CO2 Sequestration Conditions Research Articles

Introducing sodium lignosulfonate as an effective promoter for CO2 sequestration as hydrates targeting gaseous and liquid CO2

Hydrate-based CO2 sequestration (HBCS) emerges as a promising solution to sequestrate CO2 as solid hydrates for the benefit of reducing CO2 concentration in the atmosphere. The natural conditions of high-pressure and low-temperature in marine seabed provide an ideal reservoir for CO2 hydrate, enabling long-term sequestration. A significant challenge in the application of HBCS is the identification of an environmental-friendly promoter to enhance or tune CO2 hydrate kinetics, which is intrinsically sluggish. In addition, the promoter identified should be effective in all CO2 sequestration conditions, covering CO2 injection as gas or liquid. In this study, we introduced sodium lignosulfonate (SL), a by-product from the papermaking industry, as an eco-friendly kinetic promoter for CO2 hydrate formation. The impact of SL (0–3.0 wt.%) on the kinetics of CO2 hydrate formation from gaseous and liquid CO2 was systematically investigated. CO2 hydrate morphology images were acquired for both gaseous and liquid CO2 in the presence of SL for the explanation of the observed promotion effect. The promotion effect of SL on CO2 hydrate formation is optimal at 1.0 wt.% with induction time reduced to 5.3 min and 21.1 min for gaseous and liquid CO2, respectively. Moreover, CO2 storage capacity increases by around two times at 1.0 wt.% SL, reaching 85.1 v/v and 57.1 v/v for gaseous and liquid CO2, respectively. The applicability of SL as an effective kinetic promoter for both gaseous and liquid CO2 was first demonstrated. A mechanism explaining how SL promotes CO2 hydrate formation was formulated with additional nucleation sites by SL micelles and the extended contact surface offered by generated gas bubbles or liquid droplets with SL. The study demonstrates that SL as an effective promoter for CO2 hydrate kinetics is possible for adoption in large-scale HBCS projects both nearshore and offshore.

Environmentally Sustainable Large-Scale CO2 Sequestration through Hydrates in Offshore Basins: Ab Initio Comprehensive Analysis of Subsea Parameters and Economic Perspective

Immediate measures are required to tackle global warming and climate change that may require the storage of enormous quantities of anthropogenic CO2 in geological and oceanic reservoirs. In terrestrial storage sites, CO2 is buoyant due to the subsurface temperature profile. Therefore, if the reservoir is not adequately sealed, stored CO2 can escape from geological formations. Alternatively, oceanic sequestration has immense potential to facilitate long-term sequestration of CO2 beneath the seafloor, thereby assisting the wider scientific and industrial community to make the world carbon neutral. There is a far-reaching scope for discussion in this aspect that will open up new avenues for future developments in this area. This article discusses the long-term viability, environmental sustainability and economic potential of CO2 sequestration in the form of hydrates into oceanic and subsea sediments. Analysis shows that at above 2800 m depth, CO2 is denser than seawater, which offers an additional gravitation barrier for CO2 to escape. This article offers an in-depth analysis of CO2 sequestration conditions, along with challenges and outlook with relevance to hydrate-based CO2 sequestration in oceanic conditions. Moreover, the crucial role of negative buoyancy and hydrate-forming zones (NBZ/HFZs) and analysis of depth criterion in oceanic conditions have been highlighted. The review also envisages that sequestration in NBZ region offers stable storage for years even in geological perturbation and earthquakes; however the depth of HFZ and NBZ regions depends on sedimentary type, storage conditions, temperature and pressure.

