Limitations of Log-Based Wellbore Stability Analysis in an Unconventional Conglomerate-Rich Reservoir in the Southern North Sea


The reservoir section of a recent North Sea discovery consists of both sandstone and conglomerate formations. The conglomerate is comprised of a sandstone matrix with granite clasts of varying size and distribution. The field will be developed with deviated and horizontal wells. One of the risks associated with the development is the stability of inclined wells in the conglomerate. To address this concern data from the vertical exploratory wells was used to conduct a wellbore stability analysis for the deviated wells with rock deformation and rock strength properties calculated from well logs. Later triaxial tests were performed to verify these results. While the log-based strength and deformation values exhibited very good predictability for the sandstones, large variation was found between the values derived from log analysis and the rock mechanical testing for the unconventional conglomerate zones. The reason for this discrepancy was observed in the rock mechanical tests where it was clear that the limiting factor of conglomerate strength was the bonding between the clasts and the matrix. Based on the rock mechanical testing it was determined that the conglomerate had lower strength and the limiting factor of mud weight selection was the conglomerate and not the sandstone as first predicted. This demonstrates how careful consideration must be taken when extending rock mechanical correlations to unconventional rock types.

Meeting Name

46th US Rock Mechanics / Geomechanics Symposium (2012: Jun. 24-27, Chicago, IL)


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Deviated Wells; Exploratory Wells; Inclined Wells; Log Analysis; Mechanical Tests; Mud Weights; North Sea; Rock Deformation; Rock Strength; Rock Types; Southern North Sea; Well Logs; Wellbore Stability Analysis; Deflected Boreholes; Deformation; Geomechanics; Horizontal Wells; Offshore Oil Wells; Oil Field Equipment; Oil Well Drilling; Rock Mechanics; Sandstone; Well Logging

International Standard Book Number (ISBN)


Document Type

Article - Conference proceedings

Document Version


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© 2012 American Rock Mechanics Association, All rights reserved.

Publication Date

01 Jun 2012