Measurements of Non-wetting Phase Trapping in Porous Media

Saline aquifers have been identified as a preferred storage location for anthropogenic carbon dioxide emissions due to their large capacities and wide geographical spread. The storage of carbon dioxide in such formations must be carefully designed so that the carbon dioxide is trapped securely and will not escape to the surface. Of the various subsurface mechanisms, capillary trapping is a fast and secure means of rendering injected carbon dioxide immobile. The carbon dioxide is trapped on the pore scale as residual phase bubbles surrounded by formation brine. Local capillary forces prevent the movement of the carbon dioxide bubbles and retain them within individual pores or groups of pores. The trapping of a phase in this manner can be characterised by the relationship between initial and residual saturation, known as the capillary trapping curve. The maximum trapped saturation and the form of the capillary trapping curve are of key importance in characterising the underlying physics of this phenomenon, giving an important indication of system wettability. To date the form of the capillary trapping curve has not been measured for a carbon dioxide-brine-rock system and there is limited technical literature available on maximum trapped carbon dioxide saturations. The aim of this study is to measure the maximum saturation and the form of the capillary trapping curve in a carbon dioxide-brine-sandstone system at conditions representative of a storage location through coreflood experiments. Similar measurements are required on analogue systems of known wettability in order to infer the wettability of the carbon dioxide-brine-sandstone system. In addition measurements of capillary pressure are sought to better understand the migration of carbon dioxide after injection, and the influence of petrophysical properties on capillary trapping is investigated. We initially measure trapping in unconsolidated sand at ambient conditions with analogue fluids: oil-brine and gas-brine. The maximum trapped saturations are relatively low – 12.8% and 14.3% for oil and gas respectively. The form of the capillary trapping curve is shown to be slightly different in each case. We then measure the capillary trapping curve for Berea sandstone at temperature and pressure representative of a storage location (T = 343 K; P = 9.0 MPa). The maximum residual carbon dioxide saturation is measured as 35.3% and the form of the trapping curve is accurately predicted by the trapping correlation proposed by Spiteri et al., [1]. The primary drainage capillary pressure curve is also measured and it is shown that the irreducible brine saturation is 14.9% for an applied capillary pressure equivalent to a carbon dioxide column height of 37 m. For comparison oil-brine measurements are also made on the same sandstone. Trapping is shown to be different for each system. More oil than carbon dioxide is trapped in Berea sandstone – 48.3% versus 35.3%. Subtle changes in imbibition contact angle and system wettability are proposed as the cause – with the rock surface being slightly less hydrophilic in the presence of carbon dioxide. Brine remains the wetting phase however, explaining the substantial quantities of carbon dioxide trapped. A simple term to quantify trapping capacities is developed – the capillary trapping capacity – being the product of maximum trapped saturation and porosity. A capillary trapping capacity of up to 7.8% of the gross rock volume for a carbon dioxide-brine-Berea sandstone system is measured. We also investigate the influence of petrophysical properties on capillary trapping in oil-brine systems. Maximum trapped oil saturations are measured for five consolidated sandstone systems in addition to six unconsolidated sands. Strong relationships exist between trapped saturation and porosity, permeability and a characteristic geometric property (aspect ratio divided by coordination number). These relationships may prove valuable for future predictions of maximum trapped saturations when coreflood data is not available

Measurements of Non-Wetting Phase Trapping Applied to Carbon Dioxide Storage

We measure the trapped non-wetting phase saturation as a function of the initial saturation in sand packs. The application of the work is for carbon dioxide (CO2) storage in aquifers where capillary trapping is a rapid and effective mechanism to render injected CO2 immobile. We used analogue fluids at ambient conditions. The trapped saturation initially rises linearly with initial saturation to a value of 0.11 for oil/water systems and 0.14 for gas/water systems. There then follows a region where the residual saturation is constant with further increases in initial saturation

Shading Beats Binocular Disparity in Depth from Luminance Gradients: Evidence against a Maximum Likelihood Principle for Cue Combination

Perceived depth is conveyed by multiple cues, including binocular disparity and luminance shading. Depth perception from luminance shading information depends on the perceptual assumption for the incident light, which has been shown to default to a diffuse illumination assumption. We focus on the case of sinusoidally corrugated surfaces to ask how shading and disparity cues combine defined by the joint luminance gradients and intrinsic disparity modulation that would occur in viewing the physical corrugation of a uniform surface under diffuse illumination. Such surfaces were simulated with a sinusoidal luminance modulation (0.26 or 1.8 cy/deg, contrast 20%-80%) modulated either in-phase or in opposite phase with a sinusoidal disparity of the same corrugation frequency, with disparity amplitudes ranging from 0’-20’. The observers’ task was to adjust the binocular disparity of a comparison random-dot stereogram surface to match the perceived depth of the joint luminance/disparitymodulated corrugation target. Regardless of target spatial frequency, the perceived target depth increased with the luminance contrast and depended on luminance phase but was largely unaffected by the luminance disparity modulation. These results validate the idea that human observers can use the diffuse illumination assumption to perceive depth from luminance gradients alone without making an assumption of light direction. For depth judgments with combined cues, the observers gave much greater weighting to the luminance shading than to the disparity modulation of the targets. The results were not well-fit by a Bayesian cue-combination model weighted in proportion to the variance of the measurements for each cue in isolation. Instead, they suggest that the visual system uses disjunctive mechanisms to process these two types of information rather than combining them according to their likelihood ratios

