Preliminary test results of inhibitive water-based muds used to mitigate unconventional shale drilling problems

Unconventional resources include oil and gas present in shale, tight sandstone and tight limestone formations. Shale oil can be produced from deposits of shale with estimates of about 5 trillion barrels of oil in place around the world. Because of the high content of clay in the shale formations, water-based drilling fluids tend to cause wellbore instability problems when drilling this type of formations. When it comes in contact with water, clay starts to react, swell and/or disperse leading to shale disintegration and sloughing. As a result of shale sloughing down into the borehole, cleaning efficiency of drilling fluids decreases significantly. Moreover, tight hole problem is expected which may cause drillpipe to get stuck and, as a result, increases non-productive time and well construction cost. Several types of shale inhibitive drilling fluids were developed using different shale inhibitors and stabilizers. Developing an inhibitive drilling fluid with long-term inhibition can eliminate the need for unnecessary casing and reduce tripping time. Multiple formations including the shale formation can be drilled and cased in one hole section. This paper summarizes preliminary laboratory testing results for characterizing one shale sample and assessing the interactions with different water-based mud systems. Shale characterization included determination of mineralogical composition using X-ray diffraction and determination of cation exchange capacity (CEC) while shale-mud interactions evaluation included swelling, dispersion and inhibition durability tests.