Pneumatic emplacement involves the use of high-pressure nitrogen to initiate new fractures or enhance existing fractures to allow for distribution of chemistries into finer grained soils and bedrock. Following the fracture, amendments are then pumped into these zones. Typically, solid slurry chemistries like zero valent iron (ZVI), calcium peroxide, granular activated carbon, or potassium persulfate are applied.
Pneumatic fracturing is usually utilized for permeability enhancement of consolidated formations and for injecting dry and liquid amendments.
Pneumatic fracturing is achieved when high pressure air is injected and creates fractures (or fissures) in soil or rock formations. Formation fractures occur when the injection is performed at a pressure exceeding the in situ stresses, and fissure propagation occurs when injection flowrates exceed the natural conductivity of the formation.
In unconsolidated soil formations, pneumatic fracturing enhances the permeability by creating new fractures along weak or low stress pathways. Permeability increases in consolidated or fractured rock formations occur through existing fracture dilation and propagation of existing cracks. The success of the technology lies in the interconnection of the fractures during implementation.
If using pneumatic fracturing for the injection of amendments, the formation must first be fractured or fluidized using gas, typically nitrogen. Then, the amendment is introduced via the same gas stream to create homogeneous zones of remedial materials. With or without amendment injections, pneumatic fracturing utilizes either radial (360-degree) or directional (90-degree) nozzles.
Liquid media injection is initiated first by the fracturing event followed by the immediate injection of the amendment into the open fracture. In tight lithologies, alternating fracturing and injection minimizes preferential pathways by allowing amendments to follow the desiccated pathways. In non-cohesive formations, the formation is fluidized initially and the sinuous wave of alternating the gas stream and amendment provide a more uniform distribution.
Dry media injection is conducted concurrently, where the fracture is initiated followed by the immediate introduction of dry amendments carried within the injection gas stream. Initiation of the fracture temporarily displaces the groundwater allowing uniform emplacement of the remedial amendment. Contaminant groundwater immediately recedes within the treatment area, eliminating the potential of mounding. The final distribution results in stacked layers of remedial amendments.
Radial fracturing is typically utilized in consolidated formations or in tight brittle clays using straddled packer assemblies. Two packers are used to isolate a zone, allowing greater vertical intervals to be fractured without media loss to previous fracture zones. Radial fracturing is efficient and cost-effective when applied within large contaminant plumes or when used in conjunction with conventional remedial technologies such as soil vapor extraction, air sparging, or pump-and-treat systems.
Directional injection is typically used in conjunction with the injection of dry or liquid media in the overburden or unconsolidated formations. The nozzle utilized is a 90-degree directional injection nozzle. The process of injecting over 90-degree segments reduces the potential for injection “short-circuiting” or “daylighting” in one-direction and maximizes the distribution of the media radially around the injection well. Previous experience utilizing this nozzle has indicated maximum radial influence with minimal preferential pathways of injected media.
Pneumatic fracturing allows you to target intervals that don’t currently have flow paths. This distribution technology also allows you to select solid chemistries that are ideal for addressing the contaminant of concern, but are larger than the pore space available.
Contaminants can only be effectively treated if the amendment achieves sufficient contact—and in some formations, that contact can only be achieved through fracturing the subsurface.