Abstract

Gas emission is a safety concern associated with idle and orphan oil and gas wells. Among the various well-integrity barriers, cement bonding between the wellbore and the surrounding geologic formation can prevent gas from escaping to the surface. The emission of gases can contribute to groundwater contamination and surface safety hazards. As such, studying the factors that affect borehole cement bonding is important for reducing methane emissions and extending long-term well integrity. This study evaluated a series of wells in Southern California to assess cement bonding across hydrocarbon-bearing geological intervals. The results show that for these wells, hydrocarbon-bearing formations exhibit weaker cement bonding than water-bearing formations. Poor bonding can create preferential pathways for methane leakage, allowing gas to migrate outside the steel casing through compromised cement barriers over long time scales and ultimately reach the surface. These findings show the importance of doing such studies before structures are built on top of plugged and abandoned wells.

Keywords:Cement-formation bonding; Idle and orphan wells; Methane emissions; Cement bond log (CBL); Variable density logs (VDL)