Mitigating drilling hazard's balancing drilling risks against the optimum well design while preparing for unplanned drilling event's has been a challenge to cost-effective well construction for decades. The pore pressure/formation fracture gradient balancing challenges mixed with the unexpected encounters with shallow flows, unstable formations, overpressure formations and depleted formations makes AFE (authorization for expenditures) goals dim.
Excessive use of loss circulation pills and traditional contingency liners drive well costs up and jeopardize reaching total depth (TD) with an effective completion. A drilling hazard remediation solution could be as simple as using that planned contingency liner or using drill-in casing to fight sloughing formations. However, the use of conventional solidexpandable drilling liners can drive excessive risks into the ell and even cause a costly sidetracking of the well.
Operators in the Gulf of Mexico, the North Sea and in Asia Pacific have been successfully using a set of proven well construction tools with a more ‘Fit for Purpose’ application to mitigate drilling problems that have resulted in excessive nonproductive time (NPT) during drilling operations. These systems are used only when necessary to mitigate the well challenge, allowing the well construction to continue while minimizing their NPT fighting these well problems.
This paper describes some of these ‘fit for problem’ well construction tools and their applications in recent case histories.
Drilling trouble zones typically consume 10 to 25% of a well's AFE. Lost circulation zones, unstable formations, and pressure transitions events, to name a few, have typified the type of drilling challenges costing operators millions of dollars annually.
In technical drilling conferences dedicated exclusively to trouble zones, operators have reported on case histories that have shown the magnitude of these well construction costs1. Figure 1 illustrates problem incidents documented in a sampling of over 1,700 gas wells, drilled between 1993 and 2002 on the shelf in the Gulf of Mexico, representing approximately 24% of these well's NPT. The Dodson chart in Figure 2 is a detailed representation of the data that documents the trouble incidents of these shelf gas wells in the Gulf of Mexico. While half of these ‘incidents’ are not related to drilling trouble zones (i.e. weather, rig problems, etc.), about 12% of the total trouble time encountered was due to drilling trouble zones. The cost of these trouble zones to the operators drilling these wells was over 2.8 billion dollars in well construction costs.
While this data represents wells exclusively in the Gulf of Mexico, further research facilitated through interviews with operators on a global basis have resulted in 10 to 35% of well construction spent for drilling trouble zones.
Simplifying the causes of these drilling hazards reveals that most of these fall into the following categories:
Wellbore Instability - 18% of the time spent addressing trouble zones
Low Pressure Events - 22% of the time spent addressing trouble zones
High Pressure Events - 12% of the time spent addressing trouble zones