When remediating contaminated soil and ground water in situ, the mass transport rate allowed by the geologic formation is critically important. Permeable formations such as sand and gravel allow relatively high transport rates, and cleanup of certain contaminants can proceed relatively quickly.
However, geologic formations containing clay, silt and tight bedrock impede transport rates and can severely limit the efficiency of both in situ and ex situ technologies. Remediation of such formations can be difficult, if it can be accomplished at all. The limiting level varies somewhat according to the viscosity of the fluid being transported in the formation and the adsorption potential of the soil or rock minerals.
Over the last decade, fracturing techniques have emerged as a viable approach for overcoming transport limitations at some sites. The idea is to create a network of artificial fractures in the geologic formation that can be used for two principle purposes. First, fractures can facilitate removal of contaminants out of the geologic formation. Second, fractures may be used to introduce beneficial reactants into the formation. Subterranean fracturing is an established concept that has been applied in various forms within the water well and petroleum industries for more than 50 years. Currently available fracturing techniques for site remediation can be divided into three general categories:
- Pneumatic fracturing, which uses pressurized gas to propagate fractures
- Hydraulic fracturing, which uses pressurized liquids to propagate fractures
- Blast fracturing, which detonates high explosives to propagate fractures
Normally, fracturing is used as an enhancing technique that is coupled with another primary remediation technology such as vapor extraction or product recovery. In these applications, fracturing improves the efficiency of the primary technology by increasing the effective permeability of the geologic formation, which usually is manifested by an improvement in well performance to recover or deliver fluids. Fracturing also may permit extension of the primary technology to a geologic condition that would not normally be treatable with the primary technology.
The use of fracturing in site remediation is not limited to permeability enhancement. The fracturing technologies also can function as delivery systems to introduce various types of treatment media, either liquid or granular, directly into the geologic formation. For example, nutrients can be injected to enhance bioremediation, and iron powder can be injected to support reductive dechlorination.
Successful fracturing of a geologic formation with a fluid requires that two basic operational conditions be met. First, the fluid must be injected at a flow rate that exceeds the ability of the formation to receive the fluid, i.e., the flow rate must be greater than the native permeability of the formation. Second, the fluid must be injected at a pressure that equals or exceeds the in situ geostatic stresses at the depth of the injection. As long as these two operational conditions are met, fractures will propagate from the point of injection and into the geologic formation. With blast fracturing, fractures also are generated by stress waves in addition to those created by expanding gases from the explosions.
The propagation of fractures in geologic formations has been studied for more than 50 years in a variety of industrial applications. Most applications have focused on the creation of artificial fractures for beneficial purposes, as in the hydraulic fracturing techniques used in the water well and petroleum industries. Other beneficial uses of fracturing include permeability testing and pressure grouting. Studies also have focused on avoiding fracture propagation, as in the design of landfill liners and caps.
The key distinguishing characteristic of the various fracturing techniques is the kind of fracturing fluid that is injected. There also are differences in the propagation velocities of the fractures through the geologic medium. Propagation velocity is related to the rate of pressurization, as well as the viscosity of the fracturing fluid.