Publications
186 results found
Adam A, Pavlidis D, Percival J, et al., 2016, Higher-order conservative interpolation between control-volume meshes: Application to advection and multiphase flow problems with dynamic mesh adaptivity, Journal of Computational Physics, Vol: 321, Pages: 512-531, ISSN: 1090-2716
A general, higher-order, conservative and bounded interpolation for the dynamic and adaptive meshing of control-volume fields dual to continuous and discontinuous finite element representations is presented. Existing techniques such as node-wise interpolation are not conservative and do not readily generalise to discontinuous fields, whilst conservative methods such as Grandy interpolation are often too diffusive. The new method uses control-volume Galerkin projection to interpolate between control-volume fields. Bounded solutions are ensured by using a post-interpolation diffusive correction. Example applications of the method to interface capturing during advection and also to the modelling of multiphase porous media flow are presented to demonstrate the generality and robustness of the approach.
Attar A, Muggeridge AH, 2016, Evaluation of mixing in low salinity waterflooding
Copyright 2016, Society of Petroleum Engineers. The impact of heterogeneity induced mixing between the injected low salinity water and the high salinity connate water is investigated in secondary low salinity waterflooding (LSW). Although LSW has proved to be a promising EOR process in laboratory experiments and field trials, its efficiency can be reduced due to mixing with in situ connate water. This mixing is caused by molecular diffusion and dispersion and some workers suggest it can be exacerbated by heterogeneity. We investigated the impact of 1) conformance caused by layering in the sandstone and 2) mixing caused by water-saturated shale layers adjacent to the reservoir sand. The study used a commercial reservoir simulator in which low salinity flooding is modelled by varying relative permeability as a function of salinity. All simulations used very fine grids so that physical (rather than numerical) diffusion and dispersion dominated the displacement. In single phase flow, it was found that physical dispersion can only be modelled for when the grid Peclet number is less than 60, for higher values numerical dispersion dominates over physical values. This threshold Peclet number varies with the number of grid blocks, e.g.The optimum number of grid blocks was found to be 10 for (Pe <12) compared with 1000 grid blocks (Pe = 60). In two phase flow, the transverse dispersion number (NTD) originally proposed by Lake and Hirasaki (1981) is shown to be a very robust way to measure the impact of mixing on the performance of LSW. Generally, for any (NTD > 1), diffusion dominates the flow and thus the efficiency of LSW is reduced. In layered high net to gross reservoirs the impact of molecular diffusion increases as the thickness of the higher permeability layer decreases. In lower net to gross reservoirs containing thin, shaly zones filled with higher salinity connate water molecular diffusion reduces the effectiveness of LSW as the thickness of the sand layers be
Attar A, Muggeridge AH, 2016, Evaluation of mixing in low salinity waterflooding
The impact of heterogeneity induced mixing between the injected low salinity water and the high salinity connate water is investigated in secondary low salinity waterflooding (LSW). Although LSW has proved to be a promising EOR process in laboratory experiments and field trials, its efficiency can be reduced due to mixing with in situ connate water. This mixing is caused by molecular diffusion and dispersion and some workers suggest it can be exacerbated by heterogeneity. We investigated the impact of 1) conformance caused by layering in the sandstone and 2) mixing caused by water-saturated shale layers adjacent to the reservoir sand. The study used a commercial reservoir simulator in which low salinity flooding is modelled by varying relative permeability as a function of salinity. All simulations used very fine grids so that physical (rather than numerical) diffusion and dispersion dominated the displacement. In single phase flow, it was found that physical dispersion can only be modelled for when the grid Peclet number is less than 60, for higher values numerical dispersion dominates over physical values. This threshold Peclet number varies with the number of grid blocks, e.g.The optimum number of grid blocks was found to be 10 for (Pe <12) compared with 1000 grid blocks (Pe = 60). In two phase flow, the transverse dispersion number (NTD) originally proposed by Lake and Hirasaki (1981) is shown to be a very robust way to measure the impact of mixing on the performance of LSW. Generally, for any (NTD > 1), diffusion dominates the flow and thus the efficiency of LSW is reduced. In layered high net to gross reservoirs the impact of molecular diffusion increases as the thickness of the higher permeability layer decreases. In lower net to gross reservoirs containing thin, shaly zones filled with higher salinity connate water molecular diffusion reduces the effectiveness of LSW as the thickness of the sand layers between the shales decreases.
