The Macondo Well: What Really Happened and Lessons Learned

By MarEx 2011-04-19 14:26:27

by Phil Rae & Michael Economides

On the evening of April 20, 2010, the crew aboard the Transocean semisubmersible drilling rig Deepwater Horizon was at work in the Gulf of Mexico, making preparations to temporarily abandon the well they had been drilling for the past 73 days in a prospect called Mississippi Canyon 252. The well had been drilled in water over a mile deep, and its total depth from the surface was over 18,000 feet. It was the first well to be drilled in this prospect, also known as Macondo, by a consortium of three oil companies, headed-up by multinational BP.

The previous night, the final steel casing – a so-called production long string – had been cemented in place, and the crew had tested the outcome of that cement job only a few hours earlier to confirm the well’s integrity with a series of pressure tests. Such tests are routinely performed to ensure there are no leaks and critically to ensure that the well does not flow when the heavy mud, used to control the well while drilling, is removed. At around 2140 hours that night mud suddenly began erupting from the well, pouring over the drill floor and quickly shooting up above the height of the derrick. It was driven by hydrocarbon gas that had leaked into the well unnoticed and was rapidly expanding as it approached the surface. Too late, the crew realized the well was out of control. They took steps to try to contain it and divert the erupting gas, but their actions were to no avail. Within minutes the gas had spilled over the rig, reached the engine room and ignited, causing the first of two catastrophic explosions.

Those explosions and their aftermath killed 11 men, damaged vital hydraulic control lines and ultimately caused the sinking of the Deepwater Horizon. The crew was unable to close the blowout preventer (BOP) on the seabed or to disconnect the rig from the well, the last desperate measure to save it from disaster. The resulting blowout lasted 87 days, spilling nearly five million barrels of oil into the Gulf of Mexico, demonizing BP, the biggest U.S. domestic oil and gas producer since 2001, and precipitating a public backlash against the entire oil and gas industry. Indeed, the public and political hysteria that ensued bordered on circus, spurred on by sensationalist media and self-serving politicians. Even the U.S. president wanted to “know whose ass to kick.” An unsupportable and, in fact, unsustainable government moratorium on deepwater drilling was partially lifted in October. The first permit to drill was finally granted on February 28, 2011 in tacit recognition of the reality that deepwater production now accounts for 30 percent of U.S. domestic oil production and, of necessity, that share will continue to grow.


Despite the furor, the finger-pointing and investigative hearings, no satisfactory official explanation emerged for several reported events immediately prior to the blowout that appeared to defy the laws of physics. Particularly puzzling was the lack of an adequate explanation for the pressure and flow anomalies during the socalled negative test, the critical and final check to ensure the well was secure and could be circulated to seawater. Even the most extensive analyses done on the disaster yet, BP’s own Deepwater Horizon Accident Investigation Report and the National Commission Report to the President, which largely relied on BP’s analysis, made wrong assumptions and drew various erroneous conclusions by misinterpreting key events. Careful analysis of these and other data widely available in the public domain, including official investigation transcripts, however, provides the only logical explanation. In fact, it provides not just an explanation for the anomalies but also a unique insight into the genesis of the blowout.


Many individual factors almost certainly came into play and contributed to the disaster, but one final critical factor set the whole train of events in motion, sealing the fate of the Deepwater Horizon that day. That critical step was the high rate displacement of drilling mud by an unusually large volume of high-viscosity, heavyweight spacer (a water-based fluid used to displace the oil-based drilling mud from the well), followed by seawater, in preparation for the negative test and, ultimately, well suspension. During the pumping and displacement of this heavyweight spacer, a breach in pressure integrity at the casing shoe resulted in the undetected loss of about 80 barrels of drilling mud into the probably uncemented annulus (the space between the outside of the production long string and the oil/gas-bearing rock formations). This undetected loss of mud resulted in under-displacement of the heavyweight spacer and led to otherwise inexplicable pressure and flow anomalies during the negative test, induced by U-tubing and other phenomena, including flow from the well.

