Aqueous drilling through clay has always been a potentially difficult situation and, over the years, sundry solutions have been presented to combat this sticky enemy. These have ranged from simple, store-bought dish soap to high-tech and highly expensive block-co-polymer technology. In this article, I cover a few of the more commonly used anti-clay additive classes and give an overview of why each type works to improve clay drilling.
Let’s begin with the basics of why clay and shales are so difficult to drill. Clay types can cover a wide range and some are more reactive than others. While phyllosilicates like montmorillonite are certainly the worst of the lot, most of us are dealing with kaolin, illites and other less-reactive clay types. The term "clay" technically refers to a particle size rather than a distinct crystalline makeup. Shale or clay-stone and mudstone are composed primarily of clay and silt compressed over time to drive out any water, thus forming a hard, cohesive rock. However, when pulverized finely enough and allowed to hydrate with water, these supposed rocks can become quite sticky and even swell, just like the moist, garden-variety clay near the surface.