By Larry H. Flak, PE
Boots & Coots International Well Control,
Positive Impact of Stripping
In deepwater, the differential between the hydrostatic pressure developed within the riser mud column and the surrounding seawater is called the "riser margin." The vertical length of this mud column runs from the mud flow line located below the rig floor of a floating rig (above sea level) down to the LMRP (lower marine riser package) split located below the upper annular BOP (blowout preventer). The LMRP split is used as the end of the riser since this will be the likely place where the riser would be intentionally disconnected in an emergency. After the riser is disconnected, only seawater hydrostatic is applied.
The riser margin hydrostatic is also a direct function of the fluid density within the riser. Fluid density within the riser can be modified independently from the well bore fluid density via the boost line or by drill cuttings hold-up within the riser (cuttings density).
With loss in riser margin, total reduced applied bottomhole hydrostatic pressure is reduced by the magnitude of the riser margin. Seawater hydrostatic remains after riser mud column hydrostatic is lost.
Loss of Riser Margin (back to top)
Loss of riser margin results in a dramatic loss in bottomhole applied hydrostatic pressure. This loss presents a major well control problem. The deeper the water the greater the loss. Figure 1 shows the % impact of riser margin on total applied hydrostatic pressure on a formation located 6000-ft below the mud line with a 15 ppg pore pressure.
Riser Margin Loss
In deepwater, loss of riser margin likely results in a severe kick and hole collapse. In the above example, pressure under the BOPs in 9000 ft of water could be ~3000 psi after riser disconnect. If kick was to migrate after loss of riser margin, much higher pressures could be seen.
Alternately, riser can be a source of additional hydrostatic. High penetration rates with large bits in synthetic muds are possible in deepwater. If the cuttings are allowed to build-up in the riser from poor hole cleaning, a significant cutting density can develop. This author has seen >0.5 ppg equivalent in one severe case
Failure Mechanisms (back to top)
The following circumstances cause riser margin losses:
- Severe weather conditions
- Excessive current loads
- Failure in dynamic positioning systems
- Anchor systems failure
- Excessive flex joint angle and internal keyseat wear
- Failure of LMRP slip
- Planned events but well not secured as thought (ie. bridge plug failure, liner top failure, casing failure)
- Broached flow at mud line undermining conductor casings, wellhead, and BOPs causing riser failure
- Riser collapse/burst
- Riser connector failure.
Possible Hole Response
If drill pipe was hung off and then sheared prior to emergency riser disconnect. The following circumstances can be expected in the Gulf of Mexico.
- Small kick flowed into well bore (kick volume limited by compressibility of mud and hole ballooning unless leak develops or openhole breaks down)
- Hole likely collapsed around drill string (in GOM deepwater)
- Well likely bridged from hole collapse (in GOM deepwater)
- Trapped pressure under BOPs may bleed-off if well bridges
These circumstances do not seem too severe but major problems have resulted when the following are present.
- Pay sand is present just at end of last casing string (ex: horizontal well)
- Kick migrates during sustained shut-in period
If pay sand is present just at casing shoe then well bridging (hole collapse) is less likely. Kick pressure cannot be bleed-off without inducing more kick volume. Alternately bullheading as a control option is possible.
If kick migrates through mud unexpanded, gas pressure will fracture out mud. Total wellbore evacuation to dry gas has been seen from this. There is no way to volumetrically control migrating kick if riser is disconnected.
Cutting density in riser can lead to problems with establishing accurate shut-in pressures. With no kick in the well significant differential pressure into the drill pipe is seen. With a known density of fluid in the booster line, this density in the riser can be measured.
The major problem with riser margin is the fact that hydrostatic pressure is increasing from above seal level but fracture pressure is increasing from the mudline. This one factor is driving the development of "riserless drilling". Fewer casing strings would be needed if hydrostatic pressure and fracture pressure development started at the same depth (as on land).
A kick from riser margin loss can result in an underground blowout if upper zone breaks down and cross-flow begins. Mud in casing will be displaced by lower density blowout fluids and pressure under BOPs will increase. If openhole bridges during this cross-flow period, drill pipe can kick as BHP increases to shut-in conditions.
Broached flow in relatively shallow water is seen below.
The West Vangard Blowout (Northern North Sea 1986)
The worse case scenario seen by this author off of floaters was caused by the following circumstances:
- Drill pipe hung off and sheared and blind ram closed above sheared pipe.
- Emergency riser disconnect
- Well kicked
- Underground flow started
- Hole collapsed
- Drill pipe kicked (no check valve in DP)
- Drill pipe pressure reflects back on top of annulus through hang-off pipe ram (rams do not hold pressure from the top)
- Casing ruptured (shut-in well pressure against annulus mud column with bridged off open hole)
- Underground blowout re-starts.
Positive Impact of Stripping (back to top)
It is much easier to strip through an annular BOP if differential pressure is limited. In deepwater riser mud density can be increase via the booster line to overcome the well pressure under the annular BOP. It is much easier to strip if riser mud is leaking past the annular into wellbore. On risk of getting gas in riser when tool joints are popped through the BOP. The mass of drill pipe within the riser also helps to provide snub force.
- Loss of riser margin presents significant well control challenges.
- Use of drill string float (check valve) is strongly recommended.
- If possible, pull bit into casing prior to riser disconnect.
- Stripping is assisted by using an over-balanced riser margin and with the greater drill string mass above BOP providing added snub force.
- Riserless drilling is in development to eliminate many of the problems associated with riser margin.
About the author
Larry H. Flak is a petroleum engineer and Vice President Engineering of Houston's renowned Boots & Coots International Well Control. Mr. Flak recently worked with many other experts in developing the new IADC Deepwater Well Control Guideline. (back to top)
For more information: Larry Flak, Boots & Coots/IWC. 11615 North Houston Rosslyn Rd. Houston, Texas 77086. Tel: 281-931-8884, fax: 281-931-8302. Email: Lflak@bncg.com.