During operations on a drilling or producing well, a sequence of events may require tubing, casing, or drill pipe to be run or pulled while annular pressure is contained by blowout preventers; such practice is called “stripping”. Stripping is normally considered an emergency procedure to maintain well control; however, plans for certain drilling, completion, or well work operations may include stripping to eliminate the necessity of loading the well with fluid.
Stripping techniques vary, and the equipment required depends upon the technique employed. Each stripping operation tends to be unique, requiring adaptation to the particular circumstances. Therefore, the equipment and the basic guidelines discussed herein are necessarily general in nature.
Stripping requires surface equipment which simultaneously:
A. permits pipe to be pulled from or run into a well,
B. provides a means of containing and monitoring annular pressure, and
C. permits measured volumes of fluid to be bled from or pumped into the well.
Stripping requires subsurface equipment which simultaneously:
A. prevent pressure entry or flow into the pipe being stripped. This equipment should either be removable or designed so that its presence will not interfere with operations subsequent to stripping. The well site supervisor and crew must have a thorough working knowledge of all well control principles and equipment employed for stripping. Equipment should be rigorously inspected, and, if practicable, operated prior to use.
For stripping operations, the primary surface equipment consists of
- blowout preventers
- closing units
- trip tanks (or other accurate drilling fluid measuring equipment).
Blow Out Preventer
The number, type, and pressure rating of the blowout preventers required for stripping are based on anticipated or known surface pressure, the environment, and degree of protection desired. Often the blowout preventer stack installed for normal drilling is suitable for low pressure stripping if spaced so that tool joints or couplings can be progressively lowered or pulled through the stack, with at least one sealing element closed to contain well pressure.
Wear of the packing element limits the sole use of this preventer if high annular pressure must be contained while stripping. To minimise wear the closing pressure should be reduced as much as possible and the element allowed to expand and contract (breathe) as tool joint pass through.
Lubrication of the pipe with a mixture of oil and graphite or by permitting a small leakage of annular fluid will reduce wear on the packing element. A spare packing element should be at the well site during any stripping operation.
Ram type preventers or combinations of ram and annular preventers are employed when pressure and/or Configuration of the coupling could cause excessive wear if the annular preventer were used alone. Ram preventers must be opened to permit passage of tool joints or couplings. When stripping between preventers, provision should be made for pumping into and releasing fluid from the space between preventers. Pressure across the sealing element should be equalised prior to opening the preventer to reduce wear and to facilitate operation of the preventer. After equalising the pressure and opening the lower preventer a volume of drilling fluid equal to that displaced as the pipe is run into or pulled from the well should be, respectively, bled from or pumped into the space between the preventers.
Chokes are required to control the release of fluid while maintaining the desired annular pressure. Adjustable chokes which permit fast, precise control should be employed.
Parallel chokes which permit isolation and repair of one choke while the other is active are desirable on lengthy stripping operations. Because of the severe service, spare parts or spare chokes should be on location. .
A pump truck or skid mounted pump is normally employed when stripping out.
The relatively small volume of drilling fluid required to replace the capacity and displacement of each stand or joint of pipe may be accurately measured and pumped at a controlled rate with such equipment.
Well fluid from below the preventer should not be used to equalise pressure across the stripping preventer.
A trip tank or other method of accurately measuring the drilling fluid bled off, leaked from, or pumped into the well within an accuracy of one-half barrel is required.
The lowermost ram should not be employed in the stripping operation. This ram should be reserved as a means of shutting in the well if other components of the blowout preventer stack fail. It should not be subjected to the wear and stress of the stripping process.