Measurements of non-wetting phase trapping in porous media

Saline aquifers have been identified as a preferred storage location for anthropogenic carbon dioxide emissions due to their large capacities and wide geographical spread. The storage of carbon dioxide in such formations must be carefully designed so that the carbon dioxide is trapped securely and will not escape to the surface. Of the various subsurface mechanisms, capillary trapping is a fast and secure means of rendering injected carbon dioxide immobile. The carbon dioxide is trapped on the pore scale as residual phase bubbles surrounded by formation brine. Local capillary forces prevent the movement of the carbon dioxide bubbles and retain them within individual pores or groups of pores. The trapping of a phase in this manner can be characterised by the relationship between initial and residual saturation, known as the capillary trapping curve. The maximum trapped saturation and the form of the capillary trapping curve are of key importance in characterising the underlying physics of this phenomenon, giving an important indication of system wettability. To date the form of the capillary trapping curve has not been measured for a carbon dioxide-brine-rock system and there is limited technical literature available on maximum trapped carbon dioxide saturations. The aim of this study is to measure the maximum saturation and the form of the capillary trapping curve in a carbon dioxide-brine-sandstone system at conditions representative of a storage location through coreflood experiments. Similar measurements are required on analogue systems of known wettability in order to infer the wettability of the carbon dioxide-brine-sandstone system. In addition measurements of capillary pressure are sought to better understand the migration of carbon dioxide after injection, and the influence of petrophysical properties on capillary trapping is investigated. We initially measure trapping in unconsolidated sand at ambient conditions with analogue fluids: oil-brine and gas-brine. The maximum trapped saturations are relatively low – 12.8% and 14.3% for oil and gas respectively. The form of the capillary trapping curve is shown to be slightly different in each case. We then measure the capillary trapping curve for Berea sandstone at temperature and pressure representative of a storage location (T = 343 K; P = 9.0 MPa). The maximum residual carbon dioxide saturation is measured as 35.3% and the form of the trapping curve is accurately predicted by the trapping correlation proposed by Spiteri et al., [1]. The primary drainage capillary pressure curve is also measured and it is shown that the irreducible brine saturation is 14.9% for an applied capillary pressure equivalent to a carbon dioxide column height of 37 m. For comparison oil-brine measurements are also made on the same sandstone. Trapping is shown to be different for each system. More oil than carbon dioxide is trapped in Berea sandstone – 48.3% versus 35.3%. Subtle changes in imbibition contact angle and system wettability are proposed as the cause – with the rock surface being slightly less hydrophilic in the presence of carbon dioxide. Brine remains the wetting phase however, explaining the substantial quantities of carbon dioxide trapped. A simple term to quantify trapping capacities is developed – the capillary trapping capacity – being the product of maximum trapped saturation and porosity. A capillary trapping capacity of up to 7.8% of the gross rock volume for a carbon dioxide-brine-Berea sandstone system is measured. We also investigate the influence of petrophysical properties on capillary trapping in oil-brine systems. Maximum trapped oil saturations are measured for five consolidated sandstone systems in addition to six unconsolidated sands. Strong relationships exist between trapped saturation and porosity, permeability and a characteristic geometric property (aspect ratio divided by coordination number). These relationships may prove valuable for future predictions of maximum trapped saturations when coreflood data is not available.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Carbon Storage: Integrating Experiments & Modelling to Quantify Trapping Capacity & Efficiency in the Subsurface

This is a fundamental study of trapping of non-wetting fluids in porous media. When injecting CO~2~ into an aquifer for carbon storage, the non-wetting phase (CO~2~) is trapped due to capillary forces. This process is investigated in the laboratory for analogue fluids. 

We then design an injection strategy to maximise CO~2~ storage capacity and efficiency on the field scale - incorporating experimental and pore scale modelling results. A streamline based simulator is modified for this purpose

Residual CO2 imaged with X‐ray micro‐tomography

Carbon capture and storage (CCS), where CO2 is injected into geological formations, has been identified as an important way to reduce CO2 emissions to the atmosphere. While there are several aquifers worldwide into which CO2 has been injected, there is still uncertainty in terms of the long‐term fate of the CO2. Simulation studies have proposed capillary trapping – where the CO2 is stranded as pore‐space droplets surrounded by water – as a rapid way to secure safe storage. However, there has been no direct evidence of pore‐scale trapping. We imaged trapped super‐critical CO2 clusters in a sandstone at elevated temperatures and pressures, representative of storage conditions using computed micro‐tomography (μ‐CT) and measured the distribution of trapped cluster size. The clusters occupy 25% of the pore space. This work suggests that locally capillary trapping is an effective, safe storage mechanism in quartz‐rich sandstones