Gago PA, King PR, Muggeridge AH, 2016, Fast estimation of effective permeability and sweep efficiency of waterflooding in geologically heterogeneous reservoirs
Geological heterogeneity can adversely affect the macroscopic sweep efficiency when waterflooding oil reservoirs, however the exact distribution of permeability and porosity is generally not known. Engineers try to estimate the range of impacts heterogeneity might have on waterflood efficiency by creating multiple geological models and then simulating a waterflood through each of those realizations. Unfortunately each simulation can be computationally intensive meaning that it is generally not possible to obtain a statistically valid estimate of the expected sweep and the associated standard deviation. In this paper we show how the volume of unswept oil can be estimated rapidly (without flow simulations) from a geometrical characterization of the spatial permeability distribution. A "constriction" factor is defined which quantifies the effective cross-section area of the zones perpendicular to the principal flow direction. This is combined with a 'net-To-gross ratio' (which quantifies the fractional reservoir volume occupied by the zones that contribute to flow) to estimate effective permeability and the expected recovery factor for that realization. The method is tested using a range of realistic geological models, including SPE10 model 2 and its predictions are shown to agree well with values obtained using a well established commercial flow simulator.
Djabbarov S, Jones ADW, Krevor S, et al., 2016, Experimental and numerical studies of first contact miscible injection in a quarter five spot pattern
We quantify the impact of mobility, simple heterogeneities and grid orientation error on the performance of first contact miscible gas flooding in a quarter five spot configuration by comparing the outputs from experimental and numerical models. The aim is to quantify the errors that may arise during simulation and to identify a workflow for minimizing these when conducting field scale fingering studies. A commercial reservoir simulator was validated by comparing its predictions with the results obtained from physical experiments. An uncorrelated, random permeability distribution was used to trigger fingering in the simulations. The physical experiments were carried out using a Hele-Shaw cell (40x40cm) designed and constructed for this study. The impact of a square low permeability inclusion (20x20cm) on flow was investigated by varying its permeability, location and orientation. For lower mobility ratios (M=2 to M=10) the commercial numerical simulator was able to reproduce the experimental observations within the uncertainty range of the permeability distribution used to trigger the fingers, provided a nine-point scheme was used for the pressure solution. At higher mobility ratios (M=20 to M=100) the grid orientation effect meant that the simulator overestimated the areal sweep even when a nine-point scheme was used. The introduction of a square, low permeability inclusion near the injection well reduced the discrepancy between experimental and numerical results, bringing it back within uncertainty limits in some of the cases. This was mainly because the real flow was then forced to move parallel to the edges of the Hele-Shaw cell and thus parallel to the simulation grid. Breakthrough times were well predicted by the numerical simulator at all mobility ratios.
Adam AG, Pavlidis D, Percival JR, et al., 2016, Simulation of immiscible viscous fingering using adaptive unstructured meshes and controlvolume galerkin interpolation
Displacement of one fluid by another in porous media occurs in various settings including hydrocarbon recovery, CO2 storage and water purification. When the invading fluid is of lower viscosity than the resident fluid, the displacement front is subject to a Saffman-Taylor instability and is unstable to transverse perturbations. These instabilities can grow, leading to fingering of the invading fluid. Numerical simulation of viscous fingering is challenging. The physics is controlled by a complex interplay of viscous and diffusive forces and it is necessary to ensure physical diffusion dominates numerical diffusion to obtain converged solutions. This typically requires the use of high mesh resolution and high order numerical methods. This is computationally expensive. We demonstrate here the use of a novel control volume - finite element (CVFE) method along with dynamic unstructured mesh adaptivity to simulate viscous fingering with higher accuracy and lower computational cost than conventional methods. Our CVFE method employs a discontinuous representation for both pressure and velocity, allowing the use of smaller control volumes (CVs). This yields higher resolution of the saturation field which is represented CV-wise. Moreover, dynamic mesh adaptivity allows high mesh resolution to be employed where it is required to resolve the fingers and lower resolution elsewhere. We use our results to re-examine the existing criteria that have been proposed to govern the onset of instability. Mesh adaptivity requires the mapping of data from one mesh to another. Conventional methods such as collocation interpolation do not readily generalise to discontinuous fields and are non-conservative. We further contribute a general framework for interpolation of CV fields by Galerkin projection. The method is conservative, higher order and yields improved results, particularly with higher order or discontinuous elements where existing approaches are often excessively diffusive.
