1. Reactive Transport in Carbonates - Impact of Structural Heterogeneity Branko Bijeljic, Oussama Gharbi, Zhadyra Azimova, and Martin Blunt

2. Motivation: Carbon Capture and Storage

3. CCS – Trapping Mechanisms Solubility trapping: CO 2 dissolves in the brine as it migrates through the aquifer. Structural trapping: the CO 2 remains as a mobile fluid beneath an impermeable cap rock that prevents its upward movement (Bachu et al. 1994; Sengul 2006). Residual trapping: the CO 2 phase becomes disconnected into an immobile fraction (Flett et al. 2004; Kumar et al. 2004; Mo and Akervoll 2005; Hesse et al. 2006; Pentland et al. 2010). Mineral trapping: the precipitation of dissolved gases as minerals by chemical reaction (Gunter et el. 1997; Gallo et al. 2002; Pruess et al. 2003; Xu et al. 2003; Ozah et al. 2005). Figure: Trapping mechanisms and change of storage security over time (IPCC, 2005)

4. Dissolution: Reactive Transport Issues Dissolution too rapid - detrimental to reservoir integrity Significant precipitation occurs – pores become clogged, can lead to a considerable decrease in permeability Salt precipitation may occur in saline aquifers and reservoirs Dissolution coupled with precipitation lead to complex overall kinetics Coupling of flow/diffusion/reaction: time and spatial dependence

5. Dissolution: Acidization Dissolution patterns in carbonate acidizing (Fredd and Fogler, 1999) Flowrate increases from 0.04cm 3 /min (a) to 60cm 3 /min (e) Increase productivity: force acid into a carbonate or sandstone in order to increase K and  by dissolving rock constituents.

6. Importance of Calcite Dissolution Carbonate minerals are plentiful in sedimentary rocks and modern sediment (Morse et al, 2002)  60% of known petroleum reserves are located in carbonate reservoirs (Morse et al, 1990)  High potential as CO 2 sink Carbonate high reactivity may lead to changes in porosity, permeability and storage capacity during CO 2 injection There is a need to establish good understanding of mineral dissolution/precipitation for geological and reservoir model to simulate CO 2 movement and trapping (SPE ATW on CO 2 sequestration, 2006)

7. Reactive Transport in Porous Rocks Significant differences between reactive transport models results and experimental data are often noticed. It is well established that the reaction rates of many minerals observed in the field were found to be several orders of magnitude slower than those measured in laboratory (White and Brantley, 2003). Differences that arise due to reactive surface area of the fresh and weathered minerals; the effect of reaction affinity (White, 1995) the discrepancies in the mineral reaction rates over the scales can be ascribed to physical and chemical heterogeneities in soils and aquifers in which subsurface flow can exarcebate the differences (Malmstrom et al., 2004; Meile and Tuncay, 2006).

8. Batch, core/column experiments are an important tool to understand the reaction mechanism – calcite dissolution was shown to be fully limited by mass transport (Lund et al.,1974 ; Alkattan,1998; Alkattan et al., 2002) Dissolution mechanisms and limiting processes can significantly vary with system temperature, saturation, structural heterogeneity, ionic strength and pH (Morse and Arvidson, 2002; Arvidson et al., 2002). Relatively few experimental results are available that analyze the impact of such coupled effects on the spatial and temporal evolution of porous structure. Calcite Dissolution

9. OBJECTIVES Illuminate the interplay between transport and reaction mechanisms during acid dissolution of carbonate rock. Study RTD of the reactants /products in the laboratory columns packed with crushed carbonate rock – both effluent analysis and the concentration profiles along the columns provide valuable insights into the time-dependent flow/transport/reaction dynamics Scanning Electron Microscopy (SEM) imaging tool used to visualize changes in micro-morphology induced by chemical reaction. Evaluate the impact of grain size distribution and flow rates on reactive transport mechanisms in carbonate rocks thus providing a better understanding of roles of structural heterogeneity and reactive surface area on carbonates dissolution

