|Schematic chart showing included stages within OPGEE. El Houjeiri et al., Supplemental Information. Click to enlarge.|
A team from Stanford University and the California Air Resources Board (ARB) has developed a new open-source lifecycle analysis (LCA) tool for modeling the greenhouse gas emissions of oil and gas production using characteristics of specific fields and associated production pathways. The team describes the Oil Production Greenhouse Gas Emissions Estimator (OPGEE) in a paper in the ACS journal Environmental Science & Technology.
Existing transportation fuel cycle emissions models are either broad—i.e., lacking process-level detail for any particular fuel pathway—and calculate nonspecific values of greenhouse gas (GHG) emissions from crude oil production, or are not available for public review and auditing, the authors note.
Emissions of greenhouse gases (GHGs) from crude oil production vary significantly depending on production practices and crude oil qualities. The use of energy-intensive secondary and tertiary recovery technologies can have significant impacts on emissions. Other major factors are venting, flaring and fugitive (VFF) emissions, which are difficult to measure and estimate. Previous studies show that upstream, well-to-refinery gate (WTR) emissions vary by a factor of 10 from low emissions to high emissions fields. This variability highlights the importance of having the capability to assess the different types of crude oil production operations and under different conditions.
Regulatory approaches, such as the California Low Carbon Fuel Standard (LCFS) and European Fuel Quality Directive (EU FQD), seek to regulate the life cycle GHG emissions for transport fuels.
...To advance the modeling of crude oil production GHGs in a transparent manner, the Oil Production Greenhouse Gas Emissions Estimator (OPGEE) has been developed. OPGEE is built with the goals of achieving more accuracy and better transparency in the assessment of life cycle GHG emissions from crude oil production. OPGEE calculates the energy use and emissions from crude oil production using engineering fundamentals of petroleum production and processing. This allows the model to flexibly estimate emissions from a variety of oil production emissions sources.—El-Houjeiri et al.
In their paper, Hassan El-Houjeiri and Adam Brandt from Stanford, and James Duffy from ARB, introduce OPGEE and its structure, modeling methods, and data sources, then run it in default mode and on a small set of fictional fields (based on real California fields) selected to have varying characteristics and meant to represent a variety of possible operations. These serve to anchor the sensitivity analysis. The results show the GHG emissions breakdown and the sensitivity of emissions to selected input parameters.
The functional unit of OPGEE is 1 MJ of crude petroleum delivered to the refinery entrance (a well-to-refinery, or WTR system boundary), with emissions presented as gCO2 equiv GHGs per MJ of crude at the refinery gate. This functional unit is held constant across different production processes included in OPGEE. The energy content of crude oil at the refinery gate is calculated based on API gravity (no account of effects of other crude oil characteristics such as sulfur content). OPGEE defaults to lower heating value (LHV) basis for all calculations, but model results can also be presented on higher heating value (HHV) basis.
|Basic structure of OPGEE. Credit: ACS, El-Houjeiri et al. Click to enlarge.|
OPGEE calculations use a bottom-up engineering-based approach. OPGEE relies on dozens of calculations across all stages of oil production, processing and transport.
Data for the four fictional fields used in the paper (A, B, C, D) are derived from the online production and injection database and technical reports from the California Department of Conservation, Division of Oil, Gas, and Geothermal Resources (DOGGR).
Field A uses steam injection to decrease crude viscosity. Field B is characterized by very high water-oil ratio (WOR), which represents an inefficient lifting process and significant energy use to manage large amounts of water at the surface (e.g., treatment and re-injection). Field C is characterized by average depth and moderate WOR. Field D is characterized by low depth, low WOR, and higher gas−oil ratio (GOR). The “generic” case uses only the default parameters used to run OPGEE when no data are available.
The researchers explored variation in GHG outcomes due to WOR; field depth; oil production volume; steam-oil ratio (SOR); application of a heater/treater in surface oil−water separation; and flaring rate. OPGEE found that that upstream emissions from petroleum production operations can vary from 3 gCO2/MJ to more than 30 gCO2/MJ using realistic ranges of input parameters. Significant drivers of emissions variation are steam injection rates, water handling requirements, and rates of flaring of associated gas.
Results from OPGEE show clear evidence that assuming a single value for the GHG intensity of oil production is problematic because of significant variation in emissions from different operations. This is particularly the case for regulations aiming to reduce WTW GHG intensity of fuels. Future efforts to better understand and characterize this variation are clearly required. Additional efforts will also focus on improving data availability and the data basis for model defaults.
Future work on OPGEE will address scope limitations and coverage of technologies. Coverage will expand to include oil sands operations, as well as heavy oil and other EOR technologies. Supporting technologies, such as hydraulic fracturing and stimulation, will be included to better represent modern production practices.—El-Houjeiri et al.
The work was funded by ARB.
Hassan M. El-Houjeiri, Adam R. Brandt, and James E. Duffy (2013) Open-Source LCA Tool for Estimating Greenhouse Gas Emissions from Crude Oil Production Using Field Characteristics. Environmental Science & Technology doi: 10.1021/es304570m