Andrew Richard Gardiner

3D high-resolution digital models of outcrop analogue study sites to constrain reservoir model uncertainty: an example from Alport Castles, Derbyshire, UK

Advances in data capture and computer technology have made possible the collection of 3D high-resolution surface and subsurface digital geological data from outcrop analogues. This paper describes research to obtain the 3D distribution and internal sedimentary architecture of turbidite channels and associated sediments at a study site in the Peak District National Park, Derbyshire, UK. The 1D, 2D and 3D digital datasets included Total Station survey, terrestrial photogrammetry and remote sensing, sedimentary logs and a Ground Penetrating Radar (GPR) dataset. A grid of 2D GPR profiles was acquired behind a cliff outcrop; electromagnetic reflection events correlated with cliff face sedimentary horizons logged by Vertical Radar Profiling. All data were combined into a Digital Solid Model (DSM) dataset of the site within reservoir modelling software. The DSM was analysed to extract 3D architectural geometries for petroleum reservoir models. A deterministic base model was created using all information, along with a suite of heterogeneous turbidite reservoir models, using 1D, 2D or 3D information. The model suite shows significant variation from the deterministic model. Models built from 2D information underestimated connectivity and the continuity of geobodies, but overestimated channel sinuosity. Advantages of using 3D digital outcrop analogue data for 3D reservoir models are detailed.

Outcrop-based stochastic modelling of turbidite amalgamation and its effects on hydrocarbon recovery

In sandstone-dominated successions of sheet-like turbidites, erosion of thin shale horizons during deposition of the overlying turbidite may lead locally to vertical amalgamation of sandstone beds, resulting in discontinuous thin fine-grained beds or, in the extreme case, thoroughly amalgamated sandstone. Measurements of discontinuous shale lengths from very well exposed turbidite successions have enabled the development of a mixed rule-based/stochastic model for the erosion of shales. Monte Carlo realizations of 2D cross-sections were used to examine the effects of shale discontinuities on both single-and two-phase flow, at the genetic sedimentary unit scale. Results demonstrate that the flow is strongly dependent on the balance of viscous, capillary and gravity forces, which can vary according to the distribution of amalgamation surfaces. The single-phase upscaled ratio of horizontal to vertical permeability and the fraction of mobile oil recovered can be related to the fraction of shale removed (amalgamation ratio) by log-linear and linear relationships respectively.

Sand Volcanoes of the Carboniferous Ross Formation, County Clare, Western Ireland: 3-D Internal Sedimentary Structure and Formation

An exceptionally well preserved sand volcano cluster approximately 7 m (23 ft) in diameter lies on top of mass-wasting deposits in the Carboniferous Ross Formation in western Ireland. Sandstone dikes were observed near the volcanoes, emanating from sandstone units beneath the slumped intervals. The volcano cluster was the focus of sedimentary, geophysical (ground-penetrating radar), and surveying (differential global positioning system) methods, with the aim of digitally reconstructing the volcanoes in three dimensions and elucidating their origin and significance. Data analysis determined that single volcanoes were, in fact, composite, with early small cones being overwhelmed by a later dominant cone during deposition. Volcanoes were sourced through feeder dikes that were correlated through slump horizons to underlying in-situ sandstones. A four-stage origin is inferred: (1) initial overpressured sand escaped up through tension gashes in the slump horizons to deposit multiple, small sand cones on the new sea floor; (2) more widely spaced vents allowed multiphase ejection to envelop initial cones and to produce larger volcanoes with a single vent; (3) composite volcanoes loaded and subsided into the underlying substrate and local sand chamber during continued deposition before final cessation; and (4) deposition of overlying pelagic sediments and lithification. Volcanoes may be sited on local sea-floor, topographic highs.

The variability of turbidite sandbody pinchout and its impact on hydrocarbon recovery in stratigraphically trapped fields