Structural evolution in micro-calcite bearing Ca-montmorillonite reinforced oilwell cement during CO2 invasion

High concentration CO2 invades and forms carbonate layers in oilwell cement under geological CO2 sequestration conditions. Long-term invasion of CO2 makes the carbonation structure unstable and prone to damage. For the reinforcement of oilwell cement, Ca-montmorillonite (Ca-MMT) was modified using supercritical CO2 (ScCO2) as the solvent and intercalator. The micro-calcite bearing Ca-montmorillonite (MC Ca-MMT) was added to the cement slurry to reinforce the oilwell cement after the modification. Microcrystal calcite was identified in the MC Ca-MMT through transmission electron microscopy(TEM) and selected-area electron diffraction(SAED) analyses. The mechanical properties and microstructural evolution of the MC Ca-MMT reinforced oilwell cement were investigated, revealing that the carbonation was inhibited by MC Ca-MMT. Through the analysis of microcrystal formation and densified carbonation area, the CO2-resisting mechanisms of MC Ca-MMT reinforced oilwell cement were revealed in this study.

Structural evolution in Nanoclay reinforced oilwell cement during Supercritical CO2 (ScCO2) invasion

Supercritical CO2 (ScCO2) invades oilwell cement and forms carbonate layers in the cement under geological CO2 sequestration conditions. Long-term invasion of ScCO2 makes the carbonation structure unstable and prone to damage. Nanoclay was modified with ScCO2 as the solvent and intercalator for the reinforcement of oilwell cement, forming the microcrystalline Nanoclay (MC Nanoclay). MC Nanoclay is identified and added into the cement slurry to reinforce oilwell cement after the above modification. Microcrystal calcite is found in the MC Nanoclay with transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analyses. Both mechanical properties and microstructural evolution of MC Nanoclay reinforced oilwell cement are investigated, showing that the carbonation was inhibited with MC Nanoclay. With generalized analysis from microcrystal formation and densified carbonation area, the reinforce mechanisms of MC Nanoclay were revealed in this study.

Experimental investigation and mathematical modelling of the carbon dioxide sequestration of cement pastes during pressurized CO2 curing

The mineral carbonation of concrete waste is a particularly attractive option to reduce the CO2 emissions of the construction industry. The aim of this study was to investigate the behaviour of CO2 capture by concrete wastes in response to various parameters, and to build a model for predicting the CO2 uptake of concrete wastes. A series of accelerated carbonation tests were performed under varying pressures of pure CO2 (0.1–1 bar), room temperature, 40% to 70% relative humidity, and carbonation times of 6, 12, 24, and 48 h. Cement paste samples with varying W/C ratios, conventional curing ages, and particle sizes were prepared and used as the concrete waste samples to emphasize its capability of CO2 sequestration. The amount of CO2 absorbed by the samples was assessed by thermogravimetric analysis (TGA). Various parameters affected CO2 uptake, and this information helped determine the optimal CO2 sequestration conditions for concrete waste. Over 40% of the CO2 emissions from cement production could be absorbed by the pressurized CO2 curing. A model was constructed by introducing the modification fraction based on experimental data, and this model could be used to evaluate the amount of CO2 uptake by concrete waste.

Secondary magnesite formation from forsterite under CO2 sequestration conditions via coupled heterogeneous nucleation and crystal growth

