Analyzing the Dynamics of Mineral Dissolution During Acid Fracturing by Pore–Scale Modeling of Acid–Rock Interaction

Summary Hydrochloric acid (HCl) is commonly used in acid fracturing. Given that the interaction between acid and rock affects multiphase flow behaviors, it is important to thoroughly understand the relevant phenomena. The Darcy-Brinkman-Stokes (DBS) method is most effective in describing the matrix-fracture system among the proposed models. This study aims to analyze the impact of acid-rock interaction on multiphase flow behavior by developing a pore-scale numerical model applying the DBS method. The new pore-scale model is developed based on OpenFOAM, an open-source platform for the prototyping of diverse flow mechanisms. The developed simulation model describes the fully coupled mass balance equation and the chemical reaction of carbonate acidizing in an advection-diffusion regime. The volume of fluid (VOF) method is used to track the liquid- and gas-phase interface on fixed Eulerian grids. Here, the penalization method is applied to describe the wettability condition on immersed boundaries. The equations of saturation, concentration, and diffusion are solved successively, and the momentum equation is solved by pressure implicit with splitting of operators method. The simulation results of the developed numerical model have been validated with experimental results. Various injection velocities and the second Damkohler numbers have been examined to investigate their impacts on the CO2 bubble generation, evolving porosity, and rock surface area. We categorized the evolving carbon dioxide (CO2) distribution into three patterns in terms of the Damkohler number and the Péclet number. We also simulated a geometry model with multiple grains and a Darcy-scale model using the input parameters found from the pore-scale simulations. The newly developed pore-scale model provides the fundamental knowledge of physical and chemical phenomena of acid-rock interaction and their impact on acid transport. The modeling results describing mineral acidization will help us implement a practical fracturing project.

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Pore–Scale Numerical Investigations of the Impact of Mineral Dissolution and Transport in Naturally Fractured Systems During CO2–Enriched Brine Injection

10.2118/204313-ms ◽

2021 ◽

Author(s):

Jiahui You ◽

Kyung Jae Lee

Keyword(s):

Dissolution Rate ◽

Co2 Storage ◽

Mineral Dissolution ◽

Initial Time ◽

Co2 Injection ◽

Pore Scale ◽

Acid Rock ◽

Rock Interaction ◽

Connected Area ◽

The Impact

Abstract CO2 storage and sequestration are regarded as an effective approach to mitigate greenhouse gas emissions. While injecting an enormous amount of CO2 into carbonate–rich aquifers, CO2 dissolves in the formation brine under the large pressure, and the subsequently formed CO2–enriched brine reacts with the calcite. Reaction–induced changes in pore structure and fracture geometry alter the porosity and permeability, giving rise to concerns of CO2storage capacity and security. Especially in the reservoir or aquifer with natural fractures, the fractures provide a highly permeable pathways for fluid flow. This study aims to analyze the acid–rock interaction and subsequent permeability evolution in the systems with complex fracture configurations during CO2 injection by implementing a pore–scale DBS reactive transport model. The model has been developed by expanding the functionality of OpenFOAM, which is an open–source code for computational fluid dynamics. A series of partial differential equations are discretized by applying the Finite Volume Method (FVM) and sequentially solved. Different fracture configurations in terms of fracture length, density, connection, and mineral components have been considered to investigate their impacts on the dynamic porosity–permeability relationship, dissolution rate, and reactant transport characteristics during CO2 storage. The investigation revealed several interesting findings. We found that calcium (Ca) concentration was low in the poorly connected area at the initial time. As CO2–enriched brine saturated the system and reacted with calcite, Ca started being accumulated in the system. However, Ca barely flowed out of the poor–connected area, and the concentration became high. Lengths of branches mainly influenced the dissolution rates, while they had slight impacts on the porosity–permeability relationship. While fracture connectivity had an apparent influence on the porosity–permeability relationship, it showed a weak relevance on the dissolution rate. These microscopic insights can help enhance the CO2 sealing capacity and guarantee environmental security.

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