This installment of the series discusses pneumatic fracturing.

Pneumatic fracturing is performed by injecting high-pressure gas down a borehole to create subsurface fractures. Proprietary injection nozzles normally are used with or without inflatable packers. The technology can be applied in an open borehole, through specially designed well screens (no pack) or through a direct push probe.

The source of compressed gas typically is high-pressure storage cylinders. Formations are fractured sequentially by moving the injection nozzle vertically within the borehole or well, and then moving the injection nozzle to adjacent locations until the entire treatment zone is fractured. The vertical interval between fractures varies from 6 inches to 3 feet, although 2 feet is typical. Fracture propagation is relatively rapid – approximately 6 feet per second – and a single injection event lasts about 15 seconds to 20 seconds. Pneumatic fracturing equipment normally is trailer-mounted and relatively portable. Accessory equipment is available to inject a wide variety of liquid or granular supplements into the formation along with a pressurized gas to complement various remediation technologies.

Many applications of pneumatic fracturing rely on “self-propping” of the geologic formation. The basis for this approach is the Cubic Law, which demonstrates that fluid flows through open fractures can be substantial, even ones with relatively small dimensions. Brittle geologic materials such as stiff clays and bedrock exhibit good self-propping since irregularities along the fracture surface (known as asperities) and shifting of the geologic medium prevent fracture closure. In plastic clays, research has demonstrated that even if a fracture constricts temporarily due to swelling, the process is fully reversible and the fracture will recover due to natural conditions or operational controls. Proppants such as sand or ceramic beads can be used with pneumatic fracturing and most often have been applied in fine-textured cohesionless soils such as silty sand. When injecting proppants or other solid media with pneumatic fracturing, the media are transported directly into the formation by the high-velocity gas stream.

Pneumatic fracturing can be distinguished from two other technologies that also involve the injection of gas into geologic formations – air sparging and air injection. Both of these technologies force air through existing pores and fractures in the formation. In contrast, pneumatic fracturing dilates the formation and creates new or expanded pathways for transport.

Pneumatic fracturing has been applied in vadose, saturated and perched ground water zones. For permeability enhancement, the technique has been applied in fine sands, silty sands, silts, clays and various soil mixtures containing silt and clay, including saprolites. The permeability of sedimentary rock formations, including mudstones, siltstones, sandstones and shale, also has been enhanced with pneumatic fracturing. When used for injection of liquid, granular or gaseous supplements, essentially all soil grain sizes and some bedrock types are considered treatable with pneumatic fracturing, including sands, gravels and mixtures.