Geologic CO2 Sequestration Conditions Research Articles

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.

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.

Effects of Sulfate during CO2 Attack on Portland Cement and Their Impacts on Mechanical Properties under Geologic CO2 Sequestration Conditions.

To investigate the effects of sulfate on CO2 attack on wellbore cement (i.e., chemical and mechanical alterations) during geologic CO2 sequestration (GCS), we reacted cement samples in brine with 0.05 M sulfate and 0.4 M NaCl at 95 °C under 100 bar of either N2 or supercritical CO2. The results were compared to those obtained from systems without additional sulfate at the same temperature, pressure, salinity, and initial brine pHs. After 10 reaction days, chemical analyses using scanning electron microscopy with a backscattered electron detector (SEM-BSE) and inductively coupled plasma optical emission spectrometry (ICP-OES) showed that the CO2 attack in the presence of additional sulfate was much less severe than that in the system without additional sulfate. The results from three-point bending tests also indicated that sulfate significantly mitigated the deterioration of the cement's strength and elastic modulus. In all our systems, typical sulfate attacks on cement via formation of ettringite were not observed. The protective effects of sulfate on cement against CO2 attack resulted from sulfate adsorption, coating of CaSO4 on the CaCO3 grains in the carbonated layer, or both, which inhibited dissolution of CaCO3. Findings from this study provide new, important information for understanding the integrity of wellbores at GCS sites and thus promote safer GCS operations.

Chemical Reactions of Portland Cement with Aqueous CO2 and Their Impacts on Cement’s Mechanical Properties under Geologic CO2 Sequestration Conditions

To provide information on wellbore cement integrity in the application of geologic CO2 sequestration (GCS), chemical and mechanical alterations were analyzed for cement paste samples reacted for 10 days under GCS conditions. The reactions were at 95 °C and had 100 bar of either N2 (control condition) or CO2 contacting the reaction brine solution with an ionic strength of 0.5 M adjusted by NaCl. Chemical analyses showed that the 3.0 cm × 1.1 cm × 0.3 cm samples were significantly attacked by aqueous CO2 and developed layer structures with a total attacked depth of 1220 μm. Microscale mechanical property analyses showed that the hardness and indentation modulus of the carbonated layer were 2-3 times greater than for the intact cement, but those in the portlandite-dissolved region decreased by ∼50%. The strength and elastic modulus of the bulk cement samples were reduced by 93% and 84%, respectively. The properties of the microscale regions, layer structure, microcracks, and swelling of the outer layers combined to affect the overall mechanical properties. These findings improve understanding of wellbore integrity from both chemical and mechanical viewpoints and can be utilized to improve the safety and efficiency of CO2 storage.

Evaluation of experimentally measured and model-calculated pH for rock–brine–CO2 systems under geologic CO2 sequestration conditions

Reliable pH estimation is essential for understanding the geochemical reactions that occur in rock–brine–CO2 systems when CO2 is injected into deep geologic formations for long-term storage. Due to a lack of reliable experimental methods, most laboratory studies of CO2–rock–brine interactions conducted under geologic CO2 sequestration (GCS) conditions have relied on thermodynamic modeling to estimate pH; however, the accuracy of these model predictions is typically uncertain. In this study, we expanded the measurement range of a spectrophotometric method for pH determination, and we applied the method to measure the pH in batch-reactor experiments at 75°C and 100atm utilizing rock samples from five ongoing GCS demonstration projects. A combination of color-changing pH indicators, bromophenol blue and bromocresol green, was shown to enable measurements over the pH range of 2.5–5.2. In-situ pH measurements were compared with pH values calculated using geochemical models. Calculations with four different thermodynamic databases resulted in a maximum difference of 0.16pH units. Among these databases, the Phrqpitz database generally provided the most accurate pH predictions for rocks comprised of carbonate, siltstone, and sandstone. With Phrqpitz, the differences between measured and calculated pH values were within 0.03pH units for these three rocks. However, for basalt, significant differences (0.10–0.25pH units) were observed even with Phrqpitz. These discrepancies may be due to the models' failure to fully account for certain proton consuming and producing reactions that occur between the basalt minerals and CO2-saturated brine solutions.