The resulting approximately 700 psi differential pressure, which was noted but has never been officially explained, persisted throughout the negative test, confusing the crew. It caused U-tubing of fluids, due to hydrostatic differentials, decoupling the fluid column in the kill line from the surface and rendering the pressure gauge there ineffective. The partial evacuation of the kill line then allowed heavyweight spacer to flow into the lower end of the kill line when the well was shut in, rendering the line useless for pressure and flow monitoring for the duration of the negative test. The flow of heavy spacer into the kill line left the line with a combined hydrostatic pressure that was in equilibrium with formation pressure acting in the wellbore. Hence, later in the test when the kill line was left open, there was no flow from it, despite a 1400 psi differential pressure between it and the drill pipe surface pressure.


This hypothesis neatly explains all the salient anomalies during the negative test and provides new interpretations to one or two key events, in particular the much reported, and generally accepted, belief in a leak in the annular BOP at the start of the negative test. In fact, there actually never was any leak. This mistaken, but widely-accepted, belief in a leak conveniently provided a possible (but incorrect) explanation for an unexpected pressure rise on the drill pipe during the test. In fact, the pressure rise came about because the well was flowing.

Exactly why the well breached at the shoe during the displacement of the mud with viscous spacer and seawater remains a mystery. Momentum changes may have caused some transient overpressure at the bottom of the well; or it may simply have been that the well’s pressure integrity, as determined in the positive test, was unreliable. Whatever the cause, on this occasion pressures that the well had successfully contained previously under static conditions proved too much. Something gave way, and the well lost fluid through the shoe track. This loss went unnoticed because at that same time the crew was busy with other operations. According to the BP Report, the trip tank was being cleaned so flow rate data during this time was considered unreliable.

This breach at the shoe and the unnoticed and unsuspected loss of a significant volume of mud, at a time when the well had already demonstrated positive pressure integrity, was the event that was ultimately to cause disaster. Anomalies that would have raised concerns in an open hole were explained away by other suggestions and ideas in a well that was lined with steel pipe, cemented and already positively pressure tested. The rest, as they say, is history.


The E&P industry, strategically essential and crucial to our modern way of life, is a human enterprise and fallible like any other. Yet it has an overall enviable safety record considering the number of wells drilled each year and the vast quantities of hydrocarbons produced. Like every other business, it learns by experience. Further improvements can and must be made in training people, and it should be mandatory that critical procedures like the negative test are supported by clear and concise documentation that fully explains the rationale for the procedure and the criteria for acceptance. Anomalous results should mandate a temporary halt in operations to allow consultation with experts rather than relying on anecdotal explanations and unsupported hypotheses. A simple phone call made that fateful Tuesday evening last April would almost certainly have averted disaster.

Well-construction plans still need to maintain the operational flexibility to make essential changes when faced with unexpected pressures or other events. However, critical decisions, e.g., to run a long string versus a liner or to use foamed cement rather than something more conventional, should always involve consultation with relevant technical experts and a clear understanding of the implications of such decisions. It is worth noting that despite all the money spent investigating the disaster and all the academic and industry gurus consulted, nobody has commented on the unequivocal technical inappropriateness of running a production long string in a deepwater well, like Macondo, with the potential for gas migration. The premature removal of several thousands of pounds/ square inch of riser hydrostatic pressure, implicit when running a long string and hanging-off the casing immediately after cement placement, practically guarantees gas invasion of the annulus in such wells. This is of far greater significance than any popular, misguided notion that a long string was chosen to save money.

Undoubtedly, new safeguards and tighter legislation will be introduced to minimize the risk of any similar catastrophe occurring in the future, but some of these measures will increase costs without necessarily improving safety. As the Deepwater Horizon saga confirms, things can still go wrong, not necessarily because of efforts to cut corners or reduce costs, as the media and politicians alike have repeatedly trumpeted, but because components can fail unexpectedly and human beings can make mistakes. Air travel provides us with ample evidence of this. In the past 10 years there have been, on average, 29 accidents and 775 deaths every year in commercial civil aviation, and the main causes were pilot error (50 percent) and mechanical failure (22 percent). Such statistics will not stop us from flying and, despite the outcry, the Deepwater Horizon disaster will not put an end to deepwater drilling and production. Hopefully, however, the lessons learned from it will amount to more than just political grandstanding and will help us avoid such accidents in the future.


Phil Rae is the co-author of The Energy Imperative. He is also
an internationally recognized expert on well completions with
extensive industry experience. Michael Economides is a professor
at the University of Houston and the Editor-in-Chief of the
Energy Tribune.

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