Derivation of Continuous Irreducible Water Saturation and Pore Throat Aperture Distribution from Well Logs in an Offshore Brownfield

Abstract

Abstract In this case study, we propose a new petrophysical workflow to evaluate an offshore mature field. In the field studied, hydrocarbon production mainly comes from sandstone reservoirs with good permeability and low water salinity. The operator typically drills deviated wells and runs Logging While Drilling (LWD) tools to make timely completions decisions. The petrophysical properties critical for these decisions are porosity, permeability, irreducible water saturation (Swi) and total water saturation (Sw). Occasionally, Nuclear Magnetic Resonance (NMR) logs are run to characterize Swi and permeability, but are only available in select wells. The challenge for the petrophysicist is to make the most of the commonly available measurements and compute the petrophysical properties required for decision making. We propose a technique that derives continuous Swi and pore throat radius distribution from porosity and permeability logs. When the well is cored, continuous permeability can be obtained through core calibration. When only basic well logs are available, permeability is typically estimated from a porosity-to-permeability relationship, which is derived from core data in an offset well. We then usea well established relationship to compute pore throat radii from interpreted porosity and permeability logs for a range of mercury saturations. We can further construct a synthetic Mercury Injection Capillary Pressure (MICP) curve from mercury saturation and capillary pressures (Pc) corresponding to different pore throat radii. Since Swi varies with Pc, we can apply a Pc cutoff on the synthetic MICP curve to obtain Swi. This cutoff can be determined empirically or from height above free water level. From the level-by-level synthetic MICP curves, we can also construct a histogram of pore throat radius at each depth level. This plot provides valuable visual aid that helps interpreters identify good quality rocks. To validate this method, we calculated the bound water volume (BFV) from synthetic MICP and compared it to BFV derived from NMR T2 distribution. There is an excellent agreement between the two computed BFV curves. The proposed workflow is only meant to provide a rock quality evaluation when log data is scarce. For accurate characterization of Swi and rock quality in complex reservoirs, more advanced measurements are recommended.