Leak-off During Placement of Cr(III)-Partially Hydrolyzed Polyacrylamide Gelling Solution in Fractured Porous Media

During waterflooding of a fractured formation, water may channel through the fracture or interconnected network of fractures, leaving a large portion of oil bearing rock unswept. One remedial practice is injection of a gelling solution into the fracture. Such placement of a gelling mixture (referred as gelant) is associated with leak-off from the face of the fracture into the adjoining matrix. As the gelant gets more crosslinked, the gelant encounters more resistance in flowing into the porous matrix. This article addresses the build-up of flow resistance as the Cr(III)-partially hydrolyzed polyacrylamide gelant, at various stages of crosslinking flows into the matrix. Flow experiments were conducted at constant injection pressure in unfractured Berea rocks that represent a matrix adjoining a fracture. Before entering the core, gelants underwent post-mixing delays, shorter than their gel time. On continued displacement, flow resistance developed that reduced the flow rate further. More delay, after mixing of gelant hastened, the build-up of resistance to flow and the resistance was contained nearer to the inlet face. Effect of flow over fracture face on the build-up of flow resistance in the matrix was also evaluated by conducting displacement of gelant in two fractured slabs. In one case, a part of the injected fluid came out of the fracture outlet with the rest leaking off into matrix. In the other case, all the fluid that entered into the fracture leaked off into the matrix. Build-up of flow resistances in the matrix for the two cases was compared. A simple conceptual model is presented that could explain the flow of gelant and build-up of resistance in porous rock at constant injection pressure.