This article elaborates the steps that a driller can take to keep oil and gas secure in a deep seawell. The first step is to use drilling fluid to create hydrostatic pressure in the wellbore to prevent oil and gas from surging up the well. Later, when the well is completed, the wellbore is usually filled with a completion fluid also designed to have a density sufficient to prevent escape of oil and gas from the rock formation. Drilling and completion fluids must be designed in such a way that they do not create excessive hydrostatic pressure. Too much pressure will incur waste because large volumes of the fluids will leak into the rock formations penetrated by the wellbore, where they cannot be retrieved. The next step of preparation is to pump a cement slurry containing various additives through the casing. Once hardened, the cement seals off the oil- and gas-bearing rock from the wellbore until the oil company is ready to produce the well.
The sinking of the Deepwater Horizon offshore drilling platform that took eleven lives and spread a massive oil spill in the Gulf of Mexico has focused attention on safety in offshore oil and gas well drilling. Just what can cause such a massive blowout? What steps do drillers take to prevent blowouts? How must they be adjusted to be effective for deep ocean drilling? An increasing number of wells are being drilled under ocean depths of 3,000 feet or more. Water temperature at these depths in the Gulf of Mexico is about 40 ˚F. Underwater equipment must withstand pressure of more than 2,300 pounds per square inch.
Blowouts are created by natural gas and crude oil under very high pressure in rock formations. Once a reservoir has been penetrated by a drill bit, oil companies have to counteract this high pressure to prevent a blowout—a surge of oil and gas up the wellbore and into the environment. They take several steps to keep the hydrocarbons under control.
Fluid After Fluid
The first step is to use drilling fluid to create hydrostatic pressure in the wellbore to prevent oil and gas from surging up the well. Later, when the well is completed, the wellbore is usually filled with a completion fluid also designed to have a density sufficient to prevent escape of oil and gas from the rock formation.
Drilling and completion fluids are expensive and are retrieved from the wellbore and taken onshore for processing and reuse. They must be designed so they do not create excessive hydrostatic pressure. Too much pressure will incur waste because large volumes of the fluids will leak into the rock formations penetrated by the wellbore, where they cannot be retrieved.
Drilling fluid, known as mud, commonly contains additives such as barite (barium sulfate) designed to control the high hydrocarbon pressures of the rock formation and prevent excessive aqueous fluid loss from the wellbore.
The actual drilling of the Macondo well by Deepwater Horizon oil rig drillers appeared to have occurred without major problems.
When a well has been drilled to the desired depth, the drill pipe and drill bit are then removed. Tests are performed to determine if the various layers of rock penetrated by the drill bit contain sufficient quantities of oil and gas for profitable production.
Cementing the Deal
Should the well be deemed unprofitable, the wellbore is sealed with cement.
Assuming the well will be a profitable producer, which was the case for the Macondo well, casing, which is wider in diameter than drill pipe, is lowered into the well. The next step of preparation is to pump a cement slurry containing various additives through the casing. The additives include retarders to delay cement hardening until the cement slurry has been pumped to the desired location in the wellbore and chemical agents to provide the required cement slurry density, rheological properties, and fluid loss characteristics.
The cement is pumped down the casing and at the bottom it is forced into the annulus, the space between the casing and the rock. The cement slurry is displaced by high-density completion fluids. In the Gulf of Mexico, it is common to use fluids containing dissolved calcium chloride salts with densities up to 11.6 pounds per gallon or bromide salts with densities of 11.5 to 19.2 pounds per gallon (compared to about 9.3 pounds per gallon for seawater) as completion fluids. These high density brines contain no suspended solids and must be handled carefully because of their corrosive nature and acidity.
The cement, meanwhile, displaces the drilling fluid already in the well bore, which exits the well and is pumped through tubing up to the drilling rig where it is stored in tanks. When the annulus is filled with cement, fluid circulation is then terminated to allow the cement to harden. Completion fluids remain in the casing while the cement sets. Although there are other techniques for cementing a well, this is the most common practice in the Gulf of Mexico.
