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- Volume 27, Issue 2, 2021
Petroleum Geoscience - Volume 27, Issue 2, 2021
Volume 27, Issue 2, 2021
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Stratigraphic controls on hydrocarbon recovery in clastic reservoirs of the Norwegian Continental Shelf
Authors Kachalla Aliyuda, John Howell, Adrian Hartley and Aliyuda AliA number of geological and engineering parameters influence and control the performance and ultimate recovery from an oil reservoir. These are commonly interlinked and the relative importance of each can be difficult to unravel. These variables include geological parameters such as depositional environment, which has long been considered to be a key factor influencing the production characteristics of fields. However, quantifying the importance of any single factor, such as depositional environment, is complicated by the impact of the other variables (geological and engineering) and their numerous interdependencies.
The main aim of this study is to unravel the impact of the depositional environment and primary facies architecture on reservoir performance using an empirical study of oilfields from the Norwegian Continental Shelf. A database of 91 fields, with a total of 7.8 Bbbl (billion barrels) of oil in place, has been built. Within this a total of 93 clastic reservoirs were classified into three gross depositional environments: continental, paralic/shallow marine and deep marine. The reservoirs were further classified into eight depositional environments in order to provide further granularity and to capture their depositional complexities. A further 28 parameters which capture other aspects that also impact production behaviour, such as reservoir depth, fluid type and structural complexity, were recorded for each reservoir. Principal component analysis (PCA) was utilized to explore the importance of sedimentological-dependent variables in the dataset, and to determine the parameters that have the strongest influence on the overall variability of the dataset. PCA revealed that parameters associated with field size and depth of burial had the most influence on recovery factor. Gross depositional environment and other stratigraphic-dependent parameters were the most significant geological factors. Fluid properties, such as API gravity and average gas/oil ratio, were unexpectedly among the less important parameters.
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Using Monte Carlo models to predict hydrocarbon column heights and to illustrate how faults influence buoyant fluid entrapment
More LessFaults are often assumed to play a significant role in the migration and entrapment of hydrocarbons, either offering conduits for, or barriers to, fluid flow. They may also affect fluid-phase trapping and influence phase fractionation in the subsurface. A Monte Carlo modelling approach is used to model these effects for trap analysis. The aim is to show how varying fault seal capacity, the fault orientation, the regional stress tensor and the trap geometry can all affect how both oil and gas are retained within a trap. The model reduces the problem to a 1D analysis with a structural description depth-referenced to the crest of a prospect. Both juxtaposition and membrane fault seal are modelled, together with hydrodynamic effects and fault reactivation risk. The potential of a prospect to trap hydrocarbons is evaluated in a roll-up of results with the outputs including a predicted hydrocarbon column height distribution and column height control statistics. The technique also offers an insight into the potential fluid-phase partitioning that may occur dependent on the interplay between the active leakage mechanisms and spill control, enabling gas v. oil columns to be predicted.
Thematic collection: This article is part of the Fault and top seals collection available at: https://www.lyellcollection.org/cc/fault-and-top-seals-2019
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The influence of inter- and intra-channel architecture on deep-water turbidite reservoir performance
Authors Casey D. Meirovitz, Lisa Stright, Stephen M. Hubbard and Brian W. RomansBed-scale heterogeneity in channelized deep-water reservoirs can significantly influence reservoir performance, but reservoir simulation typically requires cell sizes much greater than the scale of intra-channel element architecture. Here, bed- to geobody-scale simulations elucidate the influence of bed-scale architecture and channel element stacking on flow and connectivity, informing full-field reservoir model development and evaluation.
Models consist of two channel element segments, each 300 m (985 ft) wide by 14 m (45 ft) thick and 550 m (1805 ft) long, stacked in 12 different stacking arrangements. Bed-scale architecture is captured in six deterministic element fills, highlighting interbedded sandstone and mudstone (thin bed) presence (homogeneous v. heterogeneous elements), position (symmetrical v. asymmetrical), and proportion (low v. high element net-to-gross). Each model is flow simulated to illuminate how element stacking and intra-element heterogeneity impacts reservoir performance.
