During overbalanced drilling, the main objective is to keep the wellbore pressure above the formation pressure. Once the formation pressure exceeds wellbore pressure, if a permeable and porous formation is encountered, formation fluid will flow into the wellbore. This situation is defined as a kick and when the fluid flow becomes uncontrollable, it is defined as a blowout. Underground blowout is an uncontrolled flow of formation fluids from a subsurface zone into a second subsurface zone typically by using the wellbore as a flow path (API RP 59, Recommended Practice for Well Control Operations, May 2006). Underground blowouts occur approximately 1.5 times as frequently as surface blowouts and can escalate into just as dangerous and costly situations (Schubert et al., 1990.) The underground fluid flow may never be identified and can be indiscernible or it can reach sea floor or ground surface and lead to catastrophic consequences. If it reaches the surface, underground blowout results in loss of equipment wasted rig time, mud losses, sometimes lost holes, sidetracks, lost reserves and sometimes even loss of life. Another problem is pressure charging in shallower sands which then makes drilling of other wells in the area difficult. The volume of influx and its composition are often not known. Because of all these reasons, underground blowouts can be the most dangerous and destructive situation in well control.
It is considered that most underground blowouts occur after the blowout preventers have been closed on a kick during drilling. The stress on the wellbore after blowout preventer is closed, exceeds the stress that the weakest formation in the wellbore can withstand. (Barnhill, Adams, 1979).
The next common case is when kick occurs in a deep zone and only surface casing is set in the well. This case exposes a large section of the well to high stresses that can cause formation fracture.
1. What can cause an underground blowout?
Most underground blowouts occur because of lack of sufficient kick tolerance. Kick tolerance is the kick intensity, amount of underbalance, that can be shut in without exceeding the fracture pressure of the weakest exposed formation after taking a given volume kick. When kick tolerance is exceeded it will result in lost circulation which then can lead to uncontrolled underground fluid flow. One example is lost returns due to drilling with excessive mud weight and surging the hole on a trip in which then fracture formation. This induces an underground flow to the fractured zone. Another case might be lost returns in a depleted reservoir with high-pressure permeability.
Lost circulation and later an underground blowout can be caused by drill pipe wear or defects that damage casing and result in casing holes. Internal tubing corrosion in producing wells can result in failure and sudden imposition of tubing pressure on production casing. Defects or external corrosion of this outer casing can lead to either a subsurface or surface blowout depending on the depth of the flowing zone.
A major cause of surface annular blowouts is gas flow after cementing. Less recognized is that annular bridging or top-out cement jobs can divert gas flow underground. Surveys of many multi-well gas fields indicate some underground flow likely occurred after cementing. Natural formation bridging and scale deposition shut off most of this flow.