Estimation of dynamic petrophysical properties of water‐bearing sands invaded with oil‐base mud from multi‐physics borehole geophysical measurements

Abstract

Summary We develop and successfully test a new method to estimate permeability, capillary pressure, and relative permeability of water-bearing sands invaded with oil-base mud (OBM) from multi-physics borehole geophysical measurements. The inferred petrophysical properties of water-saturated sands are used for calibration of equivalent properties in hydrocarbon-bearing sands within the same sedimentary sequence. Our estimation method simulates the process of invasion between OBM and water. We iteratively adjust porosity, permeability (mobility), capillary pressure, and relative permeability in the simulation of invasion until density, PEF, neutron, and resistivity logs are accurately reproduced with numerical simulations from post-invasion radial distributions of water saturation. Examples of application include the cases of oil- and gasbearing reservoirs that exhibit a complete capillary fluid transition between water at the bottom and hydrocarbon at irreducible water saturation at the top. We show that the estimated dynamic petrophysical properties in the waterbearing portion of the reservoirs are in agreement with the vertical variations of water saturation above the free waterhydrocarbon contact, thereby valid ating our estimation method. Furthermore, we show that the radial distribution of water saturation inferred from resistivity and nuclear logs can be used for fluid-substitution analysis of sonic compressional and shear logs.