Underground Blowouts In Deep Well Drilling
Authors: Barnhill C.C. , Adams N.J. 
Published: December 2007

ABSTRACT
A kick occurs when the wellbore pressure becomes less than the formation pressure, and formation fluid enters the wellbore. When the flow of formation fluid becomes uncontrollable it is classified as a blowout. If the uncontrolled flow of formation fluid is into a formation exposed to the wellbore, it is termed an underground blowout.
Any underground blowout is a unique situation which has inherent dangers. An everpresent danger is the possibility of the underground blowout turning into a surface blowout. This is especially true in shallow cases where the flow of formation fluid could channel up around the outside of the casing to the surface. The problem of pressure charging shallower sands is also present. This phenomena makes drilling of other wells in the area difficult.
Once an underground blowout has occurred, there may be only a partial loss of pressure at the surface which makes it difficult to determine if indeed an underground blowout is in progress. There are several surface indicators that can be observed that will aid in determining if an underground blowout is occurring.
Once it is established that an underground blowout is in progress, conventional well control alone will not suffice. There are several remedial control procedures that can be implemented. There include techniques for killing the well and techniques for plugging the thief zone. To increase the chances of these procedures working, the location of the thief zone should be known. There are several zone detection methods that are available for this purpose. Each underground blowout is a unique situation that must be studied and the action best suited for that situation must be undertaken.

INTRODUCTION
During a normal drilling operation, the wellbore pressure is held slightly above the formation pressure. As long as this condition is maintained, formation fluids cannot enter the wellbore. Once the formation pressure becomes greater than the wellbore pressure and a permeable, porous formation is encountered, formation fluid will enter the wellbore. When this situation exists it is defined as a kick. If the flow of formation fluid into the wellbore becomes uncontrollable, it is defined as a blowout. If the uncontrolled flow of formation fluid is into a formation that has been fractured in the wellbore it is termed ar underground blowout.

FLOW DIRECTIONS IN UNDERGROUND BLOWOUTS
The conditions which cause the underground transfer of fluid from one zone to another will often indicate the direction of fluid flow. The direction of the fluid flow in a blowout is an important concern when choosing a control procedure. If lost circulation is encountered at or near the bottom of the well, the fluid level in the well may fall and decrease the hydrostatic pressure sufficiently to allow an upper zone to flow. This flow can be directed from the shallower kicking zone to the lower fractured zone. See Figure 1. There may also be an upward migration of the kicking fluid associated with this situation.
If lost circulation is encountered in the upper portion of the wellbore, the fluid level in the well may fall and decrease the hydrostatic pressure sufficiently to allow a lower zone to flow. This flow can be directed from the deeper kicking zone to the shallower fractured zone. See Figure 2.
Most underground blowouts are thought to occur after the blowout preventers have been closed on a kick while drilling. The stress exerted on the wellbore associated with the kick situation exceeds the stress that the weakest formation in the wellbore can withstand. When this situation occurs, the fluid flow is from the kicking zone, usually at the bit, to the higher fractured formation. This is based on the assumption that (1) shallow zones will fracture prior to lower zones and (2) the initial kicking zone will be the primary source of fluid flow. See Figure 2. The most common case in which underground blowouts occur after the blowout preventers have been closed, is that of a kick occurring in a deep zone when only surface casing is set in the well. This situation exposes a large section of the hole to high stresses which results in formation fracture.1