Previous laboratory and field evidence have proved that completed aluminum compounds can be utilized in water-based drilling fluids to increase shale stability. This environmentally-sound chemistry has never been used to its utmost potential, primarily because the previous aluminum complex required presolubilization and pH neutralization. The low aluminum concentration resulted in high dilution volumes with associated increased drilling fluids cost, while taxing the surface mixing equipment on the drilling rig.
A newly invented approach simplifies the use of aluminum chemistry in drilling fluids. This new chemistry is the result of laboratory investigations that confirmed the inhibition characteristics on Wyoming bentonite. Further laboratory work proved the effectiveness of this chemistry on numerous shales of varying mineralogy. Field evidence in several areas of the world has reconfirmed that this aluminum chemistry can, cost effectively, be used to reduce the hydration rate of troublesome shales, stabilize wellbores, and reduce drilling problems. This chemistry has proved to have application when drilling troublesome shales with freshwater systems on land, seawater systems offshore, and high chloride systems in deepwater operations.
This approach for drilling fluid chemistry provides an environmentally sound replacement for current additives. It demonstrates increased shale stability without the addition of potassium or chloride ions. The resulting reduction in clay hydration eliminates the need for chrome lignosulfonate additions, especially in polymer-based drilling fluids.
The hydration of clay containing shale has always been a major concern when drilling oil and gas wells. Upon contact with water, hydrogen bonding occurs at the clay surface, resulting in surface hydration. A tightly bonded molecular layer of water occurs on the basal crystal surface. Additional molecular water layers bond to each other, with each layer being less tightly bound. Surface hydration is particularly strong in shales because of high surface area. Additionally, clays may have a lattice-type structure. Such a structure allows adsorption of water between layers as well as upon the particle surface. Osmotic force then draws water into the shale particle, dependent upon the salt concentrations of the shale and the liquid. This hydration of clay softens and weakens shale formations.
Hole instabilities including hole sloughing, washout, and swelling are attributable to this mechanism. Additional problems experienced while drilling include shale dispersion, resulting in fine solids buildup in the drilling fluid, and the adherence of shale to the drill pipe and bit, reducing penetration rates. When drilling for hydrocarbons, water-based drilling fluids that do not effectively limit the hydration of the drilled shale may contribute to increased cost or project abandonment. Drilling problems such as stuck pipe, hole fill, and slow penetration rates will increase drilling difficulty.