Effects of skin and hydraulic fractures on SVE wells.

Soil vapor extraction (SVE) systems are intended to cause substantial volumes of air to flow through the subsurface with the purpose of removing volatile contaminants. The effectiveness of SVE can be influenced by any effect that changes the specific gas capacity (discharge as a function of vacuum) of a well. Skins of low permeability material enveloping a well bore are widely recognized to affect the performance of wells used to recover water, natural gas, or petroleum, and skin can also significantly diminish the performance of an SVE well. Skins a few mm thick consisting of material whose gas phase permeability is 0.01 of the formation can reduce the specific gas capacity of an SVE well by factors of 2 to 10 or more. Hydraulic fractures created in the vicinities of shallow wells commonly resemble sand-filled layers shaped like flat-lying disks or gently dipping saucers. The contrast between the gas-phase permeability of the sand in the fracture and that of the formation is particularly important, with significant effects requiring the ratio to be greater than approximately 50. Shallow hydraulic fractures filled with several tenths of m3 of sand in formations that are several orders of magnitude less permeable than that of the enveloping formation should increase specific gas capacity by factors of 10 or more. Field tests of the effects of hydraulic fractures on the performance of SVE were conducted by creating four wells intersecting fractures and a suite of control wells created using conventional methods in silty saprolite. Specific gas capacities ranged over more than an order of magnitude for 10 wells completed within a small area (2 m2) and at the same depth. Specific capacities correlate to the drilling method that was used to create the bore for the well: lowest values occurred in wells drilled with a machine auger, slightly better results were obtained using a Shelby tube, and the best results were obtained from conventional wells bored with a hand auger. Skin factors determined for wells created with a machine auger could be explained by a layer 1 cm thick that has 0.007 times the permeability of the enveloping material, which could readily have been created during the drilling procedure. Specific capacities of wells intersecting hydraulic fractures were 5 to 100 times more than those of conventional wells. The large difference in performance appears to be due in part to the beneficial effects of the fracture, and in part to the detrimental effects of well skin.