Flow-only and coupled flow-diffusion experiments at 95 °C and 100 bars pCO2 carried out in micro-capillary tubes packed with forsterite mineral grains were used to constrain a coupled classical heterogeneous nucleation and crystal growth reactive transport model describing the formation of secondary magnesite. The study made use of a novel experimental setup in which one capillary tube for flow is connected via a three-way tee to a perpendicular capillary tube sealed at the distal end in which only molecular diffusion is allowed to occur—an experimental analogue of a single fracture-rock matrix system. While the high flux of CO2 bearing fluids and their low pH did not result in the formation of secondary carbonates in the flow-dominated channels, as much as 2.7% magnesite formed in a diffusion-controlled capillary tube sample after 300 hours of reaction. The precipitation of secondary magnesite was not uniformly distributed, however, but showed a distinctive peak shaped pattern along the sample reacted as quantified by RAMAN spectroscopic analysis. About 50% of the total magnesite precipitation formed within a narrow interval of a few millimeters close to the middle of the 3 cm diffusion sample. To simulate the behavior of the diffusion–reaction column, and in particular the pronounced mm scale peak at approximately 1.8 cm, an interfacial free energy of approximately 70 mJ·m−2 combined with a relatively high crystal growth rate for secondary magnesite was required. In agreement with the observations based on RAMAN spectroscopy, the simulations suggest that more than 50% of Mg2+ dissolved from the primary forsterite precipitated as secondary magnesite. The magnesite precipitation at the central peak position in the diffusion sample showed such rapid growth that it created a local minimum in pore fluids Mg2+ concentration close to the peak, creating a “nucleation shadow” that focused continued growth there as nearby regions had no nucleation seeds available for crystal growth. Continued crystal growth on the initial magnesite band acted to further increase the reactive surface area at this point, thus enhancing the spatially focused crystal growth rate and creating a positive feedback leading to pattern formation. This highlights the potentially critical role of an initial nucleation event in controlling mineral precipitation patterns in subsurface porous media, patterns that may determine how the pore structure subsequently evolves physically and chemically over time due to reactive flow and transport.

Analysis of shale property changes after geochemical interaction under CO2 sequestration conditions

This study implements a supercritical CO2 (scCO2) sequestration environment in a laboratory and investigates the geochemical effects of scCO2 on the properties of rock specimens. Shale specimens constituting caprock were kept in a laboratory reactor chamber with scCO2 for 63 days. Chemical interaction between rock surface and the scCO2 was then induced, and the changes in the properties were inspected before and after the reaction through various tests. The non-destructive tests comprised measurement of elastic wave velocity, shore hardness followed by investigation using the inductively coupled plasma mass spectrometer, and scanning probe microscope. The destructive tests include uniaxial compressive strength, Brazilian tensile strength, and X-ray fluorescence tests. Results show that mechanical properties are significantly weakened when shale specimens are exposed to both scCO2 and brine. The specimens reacting with only scCO2 induced a self-healing effect by precipitation of secondary sediments, becoming a more intact rock with increased strength and elastic modulus. The results denote that the geochemical alteration of shale occurs in a short time and are highly dependent on the reaction conditions. These results provide fundamental information for the stability of CO2 storage sites regarding the physical and chemical reactions between rocks under geological sequestration conditions.

Improved durability of Saudi Class G oil-well cement sheath in CO2 rich environments using olive waste

The physical properties of the oil-well cement, especially its strength and permeability, are changing significantly after reacting with CO2-saturated brine which will alter the cement hydration products. In this study, the effect of incorporating the olive waste into Saudi Class G oil-well cement formulation subjected to CO2 sequestration conditions at 130 °C and 10 MPa on the cement permeability, carbonation depth, compressive and tensile strength, and microstructural changes during 20 days of carbonation was evaluated. The results obtained showed that the addition of 0.1% by weight of cement (BWOC) of the olive waste to the cement enhanced the cement resistance to the carbonation process and decrease the carbonation depth from 1469 μm for the base cement to 1269 μm after 20 days of carbonation. Incorporating 0.1% BWOC of the olive waste into the cement slurry also maintained the original cement permeability of 14.3% less than that for the base cement, while the permeability of the cement samples with 0.1% BWOC olive waste after 20 days of carbonation is 33.9% lower than the base cement permeability. The permeability reduction is the main mechanism responsible for the enhancement of carbonation resistance for the olive waste-based cement. The ability of the olive waste to delay calcium leaching is attributed to the decrease in the permeability of the sample with 0.1% BWOC of olive waste.

