What is Pneumatic Fracturing?

Pneumatic fracturing can best be described as a process whereby a gas is injected into the subsurface at pressures exceeding the natural insitu pressures present in the soil / rock interface (i.e. overburden pressure, cohesive stresses, etc.) and at flow volumes exceeding the natural permeability of the subsurface. The result of this action is the propagation of fractures outward from the injection point.

How far out do the fractures go?

Fracture propagation distances of 30 - 60 feet are common in fractured bedrock formations. Unconsolidated silts and clays typically exhibit fracture propagation distances of 20 - 40 feet.

The Pressure - Time History curve is the best evidence that the cohesion bonds within the geologic matrix are broken and fractures are created in the subsurface.

How Pneumatic Fracturing is Applied?

Pneumatic Fracturing injections are applied to the subsurface in two ways:

  • Using a direct-push type drive casing
  • Within an open borehole

 

In rock formations and fine-grained unconsolidated formations, an open borehole is all that is required to effectively apply PF injections to the formation. Lowering our proprietary double or triple downhole packer systems, injections are applied to the subsurface in two to three foot intervals. In loosely consolidated geologies, PF injections are applied using a temporary driven casing.  This drill casing can be various direct-push, vibratory or sonic drilling methods.

After the drive-casing is advanced to depth the target depth, ARS lowers its injectors within this casing, which is then retracted exposing the internal packer/nozzle assembly. The packers are inflated sealing the borehole and the gas/material is injected into the formation. When the injection is complete, the outer casing is retracted farther upward with the packer nozzle system and positioned for the next fracturing event.

Under commercial production fracture projects, as many as 20 fracturing events per day can be accomplished using our new gas supply systems.

To what Type of Geology is Pneumatic Fracturing most applicable?

Pneumatic fracturing was developed with the idea that it would only be applicable to treat solely low permeability geologic formations such as silts, clays, and fractured rock. Initial testing was conducted on these formations as a way to increase the effective permeability to enhance the performance of existing remedia processes. More recently, the gas injection methods have been applied to formations containing coarser grained materials including sands and silty (non fracturable material) sands as a method of aeration, or geologic hetergeneity reduction.


What is the Self-Propping Phenomenon?

Following pneumatic fracture injections, the dialated formation settles and the fracture network constricts. Since the cohesive bonds of the geologic matrix are broken, closure of the fractures due to overburden stress will not occur at shallow depths (less than 75 feet). In rock formations a phenomenon known as self-propping has been observed. This behavior is attributed to asperities present along the fracture planes, as well as the rock block shifts during injections. Self-propping is accentuated in brittle geologic formations like siltstone.

Once opened, the open, self-propped fractures resulting from the pneumatic injections are capable of transmitting significant amounts of fluid flow. This high flow potential (even in small fractures) may be explained by the "cubic law," which states that flow rate in planar fractures is proportional to the cube of the aperture. Numerous hydrogeological studies indicates that cubic law prevails in fractured bedrock formations, and this is the principal reason why dramatic permeability increases are observed even in relatively small pneumatically created fracture networks. In summary, small non-sand filled fractures transmit greater volumes of fluid than larger sand filled fractures.  For deeper applications or specialized applications, ARS has the full capability to inject proppants within our fractures if required.