Abstract Stratigraphic trapping is an important component of many hydrocarbon fields reservoired in deep-water, turbidite deposits. The trapping may occur at channel margins, onlap surfaces and when turbidite sandbodies exhibit lateral variations in sand quality and/or bed thickness. The range of geometries occurring at these sandbody terminations has been the subject of detailed previous research and a number of classification schemes have been proposed. A single classification scheme, based only on the geometry of the sandbody and individual sandstone beds, is proposed here. The different geometries of sandbody termination or pinchout will have an impact on both the static and dynamic behaviour of hydrocarbon reservoirs. Dynamic simulation of a range of models of sandbody pinchout by onlap indicates that the recovery factor of stratigraphically trapped fields will be influenced by a range of geological and engineering parameters. Producing wells positioned too far from the sandbody termination run the risk of leaving behind significant volumes of up-dip oil. If wells are moved closer to the sandbody termination, they may intersect the onlap surface, and so not penetrate the lowest sandstone beds. In systems with a low effective vertical permeability, oil from these lower beds may not be efficiently produced. In this case, the optimum well location, in terms of recovery factor, is close to the initiation of onlap. Unfortunately, this position may be difficult to identify in the subsurface without the drilling of many appraisal wells. Variation in a range of parameters has been modelled, in order to examine their impact on hydrocarbon recovery. For layered sand/shale successions, with low effective vertical permeability, the initiation of onlap, and therefore the optimum well location, moves further from the onlap termination as the angle of onlap decreases. The maximum recovery factor is also lower for the lower onlap dips, as a greater volume of the reservoir lies up-dip of the producer at its optimum location. If individual sandstone beds thin towards the onlap, the volume of oil which might be left up-dip of producing wells is reduced, so that the risk in placing a well away from the sandbody termination is lower. The degree of trapping of hydrocarbons in the lower layers, as the producing well location moves onto the onlap surface, depends on the effective vertical permeability of the sandbody. If the vertical permeability is zero (as would be the case for a perfectly layered system with continuous sealing shales) no oil will be produced from the lower layers. As the vertical permeability is increased, fluids are able to flow vertically from these beds into higher beds and thence to the producer. The trapping potential is significantly reduced for kv:kh ratios of 2 × 10−5 or more, which is equivalent to an effective kv of 0.01 mD for a kh of 500 mD. In layered turbidites, this effective kv could be produced by 2 m thick sandstone beds, with a kv of 400 mD, interbedded with thin, non-sealing siltstones or silty shales, with permeabilities of the order of 10−4 mD. In practice, the effective kv:kh ratio of interbedded turbidite sandstones and shales is greatly influenced by the local erosion of the shales. Flow simulation through models representing various proportions of shale removal indicates that significant trapping of hydrocarbons in the lower layers may occur for proportions of shale removal below 15%. Above this value of shale removal, little trapping occurs, as fluids are able to move sufficiently easily between the individual sandstone layers. These results suggest that the risk of reduced hydrocarbon recovery, as producing wells are moved closer to the onlap termination, are significant only in the case of well-layered reservoirs with low proportions of shale removal and sand-bed amalgamation. Examination of available core should enable the proportion of bed amalgamation, and therefore the risk of reduced recovery, to be evaluated for a stratigraphically trapped reservoir of this type.

The use of vertical radar profiling (VRP) in GPR surveys of ancient sedimentary strata

Abstract Vertical radar profiling (VRP) is an application of ground penetrating radar (GPR) technologies that can extract important subsurface information from suitable outcrops. Using standard GPR equipment, a site-specific time-depth calibration can be obtained, along with correlation of observed sedimentological horizons exposed on cliff-faces. These horizons may then be correlated with subsurface reflection events imaged on fixed-offset profiles. Summaries of six GPR study sites, where the VRP technique was used, are detailed. Where possible, CMP and VRP velocities have been compared, and show good correlations. Geochemical analysis of selected sedimentary rocks shows that increasing grain size and quartz mineral percentages generally lead to increased GPR velocities. Reflection events tend to be associated with sandstone/shale boundaries.

The use of GPR to image three-dimensional (3-D) turbidite channel architecture in the Carboniferous Ross Formation, County Clare, Western Ireland

Abstract Petroleum reservoir models are currently built from two-dimensional (2-D) information. An understanding of both the large-scale and internal three-dimensional (3-D) architecture of turbidite channel deposits is important for both hydrocarbon exploration and production. A ground penetrating radar (GPR) survey was undertaken on a study site exposing Upper Carboniferous Ross Formation deposits in western Ireland. Both channel margins and intrachannel fill were imaged in 3-D. Constant-offset, 2-D reflection sections were calibrated by vertical radar profiles. GPR data were integrated with sedimentary and survey data to produce a 3-D model of the study site.

