Arne Skorstad

Modeling of fluvial reservoirs with object models

An object model for fluvial reservoirs that has been developed from 1985 to present is described. It uses a formal mathematical object model (marked point process) describing the distributions of four facies: channel, crevasse, barrier, and background. Realisations from the model are generated using the Metropolis-Hastings simulation algorithm with simulated annealing conditioning on the volume ratios and well observations. The main challenge has been to find a suitable parameterization of the geology of fluvial reservoirs, and to find and implement the generating function of the channels in the simulation algorithm. The model and simulation algorithm can be conditioned on arbitrary well paths including horizontal wells and paths with partly missing observations, well test data, well contacts, seismic data, and general geological knowledge.

Sedimentological parameterization of shallow-marine reservoirs

The key causes of heterogeneity within progradational shallow-marine reservoirs have been defined as: shoreline type (wave vs. fluvial dominated); shoreline trajectory; the presence of permeability contrasts associated with dipping clinoform surfaces within the shoreface or delta front; the presence of cemented barriers between parasequences; and the progradation direction of the shoreline (described with respect to the main waterflood direction in the simulated reservoir). These parameters were recorded from a series of 56 modern and ancient depositional systems from a variety of climatic and tectonic settings. These data were then used to build the 408 synthetic sedimentological models that formed the basis for the SAIGUP study.

Sensitivity of the impact of geological uncertainty on production from faulted and unfaulted shallow-marine oil reservoirs: objectives and methods

Estimates of recovery from oil fields are often found to be significantly in error, and the multidisciplinary SAIGUP modelling project has focused on the problem by assessing the influence of geological factors on production in a large suite of synthetic shallow-marine reservoir models. Over 400 progradational shallow-marine reservoirs, ranging from comparatively simple, parallel, wave-dominated shorelines through to laterally heterogeneous, lobate, river-dominated systems with abundant low-angle clinoforms, were generated as a function of sedimentological input conditioned to natural data. These sedimentological models were combined with structural models sharing a common overall form but consisting of three different fault systems with variable fault density and fault permeability characteristics and a common unfaulted end-member. Different sets of relative permeability functions applied on a facies-by-facies basis were calculated as a function of different lamina-scale properties and upscaling algorithms to establish the uncertainty in production introduced through the upscaling process. Different fault-related upscaling assumptions were also included in some models. A waterflood production mechanism was simulated using up to five different sets of well locations, resulting in simulated production behaviour for over 35 000 full-field reservoir models. The model reservoirs are typical of many North Sea examples, with total production ranging from c. 15×106 m3 to 35×106 m3, and recovery factors of between 30% and 55%. A variety of analytical methods were applied. Formal statistical methods quantified the relative influences of individual input parameters and parameter combinations on production measures. Various measures of reservoir heterogeneity were tested for their ability to discriminate reservoir performance. This paper gives a summary of the modelling and analyses described in more detail in the remainder of this thematic set of papers.

Assessing the effect of geological uncertainty on recovery estimates in shallow-marine reservoirs: The application of reservoir engineering to the SAIGUP project

Reservoir management is a balancing act between making timely operational decisions and the need to obtain data on which such decisions can be made. There is a further problem: estimates of recovery for prospective development plans are subject to uncertainty because of the uncertainty of the geological description within the simulation model. The SAIGUP project was designed to analyse the sensitivity of estimates of recovery due to geological uncertainty in a suite of shallow-marine reservoir models. However, although it was generic, it had the hallmarks of active reservoir management, because those members of the team responsible for deriving the notional development plans for individual models via reservoir simulation, and computing the recoveries, had to work in parallel with others under time and budget constraints. This paper describes the way the reservoir engineering was carried out to achieve these objectives, the assumptions made, the reasoning behind them, and how the principles could be used in other studies. Sample results are also presented, although the bulk of the results are presented in other papers in the project series. One surprising result was that faults that impede flow can improve recovery. The underlying physical explanation for this behaviour is provided.

Fault facies modeling: Technique and approach for 3-D conditioning and modeling of faulted grids

Faults in nature commonly affect surrounding rock volumes and can as such be described as fault envelopes with a given internal geometry and architecture. Modeling techniques currently employed when modeling faults in petroleum reservoirs are mostly two-dimensional (2-D); hence, a need is present for more accurate and realistic description and quantification of deformational architectures and properties to accurately predict fluid flow in fault zones. Fault facies (FF) modeling is a concept for three-dimensional (3-D) fault zone characterization, facies modeling of fault rocks and fluid flow simulation, which is presented here and demonstrated by the use of a synthetic fault model. FF modeling is performed by first generating a 3-D grid of the fault envelope, which includes the conventional fault plane. Second, a kinematic strain calculation is executed in the FF grid. The strain parameter is used to calculate a fault product distribution factor (FPDF), which describes the fault displacement in the fault envelope. This parameter together with strain distribution is subsequently used to condition the fault model for facies modeling. Finally, FF modeling is executed. To achieve adequate flexibility and realism, pixel-based modeling is combined with object-based modeling methods to populate the FF grid with facies. This synthetic model shows that it is possible to honor structural outcrop observations in fault zones, and FF modeling is able to produce realistic looking fault zone deformation structures in 3-D. It is possible to implement faults with varying width and displacement, although the FF grid itself has a regular fixed width. This is highly advantageous as compared to controlling the fault geometry with the grid itself. We propose that FF modeling can improve fault zone characterization and also capture fluid flow uncertainty in fault zones in a more realistic way than is possible with 2-D methods.

