Introduction to EarthVision
Geocellular Modeling
The EarthVision Geocellular Modeling package is a program designed to convert a complex but rigorous EarthVision structural or property model into an upscaled, property populated geocellular grid suitable for fluid flow calculation in the standard reservoir simulation programs. The program is fast, easy to use, and allows for easy adjustment of modeling parameters to facilitate the rapid calculation and revision of the often times non-orthogonal cellular model. The initial structural model defines the geometry for the cellgrid thus eliminating the time consuming process of incorporating structural components or relationships as part of the geocellular modeling process.
The program is specifically designed for faulted structures; those which previously posed significant difficulties and compromise in reconciling the variable angular and cross cutting character of major faults with regular cell geometry. A simplified workflow uses the EarthVision structure model to interactively establish a modeling space or volume where the I, J, and K planes follow the geologic features (faults and structure surfaces) within the model. The user defines the treatment of cells along each fault, and for each stratigraphically based cellular flow unit, and a grid is calculated. This grid may be populated with petrophysical properties using industry standard upscaling techniques.
Once constructed, the cell grid is easily exported to formats designed for Eclipse®, VIP® CORP, CMG®, FGRID Eclipse®, GRIDGENR®, RESCUE™, or as an EarthVision ASCII corner point file.
With training and practice, multiple grid models may be generated and evaluated in minimal amounts of time.
What is a Cellular Grid?
A cellular grid is a corner point grid of irregularly shaped cells. The grid is arrayed in an I, J and K dimension with the origin in the northwest corner. The I dimension increases eastward; the J dimension increases southward; and the K dimension increases downward. Each cell has an IJK address.
The grid is built in a rectangular space with each cell being connected to its neighbor via the IJK array address. For instance, the upper most cell in the northwest corner has an IJK array address of 1, 1, and 1. The cell to the immediate right is 2, 1, 1. In this fashon all the cells are related to each other in space via this IJK number. All cells that are neighbors are easily identified through their IJK addresses.
Cellular Grid With IJK Array (1,1,1) Beginning in NorthWest Corner.
Once the cell grid is built it may be populated with properties from an EarthVision property model.
What is the Purpose of the Cellular Grid?
The cellular grid is used by the reservoir engineers to simulate the flow of oil through the reservoir. The cells are populated by properties such as porosity, permeability, and oil saturation which control the location and volume of the fluid flow. The location of wells, producers and injectors, is mapped out in the cell grid together with the perforation intervals.
Given this, as well as additional information, the cellular grid is input into a Reservoir Simulation program to measure the flow of fluids within the reservoir to the production wells. The movement of fluid is tracked from one cell to the next. The movement from one cell to the next represents a fixed time period (one day, one week, etc.).
Put rather simply simulation runs are used to predict, among other things, how much oil is present in the reservoir (yes, they do their own volumetrics) and given the flow properties how much oil can be produced in a given period of time. Different simulations may be run to test parameters. These runs are then compared over time to actual performance results in a process called ″history matching.″ A principle goal of simulation is to optimize the production to extract the most oil in a given period of time. This time period may vary depending on reservoir and oil conditions.
What is Required to Create a Cellular Grid?
An EarthVision 3D model is required to build a cellular grid. The sequence file created by the WorkFlow Manager or Geologic Structure Builder is used to identify all the fault, horizon and property grids and their relationships. All these grids must be present for the cell grid to be built.
Tips on Building a Cellular Grid
Before beginning to build geocellular grids it is useful to have some basic understanding about what you are trying to achieve and what should be avoided. Often it is the geologist who will be running the cellular gridder not a reservoir engineer. Therefore, it is important to know what the reservoir engineer requires. Communicate with your reservoir engineers about some of the topics outlined here. What, for instance, are reasonable expectations, and what are desirable characteristics you want to achieve in your cellular grids.
Modeling is as much an art as a science and that is clearly the case with geocellular models. Given the nature of geologic structures and the limitations of geocellular grids it will be difficult (if not altogether impossible) to calculate a model that is ideal in all areas. Also, by definition, such models have to be generalizations since great or increasing detail causes flow calculation time in the simulator (where the grid model will be used) to increase non-linearly. But, while every modeling situation will be different, try to remember the following general rules of thumb. Keeping these in mind will facilitate building a cellular grid that is not only immediately useful in simulation but will be easier to revise and update as new production data becomes available.
The ideal cellular grid contains only cubic cells having the same volume. The more cubic and uniform the cells are (particularly in the I and J directions) the better. This is the single most important factor in cell building. The more cells vary from cubic or square the more volumes may differ resulting in a higher degree of instability in the simulation.
Faults and their orientation are the second most important variable. Cells should ideally be aligned so as to be conformal and perpendicular to fault faces in order to maintain a constant volume. Where faults are not parallel, or where either crossing faults or antithetic faults exist, stair stepping of the grid becomes necessary.
Generally the fewer the number of cells, the better. Simulation calculation times can vary from several hours to one or two days depending on the number of cells involved, the complexity of flow, and the compute power available. Typically, grid sizes vary from 50,000 up to 200,000 cells, with large cell jobs having a million or more cells. More than this has generally been too many. As compute power increases, however, this limitation is shrinking.
Wedge shaped cells are most likely to occur along unit boundaries where zones or units increase or decrease in thickness. These present a problem to simulators as they do not have a thickness at the wedge and fluid cannot flow across or through this wedge boundary and their volume is essentially lost in flow calculation. An option exists to merge wedge cells with other cells thus eliminating lost volumes.
Grid resolution in the K dimension is dependent on reservoir property heterogeneity. Where thin shale lenses or layers occur within a reservoir the K layer thicknesses will need to be relatively small. It is not uncommon for some simulations to be run with K layer thicknesses of two feet.
The pattern of existing wells in a field is also an important variable in determining I, J cell size. A cell containing a well should be surrounded by empty cells.
Where injector wells exist and are a part of field production the cell grid pattern should NOT parallel the pattern of the injector wells. Ideally the cells should be aligned diagonal to the injector and producer well pattern.
There are generally two types of simulation calculations; black oil PVT (pressure, volume, temperature) runs and composition runs. Black oil PVT runs typically use coarser cellular grids and are usually faster to calculate as a result. Composition runs are used where gas injectors exist and are more complex, often use finer grids, and require more calculation time.
Defining a good cellular grid for simulation is an iterative process and is seldom accomplished with the calculation of the initial cellular model. Similarly, there is not likely to be a single ″correct″ solution since a number of compromises must be made during construction that depend on the individual structure and the objectives of the reservoir engineer. Fortunately, the grids calculate very quickly, and there are a limited number of parameter settings. It is therefore possible to explore numerous iterations and evaluate a great number of possible cellular grids in a very short time period.
Remember, the best cell grids have the following features:
- square cells
- consistent volumes
- conform as much as possible (or desired) to the geologic features
- at least one or more cells between wells
- aligned diagonally to injector patterns
