The process of drilling horizontal wells through sand-shale interbedded sequences can present difficulties and risks due to variations in their mechanical properties, leading to unstable boreholes, especially when mud weight is optimized to mitigate against-loss events. This study aims to use a comprehensive approach involving geological, petrophysical, and geomechanical analysis to understand why and where severe borehole instabilities are occurring in a specific field located in the Norwegian North Sea.
The data utilized in this research includes high-resolution ultrasonic amplitude and density images, along with deep resistivity inversions, X-ray diffraction (XRD) analysis performed on drill cuttings, conventional log data acquired while drilling two deviated wells and drilling observations. This comprehensive approach consists of five main steps: 1) analyzing petrophysical unsupervised and supervised rock typing and rock physics from conventional log data; 2) deep resistivity inversion interpretation for identification of sub-seismic faults zones; 3) conducting XRD analysis to differentiate mineralogical compositions of various mudstones; 4) employing image texture analysis to find features related to geomechanics and their progression stages and 5) integrating drilling data from operations.
Severe borehole instabilities were observed within mudstone-dominated intervals, exhibiting various image textures: 1) highly dipping, extensively laminated mudstone intervals; 2) laminated mudstone, crosscut by vertical fractures and faults; 3) laminated and naturally fractured mudstone transitioning to hard stringers with sharp entry contacts; 4) laminated mudstone with abrupt contacts to sand bodies; and 5) dense, laminated mudstone exhibiting natural and shear fractures. Instabilities numbered 1 to 4, identified as washouts and rock failures, occur along weak bedding planes, in zones with vertical fractures and faults (confirmed through deep resistivity inversions), and during challenging drilling conditions (such as varying drilling parameters, bit pressure, and torque) encountered when penetrating hard cemented sands and stringers. Unsupervised classification shows borehole instabilities 1 to 4 were observed within different petrophysical clusters compared to instability- type5. Additionally, XRD analysis showed that borehole instabilities 1 to 4 occurred predominantly within mudstones dominated by high silica content. However, borehole instability 5 was observed within several types of mudstones, which is clearly differentiated in clusters obtained by unsupervised classifications. XRD analysis also showed that borehole instability type 5 was only (or predominantly) observed within mudstone dominated by higher concentrations of illite and smectite. The connection between borehole instabilities with lithological, structural, petrophysical, and mineralogical differences provides valuable insight into the presence of challenging intervals. This research can improve the management of drilling parameters in heterogeneous and stratigraphically and structurally complex sequences. It can additionally contribute to informing the subsurface team of the presence and distribution of these weaker intervals and to define a larger mechanical and lithological classification of heterogeneous sediments.
