Quantification of Viscous Creep Influence on Storage Capacity of Caprock

In the light of growing concerns for the climate change, it is of particular interest for governments to encourage efficient capture and safe storage of large amounts of carbon dioxide in the subsurface. In this perspective and in order to accurately predict the short and long-term response of the reservoir, a precise characterization of the geomechanical properties has to be carried out. In addition to the classical poroelastic properties, time-dependent deformation, such as viscous creep should also be considered. Storage capacity of a caprock may be seriously affected by local creep deformation allowing fast vertical fluid flow through an a priori very impermeable formation. Within this study, we investigate the ease to creep of Opalinus clay (Jurassic shale) under shallow geological storage conditions and predict the propagation of high porosity channels at operational time scales. The effective poroviscoelastic parameters of rock are inferred from the novel laboratory experiments that allow evaluation of time-dependent deformation. The bulk viscosity of the shale is found to be ∼ 1014 - 1015 Pa·s and it decreases with rise of temperature and pore fluid pressure to total mean stress ratio. Furthermore, the propagation speed of high porosity channels (porosity waves) is calculated to be on the order of the centimeters per year.

The effects of land use and climate change on the carbon cycle of Europe over the past 500 years

Jed Oliver Kaplan, Kristen Marie Krumhardt

The long residence time of carbon in forests and soils means that both the current state and future behavior of the terrestrial biosphere are influenced by past variability in climate and anthropogenic land use. Over the last half-millennium, European terrestrial ecosystems were affected by the cool temperatures of the Little Ice Age, rising CO2 concentrations, and human induced deforestation and land abandonment. To quantify the importance of these processes, we performed a series of simulations with the LPJ dynamic vegetation model driven by reconstructed climate, land use, and CO2 concentrations. Although land use change was the major control on the carbon inventory of Europe over the last 500years, the current state of the terrestrial biosphere is largely controlled by land use change during the past century. Between 1500 and 2000, climate variability led to temporary sequestration events of up to 3Pg, whereas increasing atmospheric CO2 concentrations during the 20th century led to an increase in carbon storage of up to 15Pg. Anthropogenic land use caused between 25Pg of carbon emissions and 5Pg of uptake over the same time period, depending on the historical and spatial pattern of past land use and the timing of the reversal from deforestation to afforestation during the last two centuries. None of the currently existing anthropogenic land use change datasets adequately capture the timing of the forest transition in most European countries as recorded in historical observations. Despite considerable uncertainty, our scenarios indicate that with limited management, extant European forests have the potential to absorb between 5 and 12Pg of carbon at the present day.

2012

Poroelastic behavior of Calcarenite for the purposes of geologic CO2 storage

Lyesse Laloui, Roman Makhnenko

Geologic CO2 sequestration is considered to be the most promising technique to reduce the concentration of greenhouse gases in the atmosphere. Among all the storage options, deep saline aquifers have the greatest potential and due to their worldwide occurrence can play a major role in reduction of carbon dioxide emissions. In sedimentary basins at depths below 800 meters, CO2 usually exists in supercritical condition (scCO2), which means that its temperature and pressure are above 31.1o C and 7.4 MPa, respectively, and it has a liquidlike density (500-800 kg/m3). Carbon dioxide then can be trapped in pore space of the storage formation and by reacting with minerals that form it, as well as dissolve in the in-situ fluids. Choice of a host rock is crucial for proper retention of scCO2, and sandstone reservoirs, which mostly are single-porosity systems, are usually considered. However, in some countries (e.g. Switzerland, Italy and Canada) limestone aquifers are widespread and have to be examined for the possibility of storage. The injection of significant amounts of CO2 into a limestone reservoir has to be carefully treated because it is usually a multipleporosity system with wide permeability variations. Also, because of the dissolution reactions, large pores can be created in calcite-rich limestones, which increases their permeability, but significantly decreases capillary trapping of CO2. In order to study the possibility of CO2 injection and storage in limestones, Calcarenite (Apulian limestone) was tested for full characterization of its poroelastic response under three limiting conditions: drained, undrained, and unjacketed. The rock with interconnected porosity of 33%, permeability equal to 3-5 mD, and less than 2% level of anisotropy was first tested in uniaxial compression. Dry elastic parameters were accurately measured using both strain gages and digital image correlation (DIC) data. Two types of measurements provided the same rock parameters: Young’s modulus E = 7.3 GPa and Poisson’s ratio  = 0.25. Hydrostatic compression experiments were performed on jacketed and unjacketed specimens: dry bulk modulus K was measured to be 4.8 GPa at 6 - 8 MPa hydrostatic pressure and unjacketed bulk modulus Ks˝ was found to be 42 GPa and constant throughout the loading. Biot coefficient was then calculated to be  = 0.89. Furthermore, plane strain compression experiments were performed under undrained conditions. Calcarenite’s Skempton’s B coefficient appeared to be increasing with saturation and the maximum achieved value at 6 MPa effective mean stress was B = 0.70. Undrained bulk modulus was measured to be smaller than the dry bulk modulus of the rock and the results of experiments performed under the same conditions were not reproducible. It means that the limestone behavior was not poroelastic even at low effective mean stress (up to 6 – 7 MPa) and can be related to the partial dissolution in water of the minerals forming the rock. X-ray CT scanning showed the significant increase in the rock porosity for the saturated specimens. Also, the decrease in P-wave velocity was observed with injection of water in the limestone. The experiments with water as the saturation fluid will be compared to the tests where scCO2 is injected; even stronger reduction of poroelastic parameters is expected to be observed.

2014

Constitutive analysis of shale: a coupled damage plasticity approach

Lyesse Laloui, Francesco Parisio

Shales have become increasingly important because they play key roles in modern energy and environmental geomechanics applications, such as nuclear waste storage, non-conventional oil and gas operations and CO2 geological storage. Shale behaves in a quasi-brittle manner, often exhibiting linear elasticity before reaching its peak stress. Furthermore, softening of the material leads to a residual state in which pure plastic flow is observed under constant values of deviatoric stress. Degradation of the elastic moduli and the accumulation of irreversible strains are believed to be primarily caused by the debonding and decohesion mechanisms in the structure, as well as the growth of microcracks. To capture these features, a constitutive model that couples elastic, plastic and damage theories is developed. The isotropic damage model is based on the Weibull probabilistic theory and describes the failure of brittle materials. This model is coupled with a modified version of the Lade-Duncan criterion to account for non-linear dependency of shear resistance with a mean stress typical of geomaterials. The two surfaces of damage and plasticity and the rate equations for the internal variables are postulated and thermodynamic consistency is subsequently investigated. The coupled plastic-damage constitutive model is integrated with an implicit stress return algorithm for multi-dissipative materials. Numerical back-calculations of experimental results from two quasi-brittle shales demonstrate the ability of the model to reproduce the primary features of their mechanical behavior.

Elsevier2015

Poroelastic behavior of Calcarenite for the purposes of geologic CO2 storage

Lyesse Laloui, Roman Makhnenko

2014