BP said that the choke line leak has been isolated but needs to be repaired before further progress on the integrity test. That repair work is under way and once it is done BP will resume the process of shutting in other valves prior to allowing testing to start.

The test is crucial because it will probe the integrity of the Macondo well bore and could, if results are positive, allow BP to shut in the well completely.

After the leak is fixed BP will next close the kill valve on the containment cap and finally remote-operated vehicles will begin incrementally closing the choke valve on the cap.

That final process is expected to happen over the course of a couple hours.

“At that point the shut-in test really starts,” Wells said.

Watch the live video feeds of the pressure test here.

The test is expected to run for 48 hours, at most, with experts assessing the progress at six-hour intervals.

Go-ahead

The head of the US spill response, Thad Allen, said the government gave BP the go-ahead to begin pressure testing the Macondo bore late Wednesday afternoon after requiring additional monitoring of the operation.

Wells called the changes “minor but reasonable.”

Allen announced the decision at a press briefing, saying that the government had made minor modifications to the test to guard against damaging the wellbore.

The concern is that if the well bore is not intact, building pressure in the well could force oil up to the surface outside of the bore.

The resulting blowout could prove even more difficult to stop, but Allen said experts believe the geology of the Macondo area makes that scenario unlikely.

BP will know if the bore is intact by the way the pressure builds within well, as measured by sensors within the cap.

Ideally pressure will build quickly and evenly to between 8000 and 9000 pounds per square inch, which would indicate that all of the oil is flowing up the wellbore and not escaping along the way, Wells said.

Flat-lined

“If the pressure goes flat-lined at a very low pressure or starts to decline that would be a very strong indicator we don’t have well integrity,” he said.

In that instance, BP would open up the valves on the original Macondo blow out preventer, which are attached to the Helix vessels Producer 1 and Q4000, and begin bringing the oil to the surface.

Allen said any pressure below about 6000 psi would be too low to give enough confidence to try to shut in the wellbore.

If pressures hover between 6000 psi and 8000 psi or go up and then begin to decline, BP and federal technical advisors will have to make “tough” decisions about how to proceed, he told reporters.

Even if the test is successful over the full 48-hour period, BP and the government may determine that another 48-hour test is needed before the well can be shut-in with confidence.

Concerns about the state of the Macondo bore have plagued plans to rein in the well since the failed top kill attempt last month, Allen said.

During that operation, in which BP pumped mud and debris from the surface into the well in an effort to push down the flow of oil and then cap the well, crews were only able to build the pressure in the well to about 6000 psi during constant pumping and could not keep enough mud flowing down the well.

Allen said that engineers were not sure if the mud was escaping out the top of blowout preventer, was flowing out breaches in the well bore or both.

Comfortable

“We have never been comfortable with what the 6000 psi meant,” he said.

The well integrity test had been on hold since Tuesday afternoon, when it was officially pushed back 24 hours.

The decision to postpone the test was made following a meeting with US Energy Secretary Steven Chu and a team of government scientists and “industry experts”, Allen said.

Government scientists wanted further information on the engineering standards of the Macondo well construction to ensure it could withstand the maximum pressures expected during the test, Allen said, noting that they were particularly interested in the 18-inch and 22-inch casing strings.

It also was revealed Wednesday that seismic information that had been shot Monday morning by WesternGeco’s seismic vessel Geco Topaz was not done being processed until Wednesday morning.

The shoot did not show any anomalies around the well area when compared against a survey shot in late April, shortly after the blowout.

Allen called the information “very, very key,” and said it made it “worthwhile” to wait to perform the testing.

Confident

Allen said he was confident that the test would yield important information and that, regardless of the results, the new cap system would increase BP’s ability to collect oil from the well.

But he would not say how confident he was that the test would show BP could shut in the well.

BP has stopped drilling on both relief wells during the pressure test operations.

Transocean’s semi-submersible rig Development Driller 3 ceased drilling as a “precaution,” Wells said, explaining that BP wanted to make sure there was no communication between the Macondo bore and that of the relief well.

“We’re only four feet away horisontally from the well,” Wells said, adding however that there exists a “possible scenario” that during the planned shut-in test oil flow could get to the relief well and possible complicate efforts.

Kick

“We don’t believe it actually is going to happen,” Wells said.

But in case it does, BP wanted to make sure that Development Driller 3 had its drill pipe in the bore and could circulate mud to stabilise the well if there was an unexpected kick caused by the test.

The rig was scheduled to trip pipe out of the hole and perform another ranging test to check its location in relation to Macondo, which would have made it harder to control the well if there was an unexpected change in downhole pressure.

Drilling would resume once the planned well integrity test is over, he added.

Transocean semi-submersible rig Development Driller 2 has also stopped operations and will remain stopped until the first relief well intercepts Macondo, Wells said.

He said that the rig was close enough to the Macondo bore that it too would need to begin ranging activities, but having both rigs ranging could lead to incorrect readings.

Ranging involves emitting an electric current and magnetising the Macondo wellbore in order to pinpoint its location.

Allen said he believes Development Driller 3 can intercept Macondo later this month, but the actual bottom kill and cementing operation could take as long as a few weeks.

Published: 14 July 2010 14:27 GMT  | Last updated: 15 July 2010 11:50 GMT