Integrated geophysical and hydromechanical assessment for CO2 storage: shallow low permeable reservoir sandstones

Geological reservoirs can be structurally complex and can respond to CO2 injection both geochemically and geomechanically. Hence, predicting reservoir formation behaviour in response to CO2 injection and assessing the resulting hazards are important prerequisites for safe geological CO2 storage. This requires a detailed study of thermal-hydro-mechanical-chemical coupled phenomena that can be triggered in the reservoir formation, most readily achieved through laboratory simulations of CO2 injection into typical reservoir formations. Here, we present the first results from a new experimental apparatus of a steady-state drainage flooding test conducted through a synthetic sandstone sample, simulating real CO2 storage reservoir conditions in a shallow (?1 km), low permeability ?1mD, 26% porosity sandstone formation. The injected pore fluid comprised brine with CO2 saturation increasing in steps of 20% brine/CO2 partial flow rates up to 100% CO2 flow. At each pore fluid stage, an unload/loading cycle of effective pressure was imposed to study the response of the rock under different geomechanical scenarios. The monitoring included axial strains and relative permeability in a continuous mode (hydromechanical assessment), and related geophysical signatures (ultrasonic P-wave and S-wave velocities Vp and Vs, and attenuations Qp?1 and Qs?1, respectively, and electrical resistivity). On average, the results showed Vp and Vs dropped ?7% and ?4% respectively during the test, whereas Qp?1 increased ?55% and Qs?1 decreased by ?25%. From the electrical resistivity data, we estimated a maximum CO2 saturation of ?0.5, whereas relative permeability curves were adjusted for both fluids. Comparing the experimental results to theoretical predictions, we found that Gassmann's equations explain Vp at high and very low CO2 saturations, whereas bulk modulus yields results consistent with White and Dutta–Odé model predictions. This is interpreted as a heterogeneous distribution of the two pore fluid phases, corroborated by electrical resistivity tomography images. The integration of laboratory geophysical and hydromechanical observations on representative shallow low-permeable sandstone reservoir allowed us to distinguish between pure geomechanical responses and those associated with the pore fluid distribution. This is a key aspect in understanding CO2 injection effects in deep geological reservoirs associated with carbon capture and storage practices

Water saturation effects on P-wave anisotropy in synthetic sandstone with aligned fractures

The seismic properties of rocks are known to be sensitive to partial liquid or gas saturation, and to aligned fractures. P-wave anisotropy is widely used for fracture characterization and is known to be sensitive to the saturating fluid. However, studies combining the effect of multiphase saturation and aligned fractures are limited even though such conditions are common in the subsurface. An understanding of the effects of partial liquid or gas saturation on P-wave anisotropy could help improve seismic characterization of fractured, gas bearing reservoirs. Using octagonal-shaped synthetic sandstone samples, one containing aligned penny-shaped fractures and the other without fractures, we examined the influence of water saturation on P-wave anisotropy in fractured rocks. In the fractured rock, the saturation related stiffening effect at higher water saturation values is larger in the direction across the fractures than along the fractures. Consequently, the anisotropy parameter ‘?’ decreases as a result of this fluid stiffening effect. These effects are frequency dependent as a result of wave-induced fluid flow mechanisms. Our observations can be explained by combining a frequency-dependent fractured rock model and a frequency-dependent partial saturation model

Modelling ultrasonic laboratory measurements of the saturation dependence of elastic modulus: new insights and implications for wave propagation mechanisms

Seismic time-lapse techniques are a valuable tool used to estimate the mobilization and distribution of stored CO2 in depleted reservoirs. The success of these techniques depends on knowing the seismic properties of partially saturated rocks with accuracy. It is commonplace to use controlled laboratory-scale experiments to determine how the fluid content impacts on their properties. In this work, we measure the ultrasonic P- and S-wave velocities of a set of synthetic sandstones of about 30% porosity. Using an accurate method, we span the entire saturation range of an air-water system. We show that the rocks’ elastic behaviour is consistent with patchy saturation and squirt flow models but observe a discontinuity at around 90% gas saturation which can be interpreted in two very different ways. In one interpretation, the responsible mechanism is frequency-dependent squirt-flow that occurs in narrow pores that are preferentially saturated. An equally plausible mechanism is the change of the mobile fluid in the pores once they are wetted. Extrapolated to seismic frequencies, our results imply that the seismic properties of rocks may be affected by the wetting effect with an impact on the interpretation of field data but would potentially be unaffected by the squirt flow effect. This provides strong motivation to conduct laboratory-scale experiments with partially saturated samples at lower frequency or, ideally, a range of frequencies in the seismo-acoustic range

