identify particular gases. The methane absorbance band is unique and therefore highly selective. The detection of VOCs is achieved with a Photo-Ionisation Detection Sensor (PID). The sensor uses an Ultraviolet (UV) light source which results in ionisation when absorbed by the gas molecules, and causes temporary loss of electrons and the formation of positively charged ions. The gas becomes electrically charged, leading to the generation of a current that is proportional to the concentration of the VOCs present.
As with most gas sensing, parameters such as relative humidity, temperature and pressure can affect the performance of the PID and NDIR sensors. Hence, each sensor package is equipped with auxiliary sensors to measure relative humidity, temperature and pressure so that the variations of these parameters are taken into account by correcting the readings from the two gas sensors using compensating algorithms.
The technique developed to measure the dissolved methane and VOCs’ concentrations follows the general approach of extracting the dissolved gases into the gas phase prior to transduction. This is done by using a membrane characterised by hydrophobic properties, and high permeability to methane and VOCs.
Another important parameter to monitor in groundwater is salinity, as high levels of this indicate groundwater salinization. This can occur if flow back water leaks into the aquifer, either from the surface or from the reservoir/well. Each sensor package is also equipped with an electrical conductivity sensor which measures the electric conductivity of water relative to its ions concentration and salinization. The principle is simple: higher salinity entails higher electric conductivity.
Benefits
Existing methods for monitoring the quality of underground water generally rely on manual methods which involve taking samples at hydrogeological boreholes and analysing them in a remote laboratory. The process of collecting, preparing and transporting the samples is labour intensive and prone to errors. Moreover, analysing the samples can take several weeks allowing pollution to go undetected for some time. As natural fluctuations occur in aquifers, the low sampling frequency of current monitoring methods makes it difficult to capture small changes, particularly
Author Details
Dr. Amina Boughrara Salman, Senior Project Leader, TWI • +44(0) 1223 899559 • Email:
amina.salman@
twi.co.uk • Web:
www.shale-safe.com Amina Boughrara Salman is a Senior Project Leader at TWI, Cambridge (UK). She joined TWI’s Integrity Management Group in 2016 where her work focuses on condition and structural health monitoring. Amina’s current activities include managing European publicly funded projects in the Oil & Gas and Marine industries, and investigating the use of acoustic emission for turbulent flow regime detection and monitoring. Previously she worked for Schlumberger, Abingdon Technology Centre, UK as a Senior Reservoir Engineer for nine years. Amina holds BS and MS degrees in Physics from Houari Boumediene University of Sciences and Technology, Algiers and a PhD degree in Petroleum Engineering from the University of Tulsa, Oklahoma.
Figure 2: how better evaluation of trends can be obtained using ShaleSafe
in the occurrence of methane, which could arise from pollution from fracking.
ShaleSafe uses automated, in-situ technology to continuously and reliably monitor aquifer quality specific to the shale oil and gas industry. Simultaneously it addresses the underground water quality requirements set out by the regulators which include: baseline monitoring for a period of 12 months before commencement of the sub-surface operations; ongoing monitoring during drilling and hydraulic fracturing; and finally monitoring during flow testing, production and well abandonment. As a result, ShaleSafe offers an environmental monitoring solution which is more frequent, accurate and efficient than existing shale gas monitoring systems.
In addition, a better evaluation of the baseline is achieved with ShaleSafe because the high frequency of sampling allows for better statistical analysis of the data (see Figure 2). The technology also allows for adaptive monitoring, both during and after subsurface operations, in order to better capture the divergence of the data from the baseline. The increased frequency of readings that ShaleSafe can provide over manual methods will undoubtedly allow more accurate trending to be obtained, and dramatically decrease the response time in cases of contamination.
The ShaleSafe project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691527. For further information, visit
www.shale-safe.com.
OCTOBER / NOVEMBER •
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