Xiao D, Fang F, Pain C, et al., 2016, Non-intrusive reduced order modelling of waterflooding in geologically heterogeneous reservoirs
Production optimisation and history matching are two applications that require the engineer to run numerous flow simulations of flow in the subsurface. Each flow simulation can be very computationally intensive, especially if reservoir is geologically complex. In some cases it may not be feasible to perform the optimisation sufficiently quickly for it to be useful. This has driven the development of reduced order modelling (ROM) techniques. The problem with most ROMs is that they have to be hard-coded into the flow simulator and so cannot be used with the commercial simulators that are used by most oil companies. In addition most applications of ROMs have assumed that the geological description of the reservoir is known. This is generally not the case, indeed the aim of history matching is to adjust the geological model of the reservoir until the flow through the model replicates that which is observed. In this paper a non-intrusive reduced order model (NIROM) is presented that enables the engineer to vary the permeabilities within a heterogeneous reservoir for a fixed well pattern and then estimate the resulting waterflood performance. The NIROM uses a Smolyak sparse grid interpolation method, radial basis functions (RBF) and proper orthogonal decomposition (POD) is presented. 'Non-intrusive' means that the NIROM is implemented independently of the underlying flow model. Here we use it in conjunction with an unstructured mesh, control volume finite element (CVFEM), multiphase flow model. The NIROM is demonstrated using three reservoir models: one with four material layers, one with four baffles and one with eight baffles. The results compare well with those from a high fidelity full model and reduce the CPU time by a factor of a thousand.
Ajibola J, Adam A, Muggeridge A, 2016, Gravity driven fingering and mixing during CO<inf>2</inf> sequestration
Injection of carbon dioxide into deep saline aquifers is one way to reduce greenhouse gas emissions. Carbon dioxide, usually a super critical fluid at aquifer pressure and temperature conditions, is lighter than the resident brine and so forms a gas cap above the water. However, over time it dissolves in the water, creating a density inversion which induces gravitational instability. Understanding whether the dominant mixing mechanism is convective mixing rather than pure diffusion is important as this controls the timescale over which the carbon dioxide-saturated brine mixes with the unsaturated brine. This paper presents numerical simulations, using a finite difference reservoir simulator, to evaluate the predictions of analytical solutions for stability analysis and growth rate of the fingers of different wavenumbers at different Rayleigh numbers (Ra). The effects of density difference, permeability anisotropy and diffusion (both longitudinal and transverse) on fingering behaviour were investigated through the dimensionless Rayleigh number. The density difference and the vertical permeability were found to mainly control the degree of instability. At Rayleigh numbers greater than 800, fingers are present and the degree of fingering increases with Rayleigh number. Growth rate analysis showed that growth rate is directly proportional to Rayleigh number and time. The critical time (at which flow becomes unstable) varies inversely with the Rayleigh number whilst the corresponding critical wavenumber number varies linearly with the Rayleigh number. These results are consistent with previously reported linear stability analyses providing a validation of the simulator. Numerical simulation results were also validated against experiments. These validations both show that the simulator is robust and can thus be used to investigate more complex situations (heterogeneity) that cannot be analysed mathematically.
Ajibola J, Adam A, Muggeridge A, 2016, Gravity driven fingering and mixing during CO<inf>2</inf> sequestration
© 2016 Society of Petroleum Engineers. All rights reserved. Injection of carbon dioxide into deep saline aquifers is one way to reduce greenhouse gas emissions. Carbon dioxide, usually a super critical fluid at aquifer pressure and temperature conditions, is lighter than the resident brine and so forms a gas cap above the water. However, over time it dissolves in the water, creating a density inversion which induces gravitational instability. Understanding whether the dominant mixing mechanism is convective mixing rather than pure diffusion is important as this controls the timescale over which the carbon dioxide-saturated brine mixes with the unsaturated brine. This paper presents numerical simulations, using a finite difference reservoir simulator, to evaluate the predictions of analytical solutions for stability analysis and growth rate of the fingers of different wavenumbers at different Rayleigh numbers (Ra). The effects of density difference, permeability anisotropy and diffusion (both longitudinal and transverse) on fingering behaviour were investigated through the dimensionless Rayleigh number. The density difference and the vertical permeability were found to mainly control the degree of instability. At Rayleigh numbers greater than 800, fingers are present and the degree of fingering increases with Rayleigh number. Growth rate analysis showed that growth rate is directly proportional to Rayleigh number and time. The critical time (at which flow becomes unstable) varies inversely with the Rayleigh number whilst the corresponding critical wavenumber number varies linearly with the Rayleigh number. These results are consistent with previously reported linear stability analyses providing a validation of the simulator. Numerical simulation results were also validated against experiments. These validations both show that the simulator is robust and can thus be used to investigate more complex situations (heterogeneity) that cannot be analysed mathematically
Gago PA, King PR, Muggeridge AH, 2016, Fast estimation of effective permeability and sweep efficiency of waterflooding in geologically heterogeneous reservoirs
Geological heterogeneity can adversely affect the macroscopic sweep efficiency when waterflooding oil reservoirs, however the exact distribution of permeability and porosity is generally not known. Engineers try to estimate the range of impacts heterogeneity might have on waterflood efficiency by creating multiple geological models and then simulating a waterflood through each of those realizations. Unfortunately each simulation can be computationally intensive meaning that it is generally not possible to obtain a statistically valid estimate of the expected sweep and the associated standard deviation. In this paper we show how the volume of unswept oil can be estimated rapidly (without flow simulations) from a geometrical characterization of the spatial permeability distribution. A "constriction" factor is defined which quantifies the effective cross-section area of the zones perpendicular to the principal flow direction. This is combined with a 'net-To-gross ratio' (which quantifies the fractional reservoir volume occupied by the zones that contribute to flow) to estimate effective permeability and the expected recovery factor for that realization. The method is tested using a range of realistic geological models, including SPE10 model 2 and its predictions are shown to agree well with values obtained using a well established commercial flow simulator.
