When Do Surfactant-Based Acids Work as Diverting Agents?
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AbstractThe purpose of matrix stimulation in carbonate reservoirs is to bypass the damaged zones and increase the effective wellbore area. This can be achieved by creating highly conductive flow channels known as wormholes. A further injection of acid will follow the wormhole path where the permeability has increased significantly, leaving substantial intervals untreated. This problem can be significant as the contrast in permeability increases within the target zones. Diverting materials such as surfactant-based acids play an important role in mitigating this problem. Several papers published in the literature aimed to verify the diversion capability and the performance of the self-diverting acids. However, most of the parallel-coreflood experiments, if not all, used relatively short cores (2 to 6 inches in length).In this paper, acidizing experiments were conducted using two 20-inch long cores with different permeabilities. Carbonate cores were used with a permeability of 5 to 150 md and the total flow rate was varied from 3 to 20 cm3/min. The initial contrasts in permeability between the two cores ranged from 2 to 15 fold. To characterize the wormholes generated, a computerized tomography (CT) was used to generate 3-D images to describe the shape of the wormholes in both cores.Several periods were identified from the shape of the flow rate distribution entering each core. Acid injection rate was found to influence the efficiency of surfactant to divert acid. Acid diversion was noted to be most efficient at low flow rates (3 cm3/min). No significant diversion was noted at higher flow rates (< 6 cm3/min). Also, no significant diversion was noted at high initial permeability ratios at least for the given core length. In conjunction with the experimental study, an analytical model was developed to verify the experimental results obtained with regular acids.
