Drilling EG-1 is followed by a period of well testing.
What is a well test?
The two key elements for a successful deep geothermal energy system are heat and permeability.
Drilling EG-1 has enabled us to access a naturally fractured geological structure in the granite at a depth of around 4,500 metres. In this zone, it’s very hot, and the natural fractures present in the rock enable water to flow through the granite.
Well tests provide us with more detail about the permeability and water flow characteristics of the target zone, so we can better understand the geothermal resource in this precise location. This detailed understanding enables us to work out how a two-well geothermal system will perform, and what its heat and power outputs are likely to be.
Following logging of the final well section, and when the rig has been demobilised, the site will be set up for well testing to start. The main aim of the tests is to assess the permeability of the formation around the well, by testing the ability to inject water into the natural fractures (injectivity) and to produce water from these fractures (productivity).
First, an injection test is carried out. Using bespoke pumps, cold water is injected from the surface down the well, at low pressure and low flow rates. Over the course of 2-3 days, gradual, incremental increases are made in the flow rate, to assess the well’s ability to accept fluid. Pressures and flow rates are continuously monitored and recorded.
The other aspect of well-testing is to perform production tests to assess the well’s ability to produce fluid. In an ‘air-lift’ test, compressed air is injected down 600 – 700 metres of open-ended tubing placed in the top of the well, inducing a drawdown on the water level within the borehole. This in turn encourages hot water from the formation surrounding the openhole section of the well to flow into, and up, the well. The fluid produced at surface during this test is a mixture of air and water and so it is passed through a separator unit, where the air is separated and released and the water is conducted into a storage lagoon. This is usually a relatively short test (12 – 24 hours) and is the type of test where steam may be seen being produced at surface. During the test, downhole pressure and flow rate are continuously monitored to provide data about the well’s productivity.
The production test could be followed by further injection tests to establish as much information as possible about the performance of the well. Further low-level injections can often improve the performance of the well.
Well testing and seismicity
Even low levels of injection and production can lead to adjustment and settling of the natural fractures in the rock, so we anticipate that our seismic monitoring network, installed before drilling started, will detect some low-level seismicity (microseismicity) associated with well testing.
Usually, such microseismic events are so tiny that they can’t be felt at surface. But our monitoring network is extremely sensitive and can pick up and locate the minutest of movements many kilometres below ground. Microseismic monitoring data is a key tool during well testing because it provides us with essential information about the permeability of the formations around the well, and the spatial movement of water through the rock.
Continuously monitoring the real-time data from our seismic network also helps us to manage and control our operations, so that we maintain microseismicity within limits which don’t cause annoyance or disturbance to our neighbours. While we can’t entirely rule out the possibility of an event being felt or heard at surface, in such a case we can stop operations while we investigate the source of the event, and can change our operating conditions to reduce the likelihood of further felt events.
You can find out more about seismicity – both the natural seismic events which regularly occur in the UK and so called ‘induced seismicity’ from activities such as mining, quarrying and drilling – in this short film, which also explains how our seismic monitoring system works and the levels within which we operate. And there’s also more detail in the FAQs section of our website: FAQs – Eden Geothermal