Application of Geochemistry to Assessing Refrac Jobs in the Bakken/Three Forks Hybrid Play

Objectives/Scope: With maturation of the asset, refrac pilots were conducted to extract oils that were left behind by the previous development schemes in the Bakken/Three Forks hybrid play. Understanding the effectiveness of a refrac job and its impact on offset wells are critical to creating a profitable refrac program. This study shows application of a set of cost-effective techniques integrating oil and water geochemistry to evaluate the newly stimulated rock volume and drainage rock volume change for the refrac wells. Methods/Procedures: ConocoPhillips has developed a time-lapse geochemistry technology calibrated to core-extracted oils to ascertain vertical drainage height and production allocation by stratigraphy. We expanded application of the technology to delineate to what extent a refrac job has stimulated "native" state reservoir rocks and tapped into previously undrained oils along a lateral well. We analyzed produced oil and water that were collected from the flowing wells on the drilling spacing unit (DSU) where the refrac jobs were conducted. The analytical program consists of a suite of GC, GCMS and SARA fractions for oils, and total ion chemistry (cations and anions) and stable isotope geochemistry for the produced water samples. Results/Conclusions: The results from two refrac pilot projects confirm that early wells with open-hole-sliding-sleeves completion did not stimulate and deplete the target reservoir to their full potential. A mechanical isolation refrac job could create access to the previously undrained reservoir rocks and recover oil that has been left behind by primary production. Some of the new fractures can reach the offset wells which may uplift their water and/or oil production. A refrac well produces heavier oil compared to that right prior to the refrac job, and to its offset wells. We interpret this to imply that refracs do in fact access previously undrained reservoir rocks. Salinity of water produced from a refrac well is fresher than its pre-refrac value, but saltier than its offset wells. The degree of deflection in salinity pre- and post-refrac from produced water correlates with oil gravity and is used as a metric to gauge the relative contribution from the undrained reservoir rocks. Significance/Novelty: We clearly demonstrate that integration of water and oil geochemistry can provide comprehensive insight into the efficacy of refrac jobs. It will spark research interest in water chemistry and expand its application in the development of unconventional reservoirs. With more and more refrac and infill wells, application of water and oil chemistry will provide a cost-effective way to evaluate differences between refrac/infill designs, infer communication between wells, and ultimately optimize the development plan.