Thin bed presence and position have the greatest impact on reservoir connectivity/performance when elements are laterally offset; impacts are minimal when elements are vertically aligned. Impacts are exacerbated when the thin-bed proportion is increased. Where bed-scale architecture is represented, complex flow behaviours generate a significant variability in production timing and the cumulative volumes produced. Simulations consisting of a homogenous element architecture fail to capture complex flow behaviours, producing comparatively optimistic results.
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Influence of host rock composition on permeability reduction in shallow fault zones – implications for fault seal analysis (Vienna Basin, Austria)
In order to calibrate equations for fault seal capacities to a specific basin, faults were analysed using core material from several Neogene hydrocarbon fields in the Vienna Basin, Austria. All studied specimens are siliciclastic rocks that were sampled from a depth interval of <2000 m, and share a similar depth at time of faulting, diagenetic conditions and maximum burial depth. Laboratory results showed a permeability reduction in all fault rocks compared to the host rocks. Both the highest and the lowest fault seal capacities were observed in the same fault rock type with a low phyllosilicate and clay content, and classifying as cataclastic deformation bands. Investigating the strong permeability variations within these fault rocks, microscopic analyses revealed that the fault seal potential is strongly linked to the detrital dolomite content in the host rock. Grain-size reduction processes occur preferably in the dolomite grains, accompanied by cementation. Our study suggests that – in addition to using standard fault seal analysis algorithms – accounting for host rock composition and grain-size reduction therein might help to further constrain fault seal behaviour in shallow depths. Fault seal mechanisms need to be understood on field, formation and micro scales before drawing conclusions for a full basin calibration.
Thematic collection: This article is part of the Fault and top seals collection available at: https://www.lyellcollection.org/cc/fault-and-top-seals-2019
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Ranking and selecting fault models using flow-indicator fault properties and simple streamline simulations
Authors Paul Wilson, Stewart Smith, Danny Povey and Simon HarrisFault zones in porous sandstones are commonly divided into two parts: a fault core and a damage zone. Both fault-zone elements could influence subsurface fluid flow and should be incorporated in a geologically realistic model. The fault core can be implemented in the model as a transmissibility multiplier (TM), while the damage zone can be implemented by modifying the grid permeability in the cells adjacent to the model faults. Each of the input parameters used in calculating the TM and damage-zone permeability modification is subject to geological uncertainty. Here an iterative workflow is employed to define probability distribution functions for each of the input parameters, with the result being many fault-model realizations. Here two methods are examined for ranking and selecting the fault-model realizations for further analysis: (i) calculating the flow-indicator fault properties (effective cross-fault transmissibility and effective cross-fault permeability) from the static model; and (ii) employing a simplified flow-based connectivity calculation, returning dynamic measures of model connectivity. The aims are to outline the methodology and workflow used, evaluate the impact of the different input parameters on the results, and examine the results of the static and dynamic approaches to understand how the ranking and selection of models compares between the two.
Our results are dependent on the structural model. In a strongly compartmentalized model based on the Gullfaks Field, North Sea, fluid-flow-indicator fault properties are weakly correlated with measures of dynamic behaviour. In particular, models with low fault transmissibility show a much greater range of dynamic behaviour, and are less predictable, than models with high fault transmissibility. In a weakly compartmentalized model with strongly channelized fluvial facies based on the Whitley Bay area in NE England, there was a strong correlation between flow-indicator fault properties and measures of dynamic behaviour. We ascribe these results to the greater complexity of flow paths expected when a highly compartmentalized model contains faults that are likely to be baffles to cross-fault flow.
Thematic collection: This article is part of the Fault and top seals collection available at: https://www.lyellcollection.org/cc/fault-and-top-seals-2019
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Key controls on the hydraulic properties of fault rocks in carbonates
A significant knowledge gap exists when analysing and predicting the hydraulic behaviour of faults within carbonate reservoirs. To improve this, a large database of carbonate fault rock properties has been collected from 42 exposed faults, from seven countries. Faults analysed cut a range of lithofacies, tectonic histories, burial depths and displacements. Porosity and permeability measurements from c. 400 samples have been made, with the goal of identifying key controls on the flow properties of fault rocks in carbonates. Intrinsic and extrinsic factors have been examined, such as host lithofacies, juxtaposition, host porosity and permeability, tectonic regime, displacement, and maximum burial depth, as well as the depth at the time of faulting. The results indicate which factors may have had the most significant influence on fault rock permeability, improving our ability to predict the sealing or baffle behaviour of faults in carbonate reservoirs. Intrinsic factors, such as host porosity, permeability and texture, appear to play the most important role in fault rock development. Extrinsic factors, such as displacement and kinematics, have shown lesser or, in some instances, a negligible control on fault rock development. This conclusion is, however, subject to two research limitations: lack of sufficient data from similar lithofacies at different displacements, and a low number of samples from thrust regimes.