At the Macondo site, however, seawater, which doesn’t have to be recovered from the well, was used to force the cement slurry into the well annulus.
Once hardened, the cement seals off the oil- and gas-bearing rock from the wellbore until the oil company is ready to produce the well. The cement sheath also serves to isolate the oil- and gas-bearing formation from penetration by water.
When the cement has hardened, pressure tests are performed to determine if hydrocarbons are entering the wellbore. Usually an additional couple of hours or more are allowed to pass and the well is tested again. A third test is often performed.
If there are leaks, a procedure known as squeeze cementing is designed to fill any gaps in the cement sheath. Such gaps are not uncommon. Squeeze cementing involves injecting relatively small amounts of cement slurry down the wellbore at high pressure and squeezing it into gaps in the cement sheath and sealing them closed.
After cementing, a cement plug is placed in the well as a final seal. When the well is ready for production, operating personnel will drill through the plug and perforate the cement sheath adjacent to the oil- and gas-bearing rock to let hydrocarbons flow into the wellbore.
Two tests were performed on the Macondo well that appeared to indicate a good cement seal had been formed. A third test was performed and appeared to provide anomalous results.
In offshore drilling, the oil company, BP at the Macondo site, makes the final decisions on operations. Drilling operations of the Macondo well were conducted by Transocean Ltd., the world's largest offshore drilling contractor. Cementing operations were conducted by Halliburton Services, the largest cementing services company.
Asked about the tests during congressional testimony, Lamar McKay, president of BP America, told the House Committee on Energy and Commerce that the pressure readings were “worrisome.”
During his testimony, Steven Newman, president of Transocean, which owned the drilling rig, said the tests indicated “that there was something happening in the well bore that shouldn’t be happening.”
McKay said that the well test issue “is critical in the investigation” into the cause of the accident.
Tim Probert, president of global business lines and chief health, safety, and environmental control officer for Halliburton, testified to the committee that after reviewing the test results BP decided to proceed with the original well program.
The blowout at Macondo occurred before the final concrete plug was put in place. At this point oil and gas under high pressure began surging into and up the wellbore.
Massive pieces of equipment called blowout preventers are designed to close valves and use shear rams to seal the drill pipe and well casing to block oil and gas from escaping the wellbore. They are the third and final line of defense against a blowout.
Invented in the 1920s, the blowout preventer has significantly improved safety of oilfields. The Cameron blowout preventer was named an ASME Mechanical Engineering Landmark in 2003.
In normal drilling operations, the drill pipe passes through a cylindrical channel running through the blowout preventer and into the wellbore. In the event of a blowout, shear rams cut through and crush the pipe and then form a seal. Each shear ram has two shear blades, an upper and a lower one. Powerful springs push them through the drill pipe crushing it. The ram blades also seal against each other forming a barrier blocking fluid flow.
In the Macondo blowout, the 450-ton, ten-year-old Cameron blowout preventer valves failed to function properly. The blowout preventer's shear rams designed to cut through the drill pipe and seal it also failed to function. As a result, large volumes of oil and gas reached the rig floor and resulted in an explosion, loss of life, and sinking of the drilling rig. The continued escape of large volumes of hydrocarbons has created the massive oil spill that began hit the Louisiana coast in mid-May.
Clues about why the blowout preventer failed are beginning to emerge. Maintenance records appear to indicate the blowout preventer was in good operating condition, but modifications were made over the years. It is being investigated whether the modifications could have contributed to the failure of the blowout preventer. Valves that should have closed did not. Deep-water remotely operated vehicles were used to try to shut off these valves but failed. It appeared that hydraulic fluid had leaked from the valves in sufficient quantities to possibly cause failure.
The blowout preventer's shear rams constitute the final line of defense. Drill pipe used in deep water is manufactured much thicker than standard drill pipe to allow for the high water pressures of great ocean depths. The blowout preventer shear rams may have been insufficiently powerful to cut through the thick drill pipe.
There probably wasn’t a single cause resulting in the blowout but multiple failures occurring in critical systems. It is premature at this time to parcel out blame for the blowout itself.