Adam AG, Pavlidis D, Percival JR, et al., 2016, Simulation of immiscible viscous fingering using adaptive unstructured meshes and controlvolume galerkin interpolation
Displacement of one fluid by another in porous media occurs in various settings including hydrocarbon recovery, CO2 storage and water purification. When the invading fluid is of lower viscosity than the resident fluid, the displacement front is subject to a Saffman-Taylor instability and is unstable to transverse perturbations. These instabilities can grow, leading to fingering of the invading fluid. Numerical simulation of viscous fingering is challenging. The physics is controlled by a complex interplay of viscous and diffusive forces and it is necessary to ensure physical diffusion dominates numerical diffusion to obtain converged solutions. This typically requires the use of high mesh resolution and high order numerical methods. This is computationally expensive. We demonstrate here the use of a novel control volume - finite element (CVFE) method along with dynamic unstructured mesh adaptivity to simulate viscous fingering with higher accuracy and lower computational cost than conventional methods. Our CVFE method employs a discontinuous representation for both pressure and velocity, allowing the use of smaller control volumes (CVs). This yields higher resolution of the saturation field which is represented CV-wise. Moreover, dynamic mesh adaptivity allows high mesh resolution to be employed where it is required to resolve the fingers and lower resolution elsewhere. We use our results to re-examine the existing criteria that have been proposed to govern the onset of instability. Mesh adaptivity requires the mapping of data from one mesh to another. Conventional methods such as collocation interpolation do not readily generalise to discontinuous fields and are non-conservative. We further contribute a general framework for interpolation of CV fields by Galerkin projection. The method is conservative, higher order and yields improved results, particularly with higher order or discontinuous elements where existing approaches are often excessively diffusive.
Djabbarov S, Jones ADW, Krevor S, et al., 2016, Experimental and numerical studies of first contact miscible injection in a quarter five spot pattern
Copyright 2016, Society of Petroleum Engineers. We quantify the impact of mobility, simple heterogeneities and grid orientation error on the performance of first contact miscible gas flooding in a quarter five spot configuration by comparing the outputs from experimental and numerical models. The aim is to quantify the errors that may arise during simulation and to identify a workflow for minimizing these when conducting field scale fingering studies. A commercial reservoir simulator was validated by comparing its predictions with the results obtained from physical experiments. An uncorrelated, random permeability distribution was used to trigger fingering in the simulations. The physical experiments were carried out using a Hele-Shaw cell (40x40cm) designed and constructed for this study. The impact of a square low permeability inclusion (20x20cm) on flow was investigated by varying its permeability, location and orientation. For lower mobility ratios (M=2 to M=10) the commercial numerical simulator was able to reproduce the experimental observations within the uncertainty range of the permeability distribution used to trigger the fingers, provided a nine-point scheme was used for the pressure solution. At higher mobility ratios (M=20 to M=100) the grid orientation effect meant that the simulator overestimated the areal sweep even when a nine-point scheme was used. The introduction of a square, low permeability inclusion near the injection well reduced the discrepancy between experimental and numerical results, bringing it back within uncertainty limits in some of the cases. This was mainly because the real flow was then forced to move parallel to the edges of the Hele-Shaw cell and thus parallel to the simulation grid. Breakthrough times were well predicted by the numerical simulator at all mobility ratios.
