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

Abstract

Some laboratory and qualitative studies have documented the influence of water-based mud͑WBM͒-filtrate invasion on borehole resistivity measurements. Negligible work, however, has been devoted to studying the effects of oil-based mud͑OBM͒-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 commercial adaptive-implicit compositional formulation that enables the quantification of effects caused by additional components of mud-filtrate and native fluids. The formation under analysis is 100% water saturated ͑base case͒ and is invaded with a single-component OBM. Subsequently, we perform simulations of WBM filtrate invading the same formation assuming that it is hydrocarbon bearing, and compare the results to those obtained in the presence of OBM. At the end of this process, we invoke Archie's equation to calculate the radial distribution of electrical resistivity from the simulated radial distributions of water saturation and salt concentration and compare the effects of invasion on borehole resistivity measurements acquired in the presence of OBM and WBM. Simulations confirm that the flow rate of OBM-filtrate invasion remains controlled by the initial mud-cake permeability and formation petrophysical properties, specifically capillary pressure and relative permeability. Moreover, WBM causes radial lengths of invasion 15%-40% larger than those associated with OBM as observed on the radial distributions of electrical resistivity. It is found also that, in general, flow rates of WBM-filtrate invasion are higher than those of OBM-filtrate invasion caused by viscosity contrasts between OBM filtrate and native fluids, which slow down the process of invasion. Such a conclusion is validated by the marginal variability of array-induction resistivity measurements observed in simulations of OBM invasion compared with those of WBM invasion.

Model and method of permeability evaluation based on mud invasion effects

“…The water saturation suggests that the invasion is a gradual transition zone instead of a piston-like interface ( Figure 5b). The extent of the gradual change of the water The numerical simulation of the mud invasion was run for 100 h. The results overall agree with previous work (Salazar and Torres-Verdín, 2008) and verify the numerical model. Figures 4d-4f show that the permeability and porosity of the mud cake are nearly constant at 0.01 mD and 0.2, respectively, despite the fl uctuations in the pressure drop across the mud cake.…”

“…Previous studies concluded that the outer mud cake on the well wall is much thicker than the inner mud cake that penetrates into the pores near the well in the case of water-based mud invasion (Salazar and Torres-Verdín, 2008;Wang et al, 2009). Therefore, we only consider the outer mud cake because of the assumption of water-based mud invasion in this study.…”

Rock quality assessment using the effect of mud-filtrate invasion on conflicting borehole resistivity measurements

“…These maps are used to simulate borehole array-induction resistivity measurements, which can be used to rank rock quality within a reservoir. For an extended explanation of this methodology, the reader is referred to Salazar and Torres-Verdín (2009).…”

“…In general, oil-based mud invasion is shallower than invasion caused by water-based mud. However, invasion with SOBM can be radially deep enough to affect borehole array-induction resistivity measurements (La Vigne et al, 1997;Salazar et al, 2011;Malik et al, 2008;Salazar and Torres-Verdín, 2009). Both mud and rock properties control the dynamic growth of mud cake and the time evolution of fluid displacement (Wu et al, 2004).…”

Finite element simulations of logging-while-drilling and extra-deep azimuthal resistivity measurements using non-fitting grids

“…When the formation geometry is more complex, however, the relation between apparent resistivity and the actual resistivity distribution is not obvious. In addition, borehole effects [20,22], and invasion by drilling mud [13,28] can further affect the measurements.…”