Matrix acidizing is one of the most practical stimulation technologies for carbonate reservoirs, which effectively improve the region permeability near the wellbore. In addition to solid matrix, vugs are also very common in carbonate reservoirs. However, a few studies have been addressed with existence of vugs on carbonate acidizing process. In this work, a two-scale model is developed using dual domain method and discrete vugs model to study effect of vugs on acidizing process. Darcy equation is employed in solid matrix region. Navier Stokes equation is adopted for free flow region in vugs. The two regions are coupled by modified Beavers-Joseph-Saffman boundary condition. Numerical cases are conducted to present the effect of vug characteristics on acid-rock reaction process. The results show that acid solution has the largest effective reducing distance and the smallest breakthrough volume in circular vugs. Dominant wormhole is created when acid injection direction is parallel or vertical to the azimuth angle of vugs. Increasing amount of vugs in horizontal effectively reduces the flow distance and breakthrough volume of acid solution. Vugs with random distribution increases effective flow distance and breakthrough volume of acid solution compared to vugs with orderly distribution.

Carbonate reservoirs are one of the most important fossil fuel sources, and the acidizing stimulation is a practical technique for improving the recovery of carbonate reservoirs. In this study, the improved two-scale continuum model, including the representative elementary volume (REV) scale model and the upscaling model, is used to study the acidizing process with an isolated fracture. Based on this model, a comprehensive discussion is presented to study the effect of the physical parameters of the isolated fracture on the acidizing results and dissolution images, including the isolated fracture geometry, location, and morphology. Results show that the isolated fracture system is still the target system for the acidizing stimulation. The isolated fracture provides a limited contribution to the core porosity. The permeability of the core sample with fracture can be obviously increased only when the fracture penetrates through the whole sample. The existence of the isolated fracture reduces the consumption of acid solution to achieve a breakthrough. The acidizing curve is sensitive to the change of the length, aperture, and position of the isolated fracture. The acidizing curve difference corresponding to different rotation angles has not changed significantly for clockwise rotation and anticlockwise rotation groups.

No abstract

Acidizing technology is an effective reformation method of oil and gas reservoirs. It can also remove the reservoir pollution near wellbore zones and enhance the fluid transmissibility. The optimal injection rate of acid is one of the key factors to reduce cost and improve the effect of acidizing. Therefore, the key issue is to find the optimal injection rate during acid corrosion in fractured carbonate rock. In this work, a novel reactive flow mathematical model based on two-scale model and discrete fracture model is established for fractured carbonate reservoirs. The matrix and fracture are described by a two-scale model and a discrete fracture model, respectively. Firstly, the two-scale model for matrix is combined with the discrete fracture model. Then, an efficient numerical scheme based on the finite element method is implemented to solve the corresponding dimensionless equations. Finally, several important aspects, such as the influence of the injection rate of acid on the dissolution patterns, the influence of fracture aperture and fracture orientations on the dissolution structure, the breakthrough volume of injected acid, and the dynamic change of fracture aperture during acidizing, are analyzed. The numerical simulation results show that there is an optimal injection rate in fractured carbonate rock. However, the fractures do not have an impact on the optimal acid injection rate, they only have an impact on the dissolution structure.