The detonation of explosives in the wellbore produces hazardous gas; however, these gases are not typically observed in high concentrations at the surface. Recently, during plug and abandonment (P&A) operations, carbon monoxide (CO) from perforation activities was observed in high concentrations. This paper examines these types of operations to determine root causes and mitigation methods.

The anticipated amount of CO produced by detonation is calculated by both the empirical equation and reaction-equilibrium simulation methods for cyclotetramethylene tetranitramine (HMX), as well as by the simulation method for cyclotrimethylene trinitramine (RDX), hexanitrostilbene (HNS), and 2,6-bis,bis-(pikrylamino)-3,5-dinitropyridine (PYX). The life cycle of this gas from the time of generation through its potential release to the surface is discussed with the intent to reduce its quantity or concentration throughout. Mitigation methods include the incorporation of an oxidizer in the explosive reaction, chemical scavenging in the wellbore, and controlled venting or catalytic conversion at the surface.

Significant quantities of CO are produced by perforating guns, with the proportion increasing for explosives of greater thermal stability until it is the single largest reaction product. During perforation, these gases are usually controlled by gas-handling equipment on the platform; however, the reduced availability of this equipment on the platform at the time of P&A operations is thought to be a contributing factor to the hazard. Another significant factor could be the use of a high circulation rate, which has the effect of increasing the concentration of the gas on the surface. Controlled venting, flaring, and catalytic conversion to carbon dioxide are feasible methods to help mitigate this hazard if conducted in accordance with regulations.

This paper details the life cycle of CO gas generated from perforating activities and discusses how it can be hazardous during P&A operations. In addition, several methods are discussed that can help mitigate this hazard.

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