Yet federal regulators give drillers a free hand in this crucial safety step - another example of lax regulation regarding events leading up to the April 20 explosion on the Deepwater Horizon oil rig.
Federal regulators don't regulate what type of cement is used, leaving it up to oil and gas companies. The drillers are urged to simply follow guidelines of the American Petroleum Institute, an industry trade group.
Far more stringent federal and state standards and controls exist on cement work for roads, bridges and buildings.
While the chain of failures on Deepwater Horizon is under investigation, rig owner Transocean has singled out cement work as one likely fundamental cause of the blowout.
Even before Transocean pointed to cementing, independent experts suspected it partly because faulty cement work - either badly mixed or poorly placed against well walls - is so prevalent at offshore wells.
An AP review of federal accident and incident reports on offshore wells shows that the cementing process has been implicated at least 34 times since 1978. Many of the reports, available from the U.S. Minerals Management Service that regulates offshore wells, identify the cause simply as "poor cement job."
In a November 2005 accident where the Deepwater Horizon was positioned above another well in the Gulf, faulty cement work allowed wall-supporting steel casing to come apart. Almost 15,000 gallons (56,800 liters) of drilling fluid spilled into the Gulf.
Just a week later in a nearby well at another platform, cement improperly seeped through drilling fluid. As a result of an additive meant to quicken setting time, the cement then failed to block a gas influx into the well. When the crew finally replaced heavy drilling fluid with lighter seawater, as they also did last month before the blowout at Deepwater Horizon, the well flowed out of control and much of the crew had to be evacuated.
Cementing was identified by federal investigators as a glaring cause of an August 2007 blowout, also off Louisiana. They said, "The cement quality is very poor, showing what looks like large areas of no cement."
Reports by MMS, a branch of the Interior Department, also provide evidence of the role bad cement work has played in accidents. One study named cementing as a factor in 18 of 39 well blowouts at Gulf rigs from 1992 to 2006. Another attributed five of nine out-of-control wells in the year 2000 to cementing problems.
Cementing in the oil rig business is a sensitive, involved process. Well cement constitutes an essential barrier that is difficult to install and control, said Gene Beck, a petroleum engineer at Texas A&M University at College Station, Texas.
Deepwater wells pose special challenges: severe pressures and temperatures, as well as the need for specialized equipment and lots of cement. The wellhead of the Deepwater Horizon operation sat on the ocean floor, nearly a mile (1.6 kilometers) from the surface. The drill hole itself went another 13,000 feet (3,960 meters) into rock.
All cement begins as a slurry with cement flakes and water. Contractors then add ingredients to make the cement set at the right time and to keep out gas and oil.
There are three major U.S. cementing companies: Halliburton, Schlumberger and BJ Services. Cementing is typically performed by such rig contractors as part of a broad range of drilling services that they supply.
Halliburton, which had the Deepwater Horizon job, mixes in nitrogen to make its slurry more elastic. The nitrogen also helps create a lightweight cement that resembles a gray foamy mousse and bonds better to the casing.
But the recipe also depends on the job, because cement must respond to varying pressures and temperatures. Cement contractors work closely with oil and gas companies on the formulas for individual wells. The oil and gas companies have the final say on what is used.
Once the consistency of the mix is decided on, it is pumped deep into the well, where it first sinks to the bottom and then oozes upward to fill the narrow spaces between the steel casing pipe and rock walls. When the cement sets, the casing and cement are supposed to form an impenetrable wall to keep gas or oil from pushing into the hole anywhere but the bottom, where its flow up the pipe can be controlled.
But if gas bubbles invade the setting cement, they can form a channel for pressurized gas and oil to surge uncontrollably up the well, usually around the casing. The cement must be strong enough to withstand up to 5,000 pounds (2,270 kilograms) of pressure per square inch (6.45 square centimeters), to keep the well walls from collapsing.
"Cement is cheap, and it fixes a lot of problems, but it's not a good place to cut corners," Beck said. Many oil and gas companies will scrimp, though, if they don't think they need all the ingredients in the cement, he said. Cement is often squeezed in later to try to fill gaps, but Beck said the success rate of this remedial work is low.
