SHALESAFE – A ‘FIRST ALERT’ SYSTEM FOR SAFER SHALE OIL AND GAS EXTRACTION
The ShaleSafe project aims to develop an automated, real-time, in-situ aquifer monitoring system which enables immediate detection of any contamination related to the shale gas extraction process. Key parameters such as methane and volatile organic compounds (VOCs) can be monitored several times a day, alerting the shale gas operator when any pollution is detected.
Introduction
Unconventional hydrocarbons such as shale oil and gas are found trapped in fi ne-grained sedimentary rocks, called shale, characterised by their very low permeability. Unless natural fractures exist in the shale rock, oil and gas cannot move readily within it. Therefore, they cannot be extracted at economically viable fl ow rates by simply drilling wells, as is the practice for producing oil and gas from permeable sandstone and carbonate rocks.
To access shale oil and gas, the industry currently uses two technologies, namely hydraulic fracturing and horizontal drilling. Hydraulic fracturing, or ‘fracking’, is a process in which the shale formation is mechanically stimulated to create additional permeability. This is done by pumping a mixture of water, sand and chemicals under high pressure into the rock to break it up and release the hydrocarbons. The use of horizontal drilling allows for an increase in well deliverability by maximising the amount of shale available from fracking.
A frequently expressed concern associated with shale gas extraction is that the underground water can become polluted by methane gas or the chemicals used during the fracking process. This can occur through subsurface pathways from the fractured shale layer or as a result of a loss of integrity of the wellbore. There is also the potential for contamination if the fl ow back water leaks from the surface due to it not being properly contained prior to disposal.
The oil and gas industry in Europe must demonstrate, and guarantee, safe exploration and exploitation by meeting monitoring requirements set by environmental regulators. There is, therefore, an industry wide need to develop and implement technologies that can continuously, reliably and cost effectively monitor the underground water quality, specifi c to shale oil and gas sub-surface activities. Towards this purpose, a European consortium comprising TWI Ltd (UK), Danish Technological Institute (Denmark), META Group S.R.L. (Italy), HGL Dynamics Limited (UK) and ZELENA INFRASTRUKTURA (Croatia) has embarked on a project called ShaleSafe with the aim of developing such a technology. The project started in October 2016 and will run until September 2018.
The ShaleSafe concept
ShaleSafe is an instrumentation system which will provide automated, regular monitoring of a broad range of targeted contaminants in hydrogeological monitoring wells, via an on-board sampling, analysis sensor and electronics package.
Figure 1 shows the basic concept of the system deployed in the vicinity of a typical shale gas drilling site. The system comprises four main modules: the Sub- Surface Sensor Package; the Surface Package; the Site Controller Package; and the Central Remote Server Package.
Sub-Surface Sensor packages sit within the aquifer at the bottom of monitoring wells. Wells with diameters as small as 50 mm can be monitored. The maximum drilling depth is 300m. These sensor packages contain modern sensors that continuously detect possible contamination of the aquifer by chemicals related to the shale gas industry. Each sub-surface sensor package is connected to a Surface Package at the top of the well via an umbilical cable. This is so that adequate power is supplied to the sensor package and the data is transmitted to the surface package. All surface packages, in turn, are wirelessly linked to a Site Controller Package which handles collection and storage of all the data received. The site controller package has a user interface to enable confi guration of the surface packages, which allows the recorded data to be viewed and analysed locally. Data from potentially many sites are communicated to the outside world via a Central Remote Server Package located offsite, which is able to generate warnings and alarms for each site within minutes of measurements being taken.
Figure 1: visualisation of ShaleSafe technology deployed in the vicinity of a shale gas well site. ShaleSafe sensors
The ShaleSafe sensor package employs two technologies to detect methane and volatile organic compounds (VOCs). The VOCs cover a broad range of chemicals including the biocide additives acrylamide and glutaraldehyde used in hydraulic fracturing. A Non Dispersive Infrared Sensor (NDIR) is used for methane detection. The principle of detection relies on the fact that certain molecules (such as methane) have the ability to absorb infrared light at wavelengths determined by the types of bonds present. The NDIR technique targets these wavelength absorptions as a way to
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