Geologic models have been generated across extensive University Lands acreage in the southern Midland Basin as input for reservoir simulation studies to optimize Wolfcamp development. The acreage covers about 32 blocks in Upton, Reagan, Crockett, Irion, and Schleicher counties, and crosses the southern end of the Midland Basin from the Central Basin Platform to the Eastern Shelf. Geologic models that contain enough information to characterize this region are challenging to construct, but can be justified if they reduce uncertainty and allow greater attention to be placed on simulation studies.
The distribution of individual geologic facies may extend over large distances, the distribution of petrophysical data is often irregular, and the distribution of geomechanical reservoir data is often very limited. Consequently, geologic models that cover a large area are constructed to distribute the geologic facies and to capture sufficient reservoir data to generate a suitable population size. Vertical layers must be thin enough to capture key geologic facies. The areal grid requires sufficient cells between wells to capture lateral variations.
With 8 horizontal wells typically drilled across a square mile, the ideal, areal cell size would be about 200 square feet. To capture the geologic facies vertical cells would need to be at least 6 inches thick, as limited by well log vertical resolution. However, if the area of interest is hundreds of square miles and the Wolfcamp is over 1000 feet thick, the resultant model could exceed several billion cells. Computation time for such a huge model will be slow. Thus, the evaluation interval is reduced, the lateral grid size is increased, and the layer thicknesses are increased to reduce cellular volume, while maintaining a statistically useful reservoir data population. Although hardware limitations of stand-alone workstations are being challenged, a total grid size of 800 million to 900 million cells is possible.
The reservoir simulation sector model to be used for production history matching may only cover a single well path, typically about two square miles, but the geologic model used to derive the sector model may cover several hundred square miles in size. Derivation of the geologic model assumes a good understanding of the geologic facies that are being distributed, and sufficient well data to populate the model with a statistically meaningful population size. If the geologic model can be sufficiently tied to regional outcrop studies, core, well data and seismic data, our confidence in the geologic model should be high. This would then allow more attention to be given to such variables as effective drainage volume and the hydraulic fracture model.