Propagation of viscoelastic surfactant-based acids in carbonate cores

Surfactants have been used over the past few years as a diverting material when mixed with acid solutions. Several papers are aimed to understand the overall process and the diversion capability. However, most of the coreflood experiments, if not all, used relatively short cores usually ranging between 2 to 6 inches in length and 1 to 1.5 inches in diameter. In this paper and for the first time, 20 inches long cores were used in conducting the acidizing experiments. The lithology of the rock type used was calcite, which has permeability of 50 to 120 md. In addition to the appropriate surfactant concentration, 15 wt% HCl was used. A key parameter such as flow rate, affects the behavior of the gelling material significantly. Therefore to capture that effect, flow rates chosen to be representative ranged between 5 and 15 cm /min. A further analysis included is the effluent concentration for the mineral composition mainly Ca ⁺⁺. In addition, the acid concentration in the effluent was measured and validated with an analytical model to address the acid profile along the wormhole, which becomes an important aspect in understanding the flow of VES fluids in carbonate cores. Part of this study is to characterize the wormhole propagation expected from the acidizing process. To accomplish this task, computerized tomography (CT) was used to generate 3-D images to describe the shape of the wormhole. There is a major difference in the shape of the wormhole when using the diverting material compared to conventional acids. The wormhole path observed in the first case tended to have several changes in direction as the acid tried to find its way to propagate further and avoid any possible blockage caused by the diverting material. This phenomenon would not be captured with short cores that were used in the previous studies.