Drilling Mud Filtration and Its Effect on the Electrical Resistivity of -Porous Media

The project described in this paper was undertaken to provide a more thorough understanding of the effect of contamination by drilling fluids on the electrical properties of porous systems. The basic work performed was filtration tests on both consolidated and unconsolidated sands and the determination of their resistivity profiles with the aid of screentype electrodes spaced at convenient intervals along the core. Four drilling fluids were selected, ranging in resistivity from 447.0 ohm-cm. to •5.76 ohm-cm. The permeability of three unconsolidated and four consolidated cores, on which resistivity measurements were made, varied from 1,257 md. to 4,'560 md. Five additional cores with permeabilities as low as 49 md. were tested to investigate the effect of permeability on the depth of invasion of drilling fluid s. The effect of drilling fluid invasion on resistivity profiles was determined on cores containing brine only, and on those containing 80 to 90 per cent oil and 20 to 10 per cent brine. The study included measurements of (a) clay content of different sections of the core, (b) saturation of the invaded fluids and oil in different parts of the core, and ( c) changes in resistivity as a result of gra vitational r e-dist ribution of fluids and diffusion of electrolytes. The da t a showed that the resistivity of an invaded core is, as was expected, controlled by the resistivity of the filtrate, the resistivity of residual core water, the resistivity of the clay particles, and the amount and distribution of the filtrate, clays, residual oil, and brine. In most cases there was a significant increase in resistivity owing to the presence of clay solids which had invaded the core. The change in resistivity, as a result of the diffusion of salts, was substantial after a period of five hours. There is evidence in some cases that the residual oil and core water saturations of the unconsolidated cores, after substantial invasion, are very low, indicating almost complete displacement by invading fluid.
Keywords: BRINE), drilling fluid, Mud C l, OHM-CM, resistivity, Clay, Clays, CORE, Fluids, Oil, Oils, Profiles, Resistivity, Water, Waters
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