Hamid SAA, Muggeridge AH, 2016, Characterizing the impact of heterogeneity on miscible gas injection
The performance of miscible gas injection processes is typically adversely affected by geological heterogeneity and viscous fingering. It is well known that very high grid resolutions are needed to model viscous fingering explicitly so most field scale simulations will use an empirical model such as the Todd and Longstaff (1972) model to describe the average effects of viscous fingering on recovery. In homogeneous reservoirs it is usual to assume to use a Todd and Longstaff mixing parameter with a value of 2/3 in these simulations. It is, however, unclear how to model the influence of heterogeneities on the viscous fingering. Previous work by Fayers et al. (1992) identified that there are two flow regimes associated with heterogeneity - diffusive when the permeability distribution is uncorrelated and random tending to advective as the permeability distribution has a larger standard deviation and a greater correlation length. In the former case the effect of heterogeneity can be modelled using a dispersivity whilst in the latter case it can be modelled by an effective viscosity ratio of heterogeneity factor (Koval, 1963). In this work we identify a third heterogeneity controlled flow regime -That of channelized flow. In this regime the effect of heterogeneity can best be modelled using a bypassed oil volume. We propose the use of a phase diagram for identifying which of these three flow regimes is dominant in a given heterogeneous reservoir and hence which method of upscaling flow is best suited to that reservoir. This phase diagram uses only static measures of the permeability distribution. We further provide a simple method for estimating the effective mobility ratio for use when the heterogeneity can be modelled by a Koval heterogeneity factor. The results are benchmarked against high resolution, finite-difference, first-contact miscibility simulation using SPE 10 Model 2 and synthetic permeability fields.
Holland RA, Scott KA, Florke M, et al., 2015, Global impacts of energy demand on the freshwater resources of nations, Proceedings of the National Academy of Sciences of the United States of America, Vol: 112, Pages: E6707-E6716, ISSN: 0027-8424
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, electricity) reveals that freshwater consumption associated with gas and electricity production is largely confined within the territorial boundaries where demand originates. This contrasts with petroleum, which exhibits a varying ratio of territorial to international freshwater consumption depending on the origin of demand. For example, while the USA 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. While much of the debate around energy is focussed on greenhouse gas emissions, our findings highlight the need to consider the full range of consequences of energy production when designing policy.
Xiao D, Fang F, Buchan AG, et al., 2015, Non-intrusive reduced order modelling of the Navier-Stokes equations, Computer Methods in Applied Mechanics and Engineering, Vol: 293, Pages: 522-541, ISSN: 0045-7825
This article presents two new non-intrusive reduced order models based upon proper orthogonal decomposition (POD) for solving the Navier–Stokes equations. The novelty of these methods resides in how the reduced order models are formed, that is, how the coefficients of the POD expansions are calculated. Rather than taking a standard approach of projecting the underlying equations onto the reduced space through a Galerkin projection, here two different techniques are employed. The first method applies a second order Taylor series to calculate the POD coefficients at each time step from the POD coefficients at earlier time steps. The second method uses a Smolyak sparse grid collocation method to calculate the POD coefficients, where again the coefficients at earlier time steps are used as the inputs. The advantage of both approaches are that they are non-intrusive and so do not require modifications to a system code; they are therefore very easy to implement. They also provide accurate solutions for modelling flow problems, and this has been demonstrated by the simulation of flows past a cylinder and within a gyre. It is demonstrated that accuracy relative to the high fidelity model is maintained whilst CPU times are reduced by several orders of magnitude in comparison to high fidelity models.
Muggeridge A, 2015, Barriers to improved oil recovery make for animated discussion at Dresden symposium, ISSN: 0263-5046
Maes J, Muggeridge AH, Jackson MD, et al., 2015, Modelling in-situ upgrading of heavy oil using operator splitting method, Computational Geosciences, Vol: 20, Pages: 581-594, ISSN: 1573-1499
The in-situ upgrading (ISU) of bitumen and oil shale is a very challenging process to model numerically because of the large number of components that need to be modelled using a system of equations that are both highly non-linear and strongly coupled. Operator splitting methods are one way of potentially improving computational performance. Each numerical operator in a process is modelled separately, allowing the best solution method to be used for the given numerical operator. A significant drawback to the approach is that decoupling the governing equations introduces an additional source of numerical error, known as the splitting error. The best splitting method for modelling a given process minimises the splitting error whilst improving computational performance compared to a fully implicit approach. Although operator splitting has been widely used for the modelling of reactive-transport problems, it has not yet been applied to the modelling of ISU. One reason is that it is not clear which operator splitting technique to use. Numerous such techniques are described in the literature and each leads to a different splitting error. While this error has been extensively analysed for linear operators for a wide range of methods, the results cannot be extended to general non-linear systems. It is therefore not clear which of these techniques is most appropriate for the modelling of ISU. In this paper, we investigate the application of various operator splitting techniques to the modelling of the ISU of bitumen and oil shale. The techniques were tested on a simplified model of the physical system in which a solid or heavy liquid component is decomposed by pyrolysis into lighter liquid and gas components. The operator splitting techniques examined include the sequential split operator (SSO), the Strang-Marchuk split operator (SMSO) and the iterative split operator (ISO). They were evaluated on various test cases by considering the evolution of the discretization error as
