Fracturing water-sensitive shale formations with water-based fracturing fluids requires special attention to minimize the destabilization of high-clay-content shale formations. The reaction of water with clay minerals can result in clay swelling, dispersion, and flocculation. Failure to prevent these effects can lead to fracture-face softening and sloughing, resulting in significant loss of fracture conductivity.
The use of chemical additives designed to minimize the effect of water-based fluids on clay minerals is a well-established technology for formation-matrix treatment of high-permeability formations where fluid leakoff during a treatment is significant. Because shale formations exhibit permeabilities in the nanodarcy (nD) range, high fluid loss and matrix treatment is not the dominate concern. Typical pore-throat diameters in shale can be in the nanometer (nm) range, and actual molecular size becomes paramount.
A variety of additives are advertised to prevent clay damage, but little information is provided explaining how they work, how to select the appropriate product, and how to optimize the treatment. This paper presents a comparative study, including many of the currently available clay-control materials tested on a variety of simulated and actual shale-formation materials.
Chemical mechanisms describing how these products function to stabilize clay minerals and how these mechanisms control the duration they remain in the formation to provide protection to the clay minerals are also presented to enhance the general understanding of product selection, capability, and limitations. A review of current laboratory-testing methods used to make recommendations, including limitations and precautions, is included.