This project provides a detailed example, based on a field trial, of how to evaluate a field for EOR operations utilizing data typically available in older fields which have undergone primary development. The approach utilizes readily available, affordable PC-based computer software and analytical services. The study illustrates the steps involved in: 1) setting up a relational database to store geologic, well-log, engineering, and production data, 2) calibrating logs to core and developing techniques to compute clay content, effective porosity, and fluid saturations from old electric logs, and 3) linking data and models with PC-based computer software to provide 2-D and 3-D visualizations ofthe reservoir and its attributes. The techniques were demonstrated through a field trial in the Monterey Formation on Pioneer Anticline in the vicinity of Pioneer Field.

In a reservoir study, log data is calibrated to ground truth. The preferred ground truth is whole-core data over as much continuous interval as can be obtained given economic realities. Additional information can be derived from sidewall-core data and well cuttings. For this project, one well from the Pioneer study area has modern whole-core data, several wells have sidewall-core data, and one well has a full suite of well cuttings. Because of limited core coverage of the Monterey reservoir in the Pioneer area, core was also obtained from two wells, with exceptionally complete core coverage, which produce from similar facies in the Monterey Formation in the McKittrick Front area of Cymric Field. The McKittrick Front 418 well has over 700 ft of conventional core that covers the interval from above the top-Miocene unconformity to the middle of the Opal A/Opal CT transition zone in the the Monterey Formation. A complete suite of routine core analyses and many special core analyses are available for this well. Selected core intervals from the McKittrick Front 418 well are on display at this workshop. The McKittrick Front 415 well was drilled into higher-grade diagenetic facies in the Monterey. Core from this well provides good converage of the Opal-CT and Opal-CT/Chert transition zone facies in the Monterey. Routine core analyses for these wells include porosity, fluid saturations, permeability, and grain density on a foot-by-foot basis. Special core analyses include mineralogy, grain size, relative permeability, and capillary pressure. Special thanks go to Unocal for providing our project members access to the McKittrick Front cores and core data.

At the beginning of the log-to-core calibration, core data were plotted alongside raw log traces on a large-scale hard-copy output and were compared with core photographs and with the core itself Core data were also compared with log data using a variety of statistical techniques in order to obtain correct parameters for entry into algorithms for calculating matrix density, porosity, and water saturation. Computed logs were then generated at the same scale and also compared to core data.

A number of analytical models have been presented over the years in the log analysis literature. Some are only appropriate for specific reservoir types, others require that special data be available. Many are general-purpose and are based upon the more common reservoir lithologies and log types. Diatomite is an unusual rock type, and there is little published literature on analytical models for it, so Digital Petrophysics, Inc., used an in-house model to handle diatomaceous facies. There are several important measures of reservoir quality in diatomaceous lithologies besides porosity and saturation. Clay content, diagenetic grade (Opal A, Opal CT, and Chert) and the presence of fractures are important to consider. Many diatomaceous rocks have adequate porosities and oil saturations but insufficient permeability to sustain economic production. The measurement and/or prediction of lithology and fracturing are therefore important components of a reservoir evaluation in diatomites. The models that were selected for this project can be applied to all wells with resistivity and density-neutron logs, and are capable of predicting matrix density, clay volume, diatomite facies, accessory minerals such as carbonates, porosity, permeability, water saturation, as well as providing a qualitative measure of fracturing and fracture potential.

An innovative procedure was developed for using modern log suites, calibrated to core, to evaluate old wells for which only electric logs (E-logs) were available. This is very important on Pioneer Anticline, where many wells were drilled more than 50 years ago and only possess old E-log suites. The procedure was used to calculate parameters such as porosity, fluid saturation, and clay content, which are not directly measured by old E-logs. The evaluation procedure is based on an iterative process in which petrophysical data collected on core material are calibrated to logs in wells with full modern log suites. Porosity, water saturation, clay content, etc., were calculated from the full log suites available in the McKittrick Front 415 and 418 wells in Cymric Field and the Tenneco 62x30 well in Pioneer Field using standard algorithms and then cross-checked against values measured in core. The same parameters were then calculated using only the basic electriclog suite. Special algorithms were developed and adjusted until parameters derived from the core, the full-log suite, and the E-log-only suite all showed reasonable agreement within the logged intervals. When an acceptable level of agreement was achieved, the algorithms were used to compute lithology, % clay, % shale, matrix type, porosity, and Sw from old electric-log data and plot the results in log form. Successful results were achieved for all wells in the Pioneer study area, including wells with full modern log suites, wells with intermediate-aged log suites, and wells drilled in the 1930's and 1940's for which only E-log suites are available. Computed curves were generated for all wells used in the Pioneer project (about 50 wells).