Carbonation of Portland-Zeolite and geopolymer well-cement composites under geologic CO2 sequestration conditions

Degradation of Portland cement under supercritical CO2 conditions (scCO2) is one of the major concern in geological sequestration projects. This paper presents the research on two novel types of binders considered to display a CO2 corrosion resistance potential: (1) Portland cement partially replaced with 20, 30 and 40 wt % of zeolite and blended with Styrene-Butadiene (SB) Latex and (2) geopolymer based on lime-slag and lime-pozzolan blends. Specimens were placed in an autoclave, covered with water, heated up to 100 °C and pressurized with 7 MPa CO2 in order to simulate CO2 environment in the Croatian wellbore. On behalf of tests on compressive strength, porosity, permeability and alkalinity, as well as thermogravimetry, X-ray diffraction and SEM microstructural analyses of samples at various penetration depths, carbonation mechanisms were discussed. Portland-Zeolite composites have been argued as potential CO2 resistant cement systems, with an replacement rate below the minimally tested addition of 20 wt %, as excessive replacement rates resulted in an increase in porosity and permeability. Geopolymer composites based on lime activation unfortunately haven't exhibited required properties for application in well cementing of CO2 injection wells. Calcium carbonate polymorphs precipitated throughout the whole specimen thickness of all samples, but preferably in mixtures based on Ca-richer raw materials. A re-precipitation of calcium carbonate filled in the porosity preferentially at the 0.5 mm surface layer of PC-based specimens, due to the outward diffusion of calcium at the early stage of carbonation.

Effect of nano-silica and curing conditions on the reaction rate of class G well cement exposed to geological CO2-sequestration conditions

Chemical reactions of class G oil well cement submerged in CO2-saturated brine were experimentally investigated to evaluate the effects of nano-silica and curing conditions. The progression of depths of reaction zones (depletion, carbonation, and degradation zone) with reaction time up to 62 days were quantitatively measured based on 3D X-ray CT in conjunction with the chemical analysis. The results show that there were no significant difference in the progress of carbonation zone regardless of the curing conditions and nano-silica. The measure of reaction depth for tested specimens concludes that the curing at the high pressure and temperature is more effective to prevent the depletion zone from progressing into the cement interior than the addition of the nano-silica, supported by XRD and NMR results. Image-based measurements, coupled with chemical analyses, quantified the evolution of the reactive zones in the oil well cement, providing insights into the underlying reaction mechanism.

Clay Swelling in Dry Supercritical Carbon Dioxide: Effects of Interlayer Cations on the Structure, Dynamics, and Energetics of CO2 Intercalation Probed by XRD, NMR, and GCMD Simulations

In situ X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) experiments combined with molecular dynamics simulations using the grand canonical ensemble [grand canonical molecular dynamics (GCMD)] show that the cation size, charge, and solvation energy play critical roles in determining the interlayer expansion of smectite clay minerals when exposed to dry supercritical scCO2 (scCO2) under conditions relevant to petroleum reservoirs and geological CO2 sequestration conditions. The GCMD results show that the smectite mineral, hectorite, containing interlayer alkali and alkaline earth cations with small ionic radii and high solvation energies (e.g., Na+ and Ca2+) does not intercalate CO2 and that the fully collapsed interlayer structure is the most energetically stable configuration. With Cs+ and Ba2+, the monolayer structure is the stable configuration, and CO2 should spontaneously enter the interlayer. With Cs+, there is not even an energy barrier for CO2 intercalation, in agreement with the XRD a...

Numerical investigation of Lower Tuscaloosa Sandstone and Selma Chalk caprock under geological CO2 sequestration conditions: mineral precipitation and permeability evolution

