Risk Assessment and Field Case Study on Oil/Natural Gas Explosion during High-Pressure Air Injection Process

Abstract

Summary Air injection techniques have been widely applied in oil fields, but the associated safety issue of natural gas (NG) and oil explosion has been of great concern. In this study, explosion experiments of different NG compositions were conducted under high-pressure and high-temperature conditions (up to 15 MPa and 373 K) to reveal the necessary conditions for gas explosion in the air injection process. The experimental results indicate that the lower flammability limits (LFLs) and upper flammability limits (UFLs) of NGs change logarithmically with increasing pressure, which can significantly increase the explosion risk. The rise of temperature can also expand the flammability limit range. Based on the mechanisms and necessary conditions for NG and oil component explosion, fault tree analysis (FTA) models for explosion occurring during the air injection process were proposed that can provide valuable guidelines for designing anti-explosion procedures for field applications. Several explosion incidents that have occurred in air injection operations in different oil reservoirs are described, and the explosion mechanisms are analyzed. NG explosion can occur during air injection when NG is the main component present in the gas phase that can mix with air to form a combustible gas. For heavy oils with little NG, autoignited explosion of vaporized oil components can be the main reason for the incidents during the steam and air coinjection process because the autoignition temperature of heavy oil can be greatly reduced at high pressure.