Linking borehole deformation and lithology: a study of intra-unit variation

Understanding and quantifying the mechanisms by which in-situ stresses are characterised is providing new insight into the stress field of the UK. The proposed development of unconventional hydrocarbons in the UK has accelerated the need for better understanding of in-situ stress field and in particular subsurface deformation and rock properties.

Stress related borehole deformation such as borehole breakouts and Drilling Induced Tensile fractures are commonly used to characterise aspects of the stress field. Studies of stress field orientation using borehole imaging in the UK Pennine Coal Measure Group (PCM) highlight lithological controls on breakout formation. A disproportionately high number of breakouts are observed to occur within palaeosols, located immediately below coal seams.

To study the controlling processes of this failure a 10.5 m section of core is investigated from the Melbourne 1 borehole from North Yorkshire, England. The section contains a single breakout, chosen based on borehole imaging. Multiple analytical techniques are used to investigate the interval including: scanning electron and optical microscopy, XRF, X-ray radiography, Point Load testing, non-destructive core scanning and wireline petrophysics.

Detailed sedimentological analysis shows that the breakout within this interval is confined to the upper section only of the palaeosol rather than the entire unit. Point Load Testing supports this and demonstrates that a shift of 0.1 MPa in the tensile strength constrains the borehole breakout.

The more cohesive material where the breakout terminates corresponds to the diagenetic growth of iron minerals. This is also associated with the better preservation of sedimentary structures. This demonstrates that secondary diagenetic processes, including the growth of iron minerals impose lithological controls on the formation and length of borehole breakouts within the UK PCM.