Abstract
A critical property for many water-base muds is the ability to prevent near-wellbore pore pressure increase in shales. By slowing or preventing this pressure rise, a true overbalance pressure is maintained, which promotes wellbore stability. Using a device that simulates downhole rock stress and overbalance fluid pressure conditions, and using preserved shale samples, we tested four different muds containing different additives. Direct measurements of shale pore pressure vs. time were obtained, representing the pore pressure inside the wall of the wellbore. All muds slowed the loss of overbalance pressure compared to a base-case brine fluid, but two muds were very effective at slowing and/or preventing the loss of overbalance. One active mechanism appears to be an increase in the osmotic membrane efficiency, but permeability measurements conducted during these same tests show that physical plugging of the pore space is also an important mechanism for these two fluids. Measurements of fluid-induced swelling were also conducted in this same set of tests, and all four muds resulted in less swelling than the base-case brine. The most effective fluid was repeat-tested using a completely different shale, and it again was found to be very effective at preventing near-wellbore pore pressure increase.