Thermochemistry of two calcium silicate carbonate minerals: scawtite, Ca7(Si6O18)(CO3)·2H2O, and spurrite, Ca5(SiO4)2(CO3)

Calcium silicate carbonate minerals are products of contact metamorphism and metasomatism processes. They are also possible products of low temperature hydrothermal reactions, including those in cementitious materials and possibly in the geologic CO2 sequestration environment. Two minerals in this class, scawtite, Ca7(Si6O18)(CO3)·2H2O, and spurrite, Ca5(SiO4)2(CO3), were synthesized and characterized in the present work. Their enthalpies of formation were determined by high temperature oxide melt solution calorimetry. The enthalpy of formation from the oxides is −689.5±14.3kJ/mol for scawtite and −455.1±9.7kJ/mol for spurrite, and the enthalpy of formation from the elements is −11564.5±16.8kJ/mol for scawtite and −5845.5±10.9kJ/mol for spurrite. By using an exchange reaction involving all solid phase reactants and products, the standard entropy of formation for scawtite was estimated. The energetics for several reactions in high temperature geochemical (metamorphic) processes have been determined. The calculated stability fields for these two minerals and calcium carbonate are presented for 25°C and 80°C, and the possibilities for these two minerals to precipitate under geologic CO2 sequestration conditions are discussed. Although calcium carbonate is the most likely phase during carbonation reactions in aqueous solution, scawtite and spurrite may precipitate near the surfaces of dissolving silicate minerals, clays, or cement phases, particularly in the caprocks.

Effects of organic ligands on supercritical CO2-induced phlogopite dissolution and secondary mineral formation

To evaluate the long-term and short-term risks associated with geologic CO2 sequestration (GCS), we need to understand both the reactions at supercritical CO2 (scCO2)–saline water–rock interfaces, and the environmental factors affecting these interactions. This research investigated the effects of four organic ligands—oxalate, malonate, acetate, and propionate—on the dissolution and surface morphological changes of phlogopite [KMg2.87Si3.07Al1.23O10(F,OH)2] under GCS conditions (95°C and 102atm). Phlogopite was chosen as a model clay mineral in potential GCS sites. After CO2 injection, the dissolution of CO2 should cause a decrease in saline water pH, increasing phlogopite dissolution. This effect can be lessened by the buffering capacity of organic ligands. However, in this study, the ligands that formed strong complexes with surface metals (i.e., oxalate) caused phlogopite dissolution rates to increase via ligand-promoted dissolution, although the pH increased. The experimentally observed dissolution rates of phlogopite were in the order of: oxalate>malonate>acetate≈propionate. In addition, based on results from ion chromatography, oxalate and malonate concentrations were stable in our reaction systems; however, aqueous acetate and propionate concentrations continuously decreased due to solvent extraction of acetic acid and propionic acid by scCO2 at 95°C and 102atm. After 159h, all of the acetate and propionate were removed from aqueous solutions. Although the aqueous species in the bulk solution were not supersaturated with respect to potential secondary mineral phases, interestingly, in the presence of oxalate, nanoscale precipitation of amorphous silica and fibrous illite was observed at the phlogopite surface only 3h after CO2 injection. At this early reaction time, illite fibers formed a connected, hexagonal framework on phlogopite basal surfaces, but at a later reaction time, these structures detached from the surface and triggered the formation of dissolution channels. In addition, kaolinite, boehmite, diaspore, and gibbsite were identified as secondary mineral phases. These results provide new information towards understanding organic species' interactions at scCO2–saline water–rock interfaces in deep saline aquifers.

