Coreflood Evaluation of In-Situ Gelled Acids

Abstract

Three different commercial formulations (Company A, B and C) of in-situ gelled acids for matrix acid stimulation were compared in carbonate coreflood experiments at a temperature of 100 °F (38 °C) and at acid concentrations of 5 wt% HCl. The in-situ gelled acids contained a polymer, a cross-linker and a breaker in addition to corrosion inhibitors and surfactants. The coreflood experiments were done with small acid volumes (0.3 to 1.0 PV) so that permeability could be measured before acid breakthrough occurred. After acid injection, permeability was measured in both forward and reverse directions to assess the damage induced by the polymers used in these systems.The concentration of cross-linker in the coreflood effluent samples was measured since precipitation of the cross-linker may lead to formation damage. It was found that in-situ gelled acids reduce permeability of low permeability carbonate reservoir rock. The polymer in these formulations is the primary source of the permeability reduction. Cross-linker was retained in the reservoir cores during injection of in-situ gelled acid.Detailed viscosity measurements of each in-situ gelled acid were made as a function of equilibrium pH. Only Acid C showed significant in-situ gelation behavior in 5 wt% HCl.