Recommended Practices for Core Analysis is firmly established in the American Petroleum Institute’s RP 40 Second Edition,February 1998 document. Standardized workflows for the core analysis of unconventional shale reservoirs are deeply rooted in analytical techniques that were originally developed for coal bed methane,shale gas and tight gas reservoirs. These techniques were researched and developed by the Gas Research Institute(GRI)and are outlined in the GRI final report: GRI-95/0496,”Development of Laboratory and Petrophysical Techniques for Evaluating Shale Reservoirs.”These methodologies were extremely successful in evaluating cores from gas shale reservoirs. As operators shifted their development focus from gas shale reservoirs to unconventional oil producing reservoirs,these same gas core analytical techniques were used to analyze unconventional tight oil cores. This paper will discuss the results of tests and experiments that were made on core from the Bakken petroleum system’s Middle Bakken and lower Three Forks formations. These series of tests were made following a very poor core-to-log data water saturation comparison. The core analysis measured 40% to 60% water saturation,and the logs showed the reservoir to be 100% water saturated. This investigation started by looking closely at the solvent extraction methods with Dean and Stark [3] using toluene followed by chloroform/methanol azeotrope extraction. Preliminary tests focused on the possibility of anhydrite dissolution,which would have created excess porosity,thus making the core analysis incorrect. Additional investigation of the problem included measurements with nuclear magnetic resonance(NMR),retort extraction methods and possibilities of errors in the computations due to extremely high water salinities. While this investigation has resulted in applying some new protocols in the analysis of tight oil cores,it has also demonstrated that more research is needed and new standard core analysis workflows need to be developed for unconventional tight oil reservoirs.
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