Do Pores Exist?—Foundational Issues in Pore Structural Characterisation

This work reviews a range of fundamental theoretical considerations in pore structural characterisation. The pore concept is essential for providing a better understanding of physical processes arising within porous media than purely phenomenological approaches. The notion of a pore structure is found to be independently valid and invariant during theory change concerning said physical processes, even for structural models obtained via indirect methods. While imaging methods provide a more direct characterisation of porous solids, there is often a surfeit of information beyond that which can be wielded with current computing power to predict processes sufficiently accurately. Unfortunately, the pore network model extraction methods cannot decide in advance the level of simplification necessary to obtain the optimum minimal idealisation for a given physical process. Pore network models can be obtained with differing geometrical and topological properties, but similar mass transfer rates, for reasons that are often not clear. In contrast, the ‘pore-sifting’ strategy aims to explicitly identify the key feature of the void space that controls a mass transport process of interest

Numerical simulation of the impact of geological heterogeneity on performance and safety of THAI heavy oil production process

The Toe-to-Heel Air Injection (THAI) in-situ combustion process is an efficient way to extract heavy oil and bitumen. However, such reservoirs are often geologically heterogeneous. This work studied the impact of a range of different geological heterogeneities, often found in bitumen deposits, on the performance and safety of THAI. These heterogeneities included random heterogeneity, layered reservoirs, shaly reservoirs, and semi-permeable cap-rocks. A further aim was to also develop potential remedial measures, such as selective well placement. It was found that the degree of symmetry assumed for the reservoir model had a substantial impact on the predicted level of oil production. This is of particular relevance to otherwise apparently symmetrical well placement designs such as staggered line drive. While the presence of impermeable zones resulted in the decrease in the overall oxygen utilisation for shaly reservoirs, compared to simply low permeability reservoirs, there was no evidence of oxygen breakthrough due to preferential channelling into the production well. In layered reservoirs, the development of a rich oil bank during THAI operation depended upon the distribution of permeability around the horizontal producer (HP), and did not occur when there was high permeability just above the HP. It has been shown that the proper representation of the cap-rock in reservoir models for the simulation of THAI is essential in order to accurately mimic the full pattern of heat distribution into the oil zone of the reservoir, and, thence, fuel lay-down. While THAI can operate stably with a permeable cap-rock, vertical permeabilities above ∼1–3 mD led to significant loss of combustion gases from the reservoir

Fluid flow in a porous medium with transverse permeability discontinuity

Magnetic Resonance Imaging (MRI) velocimetry methods were used to study fully developed axially symmetric fluid flow in a model porous medium of cylindrical symmetry with a transverse permeability discontinuity. Spatial mapping of fluid ow resulted in radial velocity profiles. High spatial resolution of these profiles allowed the estimating of the slip in velocities at the boundary with a permeability discontinuity zone in a sample. The profiles were compared to theoretical velocity fields for a fully developed axially symmetric flow in a cylinder derived from the Joseph and Beavers and the Brinkman models. Velocity fields were also computed using pore-scale lattice Boltzmann Modelling (LBM) where the assumption about the boundary could be omitted. Both approaches gave a good agreement between theory and experiment though LBM velocity fields followed experiment more closely. This work shows great promise for MRI velocimetry methods in addressing the boundary behavior of fluids in opaque heterogeneous porous media

Evaluation of impact of surface diffusion on methane recovery via carbon dioxide injection in shale reservoirs

Injection of carbon dioxide into shale reservoirs is a promising technology for enhancing natural gas recovery and reducing greenhouse gas emissions. Nanoscale phenomena contribute to a significant difference in mass transfer processes within shale-gas reservoirs compared to conventional gas reservoirs. Previous investigations have shown the significance of surface diffusion to gas transfer mechanisms. Surface diffusion was added to an established apparent permeability model, which was then applied for the first time to numerical reservoir simulations to model CO2 injection techniques. Most publications to date have used a theoretical model to predict surface diffusion coefficient in a low-pressure condition, whereas, in this paper, it has been estimated from gravimetric experiments. Shale reservoirs, with different reservoir and petrophysical properties, were generated to investigate the efficiency of transport of CO2 via surface diffusion. A recently proposed fractal model for surface diffusion was used to investigate the impact of rock surface roughness on CH4 production. The results show that surface diffusion plays a significant role in increasing CH4 recovery by up to 3.2% when the average pore radius is less than 2 nm. In particular, a high surface fractal dimension can potentially enhance CH4 production by up to 1.5% and should not be neglected when the average pore radius is less than 1 nm. In areas with high surface capacity, adsorption of CO2 and desorption of CH4 molecules may increase by up to 2.74% and 2.3%, respectively, when compared to models with no surface diffusion. In all the reservoirs examined, geostatistical reservoir simulations showed that reservoir heterogeneity is not favourable to methane recovery via CO2 injection techniques, except for the Barnett shale reservoir. To the best of our knowledge, this work is the first to implement an apparent model within a reservoir simulator to investigate the impact of surface diffusion on methane recovery via CO2 injection techniques at various shale reservoirs with different properties

