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.

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  • http://unofficialnetworks.com John Konrad

    Interesting facts Rob and I think the negative test remains one of the most difficult concepts for the non-driller to understand but is an important component to the error chain.

  • Mike

    ” 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”

    Ok but if the proceedures didn’t include those detailswhy did the rig crew proceed with the test, why didn’t they stop the job and demand more information?

    Ok

  • rob

    Mike, good point. You actually bring up something that I overlooked… BP states’ “The guidelines for the negative-pressure test, a critical activity, did not provide detailed steps”

    That’s a false statement. There’s no way that Transocean, or BP for that matter, would have relied “on the competancy and leadership skills of the BP and Transocean rig leaders” to perform such a task. There was most certainly a procedure in place.

    • Alan

      I’m a non-driller and I get the impression that there are many fluctuations that can take place in the steps to complete an exploration well. At a certain point there has to be a cut off in terms of very detailed procedures that accommodate all the scenarios that can take place. So to me competence is the key component here. Then we have to ask ourselves how does competence assurance work with BP and TO. Is there a proper system with criteria, KPIs, evaluations, improvement cycles, is it robust, does it follow industry practice, is it regulated like pilots etc etc. Fascinating stuff and gets to the deeper system aspects of this disaster. Sorry if I’m rambling.

  • Mike

    Good article. I really enjoyed this.

  • Carl

    “Why BP decided to continue with the negative pressure test is unclear at this point,”

    Shouldn’t that read, “Why the well site leader decided to continue with the negative pressure test is unclear at this point,”

    One thing that really, really irritates me about this and other big headline events recently is the companies don’t make decisions, people do. “BP” didn’t decide anything in this case, the well site leader did. Corporations don’t do bad things, people do. Yet people hide behind the corporation. Perhaps if we started holding people accountable for their decisions they would think more about the consequences of their decisions.

  • rob

    Point taken Carl. It’s certainly true that the Well Site Leader made the ultimate decision to continue forward at that point.

    I did make an assumption however that the Well Site Leader was not acting independently in his decision to continue forward with the negative pressure test and that consultation with shore-side experts from BP, or the other Company Man, most likely occurred.

    The purpose of this article is not to place blame, it’s to highlight areas where critical decisions were made and to hopefully shed some insight into why they were made, who made them, and how they were made.

  • yew

    Isn’t it obvious that 15 barrels gain in the mud pit an indication of kick? The mud logger should have also notice that.

  • rob

    According to the report, “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.”

    Hopefully we’ll see these guidelines soon to get more of an idea of what this test called for specifically.

  • http://maritimeaccident.org Bob Couttie

    Would that be here?:

    Appendix P.
    BP/Deepwater Horizon Rheliant Displacement Procedure “Macondo” OSC-G 32306

  • rob

    That procedure covers the displacement, but not the negative test itself.

  • http://maritimeaccident.org Bob Couttie

    The most important issue is whether the recommendendations in the BP report will reduce the chances of the errors re-occurring. What are the iews on the recommendations?

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