Determination of the optimum wormholing conditions in carbonate acidizing using NMR

Abstract

The current optimum wormhole injection rate during acid stimulation treatment in a carbonate formation is defined as the injection rate that creates a dominant wormhole with the minimum acid volume. Wormholes are created to connect the reservoir to the wellbore by bypassing the drilling fluid damage and to enhance the wellbore productivity by increasing the effective wellbore radius in carbonate formations. Currently, the pressure drop and computed tomography scan (CT scan) are used to define the acid optimum injection rate and wormhole shape in the stimulated carbonate rock cores. However, these two techniques assess the interconnectivity of the created wormhole to the rest of the pore system in the reservoir in a coarse way.In this paper and for the first time new definitions and new approach to the optimum injection rate and wormhole evaluation during carbonate stimulation are introduced. Coreflooding experiments were performed using 3-inch Indiana limestone cores at 100 °C at different injection rates. A suite of stimulation fluids such as emulsified acid, hydrochloric (HCl) acid, gelled HCl acid systems based on polymers and viscoelastic surfactant (VES), and chelating agents were used in this study. Nuclear Magnetic Resonance (NMR) was used to evaluate the efficiency of different stimulation fluids in creating wormholes and their interconnectivity with the surrounding pore distributions in the rock. A new dimensionless number, pore interconnectivity number, is introduced to describe the interconnectivity between the created wormhole and the rest of the pore size distributions in the rock. Optimum wormhole shape is determined at the highest interconnectivity number. The optimum injection rate for a specific fluid can also be determined at the highest interconnectivity number.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.