Monitoring shear deformations above compacting high-pressure high-temperature reservoirs with calliper surveys

• Localized shear deformations resulting from reservoir compaction are characterized.
• Calliper survey allows quantifying slip displacement along reactivated interfaces.
• Shear deformations are distributed along various geological discontinuities.
• Calliper surveys are an efficient tool for monitoring geomechanical changes.

Approximately thirty Multi-Finger Calliper (MFC) surveys have been acquired in Elgin and Franklin development wells between 2003 and 2013. These surveys reveal around one hundred shear deformation features characterized by a kink in the pipe. The calliper surveys were depth shifted against the open-hole reference depth used in Geoscience to accurately identify the interfaces along which the deformations occur. The deformations have been found to be distributed at the top of the Fulmar Formation reservoir, in the Late Jurassic shales, in the Early Cretaceous Cromer Knoll Group, and the Late Cretaceous Chalk Group. For each deformation feature, the relative displacement of the well was estimated using the calliper tool eccentricity resulting from the movement of the toolstring through the kink in the pipe. Maximum relative displacements were estimated at 3.7 in. in the 7 in. liner and at 1.2 in. in the 9 7/8 in. production casing. The analysis of the calliper surveys show that the deformation features result from slip displacements along various discontinuities such as bedding planes, reservoir faults extending in the overburden, and overburden faults. It is interpreted that these interfaces have been reactivated because of the strong reservoir depletion. This work clearly demonstrates that MFC surveys are an effective tool for monitoring shear deformations occurring above a compacting reservoir.