Natural gas exploration and production is proceeding in rugged and remote areas of Alberta and B.C. Wells need to be tested to evaluate reservoir parameters and this often requires flaring of sour natural gas. A combination of dispersion modelling to predict SO2 levels and ground monitoring to measure SO2 at locations of concern is used to ensure that ground level concentrations of SO2 do not exceed provincial ambient air guidelines during flaring. Accurate dispersion modelling is difficult in mountainous terrain. Also, the use of truck or fixed SO2 monitors to reliably track the plume and monitor ambient SO2 levels can be restricted by the lack of roads and the cost of installation in remote mountainous terrain. Hence more cost effective plume tracking methods and associated SO2 measurement devices are required. The objective of this project was to identify, evaluate and demonstrate a technology with the potential to be a practical and cost effective method to track plumes from well test flares and to monitor ground level SO2 to ensure compliance with environmental guidelines. The ideal system would be able to operate in remote, rugged areas with limited road access with a high level of availability and reliability. This report covers the first phase of the project, which consisted of a literature review to identify candidate technologies and the development of a proposal for testing of the candidate technologies. The Differential Absorption Light detection and ranging (DIAL) technique was the only method identified in the literature review that can remotely measure SO2 concentrations at multiple points in the vicinity of well test flares. The DIAL technique can remotely measure SO2 concentration to ppb levels at a volume point of the atmosphere at a distance up to several kilometers. DIAL instruments have been used in Europe to analyse volatile organic hydrocarbons, SO2, NO2 and ozone emissions from urban areas, industrial facilities, oil field flares and volcano plumes. As a laser is used as the light source, the system works day and night and can operate under some conditions of rain and snow. When used with tracking optics and computer data analysis, 2D and 3D concentration profiles of the plume can be measured and displayed. The DIAL equipment is expensive. A truck-mounted system with tracking optics costs in the order of $1,000,000 to $1,500,000 Canadian. Several other optical techniques measure the average SO2 concentration in a column of air between a light source and detector. The methods fall into two groups. One group measures SO2 absorption of either background sunlight (Correlation Spectroscopy) or background infrared (Image Multi-Spectral Sensing). The second group uses a remotely mounted mirror to reflect a laser or other light source back to the detector (Fourier Transform Infrared). These systems may be a less expensive option than DIAL for visualizing the plume, but cannot currently meet the requirement for analysis of SO2 at a defined point in space. Image Multi-Spectral Sensing has the most promise as a method for rapid imaging of the plume either by detecting SO2 or CH4 and would be able to operate day and night.
Saskatchewan Research Council Executive Summary
Many optical methods for environmental monitoring have become mature in the last decade. In this project, various optical remote sensing technologies were assessed for their current applicability to monitoring criteria pollutant levels of SO2 in mountainside environments. The particular application of concern was SO2 concentrations in the vicinity of well test flaring activities in Western Canada. Of the technologies available, laser systems were focused on because they have unique capabilities in addressing this monitoring problem. Lasers propagate narrow beams of electromagnetic radiation (light) which can be applied to atmospheric environmental monitoring, producing three dimensional plots of gas concentrations in the air. This is ideal for the SO2 monitoring problem near well test flaring monitoring sites. General applications of optical monitors, from monitoring real-time river water quality to air quality were reviewed in the project. This information, along with a discussion of early laser development and principles of operation of optical systems, is presented in the background section of this report. In summary, all gases absorb characteristic colors of light, either ultraviolet, visible, or infrared. Optical system using this absorption to detect the gases can operate in one of two ways: 1) Spectral systems produce a full spectrum of the air (like a prism produces a spectrum of sun light) and then analyse it to find embedded signatures of pollutant species and 2) non-dispersive (including laser-based) systems look at only one or a very limited number of wavelengths or colours and use these to identify gas concentrations. There is one spectral approach that is readily available on the market today; but is not suitable for application to the terrain of concern. This is UV-DOAS (Ultraviolet Differential Optical Absorption Spectrometry) which uses an ultraviolet (UV) lamp to transmit UV-light through the air and then analyse it after it has traversed an air parcel to identify selective atomic absorptions of airborne pollutants. The difficulty of this approach for the current problem is that the light must be transmitted through the air and then captured after propagation to do the analysis. This is not always possible in mountainous terrain. A technique that is suitable for PTAC’s SO2 monitoring purposes is a special case of nondispersive detection, the Lidara . Lidars are essentially laser radars which look at backscattered laser light just as a radar looks at backscattered radio waves. One type of Lidar, referred to as DIfferential Absorption Lidar or DIAL uses two lasers, or one tuneable laser, to look at the ratio of reflected or backscattered laser light at two closely spaced wavelengths. It measures a differential absorption of laser light between the two colours to determine the air concentration of a specific pollutant molecule. Because the lasers can be scanned vertically and horizontally, the DIAL Lidar can produce a three dimensional map of the detected gas concentrations. DIAL Lidar does not require a retroreflector, like DOAS, because it looks at backscattered light from the air itself. It can measure SO2 in the atmosphere using ultra-violet lasers. Details of DIAL Lidar operation are provided, specifically for trace gas detection in the air.