Jackson MD, Percival JR, Mostaghiml P, et al., 2015, Reservoir modeling for flow simulation by use of surfaces, adaptive unstructured meshes, and an overlapping-control-volume finite-element method, SPE Reservoir Evaluation and Engineering, Vol: 18, Pages: 115-132, ISSN: 1094-6470
We present new approaches to reservoir modeling and flow simulation that dispose of the pillar-grid concept that has persisted since reservoir simulation began. This results in significant improvements to the representation of multiscale geologic heterogeneity and the prediction of flow through that heterogeneity. The research builds on more than 20 years of development of innovative numerical methods in geophysical fluid mechanics, refined and modified to deal with the unique challenges associated with reservoir simulation.Geologic heterogeneities, whether structural, stratigraphic, sedimentologic, or diagenetic in origin, are represented as discrete volumes bounded by surfaces, without reference to a predefined grid. Petrophysical properties are uniform within the geologically defined rock volumes, rather than within grid cells. The resulting model is discretized for flow simulation by use of an unstructured, tetrahedral mesh that honors the architecture of the surfaces. This approach allows heterogeneity over multiple length-scales to be explicitly captured by use of fewer cells than conventional corner-point or unstructured grids.Multiphase flow is simulated by use of a novel mixed finite-element formulation centered on a new family of tetrahedral element types, PN(DG)–PN+1, which has a discontinuous Nth-order polynomial representation for velocity and a continuous (order N +1) representation for pressure. This method exactly represents Darcy-force balances on unstructured meshes and thus accurately calculates pressure, velocity, and saturation fields throughout the domain. Computational costs are reduced through dynamic adaptive-mesh optimization and efficient parallelization. Within each rock volume, the mesh coarsens and refines to capture key flow processes during a simulation, and also preserves the surface-based representation of geologic heterogeneity. Computational effort is thus focused on regions of the model where it is most required.After valid
Holland RA, Eigenbrod F, Muggeridge A, et al., 2015, A synthesis of the ecosystem services impact of second generation bioenergy crop production, Renewable & Sustainable Energy Reviews, Vol: 46, Pages: 30-40, ISSN: 1364-0321
The production of bioenergy from second generation (2G) feedstocks is being encouraged by legislationtargeted at addressing a number of controversial issues including carbon emissions driven by land-use changeand competition for crops used in food production. Here, we synthesise the implications of 2G feedstockproduction for a range of key ecosystem services beyond climate regulation. We consider feedstocks typical oftemperate systems (Miscanthus; short-rotation coppice, short rotation forestry) and transitions from areas offorest, marginal land and first generation (1G) feedstock production. For transitions from 1G feedstocks, studiessuggest significant benefits may arise for a number of ecosystem services, including hazard regulation, diseaseand pest control, water and soil quality. Although less evidence is available, the conversion of marginal land to2G production will likely deliver benefits for some services while remaining broadly neutral for others.Conversion of forest to 2G production will likely reduce the provision of a range of services due to increaseddisturbance associated with shortening of the management cycle. Most importantly, further research is neededto broaden, and deepen, our understanding of the implications of transitions to 2G feedstocks on ecosystemservices, providing empirical evidence for policy development, particularly for commercial deployment wherelandscape scale effects may emerge. A programme of research that mixes both the natural and social sciencesbased on an ecosystem service framework, and occurs concurrently with large scale commercial deploymentof 2G feedstocks, would address this gap, providing evidence on the effectiveness of policies to promoteproduction of 2G feedstocks on a wide range of ecosystem services.
Maes J, Muggeridge AH, Jackson MD, et al., 2015, Scaling heat and mass flow through porous media during pyrolysis, HEAT AND MASS TRANSFER, Vol: 51, Pages: 313-334, ISSN: 0947-7411
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Mostaghimi P, Kamali F, Jackson MD, et al., 2015, A dynamic mesh approach for simulation of immiscible viscous fingering, Pages: 1537-1548
Viscous fingering is a major concern in the water flooding of heavy oil reservoirs. Traditional reservoir simulators employ low-order finite volume/difference methods on structured grids to resolve this phenomenon. However, their approach suffers from a significant numerical dispersion error due to insufficient mesh resolution which smears out some important features of the flow. We simulate immiscible incompressible two phase displacements and propose use of unstructured control volume finite element (CVFE) methods for capturing viscous fingering in porous media. Our approach uses anisotropic mesh adaptation where the mesh resolution is optimized based on the evolving flow features. The adaptive algorithm uses a metric tensor field based on solution interpolation error estimates to locally control the size and shape of elements in the metric. We resolve the viscous fingering patterns accurately and reduce the numerical dispersion error significantly. The mesh optimization, generates an unstructured coarse mesh in other regions of the computational domain where a high resolution is not required. We analyze the computational cost of mesh adaptivity on unstructured mesh and compare its results with those obtained by a commercial reservoir simulator based on the finite volume methods.