AbstractA numerical model was developed using CrunchFlow to simulate reactive transport and porosity and permeability changes of sandstone and carbonate rock samples taken from the Lower Tuscaloosa Formation and the Selma Chalk Formation, Jackson County, MS, USA. The model predicted a permeability decrease from 2190 mD to 2038 mD for the Lower Tuscaloosa Sandstone sample in a static batch reactor after 180 days of exposure to CO2‐saturated brine, which is consistent with measured permeability results. The model predicted a negligible permeability change from 2.00 mD to 2.08 mD for the Selma Chalk carbonate sample after 180 days of exposure to CO2‐saturated brine. Based on model prediction, key mineral dissolution and precipitation reactions in the Lower Tuscaloosa Sandstone sample include dissolution of quartz, chlorite, and feldspar, as well as precipitation of amorphous silica and kaolinite. For the Selma Chalk carbonate sample, key predicted reactions include dissolution of calcite, quartz and chlorite, and precipitation of kaolinite and amorphous silica. Initial porosity, initial feldspar content and the exponent n value (related to pore structure and tortuosity) used in permeability calculations were three important factors affecting permeability evolution of sandstone samples under CO2 sequestration conditions. The small permeability change predicted for both the Lower Tuscaloosa Sandstone and the Selma Chalk caprock after exposure to CO2‐saturated brine suggests that poro‐permeability changes during CO2 injection into the Lower Tuscaloosa Formation are not likely to significantly affect reservoir and seal quality. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Fracture Conductivity Modelling in Experimental Shale Rock Interactions with Aqueous CO2

In large scale subsurface injection of carbondioxide (CO2) as obtainable in carbon sequestration programs and in environmentally friendly hydraulic fracturing processes (using supercritical CO2), long term rock-fluid interaction can affect reservoir and seal rocks properties which are essential in monitoring the progress of these operations. The mineralogical components of sedimentary rocks are geochemically active particularly under enormous earth stresses, which generate high pressure and temperature conditions in the subsurface. While geomechanical properties such as rock stiffness, Poisson's ratio and fracture geometry largely govern fluid flow characteristics in deep micro-fractured formations. Simulation results suggested that influx-induced mineral dissolution/precipitation reactions within clay-based sedimentary rocks can continuously close micro-fracture networks, though injection pressure and effective-stress transformation first rapidly expand the fractures. This experimental modelling research investigated the impact of in-situ geochemical precipitation on conductivity of fractures under geomechanical stress conditions. Bulk rock geomechanical hardness was determined using Vickers’ micro-indentation. Differential pressure drop data across fractured composite core were also measured with respect to time over a five day period. This was used in estimating the conductivity of the artificially fractured cores with 25μm-bore microtubings embedded. Three experimental runs per sample types were carried out in order to check the repeatability of observed changes. The results showed that most significant diagenetic changes in shale rocks after flooding with CO2-brine, reflect in the effluent fluid with predominantly calcium based minerals dissolving and precipitating under experimental conditions. Micro-indentation results suggested slight reduction in the hardness of the shale rocks and this reduction appears dependent on diagenetic quartz content. Estimated fracture conductivity indicated that reactive transport of dissolved minerals can occlude micro-fracture flow paths, thereby improving caprock seal integrity with respect to leakage risk under CO2 sequestration conditions.

Investigation of relative flow characteristics of brine-saturated reservoir formation: A numerical study of the Hawkesbury formation

Precise knowledge of the relative flow behaviour of CO2 and brine during CO2 sequestration in deep saline aquifers and its impact on the sequestration process is required to ensure the safety and efficiency of sequestration projects. This numerical study therefore aims to identify the interaction-induced relative flow behaviour of reservoir rock and its influence on the hydro-mechanical and geochemical phenomena in deep saline aquifers. COMSOL Multiphysics numerical simulator was used to develop a laboratory-scale relative flow model to simulate the CO2 movement and brine drainage processes in brine-saturated Hawkesbury sandstone samples and finally, was extended it into a field-scale numerical model which can simulate the hydro-mechanical, mineralogical and geochemical behaviours of deep saline aquifers under CO2 sequestration conditions. A 2-D axisymmetric pore-elastic model was developed using the pore-elastic module available in COMSOL and Buckely-Leverett flow theory was applied to the model using pre-defined partial differential equations. The proposed laboratory-scale model was first validated using experimental permeability data conducted under triaxial drained conditions and the model was then extended to predict relative flow characteristics, such as brine and CO2 saturation and CO2 pressure distribution along the sample length under different injection pressures, including both sub- and super-critical conditions and finally, real CO2 sequestration processes.According to the results, the developed laboratory-scale model simulates the experimental results reasonably well with less than 10% relative error. The numerical results also reveal that there is a considerable effect of CO2 phase change on the final distribution of the CO2 and brine saturation profiles. In addition, the CO2 pressure distribution along the sample length shows a non-linear relationship between the CO2 pressure and sample length. According to the results of the field-scale model, the long-term interaction of CO2 causes Hawkesbury formation's pore structure to significantly change.

