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Displacement of Drillpipe and Upper Wellbore

The process of preparing the Macondo well so that the Deepwater Horizon could depart until another rig returned is called temporary abandonment. For temporary abandonment to occur, the well needed to be sealed. U.S. regulations required cement to be placed at the bottom of the well and the installation of a surface retrievable cement plug. At Macondo, BP gained regulatory approval to set the surface plug at 3,000 ft below the seafloor, much deeper than the regulated depth of 1,000 ft for the plug. This deeper location of the surface plug ultimately affected the negative pressure test conditions to assess the integrity of the cement at the bottom of the hole.1U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Displacing the Mud from the Drillpipe and Upper Wellbore

Generally, the primary barrier used to prevent the flow of hydrocarbons (oil and gas) from the reservoir is a column of heavy fluid that fills the wellbore and the riser and essentially “pushes” back on the hydrocarbons. When a well is abandoned, some of the fluid column is replaced with lighter sea water, and the well may become underbalanced, meaning the weight of the fluid column may not be sufficient to keep hydrocarbons from entering the wellbore. If the hydrocarbon bearing zones in the well are sealed by additional barriers (e.g., cement), the well will not flow despite being underbalanced. By simulating the underbalanced condition and observing the pressure in the well (i.e., a negative test), the crew is able to test the integrity of the well in a controlled manner before removing the fluid column barrier.2U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Macondo well configuration for the negative test

The diagram above shows the planned configuration of the Macondo well for the negative test.

Loss Circulation Material

To prepare for the negative pressure test at Macondo, a drillpipe was lowered to the depth identified for the bottom of the cement plug at 8,367 ft. A viscous and dense, water-based Loss Circulation Material (LCM) followed by seawater was pumped down the drillpipe to displace the mud by pushing it above the BOP (at the seafloor) toward the rig.3U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930 The diagram above illustrates this configuration with the LCM spacer colored orange, followed by seawater in blue. At this stage, the drillpipe is full of seawater and the riser is full of drilling mud. The BOP is closed to isolate the well from the hydrostatic pressure generated by the liquids above the BOP.

Factor: BP chose to put the LCM into the well to avoid disposing the material as a hazardous waste pursuant to the Resource Conservation and Recovery Act. The LCM material had never been tested for this application, there was no operational reason to use it. 4National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling. (2011, February 14). Chief Counsel’s Report: The Gulf Oil Disaster, p. 151

Finding: The crew did not achieve the intended well conditions during the displacement of the drillpipe and wellbore. Some spacer material remained below the closed BOP. Introducing viscous, gelling spacer material (LCM) into the well and inadvertently placing it across the kill line of the BOP may have led to plugging of the kill line during the negative test, causing the zero pressure reading that the crew accepted as indication of a secure well.5U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Pressure Tests

U.S. regulations require conducting a positive pressure test to verify that no leaks are in the well. On the day of the incident, the Deepwater Horizon drilling crew conducted a positive pressure test and determined the Macondo well passed the test. A major limitation of a positive pressure test is that it does not check the cement sealing off the hydrocarbon bearing zones at the very bottom of the well for inward leakage. Even though regulations at the time of the incident did not require a negative pressure test, BP’s final temporary abandonment plan called for one.6U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Negative Pressure Test

A negative pressure test can assess the integrity of the bottom-hole cement. It simulates an underbalanced well by displacing some of the heavy drilling mud from the well and closing the BOP to isolate the bottom of the well from the hydrostatic pressure exerted by fluids above the BOP. By simulating the underbalanced condition and observing the pressure in the well, the crew is able to test the integrity of the well in a controlled manner before removing the fluid column barrier. If the hydrocarbon bearing zones in the well are sealed by additional barriers (e.g., cement), the well will not flow despite being underbalanced. Various methods are available to accomplish the test, and the specific procedure was left to those planning the process.7U.S. Chemical Safety and Hazard Investigation Board. (2016, June 5). Deepwater Horizon Blowout Animation. https://www.csb.gov/macondo-blowout-and-explosion/

