Greenhouse gas emissions from oil and natural gas production activities in the United States are reported to the U.S. Environmental Protection Agency (EPA) and other regulatory bodies to understand their environmental impact and to maintain a national inventory of emissions. In this study, continuous monitoring using a methane lidar camera was utilized to measure vented methane emissions from completions flowback operations at a natural gas production facility in the Marcellus Basin in northern Pennsylvania. Measurements showed significant temporal variations in the emission rate. Total emissions were also estimated using conventional engineering calculations established by the EPA for reporting under the Greenhouse Gas Mandatory Reporting Rule program in the US. The measured methane emissions were found to be 88% less than the estimated emissions. The overestimation from engineering calculations has direct impact on the greenhouse gas inventory and could increase the waste emissions charges incurred by the facility. This study examines assumptions made in the engineering calculations and illustrates the value of continuous monitoring to accurately report and quantify highly variable methane emissions from oil and natural gas operations.