Attar A, Muggeridge A, 2015, Impact of geological heterogeneity on performance of secondary and tertiary low salinity water injection, Pages: 2452-2463
The impact of geological heterogeneity on the oil recovery and water cut obtained from secondary and tertiary low salinity water injection is investigated as a function of the size of the low salinity slug. We have used synthetic geological models including both simple layering and more geologically realistic 2D models based on the Brent formation taken from the SPE10 Model 2. Heterogeneity was quantified using a dimensionless number based on vorticity. Two different commercial simulators were used, one which models low salinity flooding using a salinity threshold limit to modify the rock's relative permeability curves with the salinity of the injected brine. The second simulator models explicitly the ion exchange between the clay surface and the injected brine's divalent ions. Results from both simulators are compared with the outcome of conventional waterflooding. Oil recovery correlates linearly with the vorticity based heterogeneity index (where high values correspond to a more homogeneous reservoir) i.e. the additional oil recovered by low salinity water injection decreases as heterogeneity increases. A low salinity slug size of at least 0.6 PVI is beneficial in heterogeneous reservoirs increasing to 0.8 PVI in highly heterogeneous reservoirs. Significant additional oil is recovered by injecting more than 1 PV. To date there are limited publications evaluating the impact of geological heterogeneity on the outcome of low salinity. Previous work by Jerauld et al. (2008) suggested that a 0.4 PV slug of low salinity water will recover almost as much oil as continuous injection. We have found this is not the case in heterogeneous reservoirs. Significant extra oil can be recovered by using larger slug sizes, especially if the reservoir is very heterogeneous. These results can be used to guide the development strategy (choice of slug size to maximize recovery, secondary or tertiary) when implementing low salinity waterflooding in heterogeneous sandstone reservoirs.
Al-Hadhrami MM, Alkindi AS, Muggeridge AH, 2014, Experimental and numerical investigations into the effect of heterogeneities on the recovery of heavy oil by VAPour EXtraction (VAPEX), FUEL, Vol: 135, Pages: 413-426, ISSN: 0016-2361
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Dale A, John CM, Mozley PS, et al., 2014, Time-capsule concretions: unlocking burial diagenetic processes in the Mancos Shale using carbonate clumped isotopes, Earth and Planetary Science Letters, Vol: 394, Pages: 30-37, ISSN: 0012-821X
Muggeridge A, Cockin A, Webb K, et al., 2014, Recovery rates, enhanced oil recovery and technological limits, Royal Society of London. Philosophical Transactions A. Mathematical, Physical and Engineering Sciences, Vol: 372, ISSN: 1364-503X
Abubakar SY, Muggeridge AH, King PR, 2014, Upscaling for thermal recovery, Pages: 468-479
Thermal recovery methods (such as steam and hot water flooding) are increasingly been used to recover bitumen and heavy oils. These schemes are designed using numerical reservoir simulation. Unfortunately in most cases thermal simulation requires very fine grids to capture both the heat transfer and associated fluid dynamics and minimize numerical dispersion. This usually needs powerful computers with a large memory as well as meaning it takes a long time to run a simulation. The alternative is to use upscaling. As yet there are no upscaling methodologies suitable for thermal oil recovery methods. Although tihere is a significant literature on single phase upscaling and the development of pseudo relative permeabilities for waterflooding applications, these methods do not capture the heat transport or the impact of heat on the oil mobility. In this paper, we use the Buckley Leverett solution approximated for thermal EOR processes to derive analytical pseudos for both hot water and steam flooding. The methodology involves upscaling both the oil viscosity dependence on temperature and the relative permeabilities to compensate for the increased numerical dispersion that occurs in coarse grid simulations. The methodology is demonstrated by comparing ID homogeneous fine and coarse grid simulations. The approach provides significantly improved predictions compared with performing coarse grid simulations without upscaling.
