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Abstract

The key to the success of matrix acidizing in carbonate reservoirs depends on the diversion efficiency of treating fluid over the total production interval, especially for treatments on vertical wells with thick formations or horizontal wells with long and heterogeneous intervals. Foamed viscoelastic-surfactant (VES) acid combined the advantage of foam and VES can distribute acid evenly in the entire production interval. However, its diversion capability and wormhole propagation behavior has not been investigated yet. Therefore, the conventional core-flooding setup was modified by adding nitrogen source and foam generator, which was used to conduct single and parallel core-flooding acidizing experiments to investigate the effect of injection rate and initial permeability on wormhole propagation, and evaluate the diversion capability under extreme permeability contrast. Furthermore, in order to characteristic the wormhole morphology, computerized tomography (CT) scanning experiments were performed on the cores after acidizing to generate a 3D view of the wormholes inside the cores. The results of single core-flooding experiments achieved the following findings: 1. The injection rate was found to be a critical parameter in maximizing the acidizing efficiency during the foamed-VES acidizing, and the optimal acid injection rate ranges from 1 mL/min to 2 mL/min under laboratory conditions. 2. The wormhole morphology changes from conical wormhole to dominant wormhole and ramified wormhole with the increase of injection rate. 3. The initial permeability of cores hardly affects the optimal injection rate and wormhole morphology, but the pore volume to breakthrough (PVbt) decreases with the increase of the core initial permeability. 4. The foamed-VES acid has a better temporary plugging capability on the cores with high permeability due to the combined effects of foam's Jamin effect and VES self-viscosified property. The parallel core-flooding experiments show that foamed-VES acid can realize the uniform distribution of acid when the permeability contrast is about 10, and overcome the limitation of conventional VES acid. However, the foamed-VES acid will work ineffectively when the permeability contrast is greater than 20. This study deepens the understanding on the foamed-VES acidizing in carbonate reservoirs and provides fundamental for field acidizing treatment design.