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FRx FAQ

Some of the most common questions that we receive are listed below:

- How does hydraulic fracturing increase formation permeability?

This is possibly the most frequently asked question that we receive, however, it is actually derived from a misinterpretation of how hydraulic fractures work. In truth, hydraulic fracturing does not affect formation permeability at all. By contrast, the process simply emplaces a high permeability lens through the formation, the permeability of which remains the same. The confusion is derived from the fact that hydraulic fracturing increases pumping or extraction rates from wells, and well performance is typically associated with bulk formation permeability. However, the cause can best be described as an increase in “effective” permeability. See below for an explanation of how hydraulic fractures increase well performance.

- How does hydraulic fracturing improve well performance?

The presence of a hydraulic fracture can manipulate the flow field around a stressed well, essentially decreasing the required distance that fluid must flow through the formation to reach the well screen. This is accomplished by maximizing the interfacial area between sand and formation, and by emplacing the sand away from the well. Potential for flow enhancement also increases along with fracture/formation permeability ratio, therefore, greater increases in effective permeability will be obtained in lower permeability formations. Note that this rearrangement of flow physics and hydrology violates some of the mathematical assumptions made when deriving conventional pumping test analysis. Therefore, application of these methods will generate only “effective” parameter value approximations. For more information on enhanced flow applications click here.

- Can hydraulic fracturing improve remedial performance in mid to high permeability formations?

From the perspective of enhanced flow applications, hydraulic fracturing technology is definitely best suited for lower permeability formation materials due to the factors described in the previous answer. That being said, enhanced flow can still be achieved based on the relatively small permeability ratio. More importantly, hydraulic fractures effectively slice through well skin. Eliminating the effect of well skin can produce significantly improved flow rates even in relatively high permeability formations.

Hydraulic fracturing for the purpose of emplacing chemically reactive solids within, or upstream of the treatment zone also has application in mid to high permeability formations. Hydraulic fractures represent a possibly less expensive alternative to the installation of reactive barriers in the form of trenches. The same can be said for emplacement of soluble chemical compounds or biological amendments. Directly emplacing these solids into the treatment zone allows direct, time-released dissolution into native groundwater. The alternative would be to inject large volumes of solution, which could potentially displace contaminated ground water from the treatment zone.

- What ranges of fracture diameters can be expected?

The ultimate diameter of a hydraulic fracture is dependent upon the volume of slurry injected. A good approximation of average radius can typically be obtained by assuming a fracture thickness (typically 0.5 to 1 cm), assuming that the fracture is cylindrical in shape, and calculating the cylinder radius required to account for the volume of solids injected. Approximately 100-300 gallons of 60% solids by volume slurry is used to create the typical FRx hydraulic fracture. This results in estimated average fracture radii ranging from 4 to 7 m (13-23 ft)(26-46 ft diameter), which tend to correlate with estimates made based on tiltmeter and uplift surface deformation measurements. These values are only estimates however, as hydraulic fractures tend to follow some preferential growth direction and typically do not remain completely horizontal. Also, different slurry volumes and compositions can produce different radii. For more information on fracture form click here.

- How does hydraulic fracturing compare to other injection technologies?

Hydraulic fracturing, as practiced by FRx, provides the opportunity to place large masses of treatment materials at targeted in situ locations. Hydraulic fracturing affords either greater loading capacity or better placement control than alternatives.

- What type of formation materials can be hydraulically fractured?

A fracture, or a feature that looks and acts a lot like a fracture, can be propagated through almost any material that can be considered a solid. At one extreme of hardness, consider the diamond; the gem cutter fashions beautiful jewelry by cleving, or fracturing the rough stone along several planes. At the other extreme, sheet-like structures have been induced by laboratory tests of putty and like substances. FRx has deployed fracturing methods in crystalline granite, sedimentary bedrock in various states of weathering, residuum soils, glacial soils, and soils derived from marine sediments.

- What type of well completions are required for hydraulic fracturing? Can pre-existing wells be used?

The ideal method for hydraulic fracture installation involves injection through direct push rods, which is the preferred methodology when site conditions allow. However, FRx has developed methods and protocols for successfully installing hydraulic fractures through a multitude of well types, including plain PVC casings and open boreholes.

Creation of hydraulic fractures through pre-existing wells is also a possibility, but is usually a secondary resort. Previously installed wells were typically not installed with the possibility of hydraulic fracturing in mind, therefore, necessary precautions may not have been taken to ensure proper sealing of annular spaces. In this case the well annulus can introduce an unwanted preferential flow path, or may not produce a seal appropriate for confining the pressures induced during the fracturing process. Also, hydraulic fractures cannot be created through traditional well screens, therefore, cutting through the casing to provide access to the formation would be required.

- Can hydraulic fracturing damage pre-existing structures at the ground surface?

Subsurface injection can cause vertical and lateral displacements of the ground surface. Any structures present at the ground surface will be subjected to these movements. Each building or structure has its own unique set of tolerances, and therefore may respond differently. Thresholds delineating displacements that may be destructive to a building can best be defined in terms of deflection (vertical movement as a ratio of span length). We have adopted a universal tolerance value based on what we perceive to be the most brittle part of a typical construction, plaster ceilings. The US Steel manual lists a deflection limit of 1/360 as a tolerance for preventing cracks in plaster ceilings. In terms of angle (microradians), this value is equal to 5500 um. FRx uses tiltmeters to monitor ground surface deformation when creating hydraulic fractures below pre-existing surface structures. As a general rule, to prevent potential damage to structures we will locate, control, and or halt injection in order to not exceed 5500 uR of surface deflection.

- How much site space is required for hydraulic fracturing equipment?

The onsite equipment assembled for a typical FRx job include a frac rig (20ft flatbed trailer), 1000 g water tank, 20ft box trailer, and possibly a wireline truck, drill rig, and/or track mounted geoprobe. Space limitations may be incurred near the injection point, where enough space must be available to maneuver the well installation and/or wireline equipment. However, excessive space near the injection point is not typically required during the injection process, as the frac rig and water tanks can be placed up to 200 ft from the well (access achieved using hoses).

- Does hydraulic fracturing pose an environmental risk in-and-of itself?

High volume hydraulic fracturing “fracking” for the purpose of stimulating gas wells in deep shale formations has attracted considerable attention lately. This attention has resulted due to the belief that these processes may cause potential environmental problems. FRx creates hydraulic fractures that are much different than those created to recover natural gas. Our fractures are designed to remediate contaminated ground, so every aspect is tailored to improve environmental quality. None of the potentially hazardous compounds used in fracking fluids are needed for our environmental applications. We use several types of fluids, and all the ingredients are fully disclosed to our clients and regulators. On-site, our field crews maintain strict procedures for waste handling and disposal in accordance with accepted protocols in the environmental industry. For additional info please click here.