The thermal properties of set Portland cements – a literature review in the context of CO2 injection well integrity

Depleted hydrocarbon reservoirs are a promising target for CO2 sequestration. Injection of cold CO2 into such geological reservoirs will cause thermal stresses and strains in wellbore casings, cement seals and surrounding rock, which may lead to the creation of unwanted pathways for seepage. Joule-Thomson effects could potentially produce freezing conditions. The design of CO2 injector wells must be able to cope with these thermal loads. While numerical modelling can be used to develop our understanding and assess the impact of thermal processes on wellbore integrity, such analyses require reliable input data for material properties, such as those of the cement seals. This critical review provides an overview of existing lab measurements and theoretical considerations to help constrain the thermal behaviour of Portland cement under relevant subsurface conditions. Special attention is given to the i) thermal conductivity, ii) specific heat capacity, and iii) coefficient of thermal expansion. Influences on these properties of factors such as a) temperature, b) pressure, c) mixing water-to-cement ratio, d) extent of hydration, e) porosity, and f) pore fluid saturation are discussed. Our review has shown that lab datasets obtained under relevant downhole conditions are limited, constraining the input for numerical assessment of wellbore cement integrity

Effect of CO2-H2O-Smectite Interactions on Permeability of Clay-Rich Rocks Under CO2 Storage Conditions

CO2 uptake by smectites can cause swelling and self-stressing in shallow clay-rich caprocks under CO2 storage P–T and constrained conditions. However, little data exist to constrain the magnitude of the effects of CO2-H2O-smectite interactions on the sealing properties of clay-rich caprocks and faults. We performed permeability experiments on intact and fractured Opalinus Claystone (OPA) cores (~ 5% smectite), as well as on a simulated gouge-filled faults consisting of Na-SWy-1 montmorillonite, under radially constrained conditions simulating “open” transport pathways (dry and variably wet He or CO2; 10 MPa fluid pressure; 40 °C). Overall, the flow of dry CO2 through intact OPA samples and simulated smectite fault gouge caused a decrease in permeability by a factor of 4–9 or even by > 1 order, compared to dry He permeability. Subsequent to flow of dry and partially wet fluid, both fractured OPA and simulated gouge showed a permeability reduction of up to 3 orders of magnitude once flow-through with wet CO2 was performed. This permeability change appeared reversible upon re-establishing dry CO2 flow, suggesting fracture permeability was dominated by water uptake or loss from the smectite clay, with CO2-water-smectite interactions play a minor effect. Our results show that whether an increases or decreases in permeability of clayey caprock is expected with continuous flow of CO2-rich fluid depends on the initial water activity in the clay material versus the water activity in the CO2 bearing fluid. This has important implications for assessing the self-sealing potential of fractured and faulted clay-rich caprocks

Impact of Chemical Environment on Compaction Creep of Quartz Sand and Possible Geomechanical Applications

Induced seismicity and surface subsidence are adverse effects of natural gas and geothermal energy production that may present barriers to their use as low-carbon alternatives to coal and oil. The driving force for these unwanted effects is compaction of the reservoir, which can potentially be mitigated by injecting (pressurized) fluids that restore the pore pressure and chemically inhibit compaction. We conducted uniaxial compaction experiments on quartz sand aggregates to investigate the effect of pore fluid chemistry on time-dependent compaction (creep). In addition to a low-vacuum (dry) environment, supercritical fluids (N2, CO2, and wet CO2), simple aqueous solutions (three HCl solutions and a NaOH solution), and complex aqueous solutions with additives (AlCl3, AMP, and washing detergent) were employed. N2, CO2, and fluids containing scaling inhibitor (AMP), as well as wastewater (detergent solution) are generally considered for injection. Compaction creep was enhanced in fluid-saturated environments compared to dry. Wet CO2 caused more creep with faster strain rates than the relatively dry CO2 and N2 environments. Experiments conducted with simple aqueous solutions exhibited a clear pH dependency. The complex aqueous solutions enhanced creep compared to their simple solution counterpart with similar pH. Based on acoustic emission data and microstructural analyses, we inferred that compaction creep was controlled by subcritical crack growth, aided by water, hydroxyl ions, and additives. If microcracking also controls compaction in reservoir sandstones, these results indicate that injection of supercritical fluids or acidic solutions may mitigate reservoir compaction

Compaction of the Groningen Gas Reservoir Sandstone: Discrete Element Modeling Using Microphysically Based Grain-Scale Interaction Laws

Reservoir compaction, surface subsidence, and induced seismicity are often associated with prolonged hydrocarbon production. Recent experiments conducted on the Groningen gas field's Slochteren sandstone reservoir rock, at in-situ conditions, have shown that compaction involves both poroelastic strain and time independent, permanent strain, caused by consolidation and shear of clay films coating the sandstone grains, with grain failure occurring at higher stresses. To model compaction of the reservoir in space and time, numerical approaches, such as the Discrete Element Method (DEM), populated with realistic grain-scale mechanisms are needed. We developed a new particle-interaction law (contact model) for classic DEM to explicitly account for the experimentally observed mechanisms of nonlinear elasticity, intergranular clay film deformation, and grain breakage. It was calibrated against both hydrostatic and conventional triaxial compression experiments and validated against an independent set of pore pressure depletion experiments conducted under uniaxial strain conditions, using a range of sample porosities, grain size distributions, and clay contents. The model obtained was used to predict compaction of the Groningen reservoir. These results were compared with field measurements of in-situ compaction and matched favorably, within field measurement uncertainties. The new model allows systematic investigation of the effects of mineralogy, microstructure, boundary conditions, and loading path on compaction behavior of the reservoir. It also offers a means of generating a data bank suitable for developing generalized constitutive models and for predicting reservoir response to different scenarios of gas extraction, or of fluid injection for stabilization or storage purposes