And if cement was part of the cause of the Deepwater Horizon catastrophe, it also could be part of the remedy. Two relief wells are being drilled to intersect the leaking well and plug it with cement.
Halliburton was completing the final cement work on the exploratory well beneath Deepwater Horizon in the wee hours of April 20. It added an initial cement plug to the well to act as a cap until a later production phase.
Workers started running a series of tests to check if the cement and casing could stand up to sufficient pressure. The first tests of outward, positive pressure showed no problems.
In the first sign of trouble, though, the well then failed a negative pressure test, where internal fluid pressure is reduced, according to congressional testimony from a BP PLC executive. It showed different pressures in two areas, indicating an unseen leak somewhere in the well.
Despite the test, managers eventually decided to replace drilling fluid with seawater and set a final cement plug so the well could be mothballed pending a decision to possibly begin production drilling.
And while it is not yet clear what sections of the casing or cement may have failed - or why - it is known that the blowout ignited and exploded before the last plug was set.
In the aftermath of the blowout, questions have been raised about the safety of nitrogen-laced cement foam. But several cementing experts told the AP it is a sound technique. Halliburton says it has used such a mix on scores of wells and told a congressional committee that the cementing on the Deepwater Horizon job was successful.
Halliburton did not respond to AP requests for comment.
In the wake of the accident, some experts support mandatory uniform cement standards for underwater wells. "When you change the composition, it should meet a certain standard. Such standards exist for the building construction industry," said Surendra Shah, Northwestern University engineering professor and director of the Center for Advanced Cement-Based Materials at Evanston, Illinois.
Elmer Danenberger, a retired chief of offshore regulatory programs for MMS, told a congressional committee this month: "An industry standard should be developed to address cementing problems, how they can be prevented, and the actions that should be taken when they do occur."
Many construction projects use concrete hardened with sand and gravel aggregate, but cement is the glue that holds it together. On federal projects, "just about everything is regulated, from the thickness of the concrete, to the strength of the concrete, to the type of aggregate that's used," said Brian Turmail, spokesman for the Associated General Contractors of America.
Oil companies test the thickness and strength of cement in wells by shooting sound waves into the cement. This kind of test, called a sonic logging test, wasn't run on April 20 at Deepwater Horizon. A Halliburton manager said it's the most realistic way of testing the quality of the cement bond, but a BP manager said pressure tests are better and log tests are used only if there's already a sign of a problem.
Either way, these tests are not 100 percent reliable. Sometimes, oil companies don't discover a bad cementing job until it fails.
There can be early warning signs, though. Federal regulators have known for years that a condition called sustained casing pressure - usually gas caught between the casing and well wall - is a major problem that typically signals bad cement work.
In the August 2007 blowout, investigators cited tests showing high casing pressures that could have indicated suspect cement work. The platform owner reported a problem to federal regulators, but nothing was done before the blowout, the report said.
More than 8,000 of the 22,000 offshore wells on federal leases, most of them in the Gulf, show sustained pressure, according to government reports.
This month, in a move in the works long before the Deepwater Horizon explosion, regulators wrote in the Federal Register that the oil and gas industry in the Gulf has "suffered serious accidents as a result of high sustained casing pressure, and the lack of proper control and monitoring of these pressures."
New rules take effect June 3. But they take a conservative watch-and-wait approach and demand only routines already carried out around the industry: a management program with monitoring and diagnostic testing. If operators discover sustained pressure, they must notify MMS of plans to fix it.
There are no new record-keeping or reporting requirements in the new rules, which are backed by industry. In the rule-making documents, regulators - long accused of being too cozy with the industry - said the regulations would cost the entire industry only $5 million, compared with the "impracticable and exceedingly costly" $2 billion alternative of fixing the wells outright.
"Unfortunately, this is yet another crisis in a long line of accidents caused by cementing problems in drilling," said U.S. Rep. Diana DeGette of Colorado, a member of the Energy Committee looking into the cause of the blowout.
MMS refused to answer specific questions about its cementing policies, including why it took so long to craft the pressure regulations and whether MMS has issued any citations for cement problems.
"All of these questions are questions that we are reviewing," said Interior Department spokeswoman Kendra Barkoff.