Wellbore stability

Wellbore stability still remains one of the major causes of non-productive time (NPT) in drilling wells and may range from minor inconvenience to the complete abandonment of the well. Resulting issues may include, stuck pipe, increased torque and drag, loss of wireline and LWD data quality, cementing and casing difficulty, shale instability, stuck wireline and wellbore collapse. Wellbore stability and the management of these issues come down to the relationship between the borehole environment and the geomechanical environment.

Collapse pressure is represented as the minimum mud-weight required to prevent borehole breakout for a specific allowable breakout width by comparing the maximum circumferential stress with the strength of the wellbore wall. These wellbore stability analysis requires a geomechanical model constructed from one or more offset well geomechanical models. Stress magnitudes are calculated using the calibrated parameters from the offset wells for required wellbore trajectories. These models can then in turn be used to generate collapse pressures for breakout initiation and/or specific allowable breakout widths and fracture gradient to define safe mud-weight windows.  

Additionally, uncertainty analysis of the wellbore stability assessment using Monte Carlo Simulation can also be undertaken. We define probability distribution functions for the input parameters and generate response surfaces for the critical mud-weights from stochastic outputs. A Monte Carlo analysis with appropriate correlation coefficients can then generate cumulative probabilistic functions for failure modes and likelihood of success as a function of mud-weight.  Appropriate correlation coefficients would be constrained and P10, P90, P50 mud-weight window constraints generated.

Example - Zagros Fold Belt. Shear failure and collapse gradient stereo-nets for depth 4070 m using the Expected PP case