“Hope is not a strategy, particularly in this post-Macondo world,” John Wright, president and blowout engineering advisor for John Wright Company, said at the Offshore Technology Conference in Houston on 2 May, kicking off his presentation on the drilling of relief wells and how to deal with uncertainty. He believes that one of the biggest gaps in companies’ blowout contingency plans actually lies with the transfer of mud to high-pressure pumps.
After the Macondo blowout, the BJ Services Blue Dolphin ultra-deepwater stimulation vessel was used to aid the drilling of relief wells. The vessel features 23,000 hydraulic horsepower and 20,000-psi pumps, and its umbilical lines have a maximum pump rate of 80 bbl/min. “We were very fortunate to have such a vessel in the Gulf of Mexico … but in other places of the world, you may not have this,” Mr Wright said.
When performing hydraulic modeling for contingency plans, it’s one thing to determine that you need 15,000 hydraulic horsepower and 10,000 bbl of mud for the kill operation. “But where are you going to store that mud? Where will the vessel come from?” he asked. Other questions to consider include: How are you going to transfer it from the mud supply vessel to the frac boat, if you plan on using frac boats? Do frac boats in your region have both the capacity for flow rate as well as the hydraulic horsepower for pressure?
“You might be able to store 10,000 bbl of mud in the below-deck tanks, but you’re not going to be able to transfer that mud up to those high-pressure tanks at any kind of rate,” Mr Wright said. “Even the large fifth-generation floaters that have 10,000-bbl mud capacities, in a lot of cases I’ve found that you just cannot transfer mud from those pits at high enough flow rates even though the mud pumps might have the hydraulic horsepower capacity to pump it.”
Mr Wright also stressed the importance of doing hydraulic simulations for flow diagnostics and hydraulic kill design. “In my opinion, this is the most important thing that you will do in any blowout scenario, the contingency planning scenario. It drives all control strategies,” he said. For example, hydraulic simulations define what the flow problem is – what are the boundary conditions. “Particularly in a relief well scenario, you really don’t know what the flow patterns are, so you’ve got to go through and look at various potential situations and model those situations and see if that looks anything like what you’re observing.” After ranking those scenarios, the kill is then planned around the most likely scenario.
Hydraulic simulations also define the best option for the relief well – whether you can bullhead and stay under certain pressures, or if you can snub the pipe to bottom. It also defines requirements for support equipment, rig equipment and kill equipment. “What kind of kill fluids should you have? You cannot kill with fluids that are higher than the fracture gradient. If you’ve got a good dynamic simulator, you can assess those various options,” Mr Wright said.