Geostatistics: Necessary, but Far from Sufficient

Summary An explanation is given of both where and why there are several major steps in the reservoir characterisation and modelling process in which geostatistics are of little avail and for which other technologies must be used before geostatistics can then be invoked. A workflow is presented which overcomes one of the more intractable problems in reservoir characterisation: that of moving petrophysical properties, including saturation-dependent properties, from a fine scale to a coarser scale in the absence of suitable grids. Without a rigorous solution to this problem, the subsequent use of geostatistical algorithms to distribute what may be poor quality properties data is questionable. The solution, termed the CUSP workflow, uses a unique parametrisation based on Characteristic Length Variables (CLVs) which honour the principles of hydraulic similitude. A Bayesian-based Probabilistic Multivariate Clustering Analysis is used to carry out the Classification and Propagation of petrophysical properties based on the CLVs. The CUSP workflow is scale independent and has been implemented in readily available software. An example of the application of the workflow to move petrophysical properties from the core-plug scale to the wireline log scale is presented and an example for moving from the log scale to the geocell scale is provided.