Abstract

Tight gas reservoirs are often characterized by complex geological and petrophysical systems as well as heterogeneities at all scales. In addition, the operating environment is very challenging and that affects the decisions for data acquisition. The use of salt-saturated or oil based mud systems creates a contrast and uncertainty in the data. Hence, the quality of static log and core data acquired is compromised.

In May of 2008 Shell's Global Wells and Exploration Management made a strategic commitment for the use of underbalanced drilling (UBD) for reservoir characterization on all tight gas exploration and appraisal wells. UBD is unique in that, when properly applied, it completely eliminates any uncertainty of drilling or completion fluid damage from the dynamic gas mobility understanding. Specifically the technology has been used in the exploratory and appraisal campaigns to obtain the direct benefits of characterizing the reservoir while drilling, obtain the real net productive pay zones of the reservoirs, establish whether open fractures were crossed, determining reservoir volumes connectivity and obtaining other important reservoir properties in real time, which helped in calibrating the reservoir models. Gas flowing during UBD has been used to optimize the testing, logging and completion plans that were conducted after drilling each of the exploratory wells.

Since 2008 Shell has successfully deployed UBD for tight gas exploration and appraisal on 10 projects in North America, North Africa, the Middle East and Asia. This paper will evaluate the strategic learning's from this deployment of UBD. Evaluation scope will include HSE, project delivery issues, reservoir learning's and changes to sub-surface team's work flows to exploit the information. Selected project results will be presented to support the evaluation conclusions. Discussions will include a view to the future including how Shell is changing overall tight gas project development with UBD de-risking.

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