FLUID-FLOW PATTERNS AROUND A WELL BORE OR AN UNDERGROUND DRIFT WITH COMPLEX SKIN EFFECTS

A semianalytic solution is given for steady state flow around a well bore or a drift with a complex skin. The hydraulic conductivity of the skin may vary continuously as a function of the radial distance from the well (or drift) and may also be radially anisotropic. Such configurations can be found around damaged or acidized well bores or around a drift near which the stress redistribution induces changes in hydraulic conductivity. Purely radial flow, regional flow around an open or cemented hole without pumping or injection, and combined regional and radial flow are considered. Variations of hydraulic potential and Darcy velocity in various radial directions are studied for several cases and are found to be strongly affected by a complex skin. It is shown that the convergence of the streamlines toward the borehole, which is used in applications involving the point dilution method of measuring regional flow velocity, is significantly enhanced by a complex negative skin. Negative skin values may also reduce the size of the capture zone of a withdrawal well. Head distribution and tracer transport in combined radial and regional flow around a drift have also been modeled. It is demonstrated that tracer breakthrough curves from an experiment of tracer transport from an injection zone in the rock to the drift can be significantly affected by the fluid flow pattern around the drift, so the commonly used radial pattern may lead to erroneous results for tracer dispersivity.