Experimental study on the causes and critical conditions of toxic gases in hydraulically fractured oil wells

Abstract

ABSTRACT Hydraulic fracturing is the most preferred oil well stimulation technology worldwide. In fractured oil wells, toxic gases (hydrogen sulfide and carbon monoxide) present a problem when the produced water is reclaimed as base fluid for fracturing. These gases can cause equipment corrosion, environment pollution, and adversely affect the operational personnel’s health. Extant research focuses on toxic gases produced at high temperatures (>200 °C). Therefore, this paper examines the causes and critical conditions for toxic gas formation in fractured wells in conventional reservoirs at low temperatures (<160 °C) through a series of laboratory experiments that include bacterial culture and heating experiments. The results show that for temperature in the range 40–60 °C, the presence of sulfate-reducing bacteria in the produced water is the main cause of hydrogen sulfide production in fractured wells. However, when the temperature is higher than 102 °C, the sulfur compounds (potassium persulfate and sodium sulfite) in the fracturing fluid and crude oil can produce hydrogen sulfide by thermochemical sulfate reduction. In addition, when the temperature is higher than 75 °C, oxygenated compounds (aldehydes) in crude oil can produce carbon monoxide by decarbonylation. Both processes can occur simultaneously when the temperature is higher than 102 °C. This paper is of great significance in controlling the formation of toxic gases in fractured wells.