Handling Wellbore Instability in Overburden Tertiary Shales

From November 2015 to April 2017 nine infill wells were drilled in the Danish Tyra South East field. The geomechanical challenges experienced in the Tertiary shales while delivering these wells were much larger than anticipated. Limited reservoir compaction had induced stress changes in the overburden, not fully captured in the pre-drill models, which caused the delivery to be more challenging and the requirement of more comprehensive models apparent. A more detailed 3D geomechanical model was developed and associated to new 4D seismic observations after drilling more than half of the wells in the campaign. Also, real-time geomechanical evaluations and near wellbore 3D models were coupled to the 3D geomechanical model to further investigate the stress changes in the overburden. Two main failure mechanisms were identified to have contributed to the instability experienced; shear failure and bedding plane instability. The investigations and new modelling results showed that the minor stress changes in the overburden caused the MW strategy used to be connected to both shear failure and bedding plane instability. Minor changes to the MW strategy and better control of the mud pressure invasion into the shale would improve stability and the delivery of the sections through the Tertiary shales.