Wellbore-Stability Performance of Water-Based Mud Additives
- Russell T. Ewy (Chevron Energy Technology Company) | E. Keith Morton (Chevron Energy Technology Company)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2009
- Document Type
- Journal Paper
- 390 - 397
- 2009. Society of Petroleum Engineers
- 1.2.1 Wellbore integrity, 1.2.2 Geomechanics, 1.6 Drilling Operations, 5.1 Reservoir Characterisation, 6.5.4 Naturally Occurring Radioactive Materials, 4.3.1 Hydrates, 1.11 Drilling Fluids and Materials
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A critical property for many water-based muds is the ability to prevent near-wellbore pore pressure increase in shales. By slowing or preventing this pressure increase, 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 completely different shales; it again was found to be very effective at preventing near-wellbore pore pressure increase for one of the shales, but not for all shales.
One of the main shale instability mechanisms that occurs with water-based drilling fluids is that the wellbore pressure penetrates into the shale pore space. This raises the near-wellbore pore pressure and reduces the true overbalance. This reduction of true overbalance, which acts like a support pressure for the hole, can result in shale failure and wellbore instability. The pressure penetration cannot be prevented with standard filtration additives, because shale pores are extremely small (approximately 0.01 micron) and shale permeability is extremely low (typically ~0.01 microdarcy or less); therefore, a filter cake does not develop on shales.
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