Effects of sulfate and magnesium on cement degradation under geologic CO2 sequestration conditions

For safer geologic CO2 sequestration (GCS), it is important to understand CO2-brine-cement interactions, which affect wellbore integrity. However, potential effects of sulfate and magnesium ions on cement degradation under GCS conditions are not well understood. Here Class H Portland cement were reacted in brines containing 0.05M sulfate and/or magnesium ions under both GCS (50°C and 100atm CO2) and control (50°C and atmospheric pressure) conditions. Using optical microscopy and scanning electron microscope coupled with energy dispersive spectrometry and electron back scattered electron (SEM-EDS/BSE), slower cement carbonation rates were observed in the presence of sulfate under GCS conditions, because of gypsum precipitation on cement surfaces. Calcite rather than gypsum formed in both the inner layers of cement samples reacted under GCS conditions, and on cement surfaces reacted under atmospheric pressure conditions. Under GCS conditions, the dissolved CO2 lowered the pH of the solution surrounding cement surfaces, thus favoring the formation of gypsum over calcite on cement surfaces; while the high pH condition in pore solution inside cement favors the formation of calcite over gypsum. The presence of magnesium had no significant effect on cement degradation under GCS conditions, as brucite, magnesium carbonates and magnesium calcite did not form, due to the low pH at cement surface and the limited diffusion of Mg into cement inner layers.

수적(垂滴)법을 이용한 이산화탄소 지중저장 조건에서의 염수-이산화탄소 간 계면장력 측정

수적(垂滴)법을 이용한 이산화탄소 지중저장 조건에서의 염수-이산화탄소 간 계면장력 측정

CO2/brine/rock interactions in Lower Tuscaloosa formation

AbstractSaline aquifers are the largest potential continental geologic CO2 sequestration resource. Understanding of potential geochemically induced changes to the porosity and permeability of host CO2 storage and sealing formation rock will improve our ability to predict CO2 plume dynamics, storage capacity, and long‐term reservoir behavior. Experiments exploring geochemical interactions of CO2/brine/rock on saline formations under CO2 sequestration conditions were conducted in a static system. Chemical interactions in core samples from the Lower Tuscaloosa formation from Jackson County, Mississippi, with exposure to CO2‐saturated brine under sequestration conditions were studied through six months of batch exposure. The experimental conditions to which the core samples of Lower Tuscaloosa sandstone and Selma chalk were exposed to a temperature of 85°C, CO2 pressure of 23.8 MPa (3500 psig), while immersed in a model brine representative of Tuscaloosa Basin. Computed tomography (CT), X‐Ray diffraction (XRD), Scanning Electron Microscopy (SEM), brine chemistry, and petrography analyses were performed before and after the exposure. Permeability measurements from the sandstone core sample before and after exposure showed a permeability reduction. No significant change of the permeability measurements was noticed for the core sample obtained from Selma chalk after it was exposed to CO2/brine for six months. These results have implications for performance of the storage interval, and the integrity of the seal in a CO2 storage setting. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

The impact of reactive surface area on brine-rock-carbon dioxide reactions in CO2 sequestration

