Ann Muggeridge

Pore‐scale modeling of longitudinal dispersion

[1] We study macroscopic (centimeter scale) dispersion using pore-scale network simulation. A Lagrangian-based transport model incorporating flow and diffusion is applied in a diamond lattice of throats with square cross section whose radius distribution is the same as computed for Berea sandstone. We use physically consistent rules using a combination of stream-tube routing and ideal mixing to transport particles across pore junctions. The influence of both heterogeneity and high Peclet numbers results in asymptotic behavior only being seen after movement through many throats. A comprehensive comparative study of longitudinal dispersion with experiments in consolidated and unconsolidated media indicates that the model can quantitatively predict the asymptotic macroscopic dispersion coefficient over a broad range of Peclet numbers, 0 400.

Recovery rates, enhanced oil recovery and technological limits

Enhanced oil recovery (EOR) techniques can significantly extend global oil reserves once oil prices are high enough to make these techniques economic. Given a broad consensus that we have entered a period of supply constraints, operators can at last plan on the assumption that the oil price is likely to remain relatively high. This, coupled with the realization that new giant fields are becoming increasingly difficult to find, is creating the conditions for extensive deployment of EOR. This paper provides a comprehensive overview of the nature, status and prospects for EOR technologies. It explains why the average oil recovery factor worldwide is only between 20% and 40%, describes the factors that contribute to these low recoveries and indicates which of those factors EOR techniques can affect. The paper then summarizes the breadth of EOR processes, the history of their application and their current status. It introduces two new EOR technologies that are beginning to be deployed and which look set to enter mainstream application. Examples of existing EOR projects in the mature oil province of the North Sea are discussed. It concludes by summarizing the future opportunities for the development and deployment of EOR.

Three-dimensional reservoir characterization and flow simulation of heterolithic tidal sandstones

Tidal sandstone reservoirs contain significant intervals of hydrocarbon-bearing heterolithic facies, characterized by the presence of tide-generated sedimentary structures such as flaser, wavy, and lenticular bedding (millimeter to centimeter sand-mud alternations). We have characterized the reservoir properties (sandstone connectivity, effective permeability, and displacement efficiency) of these facies using three-dimensional (3-D) models reconstructed directly from large rock specimens. The models are significantly larger than a core plug, but smaller than a typical reservoir model grid block. We find that the key control on reservoir quality is the connectivity and continuity of the sandstone and mudstone layers. If the sandstone layers form a connected network, they are likely to be productive even at low values of net-to-gross (about 0.30.5). This may explain why the productivity of low net-to-gross, heterolithic tidal sandstones is commonly underestimated or overlooked. Connectivity is the dominant control on the transition between productive (pay) and nonproductive (nonpay) heterolithic facies. However, connectivity is difficult to characterize because core plugs sampled from the subsurface are too small to capture connectivity, whereas two-dimensional outcrop measurements can significantly underestimate the true 3-D value. Our results suggest that core-plug measurements of permeability and displacement efficiency are unlikely to yield representative values at the scale of a reservoir model grid block because the connectivity and continuity of sandstone and mudstone layers varies significantly with length scale.

Upscaling Permeability Measurements Within Complex Heterolithic Tidal Sandstones

We investigate numerically the effect of sample volume on the effective single-phase permeability of heterolithic tidal sandstones, using three-dimensional models reconstructed directly from large rock specimens measuring ∼45 × 30 × 15 cm. We find that both individual and averaged effective permeability values vary as a function of sample volume, which indicates that permeability data obtained from core-plugs will not be representative at the scale of a reservoir model grid-block regardless of the number of measurements taken. However, the error introduced by averaged data may be minimized using the appropriate averaging scheme for a given facies type and flow direction.

Global impacts of energy demand on the freshwater resources of nations

Significance Understanding the role of international trade in driving pressures on freshwater resources is key to meeting challenges at the water–energy nexus. A coupled trade and hydrological model is used to examine pressures on freshwater resources associated with energy production across the global economy. While the electric and gas sectors induce freshwater consumption predominantly within countries where demand originates (91% and 81%, respectively), the petroleum sector exhibits a high international footprint (56%). Critical geographic areas and economic sectors are identified, providing focus for resource-management actions to ensure energy and freshwater security. Our analysis demonstrates the importance of broadening the discourse on energy policy to address issues including freshwater scarcity, the role of international trade, and wider environmental and societal considerations. The growing geographic disconnect between consumption of goods, the extraction and processing of resources, and the environmental impacts associated with production activities makes it crucial to factor global trade into sustainability assessments. Using an empirically validated environmentally extended global trade model, we examine the relationship between two key resources underpinning economies and human well-being—energy and freshwater. A comparison of three energy sectors (petroleum, gas, and electricity) reveals that freshwater consumption associated with gas and electricity production is largely confined within the territorial boundaries where demand originates. This finding contrasts with petroleum, which exhibits a varying ratio of territorial to international freshwater consumption, depending on the origin of demand. For example, although the United States and China have similar demand associated with the petroleum sector, international freshwater consumption is three times higher for the former than the latter. Based on mapping patterns of freshwater consumption associated with energy sectors at subnational scales, our analysis also reveals concordance between pressure on freshwater resources associated with energy production and freshwater scarcity in a number of river basins globally. These energy-driven pressures on freshwater resources in areas distant from the origin of energy demand complicate the design of policy to ensure security of fresh water and energy supply. Although much of the debate around energy is focused on greenhouse gas emissions, our findings highlight the need to consider the full range of consequences of energy production when designing policy.