Impact of downhole pressure and fluid-access on the effectiveness of wellbore cement expansion additives

Autogenous shrinkage of wellbore cement widely impairs zonal isolation. MgO-based cement expansion additives (CEAs) can mitigate this shrinkage, or even impart net expansion, by creating porosity through displacive crystal growth-processes. However, both MgO hydration and autogenous shrinkage behaviour depend strongly on stress state. Evaluation of CEA performance in wellbore cements therefore requires testing under elevated pressures representative for subsurface environments. We report experiments addressing the chemical and bulk volume changes that occur in cement hydrating at 10 MPa confining pressure and 90 °C. Volumetric response was investigated as function of MgO concentration, external water supply, and pore pressure decrease through water consumption during reaction. Results show the bulk expansion achieved using MgO-based CEAs diminishes markedly with increasing effective confining pressure or, equivalently, upon restricting fluid supply. This reduced expansion-potential under pressure has profound implications for slurry design, notably regarding CEA-concentrations required to counteract micro-annulus formation while maintaining low cement permeability

Impact of Chemical Environment on Compaction Creep of Quartz Sand and Possible Geomechanical Applications

Induced seismicity and surface subsidence are adverse effects of natural gas and geothermal energy production that may present barriers to their use as low-carbon alternatives to coal and oil. The driving force for these unwanted effects is compaction of the reservoir, which can potentially be mitigated by injecting (pressurized) fluids that restore the pore pressure and chemically inhibit compaction. We conducted uniaxial compaction experiments on quartz sand aggregates to investigate the effect of pore fluid chemistry on time-dependent compaction (creep). In addition to a low-vacuum (dry) environment, supercritical fluids (N2, CO2, and wet CO2), simple aqueous solutions (three HCl solutions and a NaOH solution), and complex aqueous solutions with additives (AlCl3, AMP, and washing detergent) were employed. N2, CO2, and fluids containing scaling inhibitor (AMP), as well as wastewater (detergent solution) are generally considered for injection. Compaction creep was enhanced in fluid-saturated environments compared to dry. Wet CO2 caused more creep with faster strain rates than the relatively dry CO2 and N2 environments. Experiments conducted with simple aqueous solutions exhibited a clear pH dependency. The complex aqueous solutions enhanced creep compared to their simple solution counterpart with similar pH. Based on acoustic emission data and microstructural analyses, we inferred that compaction creep was controlled by subcritical crack growth, aided by water, hydroxyl ions, and additives. If microcracking also controls compaction in reservoir sandstones, these results indicate that injection of supercritical fluids or acidic solutions may mitigate reservoir compaction

Compaction of the Groningen Gas Reservoir Sandstone: Discrete Element Modeling Using Microphysically Based Grain-Scale Interaction Laws

Reservoir compaction, surface subsidence, and induced seismicity are often associated with prolonged hydrocarbon production. Recent experiments conducted on the Groningen gas field's Slochteren sandstone reservoir rock, at in-situ conditions, have shown that compaction involves both poroelastic strain and time independent, permanent strain, caused by consolidation and shear of clay films coating the sandstone grains, with grain failure occurring at higher stresses. To model compaction of the reservoir in space and time, numerical approaches, such as the Discrete Element Method (DEM), populated with realistic grain-scale mechanisms are needed. We developed a new particle-interaction law (contact model) for classic DEM to explicitly account for the experimentally observed mechanisms of nonlinear elasticity, intergranular clay film deformation, and grain breakage. It was calibrated against both hydrostatic and conventional triaxial compression experiments and validated against an independent set of pore pressure depletion experiments conducted under uniaxial strain conditions, using a range of sample porosities, grain size distributions, and clay contents. The model obtained was used to predict compaction of the Groningen reservoir. These results were compared with field measurements of in-situ compaction and matched favorably, within field measurement uncertainties. The new model allows systematic investigation of the effects of mineralogy, microstructure, boundary conditions, and loading path on compaction behavior of the reservoir. It also offers a means of generating a data bank suitable for developing generalized constitutive models and for predicting reservoir response to different scenarios of gas extraction, or of fluid injection for stabilization or storage purposes

Impact of downhole pressure and fluid-access on the effectiveness of wellbore cement expansion additives

Autogenous shrinkage of wellbore cement widely impairs zonal isolation. MgO-based cement expansion additives (CEAs) can mitigate this shrinkage, or even impart net expansion, by creating porosity through displacive crystal growth-processes. However, both MgO hydration and autogenous shrinkage behaviour depend strongly on stress state. Evaluation of CEA performance in wellbore cements therefore requires testing under elevated pressures representative for subsurface environments. We report experiments addressing the chemical and bulk volume changes that occur in cement hydrating at 10 MPa confining pressure and 90 °C. Volumetric response was investigated as function of MgO concentration, external water supply, and pore pressure decrease through water consumption during reaction. Results show the bulk expansion achieved using MgO-based CEAs diminishes markedly with increasing effective confining pressure or, equivalently, upon restricting fluid supply. This reduced expansion-potential under pressure has profound implications for slurry design, notably regarding CEA-concentrations required to counteract micro-annulus formation while maintaining low cement permeability