The Failed Negative Pressure Test – a BP Investigation Report Analysis

In Section 4 of BP’s Investigation report on the Horizon disaster, they discuss the negative pressure test as a possible missed indication of a “kick” in progress.

Per Transocean’s Well Control Handbook, a kick is defined as:

“The entry of oil, gas, or water into the wellbore.  When the bottom hole pressure becomes less than the formation pressure and the permeability is great enough, formation fluid will enter the wellbore causing a kick.”

The purpose of the negative pressure test on the Macondo well was to purposely lower the pressure inside the wellbore in a controlled manner to ensure that the casing and cement that separated the wellbore from the hydrocarbon-bearing formation could withstand that pressure differential without any leaks.

To do this however, it’s not a matter of simply turning a valve and watching the pressure go down.  The 5000-foot vertical column of heavy, barite-saturated mud within the choke line, kill line, mud boost, drill pipe, and production casing must be replaced by a much lighter column of seawater, which in turn reduces the total hydrostatic head pushing on the sides of the wellbore.

During the negative pressure test, 1,260 psi of pump pressure was needed to overcome the difference in hydrostatic pressure between the column of seawater and the column of mud that was being forced back to the mud pits via the riser.  Once the seawater/mud replacement was complete, the column of seawater was then isolated from the wellbore via the annular BOP and the 1,260 psi of head pressure differential was allowed to bleed off via the kill line causing a U-tubing affect within the wellbore.

BP’s investigation report states that upon bleeding off the pressure from the wellbore, 3.5 barrels of fluid should have returned.  Instead, 15 barrels came back.  BP goes on to say that this “should have indicated to the rig crew a communication flow path with the reservoir through failed barriers”…“The well site leader noticed the discrepancy and after a discussion with the rig crew, preparations for continuing the negative-pressure test were made by bleeding the kill line.”

Why BP decided to continue with the negative pressure test is unclear at this point, but in BP’s conclusion, they state:

“The guidelines for the negative-pressure test, a critical activity, did not provide detailed steps and did not specify expected bleed volumes or success/failure criteria. Therefore, effective performance of the test placed a higher reliance on the competency and leadership skills of the BP and Transocean rig leaders.

That statement seems to dictate a bit of ambiguity over who was in charge of this negative test in the first place.  Also, if BP’s procedure for conducting the negative test did not include details of expected bleed volumes from the kill line, or procedures on what to look for, it’s unclear how the rig crew could have been expected to notice a 15 barrel gain in the mud pits that ultimately would have indicated a kick was in progress.

Overall, there was a lot of interesting information in BP’s report, but this was the first of it’s kind we’ve seen and like most things lately, certainly deserves to be taken with a grain of salt.