2019-092 Gas Well Setbacks - Dispersion ModelsDate: May 3, 2019 Report No. 2019-092
INFORMAL STAFF REPORT TO MAYOR AND CITY COUNCIL SUBJECT: Gas well setbacks based on different numbers of wellheads and dispersion models of vapor clouds EXECUTIVE SUMMARY: On April 2, 2019, Council Member Meltzer stated that he had recently participated in a joint University of North Texas and Texas A&M Panel / Energy Symposium regarding gas well setbacks. Based on this panel discussion, Council Member Meltzer requested a staff report on the feasibility and desirability of site specific gas well padsite setbacks based on differing numbers
of wellheads and dispersion models of vapor clouds. BACKGROUND: Staff contacted the panelist that provided the setback information referenced above. The discussion was centered on the idea of basing reverse setbacks based on safety and health.
However, the examples provided during the panel discussion were related to acute hazards of gas well sites, and included an example of a developer that wanted to maximize the amount of developable land on a property that contains a gas well pad site. The panelist chose to use a modeling tool to analyze the amount of distance that was required to ensure future homes will not be impacted by a release of gas from the gas well pad site due to the gas concentrations exceeding
the gas / air mixture’s lower flammability limit (LFL). The lower flammability limit is the lower end of the concentration range over which a flammable mixture of gas or vapor in air can be ignited at a given temperature and pressure. Below this range, the mixture of air and gas cannot be ignited. The analyses were performed under a variety of potential scenarios to determine the minimum
reverse setback distance needed to ensure no impacts, then concluded with a recommended overall minimum distance that was based on the maximum distance potentially impacted. The modeling tool used to perform the analyses was developed by a consulting firm, and used Aloha, Gaussian, and heave gas models. The tool is intended to be used for planning and response purposes to determine the distance from the pad site that might have a flammable environment due to a gas
release. In other words, the model was used as a part of an overall a risk assessment to determine the maximum distance that would likely exceed the LFL due to a catastrophic gas release, and used the result to help establish a minimum setback. The modeling tool uses dispersion modeling to determine gas concentrations at specific distances
away from a gas release. It is important that the dispersion model for a given site is based on that
site’s gas, pressure, flow, etc. and results will differ due to site conditions and will likely change
over time as pressures and flows change at the site. Of note, the model results indicated a 100
foot reverse setback was adequate to ensure protection against a flammable environment during a
catastrophic release for the particular site analyzed by the panelist.
Staff contacted the consultant that provided the model to the panelist and asked the consultant to
provide a cost estimate for conducting lower flammability risk assessments on a per gas well pad
Date: May 3, 2019 Report No. 2019-092
site basis. These analyses would include the development of a Lower Flammability Limit graph
as a function of distance and a figure denoting a “circle of concern” at three choices of
concentrations. The costs did not include any report development or additional data analyses,
and would need the input of gas well site operators and likely other experts to determine realistic
gas concentrations to use in the analyses. The cost for just the lower Flammability Limit graph
and three circles of concern figures is $1,485 per pad site. The cost of the additional expertise
needed to determine realistic gas release concentrations under various failure models is unknown
at this time and will likely be contingent on the characteristics of each pad site.
CONCLUSION:
This informal staff report has been provided for informational purposes.
ATTACHMENT(S): None STAFF CONTACT:
Kenneth Banks
General Manager of Utilities 940-349-7165 Kenneth.banks@cityofdenton.com