Getting A Handle On The Big Numbers . . .
Remember the headlines in the summer of 2015? “100 billion barrels of oil found under Gatwick!”
Well, it’s oil, so that’s good news. And 100 billion sure sounds like a big number. So we should be excited, right? But what does it actually mean? Where does it fit in? UK crude oil consumption in 2012 was 1,520,000 barrels per day. That’s about 555 million barrels per year. Like interstellar distances, or atomic scales, millions of barrels is not a quantity that most of us are readily able to visualise.
I like to think of it in terms I can understand. The town where I live, a typical UK ‘market town’, has a recreational swimming pool. The pool is 25 metres long, 1.5 metres deep, on average, and 10 metres wide, giving a volumetric capacity of 375 cubic metres. 1 cubic metre contains 6.23 barrels, so our pool could hold 2,336.25 barrels of oil. It would keep the UK running for 2 minutes and 13 seconds, or four millionths of a year. That’s not really any more accessible. Let’s apply the same principle to something a little larger.
The northern tower of the original World Trade Centre (WTC) had a floor area of 4,020 square metres, and it was 417 meters high. A tank that size would hold 10.4 million barrels. So we can say that the UK’s oil consumption can be visualised as approximately 1 WTC (North) tower per week.
For all the wrong reasons, most people are able to visualise the WTC (North) tower, and the number of weeks in a year is something that most of us are comfortable with. Combining those two gives us a manageable handle on what 555 million barrels of oil actually represents.
It is interesting to contemplate how that quantity of oil relates to reservoir size. Remember that the oil we can hope to recover from a reservoir using today’s technology is a maximum of 25% of the oil in place. So we need a reservoir containing 2.2 billion barrels of oil in place to keep the UK going for a year. We know, of course, that there are no lakes of oil under the ground. Oil is contained in the inter-granular spaces of reservoir rocks. Let’s say this inter-granular space is 10% of the total material volume – it’s not so unrealistic, it keeps the maths simple and it tells us that we need a reservoir volume of 22 billion barrels, or 3.5 billion cubic meters.
To get an idea of the size of that reservoir in real world terms, consider the following. AnTech is headquartered in Exeter, Devon, a smallish UK city covering 18 square miles, or 47 million square metres. Whilst Devon is not renowned for its oil prospectivity, we can hypothesise a reservoir thousands of feet below Exeter’s streets. To provide enough oil to keep the UK supplied for a year, that reservoir would need to be 74.5 metres thick.
Of course the picture is never that simple in real life. A reservoir that had ‘just’ 555 million barrels recoverable would be expected to yield that total volume over a number of years, with the initial production declining to a threshold of economic viability as the total recoverable volume is approached. The initial production rate, the shape of the decline cure and the length of the tail are strongly dependent on the reservoir characteristics and the amount of reservoir exposed by the well(s) draining it.
Similarly, while initial exploration has determined that there is 100 billion barrels of oil in place below Gatwick, the proportion of that oil that will ever be recovered, and the rate at which it is extracted is strongly dependent on the mobility of the oil in that location, and the technologies that we are allowed to use to get to it. But that is a subject for another blog post.
Author: Richard Stevens