Pressure Control

•Despite efforts to understand and control formation pressures, blowouts still occur.
•A blowout is an uncontrolled flow of formation fluids as the result of failure to control subsurface pressures.
•Blowouts can occur at the surface or into an underground formation.
•Nearly every well drilled has the potential to blow out.
•Experience has shown that blowouts occur as the result of human error and/or mechanical failures.
•However, a carefully planned, continuously supervised pressure-control program will lessen the possibility of a blowout considerably.
•It is important to identify high formation pressures before drilling, to detect pressure changes while drilling, and to control them safely during drilling and completion operations.
•Pressure control can be divided into three categories:
•Primary control.
–The proper use of hydrostatic pressure to overbalance the formation and prevent unwanted formation fluids from entering the wellbore.
–The advantages of control at this level are self-evident.
•Secondary control.
–The use of equipment to control the well in the event primary control is lost.
–Formation fluids that have entered the annulus can cause a blowout quickly if not properly controlled.
•Tertiary control.
–The use of equipment and hydrostatic pressure to regain control once a blowout has occurred.
–This could involve the drilling of a relief well.
–Although tertiary control is normally handled by experts, many things can be done during the planning and drilling of a relief well to simplify the final kill procedure and regain control of the well.
A kick is an influx of formation fluid into the well.
A blowout is an uncontrolled kick.
•Failure of primary control.
–Any event or chain of events that create a negative differential pressure between the hydrostatic pressure of the drilling fluid and the formation pressure can cause a “kick.”
•The most common causes of a kick are:
–Failure to keep the hole full of mud during trips.
–Insufficient mud weight.
–Lost circulation causing the hydrostatic pressure to be reduced.
–Swabbing in when pulling out of the hole.
–Improper casing design and pore pressure prediction.
•Failure of secondary control.
–It has been estimated that 95% of the wells in which secondary control is lost arrive at that condition as the result of either poor maintenance and inadequate testing programs, which result in leaks that erode pressure-control equipment, or inadequate crew training, which results in miss-use or no use at all of pressure control equipment.