thick and economically viable for tertiary EOR techniques if there is a sufficient recoverable volume. To confirm the presence and quantify the saturation of residual oil, we used diffusion-T2 intrinsic (DT2) maps from Nuclear Magnetic Resonance NMR log. The DT2 technique was challenged with a possibility of superimposed signals from residual oil and the filtrate from Synthetic-oil-based-mud (SOBM). However, an appreciable viscosity difference between residual formation oil and SOBM-filtrate made it possible to differentiate the NMR signals from SOBM and residual oil based on different diffusion characteristics. We had all possible reasons for having a thick zone of Sro. Either mechanical (tilting of the basin) and/or compartmentalization due to re-formed seals or later movement of water to the lower part of the oil accumulation were present. Hence, looking for a thick zone of Sro that was generated by reasons beyond the capillary behavior was justifiable. However, the quantification and derivation of Sro profile based on clearly identified residual-oil signals revealed a Sroprofile that failed to justify the formation as a future EOR sweep-zone. DEFINITION We would like to define "residual oil" as the oil left in the formation that cannot be recovered by conventional means (including water-flood) under practical and field-driven average conditions (as the optimum draw down pressures and completion requirements). The residual saturation of hydrocarbons depends on the horizontal and vertical sweep efficiencies, structural and geological constraints, heterogeneities of the petroleum system and the microscopic displacement efficiency. However, including the "field-averageconditions" and "conventional means" to the definition, makes it relevant to the case under study. METHODOLOGY SELECTION Microscopic pore-scale attributes(wettability, pore throats, irreducible-wetting-phase saturations, displacement pressure, etc.) control the flow before it is affected by the macroscopic constraints (seals, tight streaks) or enhancers (fractures, channels). Core analysis can shed a light to the Sro with certain limitations. Most of the residual oil saturation from conventional cores under-call the actual residual oil saturation due to expulsion of oil (bleeding) and shrinkage of oil as pressure decreases during the removal of core from formation to the surface. There are methods of corrections, but the Sro determination from core analysis still has high uncertainties if one considers the filtrate invasion (or coring fluid) of the core. The pressure coring addresses these issues, but we did not have pressure-coring for this case. Recent "tracer testing" workflows have also been effective measures of Sro. We did not have tracer testing for the subject study either.