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

摘要

Unexpected borehole measurements are often inaccurately interpreted due to limited knowledge of the formation, particularly in tight-gas unconventional reservoirs. A novel method was applied to determine reservoir quality and the reliability of borehole array-induction resistivity measurements in a tight gas sandstone reservoir. Four exploration wells drilled with synthetic oil-based mud showed conflicting resistivity profiles. The discovery well showed a conductive invasion profile, but the appraisal wells showed resistive profiles. Simulation of oilbased mud-filtrate invasion was coupled with forward simulation and inversion of array-induction resistivity measurements to determine the difference in such resistivity profiles. Laboratory measurements on rock core and fluid samples were used to calibrate a log-based petrophysical model that was necessary to simulate the physics of fluid-flow mud-filtrate invasion. The dynamic process of invasion was simulated with a multicomponent formulation for the hydrocarbon phase and rock wettability alteration effects due to surfactants present in the mud. Simulated borehole-resistivity measurements were compared to field logs, and the rock properties were modified to secure a close agreement between simulated and field logs. The different invasion profiles corresponded to variable rock quality and mud composition. In the discovery well, good rock quality and thick surfactants in the mud caused a low mobility ratio between filtrate and native fluids, which created a water bank that moved ahead into the formation. This effect created a conductive annulus in the near-wellbore region. In turn, the deep invasion and high resistivity are due to excellent rock quality that is typical of a conventional sandstone reservoir. The shallower invasion and resistive profiles in the delineation wells suggest the tight gas sandstone reservoir we expected to find throughout the formation.