Experimental rig to improve the geophysical and geomechanical understanding of CO2 reservoirs

We intend to perform experiments that simulate real Carbon Capture and Storage (CCS) conditions in the laboratory, and hence provide the necessary knowledge to interpret field seismic surveys. Primarily, our research is focused on determining seismic rock properties (i.e., wave velocities and attenuation) of real and artificial 50 mm diameter brine-CO2-bearing sandstone and sand samples that are representative host rocks of real CCS scenarios. Accordingly, we have integrated into a new triaxial cell system both an ultrasonic pulse-echo method for accurate velocity (± 0.3%) and attenuation (± 0.1 dB cm-1) measurements, and an electrical resistivity tomography (ERT) method to monitor homogeneity of pore fluid distribution within the samples. The use of ERT provides calibration data for field scale techniques (such as marine controlled source electromagnetic surveying) but also allows measurements of bulk resistivity, fluid diffusion monitoring, flow pathway characterization, and determination of the relative permeability for different brine/brine-CO2 ratios. By simultaneously measuring ultrasonic P- and S-wave velocities and electrical resistivity, we also provide data for joint inversion of seismic and electric field data. Furthermore, the stress-strain behaviour of the sample is continuously monitored with the aid of electrical gauges, so that we deal consistently and simultaneously with the geophysical and geomechanical response of the reservoir when submitted to CO2 injections

Pressure-varying CO 2 distribution affects the ultrasonic velocities of synthetic sandstones

We performed a novel experiment in which three synthetic sandstones – manufactured using a common method but having different porosities – were saturated with brine and progressively flooded with CO2 under constant confining pressure. The fluid pressure was varied around the critical pressure of CO2 and repeated measurements were made of resistivity, in order to assess the saturation, and elastic wave velocity during the flood. The measured saturated bulk moduli were higher than those predicted by the Gassmann–Wood theory, but were consistent with behaviour described by a recently derived poroelastic model which combines “patch” and “squirt” effects. Measurements on two of the samples followed a patch-based model while those on the highest porosity sample showed evidence of squirt-flow behaviour. Our analysis suggests that the appropriate fluid mixing law is pressure dependent, which is consistent with the notion that the effective patch size decreases as fluid pressure is increased. We derive simple empirical models for the patch dependence from fluid pressure which may be used in seismic modelling and interpretation exercises relevant to monitoring of CO2 injection

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

Introduction to this special section: The role of geophysics in a net-zero-carbon world

Human activities are changing the earth's climate, causing increasingly disruptive social and ecological impacts. These impacts can be reduced if global carbon dioxide (CO2) emissions reach net zero in the near future. A net-zero-carbon world can be achieved by using energy more efficiently and responsibly; transitioning toward energy sources, products, and services that minimize greenhouse gas release; and implementing existing and novel technologies to remove and store CO2 from the atmosphere

Forensic mapping of seismic velocity heterogeneity in a CO2 layer at the Sleipner CO2 storage operation, North Sea, using time-lapse seismics

CO2 separated from natural gas produced at the Sleipner and Gudrun fields is being injected into the Utsira Sand, with around 18 million tons currently stored. Time-lapse 3D seismics have been deployed to monitor development of the CO2 plume. The 2010 seismic survey resolved, for the first time in 3D, the topmost CO2 layer into distinct reflections from its top and base. Seismic velocity is diagnostic of CO2 layer properties and a forensic interpretative approach is adopted to determine spatial velocity variation in the topmost CO2 layer. Velocity is obtained by equating absolute layer thickness, derived by subtracting a constructed flat CO2 – water contact from the topographical relief of the reservoir top, to the temporal separation of the layer top and base reflections, with appropriate correction for wavelet interference effects. Layer velocities show a systematic and robust spatial variation between a northern area with a mean velocity of 1371 ± 122 ms−1 and a central area with a much higher mean velocity of 1638 ± 103 ms−1. Recent fluid flow simulations of the topmost CO2 layer have shown that incorporating a high permeability channel in the model reservoir significantly improves the history-match. This high permeability channel corresponds remarkably closely to the low seismic velocities mapped in the northern area, with higher layer velocities of the central area interpreted as more argillaceous, less permeable overbank deposits. The new velocity analysis therefore provides independent support for including deterministic permeability heterogeneity in predictive fluid flow modelling of Sleipner

Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones

Seismic and electromagnetic properties are generally anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, VP, and attenuation 1/QP) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20–35 MPa) and pore (2–17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale, all affect the elastic and electrical anisotropies. Strong linear correlations (R2 > 0.9) between VP, 1/QP, and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore pressure changes (effective stress coefficients n > 1), whereas P-wave properties provide simultaneous information about the confining (from VP, with n slightly less than 1) and pore pressure (from 1/QP, with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities

Geophysical early warning of salt precipitation during geological carbon sequestration

