Wellbore Stability Challenges in Fractured Carbonates: Analyzing Stresses, Planes of Weaknesses, and Thermal Effects

Drilling through fractured carbonate formations often presents significant geomechanical challenges, including borehole collapse, cavings, tight spots, stuck pipe and lost circulation issues. These instabilities arise due to complex interactions between in-situ stresses, natural fractures, and thermal effects, requiring a robust geomechanical model for risk mitigation.

The LTG-01 well, drilled in the Dutch subsurface, encountered severe wellbore instability issues while drilling the 8.5-inch section through the Dinantian carbonates. The well exhibited tight hole conditions, stuck pipe events, severe mud losses, wellbore breathing, along with washouts identified from caliper logs. Borehole resistivity imaging further revealed borehole breakouts and drilling-induced tensile fractures (DIFs) within the carbonate interval. To investigate these geomechanical challenges, a 1D Mechanical Earth Model (MEM) was constructed using well log data available from the NLOG public database.

Elastic properties such as Young’s modulus and Poisson’s ratio, along with strength parameters including Unconfined Compressive Strength (UCS), tensile strength, friction angle, and cohesion, were computed using empirical correlations. Pore pressure was estimated using Eaton’s method for the clastic overburden and a gradient-based approach in the carbonate section. Vertical stress was computed via density log integration, while horizontal stresses were derived from the poroelastic horizontal strain equation, constrained by LOT and FIT data.

A key finding was that the drilling-induced fractures in the carbonate interval could be linked to thermal stress effects, caused by the temperature contrast between the borehole fluid and formation temperatures which were in order of ~160-190°C. The breakdown gradient computed from the MEM approached the equivalent circulating density (ECD) in zones where DIFs were observed, suggesting that thermal stress significantly reduced the rock’s tensile strength, leading to DIF formation. Additionally, borehole washouts observed in calipers, along with vuggy and brecciated intervals, highlighted the presence of mechanically weak zones.

Furthermore, borehole breakouts appear to correlate more strongly with plane of weakness (PoW) shear failure rather than intact rock failure. In addition to Mohr-Coulomb intact rock failure, alternative shear failure mechanisms were assessed—one using bedding planes as failure surfaces, and another considering natural conductive fractures as weakness planes. A notable correlation between PoW shear failure gradients and breakout intervals suggests that pre-existing weak planes significantly influenced wellbore instability.

Despite these insights, some uncertainties remain, particularly regarding fracture connectivity and fluid interaction effects, which merit further investigation. Nonetheless, this study provides critical geomechanical insights for future drilling in fractured carbonate formations, emphasizing the need for thermal stress considerations and plane of weakness analysis in wellbore stability assessments.