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Passive Applications

Passive applications make reference to environmental remediation approaches that do not require pumping or extraction following the creation of hydraulic fractures. In these cases, the materials utilized to destroy the target contaminants are the solid proppant and/or fracturing fluids themselves. Typically, the appropriate materials are used to create fractures in the treatment zone, the borehole is sealed, and the materials are left to do their job.

Hydraulic fractures created for passive applications are not utilized for injection or extraction. However, these fractures are created within a pre-existing fluid flow field where the properties and geometry of hydraulic fractures can produce significant benefits. The presence of a high permeability, proppant filled fracture can distort the naturally occurring fluid flow field within a low permeability treatment zone. The high permeability fracture represents a path of least resistance within the flow field, therefore, flowpaths tend to converge on the upstream side (or edges) of the fracture and diverge on the downstream side (or edges) of the fracture. The result is a “capture zone” with a cross-sectional area that is much greater than the cross-sectional area of the fracture itself. In the case of a horizontal, flat lying, circular fracture, the capture zone takes on an oval shape in the far-field.

Results of a flow simulation plotted to show the 3-dimensional shape of a hydraulic fracture capture zone. The simulation involved placing a 10 m radius, 1 cm aperture, circular, flat-lying fracture into a preexisting flow field. The fracture to formation permeability ratio is 10,000:1. The zone is delineated to show flow that enters the fracture, however, flow exits the fracture following the same pattern.

The capture zone represents all flow that enters the fracture. Ground water that enters the fracture can subsequently react with the proppant material. The degree of treatment for flow entering an individual fracture is not uniform, because the residence times vary for different flow lines. The flowlines occurring near the centroid of the oval-shaped capture zone have the longest residence time, whereas those occurring near the perimeter have the shortest residence time. The dimensions of the capture zone increase with fracture dimension and fracture to formation permeability ratio. At high permeability ratios the capture zone can reach a maximum height of 0.9*DF and maximum width of 1.5*DF (DF = Fracture diameter).

Capture zone delineations from flow simulations for different fracture/formation k-ratios. Based on a 10-m-diameter, circular, flat lying, 1 cm aperture fracture.

High permeability proppant materials are not necessarily required to produce efficient hydraulic fractures in a passive remediation system. Assuming that the proppant material is soluble in ground water, a “plume” of dissolved treatment materials will be produced downstream of the hydraulic fracture in an active flow field. Fracture placement can be selected such that the resulting amendment plume encompasses a source area or overlaps a preexisting contaminant plume.

Diagram illustrating an amendment plume resulting from installation of a soluble material-filled fracture in a pre-existing flow field.

Development of an amendment “plume” requires that the hydraulic fracture is created within a pre-existing flow field with an appreciable flow rate. However, when created below the water table, amendment distribution outside of the hydraulic fracture will still occur even when zero natural groundwater flow takes place. This distribution occurs primarily due to dissolution and diffusive processes, and can result in a relatively large active treatment zone. For instance, potassium permanganate-filled fractures were installed in silty clay soils and core samples were taken through the fractures to evaluate distribution patterns. After 3 months a purple diffusion front had migrated to 10 cm away from the upper and lower surfaces of the fracture, and after 10 months the distance had reached 23 cm in both directions.

Example of a core taken through a potassium permanganate-filled fracture. Dark faces at the break represent the upper and lower surfaces of the fracture. Purple coloration along the core represents diffusion of the amendment through the formation.

Diagram showing positions of diffusion fronts from potassium permanganate-filled fracture with time. Data obtained from core sample data.