Application of a synthesis of PHREEQC and PoreFlow: Investigating the onset and development of wormholing during CO2 sequestration in limestone

Abstract

In order to curb the effects of climate change as a result of greenhouse gas emissions, the IPCC has emphasized the role carbon-dioxide capture and storage (CCS) will need to have in the coming decades. Of the potential CCS sites are former oil fields and deep-sea reservoirs, some of which have high carbonate content. When CO2 is dissolved in water it forms an acidic solution which will react with resident carbonate minerals (e.g. limestone), resulting in a change of the pore-structure of and upscaled pore-scale properties such as permeability. To that end, pore-network models coupled with reaction modules provide a useful tool to investigate the reactive transport of such systems. In this research a novel approach to model reactive transport using a pore-network model, PoreFlow (Raoof et al., 2013), and geochemical model, PHREEQC (Appelo and Parkhurst, 2013), is proposed and investigated in the context of CO2 sequestration in limestone. The approach entails projecting a 3D pore-network to a 1D domain and coupling the two domains such that resulting changes in the 3D pore-network would be analogous to doing reactive transport directly on the network. The goal of this new approach is to greatly reduce computational costs, yet still retain physical relevance. To test the model validity, an attempt to realize the wormholing phenomenon is made, and related concepts such as optimum conditions for well-stimulation and Damköhler number are investigated. Both strengths and shortcomings are discussed, as well as possibilities and suggestions for further research to develop the model even more.