Recovery efficiency from a turbidite sheet system: numerical simulation of waterflooding using outcrop-based geological models

A series of waterflood simulations were performed to investigate the effect of basinal position and facies permeability within a turbidite sheet system on oil recovery efficiency. Simulations used three-dimensional outcrop models of the Peïra Cava system, comprising gravel, sandstone, thin-bedded heterolithic and mudstone facies. Recovery efficiency declines with increasing permeability heterogeneity and is influenced by the interaction of vertical bed-permeability trends and flood-front gravity slumping. The occurrence of gravels with permeabilities lower than overlying sandstones produces optimum recoveries. High permeability gravels act as thief zones, enhanced by downward gravity slumping, reducing normalized recovery by up to 34 %. The effect of thief zones on recovery is related to their permeability contrast, abundance, thickness, lateral continuity, vertical position within permeable units and the permeability of underlying facies. Proximal to distal stratigraphic variations produce relatively small differences in normalized recovery of up to 13 % in models with the highest permeability heterogeneity. Differences in recovery are interpreted to reflect spatial trends in facies architecture, which determine the effectiveness of high permeability gravel thief zones. The poorest recovery is recorded from the medial model where recovery is lower than distal areas because of higher gravel abundance and thicknesses and lower compared to proximal areas because of the higher lateral continuity of gravels and underlying low-permeability mudstones.

The Impact of Sub-seismic Shale Layers On the Reservoir’s Stress Sensitivity

In most clastic reservoirs experiencing pressure depletion due to production, the hydraulically connected sands in the reservoir naturally compact to some degree. As a consequence, the much lower permeability reservoir shales may experience mechanical tension. The effective seismic response of the reservoir interval is thus a mix of both hardening and softening reservoir components. This phenomenon alters the predicted overall stress sensitivity from that anticipated for a homogeneous, fully connected reservoir interval. The time period over which this effect might be observed is influenced by the rate at which the shales reach pressure equilibrium with the surrounding sands. This work indicates that sub-seismic shale layers of approximately 1m thickness take less than 12 months to equilibrate, whilst thicker shale layers of 8m can take over 10 years. It is concluded that the mechanical and dynamic response of sub-seismic reservoir shale must be considered when quantitatively assessing the 4D seismic signature from frequently shot time-lapse surveys with a periodicity of 6 to 12 months, but also perhaps, for conventional 4D seismic surveys shot over 5 to 10 years. These conclusions are strongly affected by the permeability of the shale layers, the stress state, and are also a function of net to gross and depositional environment.

Evaluation of added value in reservoir management by application of flow control with intelligent wells

Intelligent Well-systems Technology (IWsT) provides the delivery and management of production flexibility thorough downhole measurement and control. This paper uses a new workflow to evaluate the suitability of a wide range of reservoir types for IWsT application. This is achieved by a systematic study of a series of generic reservoir scenarios, based on property distributions derived from real field data and operational oil-field models. These geological scenarios were tested to determine the ‘Added Value’ from IWsT compared with standard well completions. Added value is expected through incremental oil recovery. Results show that IWsT can control uneven, invading fluid fronts, which develop along the wellbore length due to permeability differences, reservoir compartmentalization, or different strengths of aquifer or gas cap support. The degree of improvement depends on the reservoir type (whether layered, faulted, channelized, etc.) and the distribution of porosity and permeability within it. Guidelines for the optimum placement of Internal Control Valve (ICV) locations in the planned completion zone are discussed. A global methodology was developed for the initial screening of favourable geological scenarios for the implementation of IWsT, and an ‘Application Envelope’ was developed based on the formations correlation length and variability. The validity of this envelope is illustrated by its application to a real reservoir modelling case.

Reservoir Impact of Small-scale Aeolian Dune Architecture - From Acquisition to Simulation in the Wahiba Sands, Oman

Small-scale dune heterogeneity has a significant impact upon recoverable reserves within aeolian hydrocarbon reservoirs. Complex geometries exist, with bounding surfaces and primary strata types often negatively impacting fluid flow. Incorporating the effects of such architectural elements into reservoir models is essential when accurately determining their effect on development strategies. In order to assess their impact, we acquired a small pseudo-3D dataset from the Wahiba Sands, Sultanate of Oman, using ground-penetrating radar (GPR). In this paper we discuss the acquisition, processing and modelling of this dataset. Data are interpreted to be of a small linear dune. Radar stratigraphic units have been interpreted and mapped in 3D; a small-scale analogue reservoir model has been produced. Simulation studies have been conducted to assess the impact of a range of sensitivities, including the affects of permeability contrast, flow direction and capillary pressure. Results indicate that permeability contrasts have a significant impact on recovery, whilst flow direction is the dominant factor. The resulting models may not be directly transferable to a specific subsurface scenario, but the generic spatial information can be a useful guide.