Laurence North

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 Gassmanns equations explain Vp at high and very low CO2 saturations, whereas bulk modulus yields results consistent with White and Dutta–Ode 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.

Characterisation and multifaceted anisotropy assessment of Corvio sandstone for geological CO2 storage studies

ABSTRACT We present a comprehensive characterisation of the physical, mineralogical, geomechanical, geophysical, and hydrodynamic properties of Corvio sandstone. This information, together with a detailed assessment of anisotropy, is needed to establish Corvio sandstone as a useful laboratory rock‐testing standard for well‐constrained studies of thermo–hydro–mechanical–chemical coupled phenomena associated with CO2 storage practices and for geological reservoir studies in general. More than 200 core plugs of Corvio sandstone (38.1 and 50 mm diameters, 2:1 length‐to‐diameter ratio) were used in this characterisation study, with a rock porosity of 21.7 ± 1.2%, dry density 2036 ± 32 kg m−3, and unconfined compressive and tensile strengths of 41 ± 3.28 and 2.3 ± 0.14 MPa, respectively. Geomechanical tests show that the rock behaves elastically between ∼10 and ∼18 MPa under unconfined conditions with associated Youngs modulus and Poissons ratio of 11.8 ± 2.8 GPa and 0.34 ± 0.01 GPa, respectively. Permeability abruptly decreases with confining pressure up to ∼10 MPa and then stabilises at ∼1 mD. Ultrasonic P‐ and S‐wave velocities vary from about 2.8–3.8 km s−1 and 1.5–2.4 km s−1, respectively, over confining and differential pressures between 0.1 and 35 MPa, allowing derivation of associated dynamic elastic moduli. Anisotropy was investigated using oriented core plugs for electrical resistivity, elastic wave velocity and attenuation, permeability, and tracer injection tests. Corvio sandstone shows weak transverse isotropy (symmetry axis normal to bedding) of <10% for velocity and <20% for attenuation.

Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents

Turbidity currents pose a serious hazard to expensive oil and gas seafloor installations, especially in deep-water where mitigation, re-routing or repair is costly and logistically challenging. These sediment-laden flows are hazardous because they can be exceptionally powerful (up to 20 m/s), and can flow for long distances (>100s km) over several days duration, causing damage over vast areas of seafloor. Even less powerful flows (~1-2 m/s) can damage seafloor equipment, or break strategically important submarine telecommunication cables. The consequences of turbidity currents impacting seafloor structures depends on the velocity, duration, direction of impact and, perhaps most crucially, the sediment concentration (or density) of the flow. While some recent studies have successfully monitored turbidity currents in deep-water, imaging flow properties close to the seafloor has proven problematic. We present innovative approaches to the quantification of the velocity and sediment concentration of dense near-bed layers that provide new insights into this important aspect of turbidity current flow. Firstly, we describe a novel experimental setup that is capable of measuring near-bed sediment concentration in dense (>10% volume by concentration) flows. Density contrasts are measured using Electrical Resistivity Tomography – a technique initially developed for geophysical characterisation of subsurface reservoirs. Velocity is measured using Ultrasonic Doppler Velocity Profiling and concentration is characterized using an Ultra High Concentration Meter. Secondly, we outline some recently developed geophysical approaches for the quantification of sediment concentration and velocity for real-world flows based on recent work in fjords, estuaries and deep-sea canyons. This includes integrated moored deployments of Acoustic Doppler Current Profilers, Multibeam Sonars, and a novel Chirp array. We outline some limitations and advantages of these methods. Finally, we underline the value and importance of establishing multiple field-scale test sites in a variety of settings, including deep-water, that will enhance the industrys understanding of turbidity current hazards. Our results demonstrate the importance of near-bed dense layers for turbidity current interaction with seafloor structures. Density contrasts and pressure build up at the base of a flow may lead to uplift, undermining and loss of support, dragging, or pipeline rupture; hence quantification of this layer is crucial for hazard assessment. Measurements of sediment concentration within turbidity currents are incredibly rare, and yet are a vital input for any numerical model that aims to predict sediment transport by turbidity currents in deep-water settings. Currently it is necessary to infer densities and velocities; however, such inferences are poorly calibrated against experimental or real world data. Our measurements underline the importance of understanding near-bed dense layers.

