Reactive Transport in Carbonates - Impact of Structural Heterogeneity
description
Transcript of Reactive Transport in Carbonates - Impact of Structural Heterogeneity
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:
10
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
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
CO2 gas
Solution EffluentInjection Pump
Pressure Transducer
Pressure Transducer
End cap, mesh and filter paper
Column sample
12
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 II III
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
[Ca2
+] (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
15
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
16
Fine grain size (150-250µm) PRIOR TO acidic brine injection
Fine grain size (150-250µm) AFTER acidic brine injection
SEM Analysis – Medium Grains
17
Medium grain size (300-500µm) PRIOR TO acidic brine injection
Medium grain size (300-500µm) AFTER acidic brine injection
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 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
19
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
20
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