Environmental controls, Legislation, Local Impacts, Seismicity, Well integrity

“Dear Aunty, does the ovalisation of the Preese Hall well give us any cause for concern?”

This is a question I’m often asked, sweeties, and the good news is that the answer is no – we needn’t be concerned.

Although you could be forgiven for thinking otherwise considering that at least one shale gas opponent in Lancashire repeatedly reminds us that the induced seismicity associated with Cuadrilla’s Preese Hall well in 2011 caused the well casing to become “ovalised over a large interval” or words to that effect.

Where does this information come from, what does this mean to you and I, poppets, and how is it significant?

Well, my loves, the reference to ovalisation stems from a report commissioned by Cuadrilla themselves in the aftermath of those two small tremors in April and May 2011. Independent experts set out to try and determine whether Cuadrilla’s operations were the cause of those minor seismic events and to try and understand what might have caused them and what effect they may have had on the well bore and well integrity so that a future repeat could be avoided.

You might see it referred to as the ‘Pater and Baisch report’ (although it’s real title is: Geomechanical Study of Bowland Shale Seismicity –
Synthesis Report) and you can find it here on the Cuadrilla website:

http://www.cuadrillaresources.com/wp-content/uploads/2012/02/Geomechanical-Study-of-Bowland-Shale-Seismicity_02-11-11.pdf

The report concludes:

“After stage 2, the casing was ovalized by more than 0.5 in (inches) over a long interval from 8500 to 8640 ftMD.”

Now, the first thing to note here is that the ‘long interval’ is 140 ft. Or just 1.6% of the entire well bore depth at that point. So, although the wording implies otherwise, the measurements tell us that only a very, very small section of casing was affected.

It goes on to say that:

“Because of its location deep in the wellbore, the casing deformation poses no risk to the uphole wellbore integrity and the seal with respect to the overlying layers. For that reason, it is safe to proceed with stimulation treatments in the upper intervals.”

In case you missed it the first time: no risk to well integrity.

So, in essence, cherubs, what these experts tell us is that a steel tube, that was originally cylindrical, became slightly misshapen as a result of the two minor seismic events. This ovalisation occurred at significant depth, and only a very small percentage of the casing was actually affected. There is no risk to well integrity. The surrounding environment remains protected – so much so that the report authors go on to conclude that it would be quite safe to continue hydraulic fracturing operations higher up the well bore above that ovalised section.

Shale gas opponents who repeatedly cite this study as evidence of considerable damage to the well do so by cherry-picking phrases that can be used to create just such an impression – but it would be the equivalent of saying that a minor collision with a car park bollard, that puts a tiny dent in your bumper, somehow renders the entire car irreversibly damaged and unsafe to drive.

It evidently doesn’t and nor should we believe that the very limited deformation of the casing at such depth means the Preese Hall well is a problem we should all be fretting about.

And the reason I state this so confidently, pumpkins? Well, because another group of independent experts that reviewed the Synthesis Report on behalf of DECC conclude in their findings https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/15745/5075-preese-hall-shale-gas-fracturing-review.pdf#page14

“However, this (casing deformation) occurred in the lower section of the reservoir productive zone and subsequent prefrac injection test analysis did not indicate any communication problems between zones, such as cumulative stress or high tortuosity. Such indicators are what might usually be expected as indicators of containment issues due to poor cement. Therefore, well integrity was not considered a risk given the proven integrity of the upper completion, confirmed by surface gas measurements and annular pressure readings (see Figure 8). These tests demonstrate that the integrity of the casing, and the cement, in the upper completion has not been compromised.”

See, the integrity of the well has not been compromised. The surrounding environment is still protected. Agriculture is safe. Tourism is safe. We are safe.

Until next time xxx

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