RIASSUNTO
Abstract
Some of the most successful water shutoff treatments in fractured reservoirs used relatively large volumes of gel that extruded through fractures during the gel placement process. Laboratory experiments show that gel extrusion through fractures occurs at an unexpectedly low rate if the fracture conductivity or width is sufficiently small. This paper demonstrates that this low rate of gel propagation occurs because the gel dehydrates as it extrudes through fractures. Our experiments used a Cr(III)-acetate-HPAM gel that is commonly injected during field applications. In fractures with conductivities between 1 and 242 darcy-ft (effective average widths between 0.006 and 0.04 in.), the gel was concentrated (or dehydrated) and gel propagation was delayed by factors typically between 20 and 40 during the extrusion process. The gel dehydration effect became less pronounced as the fracture width increased. However, a fracture width around 0.4 in. was required to completely eliminate the effect.
For a given fracture conductivity, a minimum pressure gradient (i.e., a yield stress) was necessary to extrude gel through the fracture. A correlation was developed that provides a good estimate of the required pressure gradient for gel extrusion for a wide range of fracture conductivity and width values. For example, to extrude the gel with a pressure gradient less than 1 psi/ft, the fracture width should be at least 0.1 in. During gel extrusion through fractures of a given width, the pressure gradient and degree of gel dehydration were nearly independent of position and velocity during both radial and linear flow. During brine injection after gel placement, no significant gel washout occurred for fracture widths up to 0.4 in.
P. 487