Storage Sites for Carbon Dioxide in the North Sea and Their Particular Characteristics

This paper reviews and evaluates work on the structural complexity of the potential carbon dioxide storage sites in the North Sea, including the nature of the reservoir structures, the reservoir rocks, the presence of inter-layers, faults, and fractures, and how these factors influence carbon dioxide capacity. In particular, the review emphasises the significance of studying caprocks in detail, not just the reservoir rock’s carbon dioxide storage capacity. This work also particularly considers reservoir simulation work on North Sea sites and illustrates the importance of using fully coupled flow–geomechanical–geochemical modelling to ensure that complex feedback and synergistic effects are not missed. It includes comparisons with other sites where relevant. It also discusses recent challenges and controversies that have arisen from simulations of sequestration in North Sea reservoirs and the need for comprehensive field data to resolve these issues

Hydrogenation and dehydrogenation of Tetralin and Naphthalene to explore heavy oil upgrading using NiMo/Al₂O₃ and CoMo/Al₂O₃ catalysts heated with steel balls via induction

This paper reports the hydrogenation and dehydrogenation of tetralin and naphthalene as model reactions that mimic polyaromatic compounds found in heavy oil. The focus is to explore complex heavy oil upgrading using NiMo/Al2O3 and CoMo/Al2O3 catalysts heated inductively with 3 mm steel balls. The application is to augment and create uniform temperature in the vicinity of the CAtalytic upgrading PRocess In-situ (CAPRI) combined with the Toe-to-Heel Air Injection (THAI) process. The effect of temperature in the range of 210–380 °C and flowrate of 1–3 mL/min were studied at catalyst/steel balls 70% (v/v), pressure 18 bar, and gas flowrate 200 mL/min (H2 or N2). The fixed bed kinetics data were described with a first-order rate equation and an assumed plug flow model. It was found that Ni metal showed higher hydrogenation/dehydrogenation functionality than Co. As the reaction temperature increased from 210 to 300 °C, naphthalene hydrogenation increased, while further temperature increases to 380 °C caused a decrease. The apparent activation energy achieved for naphthalene hydrogenation was 16.3 kJ/mol. The rate of naphthalene hydrogenation was faster than tetralin with the rate constant in the ratio of 1:2.5 (tetralin/naphthalene). It was demonstrated that an inductively heated mixed catalytic bed had a smaller temperature gradient between the catalyst and the surrounding fluid than the conventional heated one. This favored endothermic tetralin dehydrogenation rather than exothermic naphthalene hydrogenation. It was also found that tetralin dehydrogenation produced six times more coke and caused more catalyst pore plugging than naphthalene hydrogenation. Hence, hydrogen addition enhanced the desorption of products from the catalyst surface and reduced coke formation

Predicting Surface Diffusivities of Gas Molecules in Shale

Carbon dioxide injection can be utilised as a means of both enhancing gas recovery from shales and sequestering carbon, and thereby simultaneously addressing the growing worldwide gas demand, as well as the challenge of greenhouse gas emissions. Greater mobility of CO2 within the shale improves the displacement efficiency of the originally present CH4, as well as, increasing the CO2 penetration of the shale formation. Previous investigations have indicated that surface diffusion is much more significant than the bulk gas transport in shale gas reservoirs because of the larger fraction of adsorbed phase found in the nanopores of shales. The surface diffusivities of CO2 on different shales, at various temperatures, have been measured. A fractal theory for predicting the Arrhenius parameters of the surface diffusivity of molecules on heterogeneous surfaces has been applied to the surface diffusion of CO2 in shales. In line with the theory, it was found that both the pre-exponential factor and the activation energy are functions of the surface fractal dimension. Hence, the surface diffusivity, at a monolayer coverage, on shales could be established from an equilibrium gas adsorption isotherm, once the Arrhenius parameters have been calibrated for the specific chemical species. To the best of our knowledge this study is the first to apply the fractal theory and effectively predict, a priori, surface diffusivity parameters for such structurally and chemically heterogeneous natural samples as shales. This theory now enables the optimization of the designs of CO2 injection in field applications since surface diffusion is of major importance in the apparent permeability, and, thus, in the gas flow mechanisms

NMR Imaging of low pressure, gas-phase transport in packed beds using hyperpolarized xenon-129

Gas-phase magnetic resonance imaging (MRI) has been used to investigate heterogeneity in mass transport in a packed bed of commercial, alumina, catalyst supports. Hyperpolarized 129Xe MRI enables study of transient diffusion for micro- scopic porous systems using xenon chemical shift to selectively image gas within the pores, and, thence, permits study of low-density, gas-phase mass-transport, such that diffusion can be studied in the Knudsen regime, and not just the molecular regime, which is the limitation with other current techniques. Knudsen-regime diffusion is common in many industrial, catalytic processes. Significantly, larger spatial variability in mass transport rates across the packed bed was found compared to techniques using only molecular diffusion. It has thus been found that that these heterogeneities arise over length-scales much larger tha