Muggeridge AH, Hongtong P, 2014, An upscaling methodology for EOR
The outcome of immiscible enhanced oil recovery (EOR) processes such as low salinity water injection and polymer flooding can be very sensitive to geological heterogeneity. In some cases the resulting reduction in macroscopic sweep (versus a waterflood) can be more than the improvement in microscopic displacement efficiency. It is therefore important to be able to model the impact of this heterogeneity during simulation studies. This may require an upscaling step if the geological heterogeneity is smaller than the simulation grid block size or simply to compensate for numerical diffusion on the coarse grid. This paper proposes an upscaling methodology that can be applied to different EOR processes and demonstrates its application to low salinity water injection examples. The methodology involves a hierarchy of upscaling steps. First the absolute permeability is upscaled with the objective of predicting the correct changes in pressure. This may also involve near well bore upscaling. Next pseudo relative permeability curves are generated to capture the shock front behaviour. In this study we compare results obtained from using traditional pore volume weighted pseudos with those obtained using pseudos determined analytically using Buckley-Leverett theory. Finally the simulator models relevant to the EOR process of interest are upscaled. In low salinity waterflooding this is the choice of low and high salinity thresholds. The upscaled models are in better agreement with the fine grid models in terms of pressure, water saturation production and production of either salinity or polymer than are the outputs of coarse grid models without upscaling. The results using the analytical pseudo relative permeabilities are comparable to those obtained from simulations using the pore volume weighted pseudo in many of the cases tested. These models are obviously less time-consuming and complex to generate as they do not need the engineer to run a full fine grid simulation of the EOR
Wen H, King PR, Muggeridge AH, et al., 2014, Using percolation theory to estimate recovery from poorly connected sands using pressure depletion
In conventional waterflooding of low to intermediate net to gross reservoirs there is always some oil unswept even in the sands connected to both injection and production wells. This is oil trapped in "dangling ends": flow units only poorly connected to the main flow path. In many cases the unswept volumes can be very large, depending on the properties of the reservoir and fluids and the well locations. In this paper we show how percolation theory can be used to estimate the volumes of oil recovered and those left behind in these dangling ends following a conventional waterflood, without recourse to large scale simulation. Percolation theory is a general mathematical framework for connectivity and has been used previously to investigate the connectivity of flow units. The structure of these connected clusters in terms of backbones and dangling ends has not been previously studied. The results are also used to estimate the recovery of the unswept oil from dangling ends by a waterflood with a voidage replacement ratio <1. We use a simple model of stochastically-distributed sandbodies to describe the reservoir. Many realizations for a range of net to gross ratio values and sandbody: system sizes were generated. In each realization the clusters connecting the injection and production wells were identified. These spanning clusters were subdivided into backbones and dangling ends. The volume fractions of the backbone and dangling end were then obtained. The statistical average and standard deviation of the volumes association with these clusters were obtained from the ensemble of realisations. These were used to determine the percolation scaling relationships in terms of simple algebraic formulae that cover the whole range of net to gross ratio and system sizes. Our results show that the fraction of dangling ends can reach 20% of the clusters, and 80% among the spanning clusters, indicating a major proportion of the oil would be unswept by conventional waterf
Maes J, Muggeridge AH, Jackson MD, et al., 2014, Modelling in-situ upgrading of heavy oil using operator splitting methods
The In-Situ Upgrading (ISU) of bitumen and oil shale is a very challenging process to model numerically because a large number of components need to be modelled using a system of equations that are both highly non-linear and strongly coupled. Operator splitting methods are one way of potentially improving computational performance. Each numerical operator in a process is modelled separately, allowing the best solution method to be used for the given numerical operator. A significant drawback to the approach is that decoupling the governing equations introduces an additional source of numerical error, known as splitting error. Obviously the best splitting method for modelling a given process is the one that minimises the splitting error whilst improving computational performance over that obtained from using a fully implicit approach. Although operator splitting has been widely used for the modelling of reactive-transport problems, it has not yet been applied to the modelling of ISU. One reason is that it is not clear which operator splitting technique to use. Numerous such techniques are described in the literature and each leads to a different splitting error. While this error has been extensively analysed for linear operators for a wide range of methods, the results observed cannot be extended to general non-linear systems. It is therefore not clear which of these techniques is most appropriate for the modelling of ISU. In this paper we investigate the application of various operator splitting techniques to the modelling of the ISU of bitumen and oil shale. The techniques were tested on a simplified model of the physical system in which a solid or heavy liquid component is decomposed by pyrolysis into lighter liquid and gas components. The operator splitting techniques examined include the Sequential Split Operator (SSO), the Strang-Marchuk split operator (SMSO) and the Iterative Split Operator (ISO). They were evaluated on various test cases by considering the ev
Mijic A, LaForce TC, Muggeridge AH, 2014, CO2 injectivity in saline aquifers: the impact of non-Darcy flow, phase miscibility and gas compressibility, Water Resources Research
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