Comparison of stress-dependent geophysical, hydraulic and mechanical properties of synthetic and natural sandstones for reservoir characterization and monitoring studies

Synthetic rock samples can offer advantages over natural rock samples when used for laboratory rock physical properties studies, provided their success as natural analogues is well understood. The ability of synthetic rocks to mimic the natural stress dependency of elastic wave, electrical and fluid transport properties is of primary interest. Hence, we compare a consistent set of laboratory multi‐physics measurements obtained on four quartz sandstone samples (porosity range 20–25%) comprising two synthetic and two natural (Berea and Corvio) samples, the latter used extensively as standards in rock physics research. We measured simultaneously ultrasonic (P‐ and S‐wave) velocity and attenuation, electrical resistivity, permeability and axial and radial strains over a wide range of differential pressure (confining stress 15–50 MPa; pore pressure 5–10 MPa) on the four brine saturated samples. Despite some obvious physical discrepancies caused by the synthetic manufacturing process, such as silica cementation and anisotropy, the results show only small differences in stress dependency between the synthetic and natural sandstones for all measured parameters. Stress dependency analysis of the dry samples using an isotropic effective medium model of spheroidal pores and penny‐shaped cracks, together with a granular cohesion model, provide evidence of crack closure mechanisms in the natural sandstones, seen to a much lesser extent in the synthetic sandstones. The smaller grain size, greater cement content, and cementation under oedometric conditions particularly affect the fluid transport properties of the synthetic sandstones, resulting in lower permeability and higher electrical resistivity for a similar porosity. The effective stress coefficients, determined for each parameter, are in agreement with data reported in the literature. Our results for the particular synthetic materials that were tested suggest that synthetic sandstones can serve as good proxies for natural sandstones for studies of elastic and mechanical properties, but should be used with care for transport properties studies

Saturation effects on frequency-dependent seismic anisotropy in fractured porous rocks

The response of Earth materials to seismic wave propagation is the most commonly used geophysical method for studying the Earth’s crust. Rocks making up the Earth’s crust are porous, with fluids occupying the pore space. The saturation of the pore space can be multiphase, for example, in gas reservoirs and gas bearing oil reservoirs where gas and liquid occupy the pore space. Additional voids such as aligned fractures are common in the Earth’s crust and are known to cause seismic anisotropy. Knowledge of the effects of pore fluids and of aligned fractures on seismic wave propagation is needed for the interpretation of seismic data in terms of these physical properties. This information is particularly useful for the hydrocarbon industry as the presence of either natural or artificially induced fractures can play a major role in the safe and efficient exploration and production of hydrocarbons. Therefore, it is important to be able to remotely characterise fractures in fluid-filled reservoir rocks.Theoretical models are used to relate seismic measurements to the physical properties of rocks such as porosity, saturation, and fracture properties. Previous studies have either focused on multiphase saturation effects in non-fractured isotropic rocks or on single fluid phase saturation effects in fractured anisotropic rocks. Therefore, the combined effect of multiphase saturation and aligned fractures is still poorly understood. This thesis focuses on improving the understanding of the effect of saturation on fracture-induced seismic anisotropy<br/

Effective medium modeling of pressure effects on the joint elastic and electrical properties of sandstones

Subsurface rocks are subjected to different stress states, and as such, physical properties of rocks including elastic modulus and electrical resistivity are stress-dependent. Joint modeling of the stress-dependence of elastic and electrical properties is important for interpreting coincident elastic and electrical measurements. We present a modeling approach for the pressure-dependence of the joint elastic and electrical properties in brine-saturated sandstones, that links both properties to the same microstructure. This approach combines the differential effective medium (DEM) and the self-consistent approximation (SCA) with the dual-porosity concept to model the pressure-dependence of P-wave velocity and electrical resistivity simultaneously measured in four sandstone samples. There is good agreement between model results and laboratory experimental results for both the elastic and electrical properties. The model parameters optimized to fit the data are: constant stiff and “soft” (or compliant) aspect ratios; a fixed critical porosity; and pressure-dependent stiff and soft porosities. These model derived pressure-dependent porosities correlate with the observed velocity and resistivity pressure-sensitivity of the rocks, for example, the higher the initial volume fraction of compliant pores, the higher the P-wave velocity pressure sensitivity. The results show that the pressure-dependence of the P-wave velocity and electrical resistivity can be jointly modeled satisfactorily using the dual-porosity concept, where the pore space is simplified to contain a single set of stiff and soft pores