Technology Focus: Formation Evaluation (August 2012)

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Technology Focus A little bit of knowledge is not always a dangerous thing. Recently, I was reviewing some simulation models with a client’s subsurface team. They were taken aback when I asked them if they were happy with the petro-physical inputs they had been provided with. “It’s not our job to question the petrophysics,” they replied in unison. However, it was obvious that they were struggling to get the simulation model to match the observed reservoir performance and could do so only by dramatically changing the input rock properties. It transpired that their employer had recently bought the asset and the team had inherited the previous operator’s interpretations, which, “to save time,” had simply been imported into their model. After much cajoling, the team agreed to review the raw core and log data and opened a Pandora’s box. The older wells had poor borehole quality with large washouts, usually in shaly zones, that adversely affected the logs so that they indicated porosity in shales. The net-pay count was, therefore, optimistic. Closer inspection of the permeability/porosity transform revealed that the core data had not been corrected for the Klinkenberg effect or for compaction or even checked against test values. Finally, the computed Corey coefficients for the relative permeability curves looked very low for the oil-wet carbonate they were supposed to characterize, which was because of unstable flooding of the core plugs in the laboratory. With these revelations in mind, the team members decided that the petrophysics for the field had to be reinterpreted, and they collaborated closely with their company petrophysicist during the review. A new simulation model was built that was more coherent, more consistent, and more realistic and that gave much better results than before. There is no harm in developing a petrophysical side to one’s discipline. As I mentioned in my Focus column in August 2009, petrophysics touches all the subsurface disciplines in one way or another. Being able to converse intelligently with log analysts and ask them pertinent questions can only be good news for employers and subsurface project managers. It can also save time that may have been wasted creating simulation models that do not reflect reality. Recommended additional reading at OnePetro: www.onepetro.org. SPE 153537 A Study of Differences in Array-Induction and Multi-Laterolog Responses in a Well Drilled With High-Salinity Water-Based Mud by Bill Corley, Baker Hughes, et al. SPE 153593 A New Method for Estimating Waterflood Oil-Recovery Efficiency Using Post-Waterflood NMR and Dielectric Well Logs, Belridge Field, California by Daniel A. Reed, Aera Energy, et al. SPE 154426 Reducing Reservoir Uncertainties Using Advanced Wireline Formation Testing by D. Loi, Eni, et al.