The current understanding on physico-chemical interactions of shales and drilling fluids is reviewed. The complicated relationship between transport in shales (e.g. hydraulic flow, osmosis, diffusion of ions and pressure) and chemical change (e.g. ion exchange, alteration of water content and swelling pressure) that governs the stability of shales is clarified. This fundamental understanding of shale-fluid interactions is then linked to the latest developments in water-based drilling fluid technology which targets improved cuttings- and wellbore stabilization and reduced bit balling. Different types of water-based shale stabilizers are discussed. In addition, a classification of shale drilling fluids system is given based on their shale-stabilizing action and efficiency. It is shown that oil-based/synthetic-based drilling performance is now achievable with economically and environmentally compatible water-based drilling fluids.
The problem of wellbore stability in shales has frustrated oilfield engineers from the start of oil- and gas well drilling. Wellbore instability is in fact the most significant technical problem area in drilling and one of the largest sources of lost time and trouble cost. A typical example of problems encountered in the field is given in Fig. 1. The 8 1/2" section of this well, drilled with a water-based mud, was enlarged up to 25" despite the presence of additives used especially for shale-stabilization purposes. Operational problems that derive from such instabilities may range from high solids loading of the mud requiring dilution, to hole cleaning problems due to reduced annular velocities in enlarged hole sections, to full-scale stuck pipe as a result of well caving and collapse.
Wellbore stability is almost a trivial issue with oil-based and synthetics-based muds. Once mud weight and invert emulsion salinity are properly established, stability can virtually be guaranteed (except for a few cases such as fractured shales). Much more problematic have been the adverse interactions of shales with water-based fluids. These are environmentally attractive alternatives for oil- and synthetic muds, but they are still outmatched by the latter in shale drilling performance.
The central issue raised in this paper is : "which chemical means can be exploited to achieve full shale stabilization and reach the desired operational performance with water-based drilling fluids?". The nature of the shale instability problem must be understood first in order to answer these questions. This requires appreciation of transport in shales, the physicochemical changes effected by this transport, and the implications of the former to shale stability. These relationships are clarified in a step-by-step approach.
Fig. 2 gives a simplistic but practical model for the forces acting on a shale system containing clays and other minerals (primarily quartz) at silt size. They can be subdivided into mechanical and physico-chemical forces. The former include:
The in-situ vertical (overburden) and horizontal stresses
The pore pressure
The stress acting at intergranular contact points, e.g. at cementation bonds
The latter, acting primarily in the clay fabric, include:
The van der Waals attraction
The electrostatic Born repulsion
Short-range repulsive and attractive forces that derive from hydration/solvation of clay surfaces and the ions that are present in interlayer spacings (adsorbed or free).
The latter forces are usually lumped together to form the "hydration stress/pressure" or "swelling stress/pressure", since they are responsible for the characteristic swelling behavior of clays and shales. The term "swelling pressure", well-accepted in oil-field practice, will be used exclusively below.