36 Air Monitoring
A TALE OF TWO RECONCILIATIONS: RECENT EXPERIENCES FROM CONDUCTING ‘TOP-DOWN’ METHANE MEASUREMENT
Accurate measurement of methane is the basis upon which real reductions in emissions can be achieved, by focusing resources where they will have greatest impact. Moreover, accurate reporting is critical if these eff orts can be confi dently demonstrated.
N
ew frameworks for reported measured methane emissions such as those developed by the Oil and Gas Methane
Partnership 2.0 (OGMP2.0) identify different types of measurement [1]. Those used for reporting emissions are colloquially referred to as ‘Bottom-Up’ and comprise an aggregation of all known sources on a site. Bottom-up measurements satisfy the expectations of OGMP2.0 Level 4 reporting and may include components such as fl aring, venting, fi red equipment and fugitives. Level 5 of OGMP2.0 reporting introduces an additional step of measurement taken at the site level as a verifi cation that reporting is accurate. Measurements of this kind are colloquially referred to as ‘Top- Down’. Comparison of bottom-up and top-down data is often referred to as the process of reconciliation.
OGMP2.0 does not specify ‘how’ top-down measurements should be performed. However, the framework does require that the top- down method be able to quantify at emission rates commensurate with those reported and both types of measurement must be accompanied with an estimate of uncertainty [2], without which quantitative comparison is not possible. Reconciliation should not be considered a one-off exercise but should be scheduled as a process of continuous improvement where the fi t between reported and verifi cation measurements should improve with each reporting cycle.
In this paper we present recent measurements taken in preparation for Level 5 reporting conducted at two complex oil and gas sites. Both measurements utilise the same methane spectrometer mounted on different types of drones.
Site 1 is a large offshore facility producing both oil and gas. It is located approximately 60 nautical miles offshore. Known emissions from the site comprise incomplete combustion from fl aring, engine slip from gas turbine power generation and fugitive emissions. Emissions vary, but are typically <25 kg/hour as estimated with current reporting methodology. At current levels of production and reporting methodology, methane intensity (emissions relative to marketed gas) is ~0.2%.
Site 2 is an onshore gas and oil processing facility. All hydrocarbons are imported and exported by pipeline. Known emissions from the site comprise incomplete combustion from fl aring, engine slip from gas turbine power generation and compressors, other fi red equipment (e.g. oil heaters), fugitives emissions and discontinuous process vents. Emissions vary but are typically <100 kg/hour. At current levels of throughput and reporting methodology, methane intensity is ~0.02%.
Both sites are typical of their kind, suffi ciently large and of distinct design and operation that for Level 5 reporting and reconciliation they can be each treated as a population of one - where they contribute material emissions relative to the operator portfolio and could be reasonably expected to be reconciled individually and not form part of a population-based study.
Methods Reported Emissions
Reconciliation requires time specifi c data for the reported emissions that can be compared to the precise time and date of the top-down survey. Production related emissions including those from fl ares, turbines and heaters were obtained from gas fl ow rate fi gures retrieved from the data historian (Pi) for the corresponding hour at which top-down measurements were taking place, using the mean average of one-minute increments. Uncertainty in gas fl ow was taken from the calibration certifi cates of the fl ow meters. Conversion to methane emission rates used United Kingdom EEMS emission factors [3]. For calculated fugitive emissions the annualised estimate was converted into an hourly emission rate. No uncertainty was assigned to emission factors. The combined uncertainty was estimated in accordance with the GUM [4].
Top-Down Measurements – methane sensor
Top-down measurements were conducted using a SeekIR methane sensor which operates on the principle of tuneable
diode laser absorption spectroscopy (TDLAS) [5]. In-fl ight limits of detection of the sensor are 150 ppbv and 40ppbv against atmospheric background levels of methane for the closed cavity (fi xed-wing sensor) and open cavity (quadcopter sensor), respectively. The method of methane emissions quantitation is based upon the application of a mass balance equation using methane concentration, wind, and location data, resulting in an accurate and deterministic estimate of the mass fl ow rate of methane for the area of interest. The limit of quantitation and uncertainty of both methods have been determined through controlled release experimentation [6]. Emissions of >1kg/h are quantifi able with a relative uncertainty of 16% (32% at the 95% confi dence interval k=2).
Top-Down measurements
For site 1 offshore measurements, the SeekIR sensor was mounted on a fi xed-wing autonomous aircraft. The fl ight was mobilized directly from shore and required no additional equipment or personnel on the offshore platform.
The fi xed wing aircraft was fl own at a constant radius from the from the survey center-point and is approximately 250 metres from the site’s furthest point. The survey started at the set radius from the asset at the highest sampling altitude. The aircraft descended in a constant radius spiral from the maximum altitude, ~210 meters (700 feet) above ground level (AGL), stepping down at a consistent vertical step of 10 metres to the lowest altitude, around 30 metres (100 ft) AGL. The aircraft was fl own at a
Figure 1: Reconciliation of emissions reported for an offshore site.
IET JANUARY / FEBRUARY 2023
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