Determination of the optimum wormholing conditions in carbonate acidizing using NMR

Detailed NMR scanning of the core was found to be a good assessing tool for the type of the stimulation fluid and locating the optimum wormholing generating conditions. For example, using a 3-inch length Indiana limestone core, conventional coreflooding experiments showed that 2 cm3/min injection rate generates the minimum acid volume HCl/VES acid system, however, NMR scan showed an injection rate of 3 cm3/min generates the highest pore interconnectivity for the wormhole. Gelled acidizing fluids such as HCl based on polymers created wormholes that were clearly identified by CT scan and pressure drop but the NMR scan showed that these wormholes are completely isolated from the rest of the surrounding pore system due to polymer residue plugging the pores. This isolation will reduce the production rate due to minimal radial fluid entry at the wormhole surface. Radial flooding experiments through the wormhole and production from the side of the core confirmed the findings of NMR scan regarding the interconnection between the wormholes and other pores in the rock. Strong relationship was found between the interconnectivity number and core radial permeability around the wormhole.