Abstract

The North Robertson Unit is a mature Permian dolostone reservoir with 259 wells penetrating 1800 to 2000 ft of gross interval. It is the subject of an integrated geological-engineering study funded by the United States Department of Energy as part of the Class II Shallow Shelf Carbonate program to improve geologically targeted infill drilling techniques and to optimize EOR techniques. The development of a geological reservoir model is a critical aspect of this investigation, because the model forms the fundamental framework for ongoing reservoir simulation and reservoir management studies.

The reservoir is characterized by a high degree of vertical layering and areal compartmentalization - a function of depositional environment and diagenesis. The reservoir interval is divided into 8 Rock Types, based on petrographic image analysis of core samples. Each Rock Type is characterized by a unique pore geometry that acts as a fundamental control on permeability, capillarity and relative permeability. Since only 8 wells are cored, it has been necessary to develop a field-wide log model based on integration of geological and petrophysical data. Rock Types have been related to log response, and a quantitative model has been developed that allows for the log recognition of each Rock Type in cored and non-cored wells. This rock-based log model provides the basic tool to predict the 3-dimensional distribution of flow units across the North Robertson Unit. Twenty four flow units have been identified and mapped. This has allowed for a detailed understanding of vertical and lateral connectivity, aided in distinction of areas of the field with the best infill drilling potential, and has improved the accuracy of predicted production performance.