Optimizing Carbonate Acid Stimulation by Minimizing Acid Injection Volume with Selective Treatment Placement

Abstract

In carbonate acidizing, highly conductive wormholes are created. The process of wormhole formation comprises two major periods: induction and wormholing. The induction period is the time from the first injection of the stimulation fluid to wormhole initiation. The volume of stimulation fluid injected during the induction phase can be more than 30% of the total volume required for the stimulation operation. Minimizing the induction period can significantly reduce the cost and time of matrix acidizing operations. Several series of core-flow experiments are conducted under the same experimental conditions, namely temperature, pressure and flow rate to investigate the effects of changes made to the geometry of the injection face of the core during acid injection. Induced holes, sometimes called notches, of various depths and locations were created into the injection face of Indiana limestone cores to evaluate their impact on the wormholing with 15% wt. HCl. The pore volume to breakthrough (PVBT) of injected acid is obtained with the notched cores are compared with ones recorded with a regular plain core (baseline). The experimental results show that inducing hole in the core can significantly reduce the PVBT of injected acid compared to regular cores. The depth and location of the notch both affect the volume of injected acid as well as the optimal flow rate at which the breackthrough in achived. Shallower notches induced in an optimal location can save about 25% volume of the injected acid while deeper ones can help to reduce the injected acid up to 50%. Moreover, notches can localize the wormhole initiation point. Based on several repeated experimental results, the creation of notch prior to acid injection can optimize a matrix acidizing treatment by reducing the wornmhole induction period and therefore reducing the volume of acid required to achieve equivalent stimulation performance. It also enables more selective treatment placement. The creation of notches in the formation method has been applied to hydraulic fracturing because it generates weak points and reduces the pressure required to fracture the formation, Kayamov et al. 2019, Aidagulov et al. 2016, and 2015. However, this proposed method is the first application in matrix acidizing treatments that demonstrates the impact of face geometry on wormhole generation.