Sequestration of industrial carbon dioxide (CO2) in deep geological saline aquifers is needed to mitigate global greenhouse gas emissions; monitoring the mechanical integrity of reservoir formations is essential for effective and safe operations. Clogging of fluid transport pathways in rocks from CO2-induced salt precipitation reduces injectivity and potentially compromises the reservoir storage integrity through pore fluid pressure build-up. Here, we show that early warning of salt precipitation can be achieved through geophysical remote sensing. From elastic P- and S-wave velocity and electrical resistivity monitoring during controlled laboratory CO2 injection experiments into brine-saturated quartz-sandstone of high porosity (29%) and permeability (1660 mD), and X-ray CT imaging of pore-scale salt precipitation, we were able to observe, for the first time, how CO2-induced salt precipitation leads to detectable geophysical signatures. We inferred salt-induced rock changes from (i) strain changes, (ii) a permanent ~ 1.5% decrease in wave velocities, linking the geophysical signatures to salt volume fraction through geophysical models, and (iii) increases of porosity (by ~ 6%) and permeability (~ 7%). Despite over 10% salt saturation, no clogging effects were observed, which suggests salt precipitation could extend to large sub-surface regions without loss of CO2 injectivity into high porosity and permeability saline sandstone aquifers

CO2‐Brine substitution effects on ultrasonic wave propagation through sandstone with oblique fractures

Seismic monitoring of injected CO2 plumes in fractured storage reservoirs relies on accurate knowledge of the physical mechanisms governing elastic wave propagation, as described by appropriate, validated rock physics models. We measured laboratory ultrasonic velocity and attenuation of P and S waves, and electrical resistivity, of a synthetic fractured sandstone with obliquely aligned (penny‐shaped) fractures, undergoing a brine‐CO2 flow‐through test at simulated reservoir pressure and temperature. Our results show systematic differences in the dependence of velocity and attenuation on fluid saturation between imbibition and drainage episodes, which we attribute to uniform and patchy fluid distributions, respectively, and the relative permeability of CO2 and brine in the rock. This behavior is consistent with predictions from a multifluid rock physics model, facilitating the identification of the dispersive mechanisms associated with wave‐induced fluid flow in fractured systems at seismic scales

Alteration of ultrasonic signatures by stress-induced changes in hydro-mechanical properties of fractured rocks

In this study, we evaluate the changes in ultrasonic signatures (i.e., frequency content, velocity, amplitude, and attenuation) due to stress-induced alteration of fracture aperture/permeability. Flow-through experiments were performed on artificially-fractured phyllite specimens along with the concurrent measurements of ultrasonic signatures under different stress conditions. Increasing pore pressure led to fracture opening, as indicated by increases in both mechanical and hydraulic apertures. In addition, we observed that increase in confining pressure (and decrease in pore pressure) led to increases in ultrasonic velocities, ultrasonic amplitudes, and fracture specific stiffness, and decrease in ultrasonic attenuations. It was found that time-frequency partitioning depends on hydraulic aperture. The higher frequency band, for both P- and S-waves, was insensitive to the changes in stress conditions; the lower band was sensitive to the changes in stress conditions, as long as the hydraulic aperture was changing. Three-Element rheological and Power-Law models successfully predicted the time-dependent fracture displacement, with the former being more accurate at higher levels of pore pressures

Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge

Serpentinized peridotites co-exist with mafic rocks in a variety of marine environments including subduction zones, continental rifts and mid-ocean ridges. Remote geophysical methods are crucial to distinguish between them and improve the understanding of the tectonic, magmatic and metamorphic history of the oceanic crust. But, serpentinite peridotites exhibit a wide range of physical properties that complicate such a distinction. We analyzed the ultrasonic P- and S-wave velocities (Vp, Vs) and their respective attenuation (Qp−1, Qs−1), electrical resistivity and permeability of four serpentinized peridotite samples from the southern wall of the Atlantis Massif, Mid-Atlantic Ridge, collected during International Ocean Discovery Program (IODP) Expedition 357. The measurements were taken over a range of loading-unloading stress paths (5 - 45 MPa), using ∼1.7 cm length, 5 cm diameter samples horizontally extracted from the original cores drilled on the seafloor. The measured parameters showed variable degrees of stress dependence, but followed similar trends. Vp, Vs, resistivity and permeability show good inter-correlations, while relationships that included Qp−1 and Qs−1 are less clear. Resistivity showed high contrast between highly serpentinized ultramafic matrix (> 50 Ω m) and mechanically/geochemically altered (magmatic/hydrothermal-driven alteration) domains (< 20 Ω m). This information together with the elastic constants (Vp/Vs ratio and bulk moduli) of the samples allowed us to infer useful information about the degree of serpentinization and the alteration state of the rock, contrasted by petrographic analysis. This study shows the potential of combining seismic techniques and controlled source electromagnetic surveys for understanding tectono-magmatic processes and fluid pathways in hydrothermal systems