Thematic collection: This article is part of the Fault and top seals collection available at: https://www.lyellcollection.org/cc/fault-and-top-seals-2019
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The effects of basaltic lava flows on the petrophysical properties and diagenesis of interbedded aeolian sandstones: an example from the Cretaceous Paraná Basin, Brazil
Authors G. Bertolini, A. J. Hartley, J. C. Marques, D. Healy and J. C. FrantzAn analysis of the petrophysical and diagenetic effects of the emplacement of Cretaceous basaltic lava flows (Serra Geral Formation) on aeolian sandstones (Botucatu Formation) has been undertaken on core samples from the Paraná Basin, Brazil. Between 0.1 and 1 m from the contact zone, acoustic wave velocities and porosities in sandstones show a significantly wider scatter than those located >1 m away from the lava contact. Higher P-wave values (average 3759.3 m s−1) occur between 0.1 and 1 m from the lava contact in contrast to those areas >1 m away (average 3376.8 m s−1), whilst the average porosity is 6.5% near the contact (0.1–1 m) and 10.7% away from the contact (>1 m). Petrographical evaluation reveals two diagenetic pathways responsible for modification of the petrophysical properties: early hydrothermal Mg-rich authigenesis (Type 1) and early chemical dissolution (Type 2). Type 3 diagenesis occurs away from the lava–sediment contact (>1 m), with the appearance of poikilitic calcite and smectite. The sandstone samples associated with Type 1 and Type 2 diagenesis display a decrease in porosity and increased acoustic velocities in relation to Type 3, while Type 3 samples show little or no variation in reservoir properties. The lava-induced diagenetic effects at the sandstone–lava contacts (0.1–1 m) may form a baffle or seal to fluids around the margins of the sandstone bodies. Therefore, whilst diagenesis associated with lava emplacement may hinder reservoir quality around the margins, the original reservoir properties are preserved within these large sandstone bodies.
Supplementary material: Petrophysical and petrographical data are available at https://doi.org/10.6084/m9.figshare.c.5244473
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Study on the effect of pore-scale heterogeneity and flow rate during repetitive two-phase fluid flow in microfluidic porous media
Authors Jingtao Zhang, Haipeng Zhang, Donghee Lee, Sangjin Ryu and Seunghee KimVarious energy recovery, storage, conversion and environmental operations may involve repetitive fluid injection and thus, cyclic drainage–imbibition processes. We conducted an experimental study for which polydimethylsiloxane (PDMS)-based micromodels were fabricated with three different levels of pore-space heterogeneity (coefficient of variation, where COV = 0, 0.25 and 0.5) to represent consolidated and/or partially consolidated sandstones. A total of 10 injection-withdrawal cycles were applied to each micromodel at two different flow rates (0.01 and 0.1 ml min−1). The experimental results were analysed in terms of flow morphology, sweep efficiency, residual saturation, the connection of fluids and the pressure gradient. The pattern of the invasion and displacement of the non-wetting fluid converged more readily in the homogeneous model (COV = 0) as the repetitive drainage–imbibition process continued. The overall sweep efficiency converged between 0.4 and 0.6 at all tested flow rates, regardless of different flow rates and COV in this study. In contrast, the effective sweep efficiency was observed to increase with higher COV at the lower flow rate, while that trend became reversed at the higher flow rate. Similarly, the residual saturation of the non-wetting fluid was largest at COV = 0 for the lower flow rate, but it was the opposite for the higher flow-rate case. However, the Minkowski functionals for the boundary length and connectedness of the non-wetting fluid remained quite constant during repetitive fluid flow. Implications of the study results for porous media-compressed air energy storage (PM-CAES) are discussed as a complementary analysis at the end of this paper.
