What happened in the Gulf of Mexico approximately 13 years ago has meant one of the worst tragedies where human lives were lost and the fauna of the entire ecosystem of that place was impacted.
Although it was something that happened 13 years ago, what happened there has always left me with the doubt of knowing what were the technical reasons of what caused this explosion, especially considering that being a petroleum engineer I can give a fairly logical interpretation of the causes that led to such an unfortunate accident.
The macondo well was the well that was being drilled by the personnel attached to the Deepwater Horizon semi-submersible offshore platform, a well that after what happened could not be recovered and was sealed so that another pressure relief well could operate to put an end to the explosion caused.
While doing some research on what happened, I found that there were events prior to the explosion that indicated that they had to be cautious in the drilling of the macondo well, and I will explain the reasons why.
The initial estimated depth to drill the macondo well was 19660 feet, however the decision was made to drill this well only to the total depth of 18360 feet, so it is worth asking the question:
Why was the decision made to drill to a shallower depth?
With the depth of 19600 feet two formations that were of interest due to their productivity were crossed, however while drilling the macondo well a loss of circulation of drilling fluid occurred, for those who do not know what a loss of circulation is, I explain, a loss of circulation is as its name suggests, ie a loss of drilling fluid from the well into the formation, the problem when this occurs is that the column of drilling fluid is going down in the well, The problem when this happens is that the column of drilling fluid is going down in the well, causing the hydrostatic pressure generated by the drilling fluid on the walls of the well to go down as the fluid is lost to the formation, making the formation pressure is greater than the hydrostatic pressure of the drilling fluid, which causes an entry of unwanted fluids into the well as oil and gas, fluids that certainly can migrate from the bottom of the well to the surface and cause an explosion.
The loss of circulation occurred in the first of the productive formations, this was an indication that the first formation crossed had a high formation pressure, which in terms of productivity is beneficial because you can have a contribution of oil and gas early, however for effects of drilling operations was very dangerous because of what was explained above that a loss of circulation can cause a blowout at the surface.
Already this first indication that the first formation is a highly porous and permeable formation and that it has a stimulus of hydrocarbon contribution, makes it necessary to have correctives for when this formation interval has to be cemented, since a good quality in the cementation guarantees that fluids such as gas and oil will not migrate through the well to the surface, so to continue advancing in the drilling of the macondo well to a depth greater than that of this formation interval, a firm and quality cementation had to be performed.
Given this scenario, the question remains: what caused the explosion of the macondo well drilled by the Deepwater Horizon semisubmersible offshore platform?
Technically according to what we are taught as petroleum engineers for those of us who study this engineering it is said that when you have this condition:
Hydrostatic pressure of the drilling fluid < formation pressure.
It is the only cause of an upwelling of fluids such as gas and oil from the producing formation to the well and then migrate from the bottom of the well to the surface, and then with some type of formation and in the presence of oxygen occurs the explosion, which is fueled by a gigantic volume of gas and oil, whose explosion will last the days it takes to drill a directional well that serves to relieve the pressure that feeds the explosion.
Under this scenario another question arises:
Was this the circumstance in which the Macondo well blowout occurred?
No, that is to say that the drilling fluid density was at the right values to generate a hydrostatic pressure that was higher than the formation pressure, however, a bad cementing was performed in the 9-7/8" casing interval, the reason was not having performed the CBL cementing test which is used to evaluate the quality of the cement in relation to its adherence, Also, the centralizers were not used when the 9-7/8" casing was lowered into the wellbore, which subsequently caused the cement to be unevenly distributed when cementing the well in the space between the wellbore walls and the 9-7/8" casing, and cracks could occur.
Since the fluid pressure of the drilling fluid could withstand the formation pressure and prevent fluids such as gas and oil from entering the wellbore through the bare wellbore space, then these fluids (gas and oil) migrated through the weak spaces left by the poor cementing and seeped to the surface.
However, before the explosion occurs, there are always indicators that give indications that an onslaught or surge of fluids such as gas and/or oil into the well is occurring, however it seems that the workers, drillers, supervisors and engineers were not aware of these indicators, and when they wanted to activate the closing of the valves of the set of BOP valves, they did not activate them because their operation was not adequate, since the respective routine tests that must be performed before starting the drilling of a well were not carried out.
Conclusion
Poor cementing to cover the annular space between the 9-7/8" casing and the walls of the well caused the hydrocarbons (gas and oil) to enter the well and migrate through the annular space and inside the casing until they reached the surface.
The pressure tests that were performed on the well did not alert BP personnel who were in charge of drilling activities in the macondo well, but instead ignored them, which resulted in the failure to establish the integrity parameters of the macondo well.
The BP personnel in charge of drilling, especially those in charge of working on the platform floor where the drilling controllers are located, did not know how to detect the indicators that unwanted fluids were coming to the well, so when they wanted to react it was already too late because the gas had already exceeded the height of the BOP valves, i.e. the riser was already influxing.
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Unfortunately, once the influx of gas and oil reached the surface there was not much that could be done with the explosion and in effect what ended up happening was that the result was significantly disastrous, with a total of 4.9 million barrels of oil spilled, thus having a contaminated ocean surface area of 180,000 square kilometers, which means to date the oil accident with the largest negative impact on the environment in history.
Recommended references for further reading
What caused the Deepwater Horizon disaster?
Offshore Accidents: What Caused the Deepwater Horizon Oil Spill?