Injection of carbon dioxide into saline aquifers is one the most promising methods for mitigating the increase in atmospheric CO2 concentrations. Reactions of CO2 with brine and formation rocks are important in determining the ultimate fate of CO2. Rates of reactions of the different relevant minerals are dependent on rate constants and mineral surface area. It is important to quantify the effect of surface area on reaction rates when different types of rock samples are employed in experiments. Batch experiments with different forms of the Berea sandstone were set up and performed at high pressure (2000 psi) and reservoir temperature (60°C) for two-week under reservoir CO2 sequestration conditions. Experiments were performed with markedly different samples (cores, rock chips, and powdered) with progressively increased surface areas. Experiments with the different forms of sandstones showed similar trends in major minerals dissolution with increase in cation concentrations of iron, magnesium, and calcium in the effluent brine. Under realistic aquifer pressure and temperature conditions, iron chemistry plays an important role in the dissolution reactions in the Berea sandstone. Reactivities increased leading to larger cationic concentrations in brine (as determined using ICP-MS) as the surface areas increased. Morphology of the reacted volume was viewed using QEMSCAN and Micro-CT for core plug samples. These experiments helped quantity the effect of surface area (higher surface area leading to more dissolution) and revealed that reactions appear to be limited more to the surface in Berea sandstone. It was shown that the factor used to obtain geometric surface area from a core decreased when samples with higher surface areas were used. This indicated that some internal surface area became available for fragmented (fractured), and powdered samples. Mineral dissolution caused the growth and expansion of pores in Berea sandstone and surface area measurements (BET) showed that the new porosity generated was characterized by a smaller pore size distribution in comparison to the unreacted one.

Investigation on porosity and permeability change of Mount Simon sandstone (Knox County, IN, USA) under geological CO2 sequestration conditions: a numerical simulation approach

AbstractA numerical model was developed to simulate reactive transport with porosity and permeability change of Mount Simon sandstone (samples from Knox County, IN, USA) after 180 days of exposure to CO2‐saturated brine under CO2 sequestration conditions. The model predicted formation of a high‐porosity zone adjacent to the surface of the sample in contact with bulk brine, and a lower porosity zone just beyond that high‐porosity zone along the path from the sample/bulk brine interface to sample core. The formation of the high porosity zone was attributed to the dissolution of quartz and muscovite/illite, while the formation of the lower porosity zone adjacent to the high porosity zone was attributed to precipitation of kaolinite and feldspar. The model predicted a 40% permeability increase for the Knox sandstone sample after 180 days of exposure to CO2‐saturated brine, which was consistent with laboratory‐measured permeability results. Model‐predicted solution chemistry results were also found to be consistent with laboratory‐measured solution chemistry data. Initial porosity, initial feldspar content, and the exponent n value (determined by pore structure and tortuosity) used in permeability calculations were three important factors affecting permeability evolution of sandstone samples under CO2 sequestration conditions.

Water Contact Angle Dependence with Hydroxyl Functional Groups on Silica Surfaces under CO2 Sequestration Conditions.

Functional groups on silica surfaces under CO2 sequestration conditions are complex due to reactions among supercritical CO2, brine and silica. Molecular dynamics simulations have been performed to investigate the effects of hydroxyl functional groups on wettability. It has been found that wettability shows a strong dependence on functional groups on silica surfaces: silanol number density, space distribution, and deprotonation/protonation degree. For neutral silica surfaces with crystalline structure (Q(3), Q(3)/Q(4), Q(4)), as silanol number density decreases, contact angle increases from 33.5° to 146.7° at 10.5 MPa and 318 K. When Q(3) surface changes to an amorphous structure, water contact angle increases 20°. Water contact angle decreases about 12° when 9% of silanol groups on Q(3) surface are deprotonated. When the deprotonation degree increases to 50%, water contact angle decreases to 0. The dependence of wettability on silica surface functional groups was used to analyze contact angle measurement ambiguity in literature. The composition of silica surfaces is complicated under CO2 sequestration conditions, the results found in this study may help to better understand wettability of CO2/brine/silica system.