Published numerical and analytical solutions for wellbore stability predict certain trends in the behavior of hole geometry as a function of time. In general, shear failure is expected to increase in severity with time to produce over-gauged hole sections. However, these solutions do not take into account many aspects of the disturbances produced throughout the drilling process. An investigation of the behavior of hole geometry of a diverse group of wells is conducted to uncover the different relationships between the time-dependent wellbore stability and the related operational practices. The highlighted relationships are then reconciled with published wellbore stability solutions.
The disturbances produced throughout the drilling process can include heat transfer between the drilling fluid and the wellbore wall, chemical interactions with the drilling fluid, and mechanical forces working on the wellbore wall. A diverse group of wells in terms of drilling fluid used, drilling practices employed, lithology of formations drilled, and time of exposure experienced is used to highlight the relationships between these disturbances and the time-dependent stability. To aid in illustrating these relationships, the mechanical properties and the stresses are estimated from open-hole wireline logging data.
The relationship between time and wellbore stability was observed to reverse from the norm in some cases. It is believed that the anticipated charging of the local pore pressure due to the hydrostatic head overbalance and the heat transfer between the drilling fluid and the formation both combined with different operational practices and drilling events to produce this reversal in the relationship. The drilling events considered include long exposure times due to logging runs and troubles, well control events, lost circulation events, and reaming operations. The observed relationship is then compared with published time-dependent wellbore stability solutions where a form of reconciliation is produced.
The presented change in the time-dependency relationships can open new doors for a more sophisticated computational modeling of wellbore stability.