Hydraulic Fracturing

Hydraulic fracturing is a method for creating lenses of granular material in soil or rock. The technology was originally developed in the late 1940's, and has long been recognized as a viable method for increasing the yields of gas and oil wells. FRx has adapted hydraulic fracturing as a technology for enhancing environmental remediation. The technology is particularly suited for application in clayey soils, glacial tills, and saprolites that are otherwise difficult to remediate due to low permeability.

Well head injection pressure log from a typical hydraulic fracturing event.

Physics of Fracturing: All solid materials will fail if enough stress is applied. Hydraulic fracturing methods use fluid pressure to create the stress needed to nucleate and propagate a fracture in soil or bedrock. Fracture nucleation is referred to as the "break". The pressure required to break the formation is primarily dependent upon depth of overburden, material toughness, and how the pressure is applied (the “art” of hydraulic fracturing). Once the break is achieved, less pressure is required to propagate the fracture through the formation. The fracture will continue to propagate until it hits an obstruction or intersects the ground surface, or until the rate of fluid loss from the fracture into the surrounding formation equals rate of injection.

Excavated face exposing fracture created using sand as proppant.

Proppants and Fracturing Fluid: The major benefits provided by hydraulic fracturing exist due to the ability to fill the fractures with "proppant". Proppants are composed of solid granular materials that are injected along with fracturing fluid and enter the open fracture during propagation. Following the creation of a hydraulic fracture the fracturing fluid dissipates into the formation or is pumped back out through the well casing. The solids are left behind and serve to permanently "prop" the fracture open. Quartz sand has traditionally been used as proppant during enhanced flow applications, however, just about any fine- to medium-grained granular material can be used. (i.e. oxidative chemicals, reductive chemicals, nutrients).

Fractures can be created by injecting plain water or even gas, but the ability to suspend significant quantities of proppant in these fluids is limited. This limitation is overcome by utilizing a high viscosity fluid during injection.

The fluid most commonly used during hydraulic fracturing is guar gum gel. Guar gum is a food additive derived from the guar bean. Guar gum forms short-chained polymers when mixed into water, resulting in a solution that has the consistency of mineral oil. Addition of a crosslinker (Borax) causes the polymer chains to link, which increases the viscosity significantly. The resulting gel is capable of suspending high concentrations (50-75% total volume) of relatively dense proppant material during injection. An organic enzyme added to the gel breaks the polymer chains and decreases the viscosity of the fluid following fracture emplacement.

Cross-linked guar gum gel

Sand loaded gel ready for injection