Supplementary material: Figures showing the distribution of water (Fig. S1) and oil (Fig. S2) at the end of each drainage and imbibition step in different microfluidic pore-network models are available at https://doi.org/10.6084/m9.figshare.c.5276814
Thematic collection: This article is part of the Energy Geoscience Series available at: https://www.lyellcollection.org/cc/energy-geoscience-series
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Optimising development and production of naturally fractured reservoirs using a large empirical dataset
Authors Shaoqing Sun and David A. PollittNaturally fractured reservoirs are important contributors to global petroleum reserves and production. Existing classification schemes for fractured reservoirs do not adequately differentiate between certain types of fractured reservoirs, leading to difficulty in understanding fundamental controls on reservoir performance and recovery efficiency. Three hundred naturally fractured reservoirs were examined to define a new classification scheme that is independent of the type of fracturing and describes fundamentally different matrix types, rock properties, fluid storage and flow characteristics.
This study categorises fractured reservoirs in three groups: (1) Type 1: characterized by a tight matrix where fractures and solution-enhanced fracture porosity provide both storage capacity and fluid-flow pathways; (2) Type 2: characterized by a macroporous matrix which provides the primary storage capacity where fractures and solution-enhanced fracture porosity provide essential fluid-flow pathways; and (3) Type 3: characterized by a microporous matrix which provides all storage capacity where fractures only provide essential fluid-flow pathways. Differentiation is made between controls imparted by inherent natural conditions, such as rock and fluid properties and natural drive mechanisms, and human controls, such as choice of development scheme and reservoir management practices.
The classification scheme presented here is based on reservoir and production characteristics of naturally fractured reservoirs and represents a refinement of existing schemes. This refinement allows accurate comparisons to be made between analogous fractured reservoirs, and trends and outliers in reservoir performance to be identified. Case histories provided herein demonstrate the practical application of this new classification scheme and the benefits that arise when applying it to the understanding of naturally fractured reservoirs.
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Pore-scale assessment of subsurface carbon storage potential: implications for the UK Geoenergy Observatories project
The growing importance of subsurface carbon storage for tackling anthropogenic carbon emissions requires new ideas to improve the rate and cost of carbon capture and storage (CCS) project development and implementation. We assessed sandstones from the UK Geoenergy Observatories (UKGEOS) site in Glasgow, UK and the Wilmslow Sandstone Formation (WSF) in Cumbria, UK at the pore scale to indicate suitability for further assessment as CCS reservoirs. We measured porosity, permeability and other pore geometry characteristics using digital rock physics techniques on microcomputed tomographic images of core material from each site. We found the Glasgow material to be unsuitable for CCS due to very low porosity (up to 1.65%), whereas the WSF material showed connected porosity up to 26.3% and permeabilities up to 6040 mD. Our results support the presence of a percolation threshold at 10% total porosity, introducing near full connectivity. We found total porosity varies with permeability with an exponent of 3.19. This provides a reason to assume near full connectivity in sedimentary samples showing porosities above this threshold without the need for expensive and time-consuming analyses.
Supplementary material: Information about the boreholes sampled in this study, additional well logs of boreholes and a summary of the supporting data plotted throughout this article from literature are available at https://doi.org/10.6084/m9.figshare.c.5260074
Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage
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Flow modelling to quantify structural control on CO2 migration and containment, CCS South West Hub, Australia
Authors Laurent Langhi, Julian Strand and Ludovic RicardIn order to reduce uncertainties around CO2 containment for the South West Hub CCS site (Western Australia), conceptual fault hydrodynamic models were defined and numerical simulations were carried out. These simulations model worst-case scenarios with a plume reaching a main compartment-bounding fault near the proposed injection depth and at the faulted interface between the primary and secondary containment interval.
The conceptual models incorporate host-rock and fault properties accounting for fault-zone lithology, cementation and cataclastic processes but with no account made for geomechanical processes as the risk of reactivation is perceived as low. Flow simulations were performed to assess cross-fault and upfault migration in the case of plume–faults interaction.
Results near the injection depth suggest that the main faults are likely to experience a significant reduction in transmissivity and impede CO2 flow. This could promote the migration of CO2 vertically or along the stratigraphic dip.
Results near the interface between the primary and secondary containment intervals show that none of the main faults would critically control CO2 flow nor would they act as primary leakage pathways. CO2 flow is predicted to be primarily controlled by the sedimentological morphology. The presence of baffles in the secondary containment interval is expected to be associated with local CO2 accumulations; additional permeability impacts introduced by faults are minor.
Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)