Thursday, 25 September 2014
Landmark shale gas study shows no groundwater problems
One of the difficulties in the current shale gas debate is that good data is hard to come by. Operators collect lots of data from around their sites, including water sampling to test for pollution, and geophysical monitoring to track where the fractures went during stimulation. However, this data is often considered commercially sensitive, so it rarely sees the light of day.
A government-sponsored project would be very useful, because it would provide a test-bed for an extensive monitoring program. All data could then be made public, and the claims of all those involved in the shale gas debate openly tested.
This is exactly what has been happened in the USA, with the final report released this week. The US National Energy Technology Lab (NETL) sponsored a monitoring program at a hydraulic fracking operation in Greene County, Pennsylvania. The monitoring program consisted of 2 parts: microseismic monitoring to track the fractures created by the stimulation, and geochemical sampling in overlying layers to test whether any contamination has occurred. Most importantly, because the data is publicly available, it's a great opportunity to talk through the anatomy of hydraulic stimulation.
The first stage of shale gas extraction is to drill horizontal wells through which the fracking will be done. The figure below shows a map of the lateral wells drilled. Those in the yellow box were the 6 wells that made up the NETL study.
The figure below shows a stratigraphic column of the geology in the study area. The Marcellus shale is at a depth of 8,000 feet. Overlying the Marcellus at a depth of 2,000-4,500 feet are Upper Devonian age rocks that contain conventional natural gas reservoirs. These reservoirs have been exploited for conventional gas, so wells penetrating these formations are available, and have been used for geochemical monitoring. Freshwater aquifers are found at depths of less than 1,000ft.
I've heard over and over again that apparently shale rocks in the USA don't have faults in them (one controversial geologist in particular springs to mind). Seismic reflection surveys show clearly that the Marcellus rocks in this area are in fact substantially faulted.
I'll cover the geochemical aspect of the monitoring program first. Geochemical tracers were injected with the hydraulic fracturing fluids. Fluid samples were taken from the overlying Upper Devonian layers, and analysed to look for the presence of these tracers, which would indicate upwards fluid migration via natural and/or hydraulic fractures. The figure below shows the monitoring well depths in relation to the shale gas operation.
I won't go into detail on the geochemical evidence, suffice to say that no evidence for upward migration of gas and/or fracking fluid was found. This concurs with the study discussed in my last post: there is no evidence that fluids can migrate from 2km down to the surface - if contamination is occurring, it occurs via faulty well bores, not through fractures in the rock.
Of more interest to me is the microseismic data, because it allows us to see what hydraulic fracture stimulation looks like. Geophones were placed in the monitoring wells that extend to depths close to the reservoir. These pick up the "microseismic events" - the pops and cracks as the fractures open up, allowing us to track where the fractures have gone.
The figure below shows the resulting microseismic map. Each blue dot represent a microseismic event - we can connect them together to see where the hydraulic stimulation fractured the rock.
The first aspect to note is that all the events are close to the well. The hydraulic fractures rarely extend more than 200-300m from the well, and we can see that to be the case here.
Microseismic event magnitudes range between -3.0 to -0.5. This is a typical range for hydraulic stimulation, and is below what could be detected even with a seismometer placed on the surface, and orders of magnitude below what people can typically feel, which is approximately magnitude 2.0.
This is interesting, because as we have seen from the reflection seismic, the rocks here are faulted. We constantly hear about how fracking in faulted rocks will inevitably lead to large earthquakes, water contamination and not much short of the end of the world. In fact, it is rare for a fault to be critically stressed such that it will trigger large events. Service providers have estimated that they see evidence for fracture-fault interaction in about 30% of stimulations, yet induced seismicity such as that seen during Cuadrilla's operations at Preese Hall are very very rare (with 1 case in the UK, and 3 cases in North America).
You can see from the microseismic that one of the stimulations did interact with one of the faults. It can be seen in the cluster of events to the left of the above figure. The figure below shows the microseismic events in more detail, and the fault interaction can be seen in the red-coloured events to the top left.
The next plot shows the same events in a map view. You can see how the events line up to demarcate the re-activated fault.
In summary - I often hear how the faulted UK geology will render hydraulic stimulation impossibly dangerous in the UK, whether because it will trigger large earthquakes or contaminate water supplies. This report shows hydraulic stimulation in the Marcellus shale, with lots of faults in evidence. The microseismic data shows that the fractures interacted with the faults. However, no large seismic events were triggered, and extensive geochemical monitoring showed no evidence of fluid and/or gas migration into shallow layers.