On April 16, 2010, BP sent a final written Forward Plan to the Transocean well operations crew for the TA plan without mention of the negative pressure test. BP provided written negative pressure test instructions to the well operations crew via a third-party contractor on the afternoon of April 20.8U.S. Chemical Safety and Hazard Investigation Board. (2016, June 5). Deepwater Horizon Blowout Animation. https://www.csb.gov/macondo-blowout-and-explosion/

Factor: Neither BP nor Transocean management provided the Deepwater Horizon crew with a negative pressure test procedure that included acceptance criteria to confirm test success or actions to take if any deviations occurred during the test, and the risks associated with conducting the negative pressure test were not formally assessed.9U.S. Chemical Safety and Hazard Investigation Board. (2016, June 5). Deepwater Horizon Blowout Animation. https://www.csb.gov/macondo-blowout-and-explosion/

Finding: BP’s Macondo Temporary Abandonment plan occurred without a formal process, and created conditions for a Temporary Abandonment that lacked assessment of decisions, including review of internal policies, standards to provide quality control, work plans, or safety-critical procedures.

Interpretation of Negative Pressure Tests

When displacement of the well was stopped to conduct the negative pressure test, as expected some trapped pressure was in the drillpipe. The crew bled the pressure from the drillpipe, which should have resulted in a zero pressure reading on the drillpipe. If the hydrocarbon bearing zone at the bottom of the well had been sealed, no subsequent increase in pressure would have resulted. Pressure did increase and was bled two more times before the crew used a lack of flow from the well as an indicator of a passed test despite a pressure increase that was detected in the well. The crew’s actions suggest they did not realize that the unexpected readings were indications of a failed negative pressure test and that integrity of the cement had not been secured.10U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Factor: The crew incorrectly rationalized pressure and flow indicators observed from the kill line and the drillpipe during the negative test. After the final negative pressure test was erroneously accepted as a “pass,” the Deepwater Horizon crew continued the process of displacement, replacing the drilling mud in the wellbore and riser with seawater in preparation to set the surface plug.11U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Finding: Replacing dense drilling mud with seawater had the effect of lowering the hydrostatic pressure on the bottom of the well. With the fluid column removed and no surface plug yet set, the only means left for preventing the hydrocarbons from entering the riser and eventually making their way up the well to the rig was detection of the kick by the crew and manual activation of blowout prevention equipment to seal the well.12U.S. Chemical Safety and Hazard investigation Board. (2014, June 5). Investigation Report Volume 1: Explosion and fire at the Macondo Well. https://www.csb.gov/file.aspx?DocumentId=5930

Factor: The bottom hole cement job was not successful and the wellbore was therefore not isolated. Contrary to what the crew thought, wellbore integrity had not been established.

Finding: The National Commission conducted stability studies on foamed cement similar to Macondo’s to further investigate a probable failure mechanism. The Chief Counsel’s Report reviewed the actions of the cement provider, Halliburton, and BP as part of its investigation. It asserts that some Halliburton personnel were aware of potential problems with the cement used at Macondo, but they did not inform BP of the issues. The National Commission attributes the lack of communication and other technical issues with the cement to management problems within the company.13National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling. (2011, February 14). Chief Counsel’s Report: The Gulf Oil Disaster, p. 151

Time Summary of Activities and Decisions

At Macondo, between 3:00 p.m. and 5:00 p.m., the crew displaced drilling mud from the drillpipe and upper wellbore by pumping a dense spacer material followed by seawater to push the drilling mud out of the drillpipe and the upper wellbore. Then they closed the BOP to isolate the well from the hydrostatic pressure generated by the liquids above the BOP. A series of pressure tests followed. The crew eventually accepts the results of the negative pressure test and begin displacing mud from the riser.14U.S. Chemical Safety and Hazard Investigation Board. (2016, June 5). Deepwater Horizon Blowout Animation. https://www.csb.gov/macondo-blowout-and-explosion/

Crew actions during the Macondo Disaster

Image Credits: U.S. Chemical Safety and Hazard Investigation Board

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