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.

Methods
This dataset uses the following methods:

Emission rates based on weekly canister samples:

We first identify the active well pads at each week based on the well pad operation timelines. The emission rates for active well pads are constrained using observed concentration and dispersion model simulations through multiple linear regression. Then we select the emission rates that can be attributed to single operations based on the well pad operation timelines and exclude emission rates associated with multiple operations on a single well pad during a given week.

Emission rates based on plume-triggered samples:

We first extrapolate VOC concentrations to 1-hour estimated concentrations using the corresponding PID readings. Then we calculate the 1-hour concentration enhancements over background concentrations from the COM site. Run AERMOD simulations with unit emission rate using wind rotation method. The 1-hour emission rates are based on the ratio of 1-hour concentration enhancements over AERMOD simulated 1-hour concentrations.

过去二十年间，得益于水力压裂（hydraulic fracturing）与水平钻井（horizontal drilling）等非常规开采技术的支撑，美国油气（Oil and Gas, O&G）开发进程显著加快。人口密集区域内油气活动产生的挥发性有机物（volatile organic compounds, VOCs）可能引发的环境与健康风险，已引发广泛关注。2017年，科罗拉多州布鲁姆菲尔德市获批开发6座全新油气井场，并于2018年10月启动空气监测计划，按周采集空气样本并针对羽流触发事件开展专项采样。

本研究针对当前非常规油气开发（unconventional O&G development, UOGD）活动特异性VOC排放速率的认知空白，依托上述监测数据与基于排放反演方法的扩散模型模拟展开研究，覆盖钻井、水力压裂、连续油管/磨铣、返排及生产等全作业环节的排放特征。

基于每周罐采样本测算的平均VOC排放速率，在不同非常规油气开发作业阶段呈现显著差异。钻井与连续油管/磨铣作业的VOC排放速率最高，其成因可归结为页岩地层烃类释放与钻井泥浆挥发。与之相对，水力压裂作业的排放速率较低，这与向井内注入流体、降低地下烃类释放概率的工艺特点相符。研究同时识别出柴油动力发动机为水力压裂阶段乙炔（ethyne）的主要排放源。生产阶段的VOC排放速率低于预作业阶段，但由于其持续周期长达数十年，仍是重要的排放来源。各作业阶段内部的排放速率差异，凸显了影响排放速率的各类因素与活动的复杂性，例如垂直钻井与水平钻井的区别、周期性维护作业等。

钻井泥浆挥发与水力压裂环节对应的VOC排放速率显示，既往公开的排放估算（美国环境保护署EPA (2022)与Hecobian等人(2019)的研究）低估了上述作业环节的VOC排放速率。返排环节的排放速率显著低于既往研究（Hecobian et al., 2019）的结果，这表明包括无罐式闭环流体处理系统在内的优化管理措施，已有效降低了此前预作业阶段主要VOC排放源的排放量。针对瞬态高浓度羽流的羽流触发采样显示，钻井与返排环节的短期VOC排放速率约为每周样本测算结果的10倍。以返排环节为例，短期排放脉冲与定期清空用于收集已压裂井返出压裂砂的砂罐作业存在关联。

## 研究方法
本数据集采用以下研究方法：

### 基于每周罐采样本的排放速率测算
首先依据井场作业时间线确定每周的活跃井场。通过多元线性回归，结合实测浓度数据与扩散模型模拟结果，约束活跃井场的排放速率。随后根据井场作业时间线，筛选出仅对应单一作业环节的排放速率，剔除单井场当周存在多作业环节时对应的排放速率数据。

### 基于羽流触发样本的排放速率测算
首先借助对应光离子化检测仪（PID）的读数，将VOC浓度外推为1小时预估浓度。随后计算来自COM监测点的背景浓度增量，采用风旋转法开展单位排放速率下的AERMOD模拟。1小时排放速率基于1小时浓度增量与AERMOD模拟得到的1小时浓度的比值计算得到。