The effect of CO2 on the mechanical properties of reservoir and cap rock

AbstractWe have investigated the effect of CO2 on the mechanical stability of the reservoir-caprock system. Castlegate and Bentheimer sandstones were used as analogues for reservoir rock. Pierre shale was utilized as an analogue material for a typical cap rock. The effect of CO2 on carbonate rocks was studied by carrying out Brazilian tests on Lixhe and Austin chalks. The tensile strengths of both salt water and CO2-salt water exposed samples were observed to decrease with sample porosity. There was a positive correlation with tensile strength and p-wave velocity. The tensile strength of sandstone, shale and chalk is not markedly affected by the presence of CO2 in our tests. This observation has important implications for modeling fracture growth due to the injection of CO2 on geological formations because geomechanical models require tensile strength as an input parameter. Future experimental work should quantify the effect of CO2 on the entire failure envelope by using preserved core material and a triaxial test setup that mimics the in-situ stress and temperature conditions at a storage site

Loading rate dependence of permeability evolution in porous aeolian sandstones

Mechanical properties of rocks are characterized by their notable dependence on the applied deformation rate. However, little is known about the strain rate dependence of fluid flow properties since most laboratory tests are conducted using a single, high strain rate. We have investigated the effect of loading rate on the permeability of porous sandstones by carrying out triaxial compression tests at four different temperatures and strain rates with continuous monitoring of permeability, acoustic emission (AE), and pore fluid chemistry. All tests are characterized by an initial permeability decrease due to inferred compaction of favorably oriented cracks. The amount of initial permeability reduction increases with decreasing strain rate, thus implying a more efficient initial compaction at slower strain rates. At a later stage of loading, permeability correlates with stress, ion concentration, or AE damage depending on the strain rate used. High strain rate tests are characterized by a positive power law or logarithmic correlation between permeability and AE damage. At slow strain rates, permeabilities decrease exponentially with mean effective stress and axial strain for the Locharbriggs sandstone. The Clashach sandstone exhibits a linear correlation between permeability and exit pore fluid concentrations (Si, Mg, Fe, Al) if a slow strain rate is used. These observations have important implications for the applicability of room temperature, high strain rate laboratory data to the conditions that prevail in the Earth's crust

Correlation of microseismic and chemical properties of brittle deformation in Locharbriggs sandstone

The time-dependent properties of ceramic materials such as rocks depend both on preexisting cracks and chemical properties acting at their tips. We have examined the direct effect of chemical processes on the growth of a crack population by carrying out triaxial flow-through compression tests on Locharbriggs sandstone. The tests were carried out at temperatures of 25-80 degrees C and at strain rates ranging from 10-5 to 10-8 s-1 under constant stress rate loading. The exit pore fluid was analyzed after the tests for the concentration of dissolved ions and acoustic emission was monitored in real time throughout the tests. The exit pore fluid silica concentration and microcrack damage derived from the acoustic emission (AE) data both exhibited an exponential increase during the strain hardening phase of deformation. Damage parameters inferred from the AE data predict the stress-strain curves adequately, or at least up to the point of strong microcrack coalescence. The damage parameters and silica signal were strongly correlated by a power law relationship. The observed environment and strain rate dependence of mechanical properties can hence be attributed uniquely to time-dependent crack growth by the stress corrosion mechanism

Stress corrosion crack growth in porous sandstones

Stress corrosion crack growth occurs when the chemical weakening of strained crack tip bonds facilitates crack propagation. I have examined the effect of chemical processes on the growth of a creack population by carrying out triaxial compression tests on Clashach and Locharbriggs sandstones at temperatures of 25-80 degrees C and at strain rates of 10-5 to 10-8/s. The axial strain, permiability, acoustic emission (AE) activity and the pore fluid chemistry were monitored continuously during these tests. Rock strength is reduced in the presence of water and on the application of a slower strain rate. Elastic modulus also decreases with decreasing strain rate. Microstructural observations indicate that microfracturing is more pervasive in the slow strain rate tests in comparison to the high strain rate tests. Damage parameters derived from the AE data predict the stress-strain curves adequately. The accumulation of damage is more rapid in the slow strain rate tests than in the high strain rate tests. The exit pore fluid silica (Si) concentrations correlate with the main microfracturing domains of the stress-strain curve. In the strain hardening phase of the Locharbriggs tests the Si concentrations and AE damage increase exponentially. The small reactive surface area and the temperature dependance of the Si concentration in the Locharbriggs tests suggest that silica is dissolving actively from the growing crack tips and that reaction rates contribute towards this signal. the Locharbriggs Si signal and damage parameters are strongly correlated by a power law relationship. the obseved strain rate and environment dependance of mechanical properties of Locharbriggs sandstone can be uniquely attributed to crack growth by the stress corrosion mechanism. In the Clashach tests the damage accumulation is best described by a powe-law. The AE activity of both sandstones exhibits clear fore- and aftershock sequences that are well modeled by the Omori law with a power law exponent that is close to unity. The Clashach Omori decay parameter correlates with test temperature, indicating a faster decay of aftershock activity at a higher temperature. The permeability evolution also displays a distinct strain rate dependence. At high strain rates permeability correlates with microcrack damage. At slow strain rate the fluid flow properties correlate with mean effective stress or pore fluid ion concentrations. These observations suggest that brittle fracturing, chemical reaction and hydraulic properties of porous sandstone are strongly coupled processes in the crust.EThOS - Electronic Theses Online ServiceGBUnited Kingdo