Presence and Consequences of Coexisting Methane Gas With Hydrate Under Two Phase Water‐Hydrate Stability Conditions

Methane hydrate saturation estimates from remote geophysical data and borehole logs are needed to assess the role of hydrates in climate change, continental slope stability, and energy resource potential. Here we present laboratory hydrate formation/dissociation experiments in which we determined the methane hydrate content independently from pore pressure and temperature and from electrical resistivity. Using these laboratory experiments, we demonstrate that hydrate formation does not take up all the methane gas or water even if the system is under two phase water-hydrate stability conditions and gas is well distributed in the sample. The experiment started with methane gas and water saturations of 16.5% and 83.5%, respectively; during the experiment, hydrate saturation proceeded up to 26% along with 12% gas and 62% water remaining in the system. The coexistence of hydrate and gas is one possible explanation for discrepancies between estimates of hydrate saturation from electrical and acoustic methods. We suggest that an important mechanism for this coexistence is the formation of a hydrate film enveloping methane gas bubbles, trapping the remaining gas inside.

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

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.

Compaction Induced Electrical Anisotropy in Sandstones

Using a novel electrical anisotropy measurement system we have measured VTI type anisotropy in clean (clay free), visually isotropic and layered sandstone samples. Ratios of mean horizontal to vertical resistivity of up to 1.2 have been observed. The minimum resistivity eigenvalue is normal to the bedding plane and normal to the mean orientation of grain axes; this is incompatible with established anisotropic grain shape effective medium models or layering models. Using finite element modelling of synthetic grain packs we show that this form of anisotropy can be simulated by gravity sorting during ballistic grain deposition and subsequent compaction during digenesis. Commonly isotropy is assumed for clean sands but may add considerable error to the estimation of fluid saturation and properties if not considered during the interpretation of well-log and CSEM data. This type of anisotropy might also prove a useful tool when monitoring the mechanical compaction response of reservoir sands.

Pressure Dependent Electrical Resistivity Anisotropy in Carbonate Reservoir Rocks

We present laboratory measurements of electrical resistivity anisotropy from a suite of 37 carbonate samples. These laboratory results show the occurrence of resistivity anisotropy in lithologies ranging from biomicritic limestone to hydrothermal dolomite. Our results also suggest that the anisotropy we observe on the centimetric scale of our samples is an intrinsic property of the rock resulting from fabric scale heterogeneity as apposed to micro-fracturing.

Pressure Sensitivity of the Joint Elastic-electrical Properties of Carbonate Reservoir Rocks

We compare newly acquired laboratory data of P-wave velocity (Vp, 700 kHz) and electrical resistivity (expressed as apparent formation factor F, 80 Hz) on 24 carbonate samples with a range of porosities and permeabilities, to similar data for shaly sandstones. Crossplots of F - Vp, and the pressure sensitivity of the F - Vp relation, reveal similar trends for carbonates and sandstone, with the carbonates following closely, and extending, the clean sandstone trend. Unlike for shaly sandstones, the carbonate trends are only partly controlled by permeability. Other possible controls include heterogeneity, pore aspect ratio and size distribution.

Laboratory Measurements of Electrical Resistivity Heterogeneity and Anisotropy in Carbonate Reservoir Rocks

We describe a novel high pressure Electrical Impedance Tomography (EIT) system capable of resolving 3D resistivity distributions within laboratory core samples at representative burial pressures. The system is also capable of measuring the full 3D resistivity tensor from a single sample using a newly developed and efficient diffemorphic finite element technique. We present results on a microbially laminated dolo-mudstone reservoir analogue from the Middle Triassic Rottweil-Formation of the South German Basin and show that what at first appears to be electrical heterogeneity is in fact the result of anisotropy caused by a parallel tubular microstructure.