Hydraulic Fracturing Process
There are a variety of methods that can be used to create hydraulic fractures. FRx has extensive experience in implementing a large number of hydraulic fracturing methods that are most relevant to environmental remediation applications. FRx fine-tunes field methods to most effectively satisfy project and site constraints. Factors that weigh-in during method selection include fracture target depths, remedial technology to be used, surface access, subsurface obstructions (utilities or existing wells), schedule, budget, and whether a fracture-intersecting well completion will be required. Since no two projects are identical, a description of fracturing methods for a hypothetical site will not be universally representative.
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The following steps describe what is perhaps the simplest method for creating hydraulic fractures. This method is especially effective in cohesive soils at depths as great as 75 feet.
Step 1: Equip rod or casing with a disposable drive-point. Utilize direct push or hammering to drive the casing to desired target depth. If convenient, a pilot hole may be drilled to within a few feet above the fracture target depth. The casing should be inserted at least 1 ft into native soil to ensure that lateral stresses can effect a sufficient seal for subsequent fracturing pressures. A small diameter soil core may be taken through the target zone prior to casing installation.
Step 2: Expose a short cylindrical wall of soil at the base of the casing. This can be accomplished by either A) Displacing the drive point downward by a few inches, or B) Retracting the casing a few inches. The exposed interval represents the elevation at which fracturing slurry will enter the formation.
Step 3: Cut a notch into the exposed formation material. Creating a notch effectively decreases the injection pressure required to break the formation, and also initiates fracture propagation on a horizontal plane. Notch creation is accomplished by rotating a high-energy water jet that is directed against the formation. Cuttings from the jet fill and overflow the casing, at which point they are collected at the surface and packaged for proper disposal.
Step 4: The notch and casing are filled with neat fracturing fluid, termed a “pad”. Sufficient pressure is applied to nucleate the fracture, or “break” the formation. The disk shape of the notch focuses stress along it’s perimeter, therefore, the fracture initiates along that edge.
Step 5: Fracturing slurry chases the pad into the open fracture following the break. The fracturing slurry is composed of proppant loaded fracturing fluid. The rheological properties of the fracturing fluid allow suspension of large proppant concentrations, yet also permit the slurry to flow when pumped. Fracture propagation occurs as slurry is pumped into the well, which is continued until desired fracture dimensions are reached, or a predetermined proppant mass has been delivered. Positive displacement pumps are generally best suited for pumping slurries, therefore, propagation is usually induced at constant volumetric rate.
Step 6: Individual fractures can be created at multiple depths from the same borehole. This is accomplished by washing fracturing slurry from the casing, pushing the casing down to re-capture the disposable tip, and then pushing the casing to the next target depth. The sequence listed above is repeated to create a fracture at the new depth, and the entire process is repeated until the number of desired fractures have been created.
Step 7: Passive treatment systems involving proppants such as oxidants, reductants, or nutrients typically do not require subsequent access to the fracture. In these instances the casing is usually removed and the borehole is grouted closed for completion. However, enhanced flow systems do require access to the fracture. There are multiple well completion options available for accomplishing this end. FRx has developed one methodology in particular that provides individual access to each fracture. This is achieved by installing nested wells within the borehole during the casing removal process.
Data can be collecting during the fracturing process that allow estimates to be made about fracture form. Descriptions of data collection and inversion techniques are available here.