D. Nicolas Espinoza

Water‐CO2‐mineral systems: Interfacial tension, contact angle, and diffusion—Implications to CO2 geological storage

[1]xa0The interfacial interaction between mineral surfaces and immiscible fluids determines the efficiency of enhanced oil or gas recovery operations as well as our ability to inject and store CO2 in geological formations. Previous studies have shown that the interfacial tension and contact angle in CO2-water-mineral systems change noticeably with fluid pressure. We compile previous results and extend the scope of available data to include saline water, different substrates (quartz, calcite, oil-wet quartz, and polytetrafluoroethylene (PTFE)), and a wide pressure range (up to 20 MPa at 298K). Data analysis provides interfacial tension and contact angle as a function of fluid pressure; in addition, we recover the diffusion coefficient of water in liquid CO2 from long-term observations. Results show that CO2-water interfacial tension decreases significantly as pressure increases in agreement with previous studies. Contact angle varies with CO2 pressure in all experiments in response to changes in CO2-water interfacial tension: it increases on nonwetting surfaces such as PTFE and oil-wet quartz and slightly decreases in water-wet quartz and calcite surfaces. Water solubility and its high diffusivity (D = 2 × 10−8 to 2 × 10−7 m2/s) in liquid CO2 govern the evolution of interparticle pendular water. CO2-derived ionic species interaction with the substrate leads to surface modification if reactions are favorable, e.g., calcite dissolution by carbonic acid and precipitation as water diffuses and migrates into the bulk CO2. Pressure-dependent interfacial tension and contact angle affect injection patterns and breakthrough mechanisms, in other words, the performance of geological formations that act as either reservoirs or seals.

Properties and phenomena relevant to CH4‐CO2 replacement in hydrate‐bearing sediments

[1]xa0The injection of carbon dioxide, CO2, into methane hydrate-bearing sediments causes the release of methane, CH4, and the formation of carbon dioxide hydrate, even if global pressure-temperature conditions remain within the CH4 hydrate stability field. This phenomenon, known as CH4-CO2 exchange or CH4-CO2 replacement, creates a unique opportunity to recover an energy resource, methane, while entrapping a greenhouse gas, carbon dioxide. Multiple coexisting processes are involved during CH4-CO2 replacement, including heat liberation, mass transport, volume change, and gas production among others. Therefore, the comprehensive analysis of CH4-CO2 related phenomena involves physico-chemical parameters such as diffusivities, mutual solubilities, thermal properties, and pressure- and temperature-dependent phase conditions. We combine new experimental results with published studies to generate a data set we use to evaluate reaction rates, to analyze underlying phenomena, to explore the pressure-temperature region for optimal exchange, and to anticipate potential geomechanical implications for CH4-CO2 replacement in hydrate-bearing sediments.

Discrete element modeling of indentation tests to investigate mechanisms of CO2-related chemomechanical rock alteration

During CO2 injection into geological formations, petrophysical and geomechanical properties of host formations can be altered due to mineral dissolution and precipitation. Field and laboratory results have shown that sandstone and siltstone can be altered by CO2-water mixtures, but few quantitative studies have been performed to fully investigate underlying mechanisms. Based on the hypothesis that CO2-water mixtures alter the integrity of rock structure by attacking cements rather than grains, we attempt to explain the degradation of cementation due to long-term contact with CO2 and water and mechanisms for changes in rock mechanical properties. Many sandstones, including calcite-cemented quartzitic sandstone, chlorite-cemented quartzitic sandstone, and hematite-cemented quartzitic sandstone contain interparticle cements that are more readily affected by CO2-water mixtures than grains. A model that couples the Discrete Element Method (DEM) and the Bonded-Particle Model (BPM) is used to perform simulations of indentation tests on synthetic rocks with crystal and random packings. The model is verified against the analytical Cavity Expansion Model (CEM) and validated against laboratory indentation tests on Entrada sandstone with and without CO2-alteration. Sensitivity analysis is performed for cementation microscopic parameters including stiffness, size, axial and shear strength. The simulation results indicate that the CO2-related degradation of mechanical properties in bleached Entrada sandstone can be attributed to the reduction of cement size rather than cement strength. Our study indicates that it is possible to describe the CO2-related rock alteration through particle-scale mechanisms.

