Using Molecular Modeling To Understand the Mechanism for Adhesion of Shale-Stabilizing Polymers to Clay Surfaces
Authors: Bell S. 
Published: December 2007

Fundamental concerns when drilling an active shale with aqueous drilling fluids are prevention of hydration, shale stabilization, and the reduction of cuttings erosion and wellbore stability. Inhibitive fluids like potassium chloride partially-hydrolyzed polyacrylamide (PHPA) fluids have proven effective for these types of drilling conditions. These systems use polyacrylamides as well as other similar polymers for their inhibitive properties. The underlying belief is that hydrogen-bonding occurs between clay surfaces and the acrylamide leads to stabilization of the shale. Use of synthetic polymer systems is rapidly increasing within drilling fluid systems. Consequently, understanding the polymer adhesion mechanism and the development of novel polymer systems that can impart improved adhesion characteristics has become ultimately important to the drilling process.

With these active shale concerns in mind, the author has undertaken molecular modeling studies to study the role of hydrogen bonding and other interactive forces in polymer bonding to clay surfaces, and then use the results to synthesize new shale-inhibition polymers. In particular, this research focused on polyacrylamide and related polymers bonding to illite and montmorillonite surfaces. Using Accelrys' Cerius2 software, finite clay and clay-aqueous systems were designed, to which polymers were added. These polymers were considered both at a distance from the surface where no significant interaction took place, and also near the surface. A comparison of these scenarios allowed for calculating comparative bonding energies for multiple polymers on the two clay surfaces.

Previous work has shown that good shale inhibitors typically have one or more of the following properties:1,2Limiting ion exchange into interlayer areas (so-called osmotic effect).Preventing hydration of surface counter-ions (typically, Na+, and K+).Sealing the clay layers such that water cannot be imbibed by the clay.

A key element to each of these properties is binding to the clay surface, from both kinetic and thermodynamic aspects. A kinetically binding polymer is one that binds quickly, perhaps preventing initial swelling of the clay, but over time, may not necessarily prevent long-term sloughing or hydration. A thermodynamically binding polymer may take longer to adhere, but will not likely be removed by other external factors; thus, being a long-term prevention of hydration or shale erosion. Of course, an ideal polymer might be one that will bind both kinetically and thermodynamically.

A qualitative understanding of the energetics involved in polymer binding to a clay surface, and what types of interactions occur, can allow insight into the thermodynamic binding effects that are present. It is important to note that the modeling described will only allow us to garner information on the thermodynamic aspect of the binding. Literature has shown that there are four types of bonding that could possibly effect significant clay-polymer interaction1,3–6: Hydrogen bonding directly to the surface.Hydrogen bonding indirectly to the surface (by means of excess water molecules).van der Waals interactions between large polymer chains and large clay entities.Electrostatic attraction between polar groups on the polymer and counter-ions present on the clay surface.