Research paperReactive transport in porous media for CO2 sequestration: Pore scale modeling using the lattice Boltzmann method
Introduction
CO2 geosequestration is widely recognized as an effective means to mitigate greenhouse gas emission and climate change for long-term environment safety. Even where the geological subsurface is well defined, however, many uncertainties still exist regarding the injected CO2 plume behavior, due to the difficulties in evaluating fluid dynamics within reacting systems. Research and prediction of reactive transport in heterogeneous porous media is topical and motivated by urgent demands in both natural resources management and industrial exploration (Gaus et al., 2005, Johnson et al., 2004; Massarotto et al., 2010; Sahimi, 2011).
There has been much observation and analysis of macroscale phenomena in systems disturbed by CO2 injection for geosequestration, but the underlying detail of the small-scale processes that dominate behavior, including transport of dissolving and precipitating species, remains challenging. This is due to the fine-scale complexity and heterogeneity of porous media, and the complicated nature of acidic fluid processes at different conditions of temperature, pressure and chemistry. Changes in pore scale flow features lead to significant variation of macroscopic phenomenon, and the potential for creation of permeability and porosity around the wellbore area as CO2 injection proceeds. This requires understanding of the pore scale processes, and development of methods to exchange, aggregate and disaggregate information across different scales.
To date, studies on the mechanisms of pore scale geochemical transport have been conducted through laboratory experiments and mathematical models. At pore scale, the dissolution of solid-phase reactants in heterogeneous porous media first occurs as surface reactions, and change the pore scale geo-structures by opening additional conductive channels and generating new or different pore throats and connectivity. Thus, porosity enhancement in reservoir rocks has been observed (Egermann et al., 2005, Farquhar et al., 2013, Farquhar et al., 2015; Pearce et al., 2012). Conversely, previously dissolved minerals (e.g. ferroan carbonates) may also be precipitated in response to chemical changes such as pH buffering, leading to occlusion of the pore throats and associated porosity reduction (Baker et al., 1995; Farquhar et al., 2015; Steefel and Lasage, 1994; Strazisar et al., 2009; Wachi and Jones, 1991). Since it takes quite a long time (~years) for geochemical systems to reach the reaction equilibrium, this makes it almost impossible for laboratory experiments to observe the whole process in tractable research time frames. Effective modeling of such coupled processes at pore scale is hence essential for predicting porosity and permeability variation through the whole sequestration sequence (Gaus et al., 2008, Massarotto et al., 2010).
Amongst numerical methods that have been proposed and applied to simulating pore scale fluid flow in the heterogeneous and fractured porous media, the lattice Boltzmann method (LBM) provides some important advantages. Gao et al., 2014, Gao et al., 2015 studied the pore scale fluid dynamics modeling through complex tortuosity with multiple mineral surface contacts in porous media. Huang et al. proposed a regularized boundary condition for solving convection-diffusion equations (Huang et al., 2011) and presented a comprehensive review of all popular multiphase Lattice Boltzmann Methods developed thus far (Huang et al., 2015). Meakin and Tartakovsky (2009) have done research on the pore scale multiphase fluid flow and reactive transport in the fractured and porous media, and Steefel et al. (2013) have focused on the pore scale process associated with the subsurface CO2 injection and sequestration (Steefel et al., 2013). Kang et al., 2010a, Kang et al., 2010b developed a pore scale LBM model for simulating the injection of CO2-saturated brine into structured porous media, focusing on the fundamental physics occurring at the pore scale for reactions involved in geologic CO2 sequestration. Recently, a further study in pore scale reactive transport was also done by Yoon and Kang (Yoon et al., 2015). A drawback of these studies, however, is the lack of model validation against experiment.
The current paper considers the acidification process in carbonate or carbonate cemented reservoirs using a LBM model and incorporating experimental validation. We present a generalized pore scale geochemical reaction computational model, for studying complex geochemical reaction systems, including mechanisms of: mutually soluble acidic fluid transport in pore networks within bulk structures; acidic fluid interactions on reactive and less-reactive mineral surfaces; dissolution/precipitation and mass transfer of the reactive minerals; and the impact of temperature on the reactivity and potential chemical reactions. In the model, simulations of reactive transport have considerable complexity due to the geometric heterogeneity and minerals diversity, contributing different reactivities and distributions that occur in reality.
