Multiscale pore network modeling of carbonate acidization

Matrix acidization is important in many applications, particularly near-wellbore stimulation in carbonate reservoirs. Ideally, injected acid creates an array of highly conductive channels that greatly enhance permeability. A pore network model of matrix acidization is developed to predict this optimal injection rate, using novel representations of pore-scale physics in the form of a mass transfer coefficient and pore-merging criterion. Results are presented for both computer generated sphere packs and networks extracted from CT scans of carbonates. Simulations return an optimal Damköhler number close to experimental optimums, but predict significantly higher pore volumes to breakthrough than most experiments. This is posited to be due to the small domain size of network models.Mortar coupling is employed as a domain decomposition technique, and is verified to accurately capture the unique physics of matrix acidization. Many networks are coupled together to extend the simulated domain size in a parallel computing environment. A hybrid modeling technique is used to simplify large-scale simulations with the inclusion of Darcy-scale grid blocks in nonreactive regions. This is shown to preserve a high degree of simulation accuracy while improving computation time substantially. A two-scale continuum model is developed for integration with the hybrid technique. A pore network model is used to develop and upscale mass transfer coefficients, porosity-permeability relationship, and other structure-property relations. The resultant two-scale continuum model is shown to preserve a high degree of accuracy at high injection rates, but falls short of capturing intermediate dissolution regimes due to insufficient description of pore-scale species channeling. Simulations on larger domain sizes show reduced pore volumes to breakthrough. Radial domains are also investigated and show the characteristic dissolution regimes, lending support to common field-scale acidizing practice. Two-scale continuum models are concluded to be accurate representations of pore scale physics that can be used efficiently at larger scales. However, pore-scale modeling is still necessary to calibrate input parameters for the two-scale continuum model without resorting to destructive acidization experiments.