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.

 
 

In the last ten years Pneumatic Fracturing has emerged as one of the most cost effective methods for enhanced remediation of contaminated soil and groundwater. The general approach of the technology is to create a network of artificial fractures in a geologic formation that serves two principal functions. First, the fractures can facilitate removal of contaminants out of the geologic formation. Second, the fractures may be used to introduce beneficial substrates into the formation. The overall objective of fracturing is to overcome the transport limitations that are inherent at many remediation sites. Fracturing is a well established concept that has been applied in various forms within the petroleum and water well industries for more than 50 years.

The result of this action is the propagation of fractures outward at rates of 2+/- m/sec. Fracture propagation distances of 30 - 60 feet are common in rock formations. Unconsolidated materials such as silts and clays typically exhibit fracture propagation distances of 20 - 40 feet. In most geologic formations the propagation is relatively uniform around the injection well. Examination of a Pressure - Time History curve provides evidence that the cohesive bonds within the geologic matrix are broken and the creation of a fracture network occurs within the subsurface.

The type of geologic formation that will be fractured has a dominant influence on the results of the fracturing technology. Fine-grained soils such as silt and clay normally respond well to permeability enhancement by pneumatic fracturing. The permeability of tight bedrock formations can also be increased by fracturing. The initial pre-fracture permeability of the formation appears to play a significant role in the amount of permeability enhancement that can be expected. The lower the initial permeability of the formation the greater the expected increase in formation permeability. Conversely, if the initial permeability is higher, the observed improvement will be less. In this respect, there appears to be an upper limiting permeability that cannot be exceeded by fracturing. When using fracturing to deliver liquid or solid supplemental media a wider variety of formation types are treatable including sands, gravels, and highly fractured bedrock, and the upper limit concept does not apply.

It is important to thoroughly evaluate the geotechnical characteristics of the formation during the design phase of a fracturing project. Exploratory borings in the proposed treatment zone with continuous sampling or coring are recommended. The borings are normally supplemented with geotechnical tests performed on collected samples of the geologic material to be fractured. The following geotechnical field and laboratory tests are used to evaluate soil and rock formations for fracturing projects:

Soil Formations:

  • Detailed visual examination to assess structure including stratification, friability,
    secondary structure, and inclusions
  • Grain size analysis
  • Evaluation of consistency if cohesive, or relative density if cohesionless
  • Natural moisture content
  • Plasticity testing, including Atterberg limits
  • Location of water table and perched water zones
  • In situ permeability testing, e.g., slug, vapor extraction, pumping

Rock Formations:

  • Detailed visual examination to assess lithology, joint frequency and orientation,
    degree of weathering, joint filling, and natural bedding
  • Computation of recovery ratios and Rock Quality Designation (RQD)
  • Strength testing (in some cases)
  • Location of water table and perched water zones
  • In situ permeability testing, e.g., slug, vapor extraction, pumping
Down-hole Packer/Nozzle Assembly
 

If the geotechnical evaluation indicates that fracturing is applicable for a site, a pilot test can relatively cheaply be performed to establish actual fracture behavior in the formation.

Depth.

There is no theoretical maximum depth limit for initiating a fracture in a geologic formation as long as sufficient pressure and flow can be delivered to the fracture zone. In hydraulic fracturing the rule of thumb for estimating the injection pressure required to “lift the overburden” is 1 psi per foot of depth (7 kN/m2 per 0.3 m of depth). In pneumatic fracturing the injection pressure required to lift the formation is typically two
to three times higher on account of gas compressibility effects in the system. Additional pressure is required to initiate a fracture, overcome the cohesive strength of the formation.

To date, the target depths of most pneumatic fracturing projects have ranged from 3 to 15 m (10 to 50 ft). The deepest applications of pneumatic fracturing for site remediation purposes have been 60 m (180 ft). For fracturing applications below a depth of around 25 to 30 m (80 to 100 ft), it may be advisable to use proppants since elevated overburden pressures can inhibit self-propping. The ability of a formation to self-prop depends both on the depth and the strength of the geologic material.

Fracturing performed at an active facility.
 

The minimum depth of fracturing is controlled by the ability to form a top seal during injection because there is a tendency for fractures to intersect or vent to the ground surface (known as “daylighting”). Whether shallow fractures will vent is usually related to the compaction history of the formation. For example, if the formation is dense and stratified, fracture injections as shallow as 1 m (3.3 ft) might be successful. However, in fill materials and loose natural soils, fractures tend to incline upwardly, even at deeper depths. The likelihood of surface venting reduces below a fracturing depth of about 3± m (10± ft). The shallowest applications of pneumatic fracturing for site remediation have been 1.2 m (4 ft).

Plasticity.

