Quantifying Methane Emissions from Oil Fields in the Central Valley of CA

Abstract

In 2024, California was the eighth-largest producer of crude oil in the U.S. by volume with ∼50,000 active wells. Working with the California Energy Commission, we measured methane (CH4) emissions from 172 actively producing oil wells (219 total observations with 47 repeat measurements) either with or without casinghead gas collection systems (GCS) in two oil fields near Bakersfield in Kern County. We also measured benzene (C6H6) emissions at a subset of wells to assess C6H6 to CH4 ratios. The mean mass-based ratio for C6H6:CH4 was 7.8 × 10–7 with a mean benzene emission rate of 0.9 mg h–1. Wells with an open-ended line (OEL) emitted ∼100-fold more methane on average than wells with a GCS: 1.26 kg h–1 and 0.01 kg h–1, respectively. The prevalence of wells with a GCS rose sharply with increasing oil and gas production, rising from 36% for wells producing <2 barrels of oil equivalent per day (boed) to 97% for wells producing ≥15 boed. The top 5.7% highest-emitting wells were responsible for ≥50% of total methane emissions from casinghead gas. Methane emissions were overwhelmingly dominated by wells without a GCS, accounting for 98.5% of total casinghead gas emissions across both fields. Capturing gas from these wells would eliminate almost all of the methane emissions we detected.