Fluid Density and Viscosity Effects on Borehole Resistivity Measurements Acquired in the Presence of Oil-Based Mud and Emulsified Surfactants

Abstract

AbstractWe quantify the influence of oil-base mud-filtrate invasion and formation fluid properties on the spatial distribution of fluid saturation and electrical resistivity in the near-wellbore region. The objective is to appraise the sensitivity of borehole resistivity measurements to the spatial distribution of fluid saturation resulting from the compositional mixing of oil-base mud (OBM) and in-situ hydrocarbons.First, we consider a simple two-component formulation for the oil phase (OBM and reservoir oil) wherein the components are first-contact miscible. A second approach consists of adding water and surfactant to a multi-component OBM invading a formation saturated with multiple hydrocarbon components. Simulations also include presence of irreducible, capillary-bound, and movable water. The dynamic process of OBM invasion causes component concentrations to vary with space and time. In addition, the relative mobility of the oil phase varies during the process of invasion because oil viscosity and oil density are both dependent on component concentrations.Presence of surfactants in the OBM is simulated with a commercial adaptive-implicit compositional formulation that models the flow of three-phase multi-component fluids in porous media. Simulations of the process of OBM invasion yield two-dimensional spatial distributions of water and oil saturation that are transformed into spatial distributions of electrical resistivity. Subsequently, we simulate the corresponding array-induction measurements assuming axial-symmetric variations of electrical resistivity.We perform sensitivity analyses on field measurements acquired in a well that penetrates a clastic formation and that includes different values of density and viscosity for mud-filtrate and formation hydrocarbon. These analyses provide evidence of the presence of a high-resistivity region near the borehole wall followed by a low-resistivity annulus close to the non-invaded resistivity region. Such an abnormal annulus is predominantly due to high viscosity contrasts between mud-filtrate and formation oil. The combined simulation of invasion and array-induction logs in the presence of OBM invasion provides a more reliable estimate of water saturation, which, in turn, improves the assessment of in-place hydrocarbon reserves.