Techniques for Handling Upward Migration of Gas Kicks in a Shut-In Well

Abstract

Abstract This paper discusses the experimental evaluation of various proposed well control techniques which may be implemented when normal kill operations are delayed. Two 6000-ft wells, one containing the flow geometry present in a surface blowout preventer (BOP) stack and the other modeling the preventer (BOP) stack and the other modeling the flow geometry of a subsea BOP stack in 3000 ft. of water, were used to evaluate (l) the constant drill pipe pressure method and (2) the static and dynamic volumetric methods that can be employed when a meaningful drill pipe pressure is not available. It was found that any of the three methods could be used to maintain an acceptable bottom hole pressure in a surface BOP system with a uniform annular geometry. However, the static volumetric method generally resulted in a decreasing bottom hole pressure once gas reaches the sub sea choke line pressure once gas reaches the sub sea choke line and is not recommended for deep-water floating drilling operations. Introduction After a well has been shut in due to the presence of a kick, there may be a considerable lapse of time before normal well control procedures can be implemented. These delays may be caused by (1) time to increase the mud density to the desired kill weight or (2) mechanical problems with drilling equipment or high pressure flow conduits. If the formation fluids are predominantly gas, the large density difference between the gas and the drilling fluid will cause the gas to migrate up the hole during this shut-in period. As the gas migrates upward, wellbore pressures continually increase until such time that the pressure opposite the weakest formation exceeds that formation's fracture pressure, resulting in the breakdown of that formation pressure, resulting in the breakdown of that formation and possibly an underground blowout. If the formation fluid does not contain a gaseous phase, upward migration of the fluid is generally not significant and kick removal is relatively simple. This paper deals exclusively with gas kicks. Under normal conditions, the excessive pressures resulting from upward gas migration in a shut-in well can be alleviated by allowing the gas to expand through periodic bleeding of mud at the surface. The expansion is best controlled by maintaining the drill pipe pressure at a value slightly in excess of its initial shut-in value through the use of an adjustable surface choke. This maintains the pressure held against the "kicking" formation above its pore pressure and additional fluids cannot enter the well. As the drill pipe pressure tends to increase due to the upward gas migration, mud is bled from the annulus using the surface choke, giving the kick room to expand in the annulus and thereby reducing the kick's pressure. However, certain situations arise when a meaningful drill pipe pressure is not available. These situations can arise if:the drill bit is plugged, shutting off pressure communication between the drill pipe and formation;the drill string has failed, allowing communication between fluids in the drill string and annulus, andthe drill string is off-bottom, causing the drill pipe and casing pressures to read the same until the kick has migrated above the bit. In addition, the drill string could be out of the hole entirely. For kicks taken with the drill string off-bottom, the pipe could be stripped back to the bottom of the hole if proper precautions are taken, but this is time-consuming and significant gas migration will occur during the stripping operations. To cope with the above situations where a meaningful drill pipe pressure is not available, volumetric methods of well control have been suggested by various authors as an alternate method to handle upward gas migration. Volumetric methods are based on observed changes in casing pressure and metered volumes of drilling fluid pressure and metered volumes of drilling fluid bled from the well. The method is based primarily on theoretical considerations and largely without documented experimental verification.