10. Calcite Properties and Reaction Rock sampleGuiting limestone OriginGuiting Quarry, Gloucestershire, UK AgeMiddle Jurassic Epoch Rock GroupInferior Oolitic Limestone MineralogoyCalcite: 98 % Quartz: 1.5% Others: 0.5% Consolidated sample porosity [%] 27.95(±0.73) 1 Consolidated sample saturated brine permeability [mD] 2.67(±0.62) 1 1-Lamy et al 2010 SPE 130720 CaCO 3(S) +2H + ↔ Ca 2+ + CO 2 (aq) +H 2 O CO 2 +H 2 O ↔ H 2 CO 3 H 2 CO 3 ↔ HCO 3 - +H + HCO 3 - ↔ CO 3 2- + H + Calcite dissolution in HCl acid: Dissolution of CO 2 in formation water: 10 1.Transport of acid through solution to the calcite surface (advection and diffusion) 2.Transport of the acid within the grains 3.Dissolution reaction at the grain surface and within the grains 4. Transport of the created products out of the grains 5. Transport of the products away from the grain surface Mechanisms:

11. Experimental Set-up and Methodology 11  Flow is monitored through pressure difference measurements  Effluent is collected for concentration analysis and pH measurements  SEM imaging tools are used to characterize micro-morphology changes Unique experimental approach providing information within the column Uniformly pack the column with crushed and sieved carbonate grains Acidic Brine injection at constant flow rate Stop injection and collect the last effluent sample Section column into parts. Near the inlet, fine size sections are considered Extract the liquid using centrifuge ICP-AES analysis for cations Saturate the column with vacuum- degassed saturated brine Flush the dry column with CO 2 gas Solution Effluent Injection Pump Pressure Transducer End cap, mesh and filter paper Column sample

12. 12 Wentworth grain size classification “Geology of Carbonate Reservoirs” Wayne M.Ahr FineCoarse Size range [µm] 150 – 250600 - 850 Total Column porosity [%] 46.66±0.1151.01±1.60 Grain Size Surface area Porous Grain Size – Classification

13. Effluent Ca 2+ - Fine Grain size (150-250µm) A time dependent regime where chemical reaction at the grain surface and intra- granular flow occur simultaneously III III The error in measured concentrations using ICP-AES in all cases is less than 2%

14. Column Experiments: COUPLING Dissolved Ca 2+ concentration increases along the column but gradually flattens towards the outlet. Significant increase of pH near the inlet but gradual decrease towards the outlet CaCO 3(S) +2H + ↔ Ca 2+ + CO 2 (aq) +H 2 O

15. In-situ vs. Effluent Concentration 15 Only a proportion of the Ca 2+ cation is mobile – Relatively high concentration of Ca 2+ remains in the sample -This is a sign of a more heterogeneous porous medium.

16. SEM Analysis – Fine Grains 16 Fine grain size (150-250µm) PRIOR TO acidic brine injection Fine grain size (150-250µm) AFTER acidic brine injection

17. SEM Analysis – Medium Grains 17 Medium grain size (300-500µm) PRIOR TO acidic brine injection Medium grain size (300-500µm) AFTER acidic brine injection

18. Impact of Grain Size 18 Different times are needed to the formed products to reach steady state. This implies a transport-limited reaction Same injection Flow Rate 2 cm 3 / min Fine grains in comparison to coarse grains:  More surface available to reaction However:  More heterogeneous flow paths  More surface area delays access to the surface of reactants  longer unsteady state regime

19. SEM Analysis – Coarse grains 19 Coarse grain size (600-850µm) PRIOR TO acidic brine injection Coarse grain size (600-850µm) AFTER acidic brine injection

20. Impact of Flowrate 75 min135 min 20 Coarse grain size distributionFine grain size distribution Decreasing the flow rate will increase the diffusive transport, more tortuous diffusive paths will take longer times in finer grains.

21. Column Experiments: CONCLUSIONS interplay between transport and reaction mechanisms during acid dissolution of carbonate rock Illuminated – unsteady-state regime identified Both effluent analysis and the concentration profiles along the columns provided valuable insights into the time-dependent flow/transport/reaction dynamics SEM analysis showed c alcite dissolution as complex:additional surface roughness and wormholes (in single grains)  creation of a more heterogeneous porous medium The in-situ Ca 2+ concentration is greater than the effluent concentration :Ca 2+ resides in the stagnant regions of the pore space. The impact of grain size distribution and flow rates on reactive transport indicated that of calcite dissolution at the column scale is transport limited (under the experimental conditions)

22. Future work

23. Acid Injection at Pore-scale Mt Gambier micro-CT Image