A Method for Handling Gas Kicks Safely in High-Pressure Wells

Abstract

Summary The risk of overloading an atmospheric mud/gas separator (MGS) during high-volume gas kill operations can be minimized. This paper presents the theory and procedure for minimizing this risk for high-temperature/high-pressure (HT/HP) wells. The method can be used to set criteria for selecting the kill method and for separator sizing. Introduction The number of HT /HP wells being drilled in the North Sea has increased rapidly. Pressures >15,000 psi are common with associated temperatures>350°F. The drilling and exploitation of these reservoirs have been described as the new challenge to the drilling industry. However, serious well-control incidents have compelled the industry to re-examine the equipment and procedures used to drill these wells. This paper examines a major shortcoming identified recently: the ability of rig equipment to separate and vent large gas volumes from mud safely. Venting rates from kicks in high-pressure wells can approach the equivalent production of a commercial gas well. If the separator is overloaded, gas will be blown back into the shale shaker room and other mud-processing facilities, creating an extremely hazardous environment with a high explosion risk. Also, live crude, condensate, and drilling mud will be expelled by the gas through the gas vent line. On offshore rigs, where the vent line often discharges at the top of the derrick, the liquids will fall back to the drilling rig or platform. This creates a fire or explosion hazard. The volatile condensate/oil mixture will accumulate close to the deck in any void space and can be ignited by a spark. The hazards of MGS blowdown are apparent. High reservoir pressures and temperatures increase the surface gas volume per unit volume of gas influx. The ability of rig gas-processing equipment to handle such increased volumes concerns the industry. Normal practice does not take into account the limitations of the separator but instead concentrates on well control, assuming that the MGS can handle whatever gas volume is present. The problem is clear. As we continue to explore for oil and gas in deeper and higher pressure reservoirs, we often exceed the safe operating limits of the MGS. Separator modification and redesign has improved its capacity considerably, but space restrictions, especially on mobile rigs, impose an inherent limitation. Our lack of understanding of the complex separation process has done little to improve operating procedures and practices.