Pneumatic fracturing (PF) is a technology that was also experimented in the 1980’s for environmental application at New Jersey Institute of Technology. The initial demonstration was performed in weak sedimentary rock formations and evaluated by EPA under the SITE (Superfund Innovative Technology Evaluation) program. Originally used for permeability enhancement of consolidated formations, the technology subsequently spun-off to other pneumatic enhancements including liquid injection and dry injection of remedial amendments. Five patents were granted for the technology.
Pneumatic Fracturing or Pneumatic Inclusion Propagation (PIP) enhances the permeability of geologic formations by injecting high pressure air; thereby creating 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, PF 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 the implementation of PF.
The immediate benefit of Pneumatic Fracturing is improved access to subsurface contaminants so that liquids and vapors can be transported and extracted rapidly, which represents a cost savings in the installation and operational phases of a remediation project. Another advantage of PF is the combined implementation of remedial amendments during fracturing; thereby, reducing the need to remobilize for further injections. The technology has been successfully applied within existing remedial systems as an enhancement and beneath or adjacent to existing structures and/or utilities.
Dry/Liquid Amendment Injection
The PF technology was expanded in the 1990s to include injection of dry and/or liquid remedial amendments. The formation is first fractured or fluidized using gas, typically nitrogen, and the amendment is introduced via the same gas stream to create homogeneous zones of remedial materials. A distinct advantage to this method is the initial introduction of the gas, which desiccates the formation and dilates the pore space, thereby allowing the passage of the injected media through the fluidized or expanded geologic matrix.
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.
The following flowchart provides the full suite of technologies complemented by the PF methods:
PF with or without amendment injections utilize either radial (360-degree) or directional (90-degree) nozzles. 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.
A distinct advantage to utilizing directional injection is the volume and type of media may be adjusted based on the isocontours of the plume. For example, the highest volume of media can be injected along the centerline of the plume while reducing perimeter injection points to address lower concentrations. Minimal potential of source area plume migration can also be accomplished by directionally injecting inward from perimeter points and multi-directional injections in the core. Other advantages include extending depth limitations of conventional trenching or excavation to essentially any depth and the ability to fracture and/or inject beneath or adjacent to structures and/or utilities.
Quality Assurance/Quality Control
GeoSierra Environmental maps the fractures and media distribution using several methods. Pressure-time history curves are recorded to identify the initiation pressure (or formation break pressure), the maintenance pressure for extension of the radius of influence, and the injection pressures if combined with an amendment. Additional performance monitoring during both dry and liquid media injections can be utilized to confirm consistent and overlapping zones of influence. Biaxial tiltmeters can be used to record subsurface deflection from surface ground movement as a result of deflection. Tiltmeters are arranged around the injection plot and monitored in real-time for confirmation of radius and direction. The cumulative data is used to generate a surface heave map, representative of the subsurface heave observed within the tiltmeter matrix.
Additional monitoring performed during consolidated formation fracturing include downhole pressure transducers in monitoring wells within the expected radius of influence. The transducers measure both pressure and temperature, and we can easily discern between connectivity during a fracture event or simple groundwater movement through existing fractures. Dye tracers have also been used in conjunction with preliminary borehole geophysics and downhole transducers to provide a 3-D map of fracture connectivity.
Alternatively, GeoSierra Environmental can utilize a surface resistivity array to monitor the extent of injections in the X-Y axis. As opposed to tiltmeters which rely on ground movement, the active resistivity system is a direct measurement of amendment propagation based upon changes in electrical resistivity, similar to its use in PRB real time monitoring.
Both tiltmeters and the surface active resistivity array can be used in conjunction with crack gauges. heave rods, transits and strain gauges to monitor potential building and/or utility movements during fracturing events. Continuous monitoring assists in recognizing and preventing structural impacts during injections.
Application Near Structures/Utilities
GeoSierra Environmental specializes in the successful application of the pneumatic fracturing technology near structures or utilities. As part of the initial site evaluation, the proximity to existing structures is examined as it relates to expected radius of influence and target depth. GeoSierra Environmental conducts their due diligence in ensuring no adverse effects occur during field implementation. A stepwise process is followed with the input and evaluation from a licensed professional engineer who has over 15 years of experience in the application of fracturing beneath structures.
A baseline structural and geotechnical evaluation is conducted, including the review of available as-built drawings and a site reconnaissance of the facility. Calculations of the structural tolerance as it relates to allowable movement are performed and a baseline report is completed, which includes structural monitoring recommendations. Once GeoSierra Environmental mobilizes, a video and photographic survey are conducted of the facility and the structural monitoring is installed. Structural monitoring equipment that may be used includes biaxial tiltmeters, crack gauges, heave rods, survey and/or strain gauges. Once field activities are completed, GeoSierra Environmental conducts another survey to document final site conditions.
Over 70% of our sites have been under structures or utilities with no adverse effect on the structural integrity.