Three general categories of fracturing technologies currently are available for site remediation. One is hydraulic fracturing, which creates subsurface fractures by pumping liquid into the formation. Another is pneumatic fracturing, which creates subsurface fractures with controlled bursts of high-pressure air or other gas. Blast fracturing is the other technique, which propagates subsurface fractures by detonation of high explosives. All three techniques propagate fractures by forcing a fluid into the geologic formation at a flow rate that exceeds the natural permeability and at a pressure that exceeds the normal geostatic stress. In blast fracturing, fractures also are generated by stress waves. The velocity of fracture propagation varies considerably among the three techniques, with hydraulic fracturing exhibiting the slowest velocity and blast fracturing the fastest. Pneumatic fracturing exhibits an intermediate propagation velocity.
In ConjunctionNormally, fracturing is coupled with another primary in site or ex site remediation technology such as vapor extraction, pump and treat or bioremediation. Over the last decade, several benefits of fracturing have been defined. 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 normally are 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.
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 or hydraulic fracturing, the supplements can be injected either during the fracturing event or after the fractures have been created. With blast fracturing, the supplements are introduced after the creation of the blasted bedrock zone. Examples include nutrients, buffers and inocula to support bioremediation; reactive solid media such as iron powder to support chemical reactions; liquid or solid oxygen-generating chemicals; electrically conductive media to support in situ electrokinetics and in situ vitrification; and solid proppants.
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. In some cases, it is possible to conduct fracture injections directly inside existing wells. Alternatively, new fracture wells can be installed between an existing array of treatment wells.
Two restrictions in the use of fracturing technologies have become apparent during the past decade of experience. The first is localized mobilization of contaminants in 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. Also, it is extremely important to employ consultants and fracturing vendors who have specific experience with hazardous site applications and fracturing techniques. A second restriction 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 evaluated carefully. 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. Also, blast fracturing produces transient vibrations that must be evaluated because they may limit blasting’s appropriateness as an application in areas in close proximity to sensitive structures. The number of projects involving fracturing in the vicinity of active structures and utilities is increasing rapidly. Recent experiences suggest that is feasible in most cases – long as sound geotechnical design practices are followed.
Geotechnical ConsiderationsA number of geotechnical factors must be considered in evaluating potential sites for fracturing applications. The most important factor is the type of soil or rock that is to be fractured. Other important considerations include depth, soil plasticity, soil density/consistency, secondary structure, rock fracture frequency, intact rock strength, rock weathering and water table configuration. Exploratory borings in the proposed treatment zone with continuous sampling or coring always are recommended. The borings are supplemented with both field and laboratory geotechnical tests. A fracturing pilot test normally is recommended to establish actual fracture behavior in the formation.
To date, pneumatic fracturing and hydraulic fracturing projects most frequently have been applied in soil formations containing silt and clay for the purposes of permeability enhancement and improving the performance of wells. Pneumatic and hydraulic fracturing also have been used for delivery of liquid or granular supplements into the subsurface, and in these applications, essentially all soil and bedrock types are considered treatable — including sands, gravels and mixtures. Blast fracturing has been exclusively applied in bedrock formations for permeability enhancement, and some applications of pneumatic and hydraulic fracturing also have been performed in bedrock. There is no theoretical depth limit for the fracturing technologies as long as sufficient pressure and flow can be delivered to the fracture zone. Applications of fracturing at shallow depths (<10 ft.) can lead to venting of the fractures to the ground surface, otherwise known as “daylighting.”
The Ground-Water Remediation Technologies Analysis Center (GWRTAC) has compiled 86 project summaries of pilot and full-scale fracturing projects in its “S” Series Report TS-00-01, “Technology Status Report Hydraulic, Pneumatic, and Blast-Enhanced Fracturing.” In addition to providing project summaries, the “S” Series report analyzes various trends, including integrated technologies, contaminant classes, site geology and fracturing results. This “S” Series report can be downloaded in PDF format from the GWRTAC Web site at www.gwrtac.org.
Measuring EffectivenessThe most common method to evaluate fracturing effectiveness 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. A review of the 86 case studies reported in the companion “S” Series report was undertaken to assess the general capabilities of the three fracturing technologies. The overall similarity of results produced by the three techniques was notable. For example, the reported increases in permeability/conductivity and specific capacity for pneumatic, hydraulic and blast fracturing ranged from 1.5 to 175 times, 5 to 153 times, and 0.7 to 100 times, respectively. Due to diffusion limitations, reported enhancements of mass transport rate were consistently less than permeability enhancements. When pneumatic fracturing or hydraulic fracturing is used as a delivery system for liquid or solid media, effectiveness is measured by distribution accuracy and delivery rate. The main determining factor for success of any fracturing application is properly matching process design and layout with the geologic conditions of the site.
All three of the fracturing technologies — pneumatic fracturing, hydraulic fracturing and blast fracturing — are commercially available for site remediation. Fracturing has been most often applied to enhance various physical treatment processes including vapor extraction, product recovery, dual phase extraction and pump and treat. Pneumatic fracturing and hydraulic fracturing also have been coupled with bioremediation and permeable reactive walls and zones. Pneumatic fracturing has been field pilot tested with in situ vitrification, and hydraulic fracturing has been field pilot tested with in situ electrokinetics.
Costs InvolvedThere are specific areas in remediation projects where fracturing can reduce costs. First, fracturing can reduce the number of wells that must be drilled, thus providing savings on initial capital costs and operating costs. Fracturing also can 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 an ex situ method that is feasible for the site.
The cost of fracturing varies according to the size of the project, depth of fracturing and how it will be integrated with the primary integrated technology. Recent data suggest unit costs for hydraulic fracturing range from $1,000 to $1,500 per well (assuming 3-5 fractures per well), and the unit costs for pneumatic fracturing range from $250 to $400 per fracture. The unit costs for hydraulic and pneumatic fracturing normally do not include drilling, well installation or long-distance mobilization. When these fracturing technologies are used to deliver supplemental media into the geologic formation, the costs are higher since materials and special equipment are involved. Blast fracturing normally is applied in a trench configuration so it is estimated on a linear foot basis. The unit cost of blast fracturing normally ranges from $120 to $200 per linear foot of blast trench. The cost includes drilling and blasting, but does not include well installation. The preceding costs for all of the fracturing technologies do not include those associated with the primary integrated technology (e.g., soil vapor extraction, product recovery). Unit fracturing costs also do not include engineering controls that may be required to mitigate potential effects on nearby structures and utilities.