Geologically Constrained Grid Design in Shallow-Marine Reservoir Models: An Example from the Flounder Field, Gippsland Basin, Australia
Sarah J. Riordan, Simon C. Lang, Tobias H.D. Payenberg, 2008. "Geologically Constrained Grid Design in Shallow-Marine Reservoir Models: An Example from the Flounder Field, Gippsland Basin, Australia", Recent Advances in Models of Siliciclastic Shallow-Marine Stratigraphy, Gray J. Hampson, Ronald J. Steel, Peter M. Burgess, Robert W. Dalrymple
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Despite increasing computer power, the need to upscale 3D geologic models for reservoir simulation is likely to continue in many commercial environments for some time. Therefore an understanding of the depositional environments in combination with modern and ancient analogs can provide the modeling team with geologically sensible scales for upscaled cell sizes in both geologic (static) and simulation (dynamic) models. A case study from the Flounder Field in the Gippsland Basin, southeastern Australia, is used to demonstrate the differences that understanding depositional environments can make in the approach to building 3D geologic models.
The interpretation of depositional environments at an early stage can aid in the planning and building process of static and the resultant dynamic models. The interpretation of a certain depositional environment has implications for grid orientation, grid cell size, and the population of grids with facies and ultimately petrophysical properties. Placing the geologic interpretation into a sequence stratigraphic context can assist in the recognition of potential flow units and aid the prediction of flow-unit boundaries, facies distribution, and sandbody dimensions.
Understanding the dimensions of the smallest sandbodies that are likely to influence fluid flow through the model is critical to the upscaling of facies models. Upscaling a model beyond half the length and/or width of the smallest influential depositional feature can result in changes to sand-body morphology. In addition, the relative influence of these bodies can be either magnified or decreased. In the Flounder Field a transgressive barrier-island-tidal-delta complex is interpreted. The smallest influential depositional features in this barrier-island system are the tidal channels, which are on average 600 m wide. Thus the side of the cells that cuts across a tidal channel should not be more than 300 m wide if the width and morphology of the tidal channels is to be modeled accurately. This places limitations on the maximum extent to which a model of this environment should be upscaled for reservoir simulation if the porosity and permeability trends of the reservoir are to be maintained.
One of the keys to successful modeling is a clear understanding of the purpose of the models. A simple nomogram can be used to calculate the approximate number of cells in a 3D model before a model is built, enabling discussion between all interested parties about the potential dimensions of both static and upscaled dynamic models during the planning stage.