Plasticity is a measure of the cohesion of a soil, i.e., the tendency of the soil particles to stick to one another. A highly plastic soil will deform without cracking and retain its deformed shape, for example. Soil plasticity is attributable to the presence of clay minerals, and the degree of plasticity depends on the proportion and kind of clay minerals present. As a rule of thumb, if the percentage of clay in a mixed soil exceeds 30%, then clay will dominate its overall behavior. Soils that are rich in the clay mineral montmorillinite exhibit the highest plasticity, while the clay minerals illite and kaolinite impart medium and low plasticity, respectively.

 

Inflated gloves show influence of fracture propagation.
 

Plastic soils are moisture sensitive with regard to their physical properties. For example, the strength of a plastic soil decreases with increasing moisture content. Also, a plastic soil will exhibit volume change in response to changes in moisture content: they shrink upon moisture loss and swell upon moisture gain. Soil plasticity is measured in the laboratory using the Atterberg Limits Tests (ASTM D4318-93), which involves varying the moisture content over a range and observing the behavior of the soil as it passes through semiliquid, plastic, semisolid, and solid states.

A common misconception about the technology is that high pressures are required to initiate fracturing. In fact, relatively low pressures (e.g. 100 psig or less) are sufficient at most sites.  The key to the technology is the flow volume. Typical injection events require more than one thousand standard cubic feet per minute (SCFM) or gas flow rate. The result of this low pressure, high volume process is the creation of a dense fracture network emanating from each injection location.

Normally, fracturing is coupled with another primary in situ or ex situ remediation technology such as vapor extraction, pump and treat, or bioremediation. A principal use of fracturing is to increase the effective hydraulic and pneumatic conductivity of the geologic formation being treated. This is important when treating fine-grained soils such as clay or silt, as well as tight bedrock. In such formations the movement of vapors and liquids is predominantly diffusion-controlled, so transport occurs rather slowly. By establishing a network of artificial fractures in the formation, advection increases and diffusive paths become shortened. The result can be quicker removal and/or treatment of contaminants, as well as access to pockets of contamination that could not be reached previously. The increase in formation permeability is accompanied by an increase in influence radius of treatment wells, so fewer wells are normally required. A related benefit of fracturing is that it can reduce the heterogeneities that are present in essentially all geologic formations. This makes pressure gradients more uniform throughout the formation, and operational control of the remediation process is improved.
For small projects, bulk cylinder packs are used for the fracturing source gas.
 

Another beneficial use of fracturing is for delivering various types of liquid and granular supplements into the geologic formation to support contaminant treatment. With pneumatic fracturing the supplements are injected either during the fracturing event or after the fractures have been created.

Another potential benefit of fracturing is that it may be retrofitted to sites already under active remediation in order to enhance recovery rates or treatment rates. New fracture wells or temporary boreholes can be installed between an existing array of treatment wells.

Two risks in the use of Pneumatic Fracturing have become apparent during the past decade of experience. The first is localized mobilization of contaminants within the fracture-enhanced zone that results from increased transport rates. It is therefore important that the coupled treatment process, e.g., soil vapor extraction, product recovery, be installed in a timely manner. A second risk is that fracturing has the potential to cause vertical movement or heaving at the ground surface. Therefore, if fracturing is performed in the vicinity of buildings or utilities, the effects of fracturing on these structures must be carefully evaluated. The amount of ground surface movement that may be caused by fracturing is related to a number of factors including the type of fracturing, depth of injection, number of fractures, and the geologic characteristics of the formation under treatment. Fracturing applications involving injection of solid media or proppants will cause the largest permanent vertical heave and potential structure movement. The number of projects ARS has performed in the vicinity of active structures and utilities is in excess of 50 sites and the technology is feasible in most cases as long as sound geotechnical design practices are followed.

The most common method to evaluate the effectiveness of Pneumatic Fracturing is to measure increases in effective permeability/conductivity and contaminant mass removal rate. Alternatively, the change in the specific capacity (discharge/drawdown) of installed wells may be used. Other evaluative methods include increase in the radius of well influence, measurement of fracture radius, and delivery rate of supplemental media.
Iron fracture
 

There are specific areas in remediation projects where Pneumatic Fracturing provides the greatest cost reductions. First, fracturing will reduce the number of treatment wells that must be drilled, and thus provide savings on initial capital costs and operating costs. Fracturing will also reduce treatment time, which proportionally reduces project operational costs. At some sites, the improvements manifested by fracturing may provide the only feasible way to use in situ remediation methods. In these situations the potential savings are more difficult to define, although one approach is to compare fracturing with ex situ treatment.

In many geologic materials pneumatic fracturing relies 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 selfpropping 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. Inert proppants such as sand or ceramic beads, or "reactive" proppants such as zero valent iron can be used with pneumatic fracturing and have most often 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.

Fluorescent tracer
 

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 the vadose, saturated, and perched groundwater 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, have also 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.

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