Towards Geochemically Accurate Modeling of Carbonate Acidizing with HCl Acid

Modeling of wormholing phenomenon is one of the challenging research problems due to the complexity of the process which requires coupling of multi-phase fluid flow, species transport, geochemistry, and geomechanics. There is not a single model that captures all of these aspects of the wormholing phenomenon which requires further development of more accurate 3D multiphase/multiphysics models. This challenge is the principal motive for this work. The objective of this paper is to model carbonate acidizing more accurately from a geochemical point of view. All previous numerical models consider only the acid/rock reaction and the transport of hydrogen ions only. In this work, we also account for chemical reactions between the aqueous species, including the dissolved CO2, under full-speciation transportation. This is done by solving the Reaction-Advection-Diffusion (RAD) equation not only for the Hydrogen ions, but also for all other primary species. Coupling of transport and reaction equations is done using a Sequential Non-Iterative Approach (SNIA). Aqueous kinetics are assumed at equilibrium, while rock/acid reaction is kinetically constrained. Single-phase 2D & 3D simulations of HCl injection in limestone rock are performed under a linear flow geometry. Results of the full speciation simulations are validated with previous experimental work, and compared with results from previous numerical models. This has been accomplished with special focus on the treatment design parameters; optimum injection rate, volume of acid injected to achieve breakthrough. The simulations show that; 1) the full-speciation model captures the different dissolution patterns reported experimentally, 2) confirms the existence of an optimum injection rate that corresponds to minimum volume of acid injected, 3) aqueous kinetics affect the treatment design parameters. We have performed a more geochemically accurate simulation of carbonate acidizing with HCl that takes in to account aqueous kinetics and effect of other species other than hydrogen ions. We believe that this is another step forward towards fully capturing the complex wormholing phenomenon.