Cuadrilla claims that they’re ceasing operations at Balcombe because they didn’t realise they should first have notified local inhabitants living above the location of their horizontal drilling. Hard to believe? Yes. But on the other hand, their plans and operations to date seem to be so inept that perhaps there is a grain of truth there. They are attempting a challenging technical drilling operation in heavily faulted geology which could act for a fast track for leakage of drilling fluids. To avoid this a full 3D seismic survey of the area would be required, and has not been undertaken.
Two weeks ago I published a technical critique on Cuadrilla at Balcombe. In their original planning application of 2010 they got the boundary of their licence wrong by up to 1200 m. They said then that “Testing the shale layers ... will be the main purpose of the [Balcombe] exploratory drilling operation".
But now they claim that they are after oil in a very thin layer of muddy hard limestone known as micrite. So no gas, no shale (and no fracking)! But the micrite layer, 33 m thick, is embedded in the shale called the Kimmeridge Clay. This clay, which underlies the whole of the Weald, is judged by DECC to be the second most important shale gas resource after the Bowland Shale of northern England. It is twenty times thicker under West Sussex than the micrite layer.
Cuadrilla seems to be unaware of the myriad faults cutting the Kimmeridge Clay. These extend to the surface, and could act as fast-track pathways for leaking methane and drilling fluids if the drilling strayed into the clay. Surrounding the wellsite there are faults big enough to shift the micrite layer vertically by tens of metres. So when drilling along this layer, if you drill through a fault (which is nearly vertical), the layer will be 'missing' on the other side of the fault. Is it higher up, or lower down? We don't know. So my questions to Cuadrilla are: How are you going to keep the horizontal drill bit confined within this thin layer? What happens if you lose track of the micrite and stray 'accidentally' into the shale above or below?
Cuadrilla intended to drill horizontally in a generally westward direction, for up to 2000 feet, or about 600 m. But the circle drawn on the map below, of 600 m radius, shows that there are no inhabitants above that zone that need to be notified. So their excuse for stopping seems somewhat thin. I suspect that they either failed to find the micrite when turning the wellbore from vertical to horizontal, or else found it and lost it again, because of folding and/or faulting of the layers.
A full three-dimensional seismic survey of the whole area is really required. This will take many months to plan, and will cost several million pounds. At the moment Cuadrilla is drilling in the dark, with no reliable image of the underground geology to go on. Such a survey requires a grid of survey traverse lines laid out in rows and columns, spaced at only 25 m apart. But the method doesn't work properly if the grid has too many missing areas. Naturally if locals don't wish to allow the necessary vibrator convoys and cables onto their property for such a survey, they have only to say no, and the survey will end up looking like a piece of Swiss cheese.
When Cuadrilla submits its new proposals, West Sussex County Council needs to ask some searching questions about Cuadrilla's sketchy understanding of both the geology and the technology. Nodding through a planning application based on faulty or incomplete data, as the Council seems to have done in 2010, will no longer suffice.
Fig 1: Map of area surrounding Balcombe drilling site. Central drilling site shown as the purple polygon, geological faults marked as red dashed lines and 600m radius as solid black line.
