Creep of CarbFix basalt: influence of rock–fluid interaction

Geological carbon sequestration provides permanent CO2 storage to mitigate the current high concentration of CO2 in the atmosphere. CO2 mineralization in basalts has been proven to be one of the most secure storage options. For successful implementation and future improvements of this technology, the time-dependent deformation behavior of reservoir rocks in the presence of reactive fluids needs to be studied in detail. We conducted load-stepping creep experiments on basalts from the CarbFix site (Iceland) under several pore fluid conditions (dry, H2O saturated and H2O + CO2 saturated) at temperature, T≈80 ∘C and effective pressure, Peff=50 MPa, during which we collected mechanical, acoustic and pore fluid chemistry data. We observed transient creep at stresses as low as 11 % of the failure strength. Acoustic emissions (AEs) correlated strongly with strain accumulation, indicating that the creep deformation was a brittle process in agreement with microstructural observations. The rate and magnitude of AEs were higher in fluid-saturated experiments than in dry conditions. We infer that the predominant mechanism governing creep deformation is time- and stress-dependent subcritical dilatant cracking. Our results suggest that the presence of aqueous fluids exerts first-order control on creep deformation of basaltic rocks, while the composition of the fluids plays only a secondary role under the studied conditions.

Impact of Particle Sizes, Mineralogy and Pore Fluid Chemistry on the Plasticity of Clayey Soils

The current classification of clayey soils does not entail information of pore fluid chemistry and particle size less than 75 µm. However, the pore fluid chemistry and particle size (at given mineralogy) are critical in the plasticity of clayey soils because of their impact on negative charge density. Therefore, this study extensively discusses the description of clay with respect to mineralogy, particle sizes, and pore fluid chemistry based on liquid and plastic limits of kaolinite, illite, and bentonite, and estimates undrained shear strength from the observed liquid limits. The liquid limits and undrained shear strength estimated from the observed liquid limits as a function of mineralogy (clay type), particle size, and ionic concentration reveal the need of incorporating pore fluid chemistry and particle size into the fines classification system. Furthermore, multiple linear regression models developed in this study demonstrate the importance of particle size and ionic concentration in predicting the liquid limit of clayey soils. This study also discusses the need for a comprehensive understanding of fines classification for proper interpretation of natural phenomena and engineering applications for fine-grained sediments.

Creep of CarbFix Basalt: Influence of Rock-fluid Interaction

Geological carbon sequestration provides permanent CO2 storage to mitigate the current high concentration of CO2 in the atmosphere. CO2 mineralization in basalts has been proven to be one of the most secure storage options. For successful implementation and future improvements of this technology, the time-dependent deformation behavior of reservoir rocks in presence of reactive fluids needs to be studied in detail. We conducted load stepping creep experiments on basalts from the CarbFix site (Iceland) under several pore fluid conditions (dry, H2O-saturated and H2O+CO2-saturated) at temperature, T≈80 °C and effective pressure, Peff = 50 MPa, during which we collected mechanical, acoustic and pore fluid chemistry data. We observed transient creep at stresses as low as 11% of the ultimate failure strength, well below the stress level at the onset of bulk dilatancy. Acoustic emissions (AEs) correlated strongly with strain accumulation, indicating that the creep deformation was a brittle process in agreement with microstructural observations. The rate and magnitude of AEs were higher in fluid-saturated experiments than in dry conditions. We infer that the predominant mechanism governing creep deformation is time- and stress-dependent sub-critical dilatant cracking. Our results suggest that the presence of aqueous fluids exerts first order control on creep deformation of basaltic rocks, while the composition of the fluids plays only a secondary role under the studied conditions.

Improvement and Soil Consistency of Sand–Clay Mixtures Treated with Enzymatic-Induced Carbonate Precipitation

Recently, microbially induced carbonate precipitation (MICP) has been studied as an alternative for the improvement of sand–clay mixtures. However, the cementing uniformity of MICP-treated sand–clay mixtures cannot be guaranteed. In this present study, enzymatic-induced carbonate precipitation (EICP) was used to deal with it. The ions used in kaolin clay was predicted to affect the production rate for calcium carbonate (CaCO3), which was studied using the calcification test. The solidification test was conducted using two different methods (the premixing method and the diffusion method). The permeability, unconfined compressive strength and the content of CaCO3 of treated samples were obtained to evaluate the solidification effect of the EICP method. Moreover, in EICP treatment, the particle aggregation decreased the liquid limit, but the addition of solution increased it. Therefore, there were contrary effects to the soil consistency. In this study, the two types of liquid limits of treated samples were measured with deionized water and 2M-NaCl brine, respectively. The results show that the Al2O3, NaCl and MgCl2 in the kaolin clay had a slight impact on the production rate for CaCO3, while FeCl3 significantly inhibited it. The EICP method can improve sand–clay mixtures and decrease their permeability. Different from MICP, the EICP method can guarantee the uniformity of treated samples. Moreover, the liquid limit of the sample treated with the premixing method decreased, while that of the sample treated with the diffusion method increased firstly and then decreased with the increasing treatment cycles. Different from the deionized water, the pore-fluid chemistry had a larger effect on the liquid limit with 2M-NaCl brine.

Physicochemical effect on shear strength characteristics of clayey soils based on ring-shear experiment

Pore fluid chemistry can significantly influence the shear strength characteristics of a clayey soil. To explore the underlying mechanisms, a series of ring shear experiments are performed on two natural clays, which represent two typical types of clayey minerals, i.e., expansive montmorillonitic clay and low-plasticity kaolinitic clay. The effects of pore solution concentrations on the shear strength of the two clays are experimentally characterized. It is shown that the shear strength of the expansive clay can be significantly influenced by the pore solution chemistry, whereas that of the low-plasticity clay proves to be relatively insensitive to it. To capture the main features of the shear strength behavior of clayey soils, the concept of intergranular stress, which is an extension of the Terzaghi’s effective stress to incorporate physicochemical effect, is introduced to interpret the experimental data. It is found that the evolution of residual shear strength can be very well characterized by using the intergranular stress, showing that the proposed intergranular stress formulation can be used alternatively to describe the stress state of clayey soils saturated with various pore solutions.