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Received: April 29, 2017 Revised: July 01, 2017 Accepted: August 30, 2017 Abstract: Introduction: Unloading phenomenon happens in the beginning of drilling and is able to change stress state around borehole. This change of stress state causes impact on rock structure and strength, thus affecting the evaluation of wellbore stability. Especially for determining initial mud pressure, unloading is a significant influence factor. Clay-rich shale formation is well-known for high risk of borehole collapsing, appropriate mud pressure is necessary to stabilize wellbore. Therefore, the unloading influence needs to be considered when it comes to selection of initial mud pressure.

The Research on Influence of Unloading on Borehole Stability in Clay-Rich Shale Formation