Reactive Transport in Carbonates - Impact of Structural Heterogeneity

Branko Bijeljic, Oussama Gharbi, Zhadyra Azimova, and Martin Blunt

Motivation: Carbon Capture and Storage

CCS – Trapping Mechanisms

• Solubility trapping: CO2 dissolves in the brine

as it migrates through the aquifer.

• Structural trapping: the CO2 remains as a

mobile fluid beneath an impermeable cap rock that prevents its upward movement (Bachu et al. 1994; Sengul 2006).

• Residual trapping: the CO2 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)

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

Dissolution: Acidization

Dissolution patterns in carbonate acidizing(Fredd and Fogler, 1999)Flowrate increases from 0.04cm3/min (a) to 60cm3/min (e)

• Increase productivity: force acid into a carbonate or sandstone in order toincrease K and e by dissolving rock constituents.

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 CO2 sink

• Carbonate high reactivity may lead to changes in porosity, permeability and storage capacity during CO2 injection

• There is a need to establish good understanding of mineral dissolution/precipitation for geological and reservoir model to simulate CO2 movement and trapping

(SPE ATW on CO2 sequestration, 2006)

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).

• 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

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

Calcite Properties and Reaction

Rock sample Guiting limestone

Origin Guiting Quarry, Gloucestershire, UK

Age Middle Jurassic Epoch

Rock Group Inferior Oolitic Limestone

Mineralogoy Calcite: 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

CaCO3(S) +2H+ ↔ Ca2+ + CO2 (aq) +H2O

CO2 +H2O ↔ H2CO3

H2CO3 ↔ HCO3- +H+ HCO3

- ↔ CO32- + H+

Calcite dissolution in HCl acid:

Dissolution of CO2 in formation water:

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1. Transport of acid through solution to the

calcite surface (advection and diffusion)2. Transport of the acid within the grains3. Dissolution reaction at the grain surface

and within the grains4. Transport of the created products out of the

grains5. Transport of the products away from the grain

surface

Mechanisms:

Experimental Set-up and Methodology

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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

CO2 gas

Solution EffluentInjection Pump

Pressure Transducer

Pressure Transducer

End cap, mesh and filter paper

Column sample

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Wentworth grain size classification “Geology of Carbonate Reservoirs” Wayne M.Ahr

Fine Coarse

Size range [µm]

150 – 250 600 - 850

Total Column porosity [%]

46.66±0.11 51.01±1.60

Grain Size

Surface area

Porous Grain Size – Classification

Effluent Ca2+ - Fine Grain size (150-250µm)

A time dependent regime where chemical reaction at the grain surface and intra- granular flow occur simultaneously

I IIIII

The error in measured concentrations using ICP-AES in all cases is less than 2%

Column Experiments: COUPLING

0 5 10 15 20 25 30 35 40 450

1

2

3

4

5

6

7

8

9

0

20

40

60

80

100

120

Distance from Injection Point (cm)

pH

[Ca

2+

] (p

pm

)

pH column

[Ca2+] column

Dissolved Ca2+ concentration increases along the column

but gradually flattens towards the outlet.

Significant increase of pH near the inlet but gradual

decrease towards the outlet

CaCO3(S) +2H+ ↔ Ca2+ + CO2 (aq) +H2O

In-situ vs. Effluent Concentration

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Only a proportion of the Ca2+

cation is mobile – Relatively high concentration of Ca2+ remains in the sample -This is a sign of a more heterogeneous porous medium.

SEM Analysis – Fine Grains

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Fine grain size (150-250µm) PRIOR TO acidic brine injection

Fine grain size (150-250µm) AFTER acidic brine injection

SEM Analysis – Medium Grains

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Medium grain size (300-500µm) PRIOR TO acidic brine injection

Medium grain size (300-500µm) AFTER acidic brine injection

Impact of Grain Size

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Different times are needed to the formed products to reach steady state. This implies a transport-limited reaction

Same injection Flow Rate 2 cm3 / 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

SEM Analysis – Coarse grains

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Coarse grain size (600-850µm) PRIOR TO acidic brine injection

Coarse grain size (600-850µm) AFTER acidic brine injection

Impact of Flowrate

75 min 135 min

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Coarse grain size distribution Fine grain size distribution

Decreasing the flow rate will increase the diffusive transport, more tortuous diffusive pathswill take longer times in finer grains.

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 calcite dissolution as complex:additional surface roughness and wormholes (in single grains) creation of a more heterogeneous porous medium

• The in-situ Ca2+ concentration is greater than the effluent concentration :Ca2+ 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)

Future work

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