Challenges of Laminated Shaly Rocks Evaluation and Fit-for-Purpose Core Analysis Workflow

Abstract

Evaluation of thinly laminated shaly sand rocks in the laboratory is challenging due to rock heterogeneity and its complex minerology, for fine scale heterogeneities may result in discrepancies of results obtained from different core analysis methods. In this laboratory study, we address those challenges and present a technique to properly evaluate laminated shaly rock properties for more accurate formation evaluation. Reservoir cores were cut from a US study Beta well. Lithologies are clean and shaly sandstones. Porosity was measured at dry condition with a gas porosimeter. Effects of drying temperature and time on porosity measurements were systematically investigated. The samples were then saturated with brine and porosity was re-measured using a bench top NMR machine. Results of both laboratory measurements were compared and integrated with log interpretations for better understanding of the formation. Data obtained in this study show that the measured gas porosity is sensitive to drying conditions for core samples of laminated shaly sands. Comparing measurements of gas porosimetry and NMR indicate that some clay and capillary bound water are difficult to remove even at temperature as high as 300 Deg C, while the NMR porosity showed consistent trend with total porosity derived from logs. While total porosity from cores across the cleaner sand and moderately laminated zones showed good agreement with total porosity from logs, microfractures which are often invisible did affect the quality of core porosity measurements, thus core-log integration. In summary, although porosity is a fundamental rock property, accuracy and representativeness of porosity measurement may not be trivial, especially for laminated shaly rocks which are geologically more complex, mechanically more sensitive to stress, and thus more difficult to handle and prepare from coring to core analysis. As the most commonly used porosity measurement technique, gas porosimeter may not be the best method for porosity measurement of laminated shaly rocks because of difficulties in sample drying, and also cleaning. In this case, NMR porosity may be a better choice. In addition, because of the stress sensitiveness of laminated core samples, measurements under formation stress will ensure core analysis data more in situ representativeness.