GOM Oil Spill Concerns

From following the updates at the official site of the Deepwater Horizon Unified Command (which has been at http://www.deepwaterhorizonresponse.com) I expect that the well will be permanently plugged within a matter of weeks at the most, all going well. Nonetheless, as an ordinary engineer and academic who is following what is going-on out of interest, I consider there are a lot of open questions and things about which to be somewhat concerned.

The pressure at the well head at the sea floor has been rising by from 10 to 20 psi per day. From officially quoted figures I calculate that from 00:00 hours on July 24 until about 14:00 hours on August 1 the measured pressure has risen by 10.3 psi per day on average. The following is a quote about the well head pressure from a transcript of Sunday’s (August 1st) press briefing by National Incident Commander Admiral Thad Allen: ‘It continues to rise and give indications of the same type of pattern we would expect with a well with integrity’.

I do not understand why the pressure in a well with integrity should rise continuously. I would like to be reassured about this, but I suspect it remains a matter of debate among the engineers who are involved directly. It is easy to say ‘this is what we would expect’ in retrospect and perhaps find a justification for that expectation such as: while the oil spill was occurring the pressure within the reservoir was relieved and, now that the flow has been halted, the pressure is moving back to an equilibrium value. Perhaps the well is being replenished from a deeper level. These things would be difficult to verify, although they could be modelled. An oil reservoir containing a significant proportion of dissolved methane under enormous pressure is a complex system. An update of 2 August, which gives a revised estimate of total leakage (4.9 million barrels, of which 0.8 million were contained so that 4.1 million barrels flowed into the ocean) indicates that detailed modelling and analysis have been taking place.

Another possible explanation for a continually increasing well-head pressure is that the density of the column of oil within the well from the sea bed down to the reservoir is decreasing. Gaseous or lighter components may be rising to the top within the well bore, while heavier components may be sinking back into the reservoir. I have the impression that the original blow-out may have been linked to a lightening of the column of crude oil within the well bore. A very significant slug of gas within the well bore could have caused a huge increase in pressure at the well-head.

To put this in perspective I have estimated that the pressure in the oil reservoir is about 13,600 psi or 936 bar. The presence of the column of crude oil within the well itself counteracts this pressure, reducing it to the measured 6,928 psi (478 bar) at the well head. The weight of the oil within the well bore provides a counterbalancing pressure of about 6,672 psi (460 bar). A significant gas slug within the well bore could increase the well-head pressure by a significant fraction of this amount.

I am a little uneasy that a collapse of the relief well bore occurred over 40 feet close to the interception site. Those in the business may be comfortable enough with this, but It makes me feel wary.

When disasters occur, the general public must often wait for a very long time for the explanations of what went wrong. I wonder whether BP can give a clear and precise explanation of exactly what things went wrong in the original accident. Can BP give an assurance that the blow out preventer on the relief well can be relied upon should it be called into action?

It does appear that considerable care is being taken in preparation for plugging the well. For instance, it is planned to pump mud into the original well at a very slow rate for a static kill. This sounds sensible to me. My own rough calculations indicate that a ballast mud density of up to 1363 kg/m3 (11.4 pounds per US gallon) would be required. However, while waiting this long to finally plug the well has probably been wise and fully justified it seems to me that completing the job cannot be delayed too much as that would increase the risks of a blow-out.

I do not believe that the exploitation of deep undersea oil reserves should be abandoned because of this Gulf of Mexico oil spill disaster. However, there may well be a need for a moratorium until all the issues can be addressed appropriately. Clearly also, on the basis of what has happened, much tighter regulation is required. Thinking ahead also, it may not always be wise to allow a commercial enterprise that causes an environmental disaster manage the arresting of the incident and the solution of the causative technical issues, even under close supervision. I am appreciative of the fact that the US Government has done a lot to maximize openness and transparency, but I would urge even greater technical openness. For example, I would like to have all the data (e.g. composition, densities, continuous pressure readings, viscosities, known or estimated temperature profiles within the formation, drawings of the BOP etc. so that I can calculate the leakage rates and perhaps try to understand what has been going on in terms of applied thermodynamics and fluid mechanics).

I have carried out my rough calculations using Maxima, a free and open-source computer algebra system. A printout is available at this link: GOM_Oil_Leak_Calcs_r008.pdf and the original Maxima file is available at GOM_Oil_Leak_Calcs_r008.wxm. Others are free to make use of these documents in accordance with the Creative Commons licence http://creativecommons.org/licenses/by-sa/3.0/.