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-
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-
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.
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.