Wellbore-Stability Performance of Water-Based-Mud Additives

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 116139, "Wellbore Stability Performance of Water Base Mud Additives," by R.T. Ewy, SPE, and E.K. Morton, SPE, Chevron Energy Technology Company, originally prepared for the 2008 SPE Annual Technical Conference and Exhibition, Denver, 21–24 September. The paper has not been peer reviewed. A critical property for many water-based muds (WBMs) is the ability to prevent a near-wellbore pore-pressure increase in shales. By slowing or preventing this pressure increase, an overbalance pressure is maintained that promotes wellbore stability. Using a device that simulates downhole rock stress and overbalance fluid-pressure conditions and preserved shale samples, four different muds containing different additives were tested. Direct measurements of shale pore pressure vs. time were obtained, representing the pore pressure inside the near-wellbore region. Introduction One of the main shale-instability mechanisms that occurs with water-based drilling fluids is the wellbore pressure penetrating into the shale pore space. This increases the near-wellbore pore pressure and reduces the overbalance. This reduction of overbalance, which acts like a support pressure for the hole, can result in shale failure and wellbore instability. This pressure penetration cannot be prevented with standard filtration additives because shale pores are extremely small, approximately 0.01 µm, and shale permeability is extremely low, typically approximately 0.01 microdarcy or less. Experimental Each sample was cut from preserved downhole shale core and had never been contacted by water or brine. By avoiding "premature" contact with aqueous fluids, a meaningful swelling response on the shale as caused by the drilling fluid can be measured. Each mud test was performed on a new, preserved shale sample. The shale used for this study contains approximately 70% clay, approximately one-third to one-half of which is kaolinite, with the remainder being mostly mixed-layer illite/smectite. All samples were equilibrated to 96% activity in a vacuum desiccator before testing. This downhole shale core was cut with an oil-based mud and was kept fully preserved. The samples were cut and end-ground using decane, and they were not contacted by any aqueous fluid. Fig. 1 in the full-length paper illustrates the inner part of the shale/fluid-interaction test apparatus. The samples were right-circular cylinders with a 0.75-in. diameter and 0.5-in. length. The sample was surrounded by an impermeable jacket, which also extends to the top and bottom end caps. This allows a confining pressure to be applied to the sample using hydraulic oil (the entire illustrated section in Fig. 1 sits inside a pressure vessel). The confining pressure represents the in-situ stress acting on the shale near the wellbore.