Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Adam T. Bourgoyne


Current well control practice for land or bottom-supported marine rigs usually calls for shutting-in the well when a kick is detected if sufficient casing has been set to keep any flow underground. In addition, the working pressure of the casing and surface equipment is maintained high enough to insure that formation fracture occurs before a failure of these items. Even if high shut-in pressures are seen, an underground blowout is preferred over a surface blowout. On the other hand, an operator will put the well on a diverter if he believes that the casing is not set deep enough to keep the underground flow outside the casing from breaking through the sediments to the surface. Once the flow reaches the surface, craters are sometimes formed which can lead to loss of the rig and associated structures. Cratering also increases the difficulty and time required to kill the blowout. The principal objective of this dissertation is to examine cratering mechanisms with the purpose of better understanding the processes involved. This work reviews various failure mechanisms that can lead to cratering and the shallow sediment conditions which are favorable for each mechanism. The cratering mechanisms discussed include borehole erosion, formation liquefaction, piping, and formation caving. Several mechanisms for upward fluid migration are also discussed. Improved methods to estimate overburden pressure and fracture pressure gradient are also proposed. Several case histories are presented to illustrate some of the more common failure mechanisms and situations that should be avoided through improved well planning. Finally, suggestions and conclusions are presented.