Quantification of volatile organic compound emissions from unconventional oil and gas development

Oil and gas (O&G) development in the U.S. has accelerated in the
past two decades, aided by unconventional extraction techniques including
hydraulic fracturing and horizontal drilling. Potential environmental and
health impacts of volatile organic compounds (VOCs) originating from
O&G activities in populated regions have raised concerns. In
Broomfield, Colorado, six new O&G well pads were approved for
development in 2017 and an air monitoring program was established in
October 2018 to collect weekly and plume-triggered air samples. This study
addresses the limited existing knowledge of activity-specific VOC emission
rates from unconventional O&G development (UOGD), utilizing these
observations and dispersion model simulations through emission inversion
methods. Emissions are characterized from well drilling, hydraulic
fracturing, coiled tubing/millout, flowback, and production operations.
Substantial variations in average VOC emission rates, determined using
weekly canister observations, are observed across different UOGD phases.
Drilling and coiled tubing/millout operations exhibit the highest VOC
emission rates, attributed to hydrocarbon release from shale formations
and drilling mud. In contrast, hydraulic fracturing gives lower emission
rates, consistent with injection of fluids into the well, minimizing the
probability of subsurface hydrocarbon emissions. Diesel-powered engines
are identified as the primary ethyne sources during hydraulic fracturing.
Production was characterized by lower VOC emission rates than
pre-production phases but remains an important emission category due to
its long duration (decades). Internal variations of emission rates within
each phase highlight the complexity of factors and activities influencing
emission rates, including, for example, vertical vs. horizontal drilling
and periodic maintenance activities. VOC emission rates associated with
drilling mud volatilization and hydraulic fracturing suggest that
previously published emission estimates (EPA (2022), and Hecobian et al.
(2019)) underestimate VOC emission rates during these activities.
Significantly lower emission rates during flowback compared to previous
work (Hecobian et al., 2019) reveal how improved management practices,
including tankless, closed-loop fluid handling systems have effectively
reduced what used to be a dominant source of pre-production VOC emissions.
Plume-triggered samples, capturing transient high-concentration plumes,
reveal short-term VOC emission rates approximately ten times higher for
drilling and flowback than determined from weekly samples. In the case of
flowback, short-term emission pulses have been linked to periodic emptying
of sand canisters used to trap fracking sand emerging from previously
fracked wells.