Upscaling uncertainty analysis in a shallow-marine environment

Geological models are often created at a scale finer than is suitable for flow simulation and also ignore the effects of sub-cellular heterogeneities. Upscaling of static and dynamic reservoir properties is an important process that captures the impact of smaller scales, ensuring that both heterogeneity and the flow physics are represented more accurately. A Geopseudo upscaling approach for shallow-marine reservoirs is presented, which captures the essential flow characteristics across a range of scales from laminae to the simulation grid. Starting with a base-case set of minimum assumptions enables generation of one set of pseudo-relative permeability and capillary pressure curves per facies. This is then expanded to investigate the limitations of these assumptions and compare their impact against variations in large-scale geological and structural parameters. For the analysis, two-level full factorial experimental design is used to determine important parameters. A comparison of upscaling effects is also performed. The most important upscaling and fine-scale parameters identified by the analysis are the shape of the capillary pressure curve, lamina-scale permeability variation and upscaling flow speed. Of similar importance are the sedimentological parameters for shoreline aggradation angle and curvature. Fault direction (perpendicular and parallel to the shoreline) and the fine-scale upscaling method are of moderate to low importance. The shallow-marine parameter for clinoform barrier strength and the direction of flow considered when upscaling are unimportant. Analysis of upscaling effects suggests that the algorithm used at the intermediate scale is not important, while the assumed flow speed is very important, typically resulting in a 10% maximum variation in cumulative recovery. Fine-scale properties and upscaling methods affect recovery mostly due to increased initial water saturations but also because of early breakthrough.

A study of the structural controls on oil recovery from shallow-marine reservoirs

The differences in oil production are examined for a simulated waterflood of faulted and unfaulted versions of synthetic shallow-marine reservoir models with a range of structural and sedimentological characteristics. Fault juxtaposition can reduce the economic value of the reservoirs by up to 30%, with the greatest losses observed in models with lower sedimentological aggradation angles and faults striking parallel to waterflood direction. Fault rock has a greater effect than fault juxtaposition on lowering the economic value of the reservoir models in the compartmentalized cases only – and only when the fault rock permeability model is based on the least permeable published laboratory data. Moderately sealing faults can increase the economic value of reservoirs except when the main flow direction is parallel to the faults. These results arise from the dependence of economic value on both sweep efficiency and production rate. Simple predictors of fault juxtaposition and fault-rock heterogeneity have been established and combined with two-dimensional considerations from streamline theory in an attempt to capture quantitatively the change in economic reservoir value arising from faults. Despite limitations associated with the three-dimensional role of juxtaposition, the results are encouraging and represent a step towards establishing a rapid transportable predictor of the effects of faults on production.

Well Conditioning in a Fluvial Reservoir Model

This paper describes a method for conditioning an object model of a fluvial reservoir on facies observations. The channels are assumed parametrized at sections normal to their main channel direction. Projections of the observations on these sections generates a map suitable for drawing conditioning values. This map contains the information from every facies observation between two adjacent sections, enabling handling of any well path. Coupling between well observations is also discussed. The methodology is implemented and demonstrated in examples with complex wells.

Combined effects of structural, stratigraphic and well controls on production variability in faulted shallow-marine reservoirs

Several key parameters that describe a prograding shallow-marine reservoir are investigated for their relative importance on hydrocarbon production variability. Sedimentological parameters are aggradation angle, progradation direction relative to the waterflood, continuity of cemented surfaces and shoreline curvature. Structural parameters are the fault pattern, the density (throw) of the faults and the fault-rock permeability. The last component investigated is the effect of well placements. Having three distinct levels for all sedimentological and structural parameters in addition to a non-faulted case gives a dataset of 2268 reservoir models. Four different sets of well locations produce 9072 production datasets. The variability of the production data is decomposed into its explanatory factors in order to see the relative importance of the chosen parameters. The production data include the total production, the discounted production and the recovery factor. The sedimentological parameters dominate both the production and the discounted production variability, especially the aggradation angle and progradation direction, whereas the fault pattern is equally significant for the recovery factor. Continuity of sedimentological barriers were found to contribute less than expected to the production variability for these reservoir models, and the well placements also showed a low effect.

Use of Well Test Data in Stochastic Reservoir Modelling

The aim of the study is to condition stochastic generated realizations on well test data in order to improve simulation of facies and petrophysics in fluvial reservoirs. First we have used the pressure data to estimate the shortest distance from the well to a possible channel boundary and thereby simulate the channel structures. The well test also provides the permeability average in the part of the channel intersected by the well. Together with core/log data and general knowledge of the reservoir this have been used to simulate permeability. These permeability realizations is input to a numerical flow simulator and compared with experimental results of the well test.