Thin layers of less permeable materials can occur in cross‐bedded rocks and may function as flow baffles, thus influencing the bulk fluid flow. However, it can be challenging to model their flow impact using standard grid‐based techniques that can only capture the topology and geometry of the layers accurately using a large number of small cells. In this paper, a numerical method, recently developed by the authors for modeling fluid flow in fault damage zones that contain thin low‐permeability fault strands, is demonstrated as being applicable to modeling the flow impact of thin baffle laminae in cross beds. This method does not require thin baffles to be discretized explicitly. For a range of permeability contrasts between the thin baffle layers and the rest of the matrix, upscaled permeability values are derived for models that have the same volumetric fraction of baffle material. The results show that when the baffles are completely connected, the upscaled permeability is less than for cases where the baffles do not form a continuous impediment for all levels of permeability contrast and declines more steeply with the increase of the permeability contrast. The flow effect of the layer configuration becomes more apparent for the disconnected situation when the permeability contrast is high. The method is shown to be accurate and efficient for this type of work. These results highlight the importance of capturing the topology and volume of the thin baffle layers in flow modeling of cross beds and the necessity of using appropriate numerical techniques.
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.
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.
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.
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