Discrete Element Modeling of Micro-scratch Tests: Investigation of Mechanisms of CO 2 Alteration in Reservoir Rocks

The injection of CO2 into geological formations leads to geochemical re-equilibrium between the pore fluid and rock minerals. Mineral–brine–CO2 reactions can induce alteration of mechanical properties and affect the structural integrity of the storage formation. The location of alterable mineral phases within the rock skeleton is important to assess the potential effects of mineral dissolution on bulk geomechanical properties. Hence, although often disregarded, the understanding of particle-scale mechanisms responsible for alterations is necessary to predict the extent of geomechanical alteration as a function of dissolved mineral amounts. This study investigates the CO2-related rock chemo-mechanical alteration through numerical modeling and matching of naturally altered rocks probed with micro-scratch tests. We use a model that couples the discrete element method (DEM) and the bonded particle model (BPM) to perform simulations of micro-scratch tests on synthetic rocks that mimic Entrada sandstone. Experimental results serve to calibrate numerical scratch tests with DEM–BPM parameters. Sensitivity analyses indicate that the cement size and bond shear strength are the most sensitive microscopic parameters that govern the CO2-induced alteration in Entrada sandstone. Reductions in cement size lead to decrease in scratch toughness and an increase in ductility in the rock samples. This work demonstrates how small variations of microscopic bond properties in cemented sandstone can lead to significant changes in macroscopic large-strain mechanical properties.

Assessment of Mudrock Brittleness with Micro-scratch Testing

Mechanical properties are essential for understanding natural and induced deformational behavior of geological formations. Brittleness characterizes energy dissipation rate and strain localization at failure. Brittleness has been investigated in hydrocarbon-bearing mudrocks in order to quantify the impact of hydraulic fracturing on the creation of complex fracture networks and surface area for reservoir drainage. Typical well logging correlations associate brittleness with carbonate content or dynamic elastic properties. However, an index of rock brittleness should involve actual rock failure and have a consistent method to quantify it. Here, we present a systematic method to quantify mudrock brittleness based on micro-mechanical measurements from the scratch test. Brittleness is formulated as the ratio of energy associated with brittle failure to the total energy required to perform a scratch. Soda lime glass and polycarbonate are used for comparison to identify failure in brittle and ductile mode and validate the developed method. Scratch testing results on mudrocks indicate that it is possible to use the recorded transverse force to estimate brittleness. Results show that tested samples rank as follows in increasing degree of brittleness: Woodford, Eagle Ford, Marcellus, Mancos, and Vaca Muerta. Eagle Ford samples show mixed ductile/brittle failure characteristics. There appears to be no definite correlation between micro-scratch brittleness and quartz or total carbonate content. Dolomite content shows a stronger correlation with brittleness than any other major mineral group. The scratch brittleness index correlates positively with increasing Young’s modulus and decreasing Poisson’s ratio, but shows deviations in rocks with distinct porosity and with stress-sensitive brittle/ductile behavior (Eagle Ford). The results of our study demonstrate that the micro-scratch test method can be used to investigate mudrock brittleness. The method is particularly useful for reservoir characterization methods that take advantage of drill cuttings or whenever large samples for triaxial testing or fracture mechanics testing cannot be recovered.

Carbon Geological Storage: Coupled Processes, Engineering and Monitoring

Abstract Today’s energy concerns reflect the large anticipated increase in demand within the next generation, the current dependency on fossil fuels and climate implications, the geographic mismatch between resources and demand, and the disparity in associated time scales. The long-term geological storage of vast quantities of CO2 is a relatively new scientific and technological challenge, plagued with underlying coupled hydro-chemo-mechanical processes and potential emergent phenomena. Processes include: capillarity, density and viscous effects on flow; acidification, mineral dissolution, and ensuing changes in permeability; phase transformations (and CO2-CH4 exchange in hydrates); and stress changes. These processes are involved in the analysis of CO2 storage in saline aquifers, coal seams, depleted reservoirs, and in clathrates. Furthermore, the understanding of underlying processes guides monitoring (active: seismic and electromagnetic; passive: seismic, deformation, thermal) and may lead to improved efficiency and leakage-sealing strategies. Dimensionless ratios help identify the domain for the various dominant processes that govern CO2 geo-storage.

EMERGENT CHEMO-HYDRO-MECHANICAL PHENOMENA IN CARBON GEOLOGICAL STORAGE

The combustion of fossil fuels produces carbon dioxide (CO2). Carbon capture and geological storage (CCS) has been proposed to reduce the emission of greenhouse gases to the atmosphere. The properties of CO2 depend on pressure and temperature: it can be found as a gas, liquid, or supercritical, and it reacts with water to produce carbonic acid lowering the water pH and can form a solid hydrate mass. These characteristics give rise to complex chemo-hydro-thermo-mechanical coupled processes and emergent phenomena that can condition the long-term geological storage of CO2. Processes include wettability, leakage, mineral dissolution, and CH4-CO2 replacement in hydrate-bearing sediments.