The effects of initial acetate concentration on CO2–brine-anorthite interactions under geologic CO2 sequestration conditions

Acetate is one of the most abundant organic compounds in many formation waters and is likely to be present in deep saline aquifers suitable for geologic CO2 sequestration (GCS). This work studied the effect of initially present acetate on the dissolution of anorthite (CaAl2Si2O8) and on subsequent secondary mineral precipitation under GCS conditions (35 °C and 74.8 atm). Anorthite was chosen as a model mineral because of the abundance of feldspar in clayey sandstones and the possibility of metal carbonation. In this study, acetate was found to decrease the cumulative aqueous concentrations of Al, Si, and Ca upon CO2 injection by inhibiting anorthite dissolution and increasing the amount of secondary mineral precipitates. The extent of the effect of acetate on metal concentration changes was element-specific (Al > Si > Ca), and the effect was found to be more significant in systems with lower salinity and lower pH. For anorthite dissolution, acetic acid inhibited the proton-mediated decomposition of the Al/Si-containing feldspar framework, while acetate anions may have facilitated the ion-exchange between interstitial Ca and aqueous cations. For secondary mineral precipitation, stoichiometry analysis of aqueous metal concentrations suggested the formation of Al-containing mineral(s). The presence of kaolinite as a secondary mineral was confirmed using high resolution transmission electron microscopy's electron diffraction data. An increase in the relative amount of precipitation due to the initial presence of acetate was suggested by mass balancing and verified on the cleaved anorthite surfaces by atomic force microscopy analysis. These results provide new insights for understanding and predicting GCS system evolution upon scCO2 injection in the initial presence of acetate.

Dissolution and Precipitation of Clay Minerals under Geologic CO2 Sequestration Conditions: CO2−Brine−Phlogopite Interactions

To ensure efficiency and sustainability of geologic CO2 sequestration (GCS), a better understanding of the geochemical reactions at CO2-water-rock interfaces is needed. In this work, both fluid/solid chemistry analysis and interfacial topographic studies were conducted to investigate the dissolution/precipitation on phlogopite (KMg3Si3AlO10(F,OH)2) surfaces under GCS conditions (368 K, 102 atm) in 1 M NaCl. Phlogopite served as a model for clay minerals in potential GCS sites. During the reaction, dissolution of phlogopite was the predominant process. Although the bulk solution was not supersaturated with respect to potential secondary mineral phases, interestingly, nanoscale precipitates formed. Atomic force microcopy (AFM) was utilized to record the evolution of the size, shape, and location of the nanoparticles. Nanoparticles first appeared on the edges of dissolution pits and then relocated to other areas as particles aggregated. Amorphous silica and kaolinite were identified as the secondary mineral phases, and qualitative and quantitative analysis of morphological changes due to phlogopite dissolution and secondary mineral precipitation are presented. The results provide new information on the evolution of morphological changes at CO2-water-clay mineral interfaces and offer implications for understanding alterations in porosity, permeability, and wettability of pre-existing rocks in GCS sites.

Experimental evaluation of interactions in supercritical CO2/water/rock minerals system under geologic CO2 sequestration conditions

The hydrothermal autoclave experiments were conducted to simulate the interactions in the scCO2/water/rock minerals (quartz, biotite and granite) reaction systems using a Hastelloy C reaction cell at 100 °C. The dissolution characteristics of rock minerals and their surface texture alternation after hydrothermal treatment were examined by ICP-AES and SEM/EDX investigation, respectively. The results suggested that the hydrolysis of plagioclase phase should be mainly responsible for the elements dissolved from the Iidate granite samples. The dissolution was encouraged by the introduction of CO2 in the water/granite system, and generated an unknown aluminosilicate. No distinct chemical alternations occurred in the water-free scCO2/granite system, which indicated that rock minerals should be chemically stable in the water-free scCO2 fluids under the current mild experimental conditions. Both the highest concentration of Ca existing in the scCO2/vapor/granite system and the SEM observation results of calcite deposit, suggested that a meaningful CO2 minerals trapping process should be potential in the CO2-rich field during a short physicochemical interaction period.