Effect of Discontinuous Shales on Reservoir Performance During Horizontal Waterflooding

We investigate quantitatively the effect of inclined and intersecting discontinuous shales on horizontal waterfloods, using generic, dimensionless twoand three-dimensional models. We focus upon whether the shales alter the sweep efficiency. We find that they have a significant effect only if they are continuous over large areas of the bedding surfaces and are steeply inclined to the flow, or if the displacement is highly unfavorable. We also find that two-dimensional simulations can significantly overestimate the effect of intersecting shales, because they assume that the shales are entirely continuous in the third dimension.

The rate of pressure dissipation from abnormally pressured compartments

An improved analytic solution is presented that describes the dissipation of abnormal pressures via Darcy flow out of a compartment formed of high-permeability rock capped top and bottom by lower permeability rock. The solution tends to the solutions presented by previous workers in the limit of (1) a thick compartment and thin barrier and (2) a thick barrier and a thin compartment. The importance of including fluid compressibility when analyzing hydrocarbon reservoirs is demonstrated. Previous workers in this field have only included the effect of bulk rock compressibility and have neglected fluid compressibility.The solution is applied to pressure compartments on the scale of a typical hydrocarbon reservoir and overpressured regions on the basin scale. It is shown that the pressure gradient in a typical hydrocarbon reservoir compartment will return to the hydrostatic gradient over timescales of the order of hours or days, but that abnormal pressures are likely to dissipate over periods of tens of thousands to hundreds of thousands of years. Thus, any reservoir compartment with a different pressure from its neighbors at discovery can be considered as an independent unit during production. However, abnormal pressures on the basin scale may take tens or hundreds of millions of years to dissipate. There is thus no need to invoke zero-permeability seals or capillary-pressure seals in order to explain the existence of abnormal pressures over geological time.

Investigation of longitudinal and transverse dispersion in stable displacements with a high viscosity and density contrast between the fluids

Transverse and longitudinal dispersion in gravity stable, favourable viscosity ratio flows are investigated and compared with earlier data obtained for miscible fluids and for tracer flow. Data from laboratory measurements of longitudinal dispersion in low viscosity ratio (8.63×10(-)(4)) and high density contrast (471 kg m(-3)) displacements are compared with literature data for more modest viscosity ratios and density differences and with earlier theoretical analysis. The longitudinal dispersivity was reduced by a factor of 2 for flows influenced by gravity. This reduction was relatively insensitive to the magnitude of the density contrast and the flow rate, for Peclet numbers less than 100 and found to be consistent with earlier theoretical predictions. Additional transverse dispersion data was obtained for fluids with a density contrast of 225 kg m(-3) and a matched viscosity ratio over a range of Peclet numbers (1<Pe<1000). A similar reduction in transverse dispersivity in gravity stable flow, independent of Peclet number, was observed and found to be consistent with observations in the literature for more modest density contrasts.

Hydrodynamic aquifer or reservoir compartmentalization

Changes in oil-water contact (OWC) depth across a field are often a signature of reservoir compartmentalization as a result of faulting or stratigraphic barriers, such as shales, but may equally well be caused by an underlying hydrodynamic aquifer. In particular, the pressure-depth data obtained from an aquifer whose flow is changing over time can look very similar to that obtained from a compartmentalized reservoir. Misunderstanding which of these mechanisms causes the observed changes in OWC across the field may result in poor estimates of oil in place and reduced recovery. To address this problem, an analytic expression is presented to estimate the time taken for a steady state tilted OWC to be established once an aquifer starts flowing. A comparison with simulations of hydrodynamic aquifers in homogeneous, compartmentalized, and heterogeneous reservoir models shows that this expression can be used in combination with the one derived by M. K. Hubbert, for the steady state tilt of the OWC, to clarify whether a reservoir contains barriers or baffles to flow or may simply have not yet reached equilibrium.

Outcrop studies of tidal sandstones for reservoir characterization (Lower Cretaceous vectis formation, isle of wight, Southern England)

The Vectis Formation (Lower Cretaceous) in the Isle of Wight, southern England serves as an excellent outcrop analogue for tidal sandstone reservoirs. It consists of two juxtaposed marginal-marine depositional systems separated by an erosional, regionally extensive sequence boundary. The lower system is characterized by a muddy succession which overlies fluvial strata and lacks tidal indicators. This is interpreted as a low-energy, broad lagoonal or embayment complex. The upper, sandstone-dominated system is characterized by a wide range of heterolithic deposits, with abundant evidence of tidal processes, as indicated by sedimentary structures and palaeocurrent patterns. It is interpreted as a laterally migrating tidal bar and channel complex within the inner part of a mixed-energy estuary (sensu Dalrymple et al., 1992). The Vectis Formation sandstones contain a highly variable array of sedimentary structures and heterogeneities, ranging from the regional scale (tens of kilometres wide, tens to hundreds of metres in thickness) to individual laminae scale (mm in thickness). These have been described and quantified in order to provide a basis for evaluating their impact on fluid flow and hydrocarbon recovery. The large-scale heterogeneities and facies variations are explained within a sequence stratigraphic framework and reflect the transgressive infill of a mixed-energy estuarine complex. The intermediate- to small-scale heterogeneities (mm-m in thickness, representing the scale of individual bedforms and laminae) reflect autocyclic tidal processes and have been examined in detail by ( 1 ) constructing a detailed 2-D facies model directly from the outcrop (measuring 400 m × 6 m), (2) photographing and digitizing small, representative areas (ca. 1 m × 1 m) of 2-D outcrops, and (3) using serial sectioning techniques to reconstruct the 3-D geometry of tidal sedimentary structures (wavy-, lenticular- and flaser-bedding) directly from large (ca. 60 cm × 60 cm × 20 cm) rock specimens. These reconstructed rock models have been used to investigate the validity of using core-plug and well-log derived permeability data to represent heterolithic facies in tidal sandstone reservoir models.