Downhole Blowout Preventer

Abstract

Abstract A downhole blowout preventer (DHBOP) for improved well control while drilling has been developed and tested. The tool is not meant to replace the traditional BOP, but to be an additional barrier when the mud is out of control. The tool is located in the bottom hole assembly. The wellbore can be sealed off in case of an influx or lost circulation. Mud of adequate density is circulated through the drillstring and into the annulus just above the downhole seal until wanted wellbore pressure is achieved. The field test has demonstrated the functions of the tool, but also identified needs for improvements. Tool modifications may also introduce additional functions like multi-operational circulation port, drillstring choke and test plug for traditional BOPs. Introduction Drillers always try to keep the well pressure between the pore pressure and the fracture gradient. Lost circulation may occur when the well pressure is higher than the fracture gradient or when drilling into permeable low-pressure formations. The problem increases in deepwater wells, deviated wells and extended reach wells. Influx may occur when the well pressure becomes lower than the pore pressure. The pore pressure is not exactly known when drilling. The uncertainty is special high in exploration wells where no experience is available. That increases the probability for having a kick in exploration wells. Drilling into shallow gas accumulations and highpressure reservoirs may give a sudden increase of the pore pressure resulting in a heavy influx. Another situation may occur when floating vessels are used in deepwater drilling. In drift-off situations, due to failure in the dynamic positioning system, the marine drilling riser has to be disconnected at the seabed. The well pressure will be reduced since the density of seawater is less than the density of mud. Influx will be the result if the pressure reduction is larger than the riser margin. Two independent pressure barriers are normally required during all well operations. These are normally heavy mud and the conventional BOP stack. As described above there are several situations where the mud is out of control as a safety barrier. A downhole blowout preventer has been developed. The concept includes an inflatable packer to seal off annulus and a double acting poppet valve to seal of the drill string and to open a circulation port. The DHBOP is located below the MWD and actuated by mud pulse telemetry. When actuated, both the drillstring and the annulus are sealed off. The circulation port allows displacement and replacement of annular fluid with mud of sufficient density. Temporary control is achieved immediately. The well is fully stabilized after the well is circulated with mud of adequate density. The main advantage by using the DHBOP during influx and lost circulation is to have the well sealed off close to the bit when the well is circulated. This paper describes the tool and the field test. Modifications based on experiences are also discussed. History The downhole blowout preventer is based on a concept developed at NTNU Department of Petroleum Engineering and Applied Geophysics. The first prototype went through several field tests in 1993 and 1994. The need for improved reliability of the actuator inside the DHBOP (Fig 1a) and increased interest from operators resulted in the work described in this paper.