Pore pressure, fracturing gradient and other formation-determined parameters for well stability are critical inputs to a safe well design, safe execution of drilling and well construction operations, also related to safe plugging and abandonment operations (P&A and PP&A).

Uncertainty and incidents

Despite a high focus on safety, the number of well control incidents on the Norwegian shelf related to uncertainty in the estimation of formation pressure remains high. Extensive studies of causality show this, such as a large qualitative study conducted under the auspices of external specialists in 2022 (Proactima), published as part of the RNNP main report for 2022.

This report on uncertainty in pore pressure estimation and communication, prepared by Geoprovider, was therefore initiated to gain better insight into how uncertainty in processes, methodologies and use of different technologies for pore pressure estimation can be identified and reduced. It also addresses the importance of clear communication of uncertainty in estimates between the various professional communities and operational teams involved in planning and executing phases of drilling and well operations.

Improved methods and technology

While comprehensive industry studies assessing the causality of events may be limited, best practices indicate that uncertainty in pore pressure and stress predictions can be reduced by:

  • Multi-method pore pressure calibration against nearby wells, combined with high-resolution seismic velocity analysis for predictions.
  • Integration of coupled pore pressure, well stability and geomechanical stress-predictions to understand stability issues, losses and well control events.
  • Pore pressure and rock parameters calibration with in-situ and laboratory tests from offset wells and during execution.

Best available techniques (BAT) developed for “high-end” wells like HPHT or extended reach drilling demonstrate that uncertainty can be reduced by:

  • MWD formation pressure measurements.
  • In situ stress calibration, including XLOT and DFIT.
  • P- and S-wave MWD velocity measurements to determine rock physics.
  • Accurate mud-gas systems providing early warning of underbalance.
  • Accurate inflow and outflow control providing early warning of inflow and borehole issues.
  • Managed pressure drilling to safely drill narrow drilling windows.

Communication

To improve communication with the well site the following recommendations can be considered:

  • Pressure and subsurface stress predictions can be presented in measured depth (MD) in drilling documentation to allow all personnel involved in operations to compare real-time information with pre-drill predictions.
  • Best Estimate (BE), Lower (LB) and Upper Bound (UB) of predictions can be presented in line with engineering practices (e.g. DNV-RP-C207), while avoiding probabilistic language (e.g. P95). Probabilistic labelling is often not statistically valid, confusing and poorly understood.
  • A clearly communicated safe drilling window for static and dynamic mud weights for each operational step that can be clearly identified in plots and drilling programs.
  • Established industry system sharing of depletion and injection pressure data across licenses to avoid inflicting major hazards to other teams or operators.
  • Clearly communicated company standards, standardized terminology and plots with clear legends to improve the quality of communication. A traffic light color-scheme is understood by all and examples suggested. Standardization of plots, legends and symbols across the industry would be a significant improvement.
  • A pore pressure and borehole stress checklist maintained by the well delivery team to improve consistency and transparency. A summary of the checklist can be used to communicate the uncertainty to the drilling team and highlight the importance of calibration. The checklist can be used in dedicated risk management sessions. An example is presented in this report.

The discussions and assessments in this report are validated by extensive experience, through referral to a number of references from reputable sources. The intention of the report is to contribute to greater awareness of uncertainty in formation pressure estimation and possible measures to qualify and reduce such uncertainty. This may provide the industry with ideas to further develop methodologies for improved qualification and selection of appropriate drilling parameters, to reduce the risk of well control incidents caused by pore pressure uncertainties and unclear communication of critical parameters.