Organic acid concentration thresholds for ageing of carbonate minerals: implications

Organic acid concentration thresholds for ageing of carbonate minerals: implications 1 for CO2 trapping/storage 2 Muhammad Ali, Sarmad Al-Anssari, Muhammad Arif, Ahmed Barifcani, 3 Mohammad Sarmadivaleh, Linda Stalker, Maxim Lebedev, Stefan Iglauer 4 a Department of Petroleum Engineering, Curtin University, 26 Dick Perry Avenue, 6151 5 Kensington, Western Australia 6 b Department of Chemical Engineering, Curtin University, Kent Street, 6102 Bentley, 7 Western Australia 8 c Commonwealth Scientific and Industrial Research Organisation (CSIRO), 26 Dick Perry 9 Avenue, 6151 Kensington, Wester Australia 10 d Department of Exploration Geophysics, Curtin University, 26 Dick Perry Avenue, 6151 11 Kensington, Western Australia 12 e Department of Petroleum Engineering, University of Engineering and Technology, G. T. 13 Road, Lahore 54890, Pakistan. 14 f Department of Chemical Engineering, University of Baghdad, Baghdad, 10071, Iraq. 15 g School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 16 6027 Australia 17 corresponding author (Muhammad.ali7@postgrad.curtin.edu.au) 18


46
Depleted hydrocarbon reservoirs and deep saline aquifers are potential CO2 sinks in which 47 anthropogenic CO2 emissions can be stored, thus mitigating global warming. [1,2] Efficient and 48 safe CO2 geological storage involves a qualitative and quantitative assessment of the 49 contribution of the different functional trapping mechanisms which prevent the buoyant CO2 50 from migrating back to the surface. [1] 51 In this context it has been shown that CO2-wet surfaces drastically reduce structural [3][4][5] and 52 capillary trapping capacities. [6][7][8][9] Furthermore, it is clear that organic acid content on the rock 53 surface is the main factor which renders (originally strongly water-wet) mineral surfaces to 54 become CO2-wet. [4] While clean mineral surfaces are weakly, strongly or completely water- 55 wet, [10][11][12][13] organic acid surfaces, e.g. alkylated or arylated minerals, minerals aged in crude oil 56 or coal [4,5,14,15] are weakly, strongly or even completely CO2-wet depending on pressure, 57 temperature and brine salinity. [4,16,17] 58 However, chemically clean mineral surfaces are artificial in the sense that they can only be 59 prepared and persist in strongly oxidising environments (e.g. in oxygen plasma or in UV-ozone 60 atmosphere), [18,19] while it is well established that in the subsurface anoxic or reducing 61 conditions prevail. [20,21]

106
The calcite samples were immersed for 30 mins in calcite-equilibrated 2 wt% NaCl brine (NaCl 107 purity ≥ 99.9 mol%; from Scharlab) at ambient conditions, while the acidity was maintained at times. [39,40,51,52] Note that it is also shown that carboxylic acids and hydrocarbons both exist in 116 deep saline aquifers [53] , as a result of biodegradation and organic matter diagenesis and 117 subsequent migration into the water zones. [54] 118 Mechanistically, the stearic acid esterifies the hydroxyl groups on the calcite surface in a 119 condensation reaction (Scheme 1). [55][56][57][58][59] 120  The substrate was placed in a tilted angle of (17º) inside the cell. Two separate high precision 168 syringe pumps (Teledyne D-500, pressure accuracy of 0.1%) adjusted the CO2 pressure, or 169 injected the brine. The detailed setup has been described earlier. [61,64] 170 Experimentally, the sample was placed inside the pressure cell on the tilted plate and the cell

186
In this context, the water receding contact angle (i.e. CO2 displacing water) is related to 187 structural trapping (below an impermeable caprock). [13] The advancing contact angle (water 188 displacing CO2) determines the capillary trapping capacity [66] and thus the amount of residually 189 trapped CO2. [6][7][8] It has also been shown in previous studies that dissolution trapping is 190 significantly affected by the wettability, and it is thus necessary to know the wettability for 191 accurate reservoir simulations and storage capacity predictions. [9,34] 192 We thus conducted contact angle measurements with different mineral surface chemistry 193 scenarios at various thermo-physical conditions. The minute concentrations of stearic acid  The results show that calcite rapidly loses its water-wetness with increasing stearic acid surface 198 coverage (Figure 1), i.e. higher organic acid concentration led to significantly higher CO2-

202
The optimal capillary trapping limit, which we define here as the point where primary drainage 203 is unaffected by wettability is at θa = 50°. [68] a is even more affected by the carboxylic acids where 'h' is the height of the CO2 column immobilized beneath the seal layer, 'γ' is CO2-brine 268 interfacial tension, 'θr' is the receding contact angle, 'Δρ' is the CO2-brine density difference,

269
'g' is the gravitational acceleration, and 'R' is the caprock's average pore throat radius.

270
Thus, for a limestone storage formation at 10 MPa and 323 K, three main cases can be 271 distinguished: a) pure calcite, b) storage rock exposed to 10 -6 M stearic acid, and c) storage 272 rock exposed to 10 -2 M stearic acid.

314
Deep saline aquifers and depleted hydrocarbon reservoirs are the most important sinks for CO2 315 geological storage. [1,73] However, it has been shown that the CO2-wettability of the storage and 316 seal rock dramatically influences the injectivity, storage capacity, containment 317 security, [4,5,9,34,35,64,65] and thus project economics and technical feasibility.

318
Realistic subsurface conditions have, however, not been tested, and the focus was on clean 319 mineral substrates, which, however, do not exist in the subsurface (as in the subsurface anoxic 320 or reducing conditions prevail, while clean mineral surfaces can only exist in strongly oxidising 321 conditions). [18,20] 322 We therefore measured the CO2-wettability on carbonate mineral surfaces which mimic these 323 subsurface storage conditions of carbonate reservoirs more realistically. This increased the water contact angle (θ), and thus CO2-wettability. This effect drastically increased 328 with higher organic acid concentration and pressure. 329 We thus conclude that CO2 geological storage capacities and containment security can be 330 significantly lower than previously thought. Reservoir-scale models thus need to take these 331 effects into account so that accurate storage predictions are obtained thus de-risking carbon 332 geological storage (CGS) projects.