Jesús M. Salazar

Quantitative comparison of processes of oil- and water-based mud-filtrate invasion and corresponding effects on borehole resistivity measurements

Some laboratory and qualitative studies have documented the influence of water-based mud(WBM)-filtrate invasion on borehole resistivity measurements. Negligible work, however, has been devoted to studying the effects of oil-based mud(OBM)-filtrate invasion on well logs and the corresponding impact on the estimation of petrophysical properties. We quantitatively compare the effects of WBM- and OBM-filtrate invasion on borehole resistivity measurements. We simulate the process of mud-filtrate invasion into a porous and permeable rock formation assuming 1D radial distributions of fluid saturation and fluid properties while other petrophysical properties remain constant. To simulate the process of mud-filtrate invasion, we calculate a time-dependent flow rate of OBM-filtrate invasion by adapting the available formulation of the physics of WBM-filtrate invasion. This approach includes the dynamically coupled effects of mud-cake growth and multiphase filtrate invasion. Simulations are performed with a commercia...

Fast 2D inversion of large borehole EM induction data sets with an efficient Fréchet-derivative approximation

In addition to reliability and stability, the efficiency and expediencyofinversionmethodshavelongbeenastrongconcernfor their routine applications by well-log interpreters. We have developedandsuccessfullyvalidatedanewinversionmethodtoestimate2Dparametricspatialdistributionsofelectricalresistivity from array-induction measurements acquired in a vertical well. The central component of the method is an efficient approximation to Frechet derivatives where both the incident and adjoint fields are precomputed and kept unchanged during inversion.To further enhance the overall efficiency of the inversion, we combined the new approximation with both the improved numerical mode-matching method and domain decomposition. Examples of application with synthetic data sets show that the new method iscomputerefficientandcapableofretrievingoriginalmodelresistivities even in the presence of noise, performing equally well in both high and low contrasts of formation resistivity. In thin resistive beds, the new inversion method estimates more accurate resistivities than standard commercial deconvolution software. We also considered examples of application with field data sets that confirm the new method can successfully process a large data set that includes 200 beds in approximately 40 minutes of CPU time on a desktop computer. In addition to 2D parametric spatial distributions of electrical resistivity, the new inversion methodprovidesaqualitativeindicatoroftheuncertaintyofestimated parameters based on the estimator’s covariance matrix. The uncertainty estimator provides a qualitative measure of the nonuniqueness of estimated resistivity parameters when the data misfitlieswithinthemeasurementerrornoise.

Combined simulation and inversion of SP and resistivity logs for the estimation of connate-water resistivity and Archie’s cementation exponent

Knowledge of initial water saturation is necessary to estimate original hydrocarbon in place. A reliable assessment of this petrophysical property is possible when rock-core measurements of Archie’s parameters, such as saturation exponent n and cementation exponent m , are available. In addition, chemical analysis of formation water is necessary to measure connate-water resistivity Rw . Such measurements are seldom available in most applications; if they are available, their reliability may be questionable. We describe a new inversion method to estimate Rw and Archie’s cementation exponent from the combined use of borehole spontaneous-potential (SP) and raw array-induction resistivity measurements acquired in water-bearing depth intervals. Combined inversion of resistivity and SP measurements is performed assuming a piston-like invasion profile. In so doing, the reservoiris divided into petrophysical layers to account for vertical heterogeneities. Inversion products are values of invaded and virgin format...

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

We 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 axialsymmetric 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 mudfiltrate 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 mudfiltrate 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. Introduction In general, OBM increases drilling rates and provides better quality boreholes than when drilling with water-base mud (WBM). In oil-base muds, the continuous fluid phase is a mixture of liquid hydrocarbons. This continuous phase dominates the process of invasion and mixes with formation fluids. Water is also present in the form of an emulsion. Chemical emulsifiers/surfactants are added in the mud to prevent water droplets from coalescing and leaving the emulsion (Bourgoyne Jr. et al., 1986) and to ensure that the weighting material is wetted by oil (Skalli et al., 2006). Calcium or magnesium fatty acid soaps are typically used as emulsifiers. Array-induction resistivity measurements are almost invariably acquired in OBM environments. Such measurements exhibit several radial lengths of investigation, typically from 10 inches to 90 inches and, in some cases, 120 inches. The variability of resistivity curves exhibiting different lengths of investigation is, in most cases, a footprint of mudfiltrate invasion. When array-induction measurements are acquired in the presence of WBM, large separation of resistivity curves and deep invasion can be expected due to monotonic change of formation resistivity. Ideally, we would not expect such a behavior in the presence of OBM. However, in hydrocarbon zones OBM-filtrate can replace native hydrocarbon and movable water, thereby resulting in an invasion front with different resistivity values in the nearwellbore region (La Vigne, et al., 1997). Moreover, invading mud filtrate is miscible with native oil. In the mixing process, OBM causes changes of fluid density and fluid viscosity, thereby modifying the apparent oil phase mobility in the invaded zone (Malik et al., 2007). The objective of this paper is to quantify the effect of fluid properties (density and viscosity) and composition on arrayinduction measurements acquired in the presence of OBMfiltrate invasion. Special attention is given to the presence of surfactants in the OBM. Previous laboratory experiments with core samples (Van, et al., 1988; Yan and Sharma 1989) show SPE 109946 Fluid Density and Viscosity Effects on Borehole Resistivity Measurements Acquired in the Presence of Oil-Based Mud and Emulsified Surfactants Jesús M. Salazar, SPE, Mayank Malik, SPE, Carlos Torres-Verdín, SPE, Gong Li Wang, SPE, and Hongyan Duan, SPE, The University of Texas at Austin

Fast 2D inversion of large-borehole EM induction data sets with a domain-decomposition method

Inversion methods have become increasingly important to estimate formation resistivity from borehole electromagnetic (EM) measurements. Recent advances in hardware computer resources as well as the formulation of new minimization algorithms make it feasible to approach the nonlinear inversion of large data sets in an efficient and reliable manner. When the number of formation beds and radial zones is large, the difficulty of the inverse resistivity problem significantly increases due to the increasing demand of computer resources. This situation has hampered the routine application of inverse methods by the borehole geophysics community to estimate spatial distributions of electrical resistivity in the near-borehole region. Several methods have been advanced to approach this problem with relative success. We develop and successfully validate a new inversion method to estimate 2D spatial distributions of electrical resistivity from array-induction measurements acquired in a vertical well. The inversion is approached with a domain-decomposition method coupled with approximate calculation of measurement sensitivities. Examples of application with noisy synthetic and field data sets confirm the efficiency and reliability of the inversion method introduced in this paper.