Towards a better understanding of wormhole propagation in carbonate rocks: Linear vs. radial acid injection

Understanding acid reactive flow and the formation of wormholes in carbonate rocks is important for designing successful acidizing operations. Linear coreflooding experiments have been widely carried out to gain insights about the dissolution process for varying rock types and acid systems. However, carbonate reservoir stimulation is dictated by radial flow configuration. As such, it is critical to bridge the gap between linear acid injection and radial acid injection. In this work, a series of radial acid injection and linear acid flow experiments are conducted at various injection rates while maintaining the same operating conditions. The objective is to develop a better understanding of the acidizing process under radial flow conditions at larger scale while applying high confining stresses as only a limited number of radial flow experimental studies have been reported in the literature. This is expected to provide guidance for the optimization and control of the matrix acidizing process for the field operations. The radial acidizing tests are performed on rectangular block samples with dimensions of 20 × 16 × 16 in. We use a quantitative acidizing model to select few representative acid injection rates due to the complexity of the experiments and limited availability of large block samples. This model shows good predictive capability when compared to the experimental results. Pore volume to breakthrough (PVBT) and optimum injection rate are determined for both radial and linear acidizing experiments. Lower PVBT values are obtained for the radial acidizing case. This is mostly associated with the larger rock pore volume exposed to acid for the radial block in comparison to the linear cylindrical core. High-resolution CT scan images of the acidized radial carbonate blocks and cylindrical cores are generated and analyzed. The CT scan images show different wormhole morphology when varying the acid injection rates. Similar branching features are obtained for the optimal wormholes in linear and radial tests. Injecting the acid at the optimal rate leads to fewer transverse branches but thicker when compared to the experiment at higher injection rate that requires more acid volume to breakthrough. The low injection rate results in the largest diameter wormholes with high tortuosity. The high-resolution nondestructive imaging and analysis has enabled to gain insight on the interaction between the acid and the rock sample under radial flow conditions. This analysis has revealed that the wormhole propagation direction is not only governed by the effective stress differential but also can be aligned with the fabric layers due to the sedimentation conditions of the rock.