The Surat Basin in Queensland, Australia (Fig. 1), has been identified as a highly prospective site for CO2 storage, estimated to have a capacity of 2962 Mt (Hodgkinson et al., 2010). The Precipice Sandstone, a low salinity aquifer, is a target reservoir for CO2 injection (or brine reinjection from coal seam gas operations) with a mean reported porosity of 16±5% (Kellett et al., 2012). The Evergreen Formation is an aquitard, overlying the Precipice Sandstone, with variable porosity, which would act as an unconventional seal or cap-rock. The overlying Hutton Sandstone (Fig. 1) that may act as a secondary seal, has been suggested as a reservoir in its own right, or represents an area for sub-surface monitoring for potential CO2 leakage from the underlying Precipice Sandstone (Hodgkinson et al., 2010, Horner et al., 2015). Mean porosity in the Hutton Sandstone is 22±6% as reported (Kellett et al., 2012). Several geochemical studies have recently reacted core from all three formations from the GSQ Chinchilla 4 well (Fig. 1) with CO2 and water or CO2-SO2 and water at simulated reservoir conditions (Farquhar et al., 2015, Horner et al., 2015; Pearce et al., 2015). The experimental data used for model verification here are from a laboratory CO2-water-rock experiment using Hutton Sandstone core (Farquhar et al., 2015). The focus of this case is on the dissolution reactions that are likely to occur in the immediate wellbore environment (Bacci et al., 2011, Massarotto et al., 2010). This region is the most influential with respect to controlling the possible injection rates, since the fluid flux here is highest.
Section snippets
Methods
The subsurface system typically encompasses multiple minerals, some of which are reactive, and a complex network of flow paths. Thus, a model must encompass many aspects including fluid transport, the representation of tortuous physical geometries with heterogeneous chemical surfaces, the fluid-solid interactions and chemical reactions, and the coupling of these multiple processes. As introduced in our previous work (Gao et al., 2014, Gao et al., 2015), fluid transport model (including fluid
Materials for experimental validation
The model was compared with a laboratory experiment carried out previously and reported elsewhere, which studied the pore scale geometry and geochemistry changes of a fresh water wetted Hutton Sandstone core sample sub-plug when exposed to CO2 injection at reservoir conditions of 60 °C and 120 bar (Farquhar et al., 2013, Farquhar et al., 2015). The experimental batch-soak reactor has been described in detail in Pearce et al. (2015). The core sample sub-plug was reacted over a period of 384 h, and
Specialized carbonate acid reaction LBM model
In the CO2-water-rock application, the reaction LBM model is used to study the transport of bulk CO2-water mixture (modeled as a single-phase aqueous solution with an elevated concentration of carbonic acid), carbonic acid interactions on assumed non-reactive solid surfaces, and carbonic acid reactions on calcite surfaces and the resultant dissolution of reactive minerals. In the simulation, the temperature and pressure were set as 60 °C and 120 bar for the reaction modeling.
Generally, dry CO2 is
Discussion and implications
One of the main difficulties associated with geological reactive transport modeling is model validation, associated with the difficulties of sufficient sample characterization and experimental conditions. This also reflects the value of a good model, insofar as it provides a way to probe and gain detailed insight from different process strategies, without the laboratory burden. The current reactive model has been validated against a single, but well characterized experimental sample data set
Conclusions
A geochemical reaction LBM model for pore scale reactive transport in heterogeneous porous media has been developed and validated against some experimental data.
The following conclusions can be drawn:
- (1)
A general geochemical reaction model acting at the pore scale is presented that describes coupled mass transport and dissolution processes, set up to include heterogeneity in the porous media and its grain structures, fluid transport and reactive mineral dissolution, and linked with the associated
Acknowledgments
The authors wish to acknowledge financial assistance provided through Australian National Low Emissions Coal Research and Development (ANLEC R&D) Project 3-1110-0101. We also wish to acknowledge Australian Research Council (ARC) Project DP110103024, and National Natural Science Foundation of China (NSFC) Project 11232003. The authors are grateful to Dr. Alexandra Golab and FEI for access to the micro-CT data sets.
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