In addition to enhancing drilling safety and performance, MPD offers new options for well control. Because MPD allows rapid changes in bottom hole pressure, the response to an unwanted influx during MPD operations is not necessarily to shut-in and weight-up. Instead, the required overbalance can be regained dynamically to stop an influx, as well as circulate it from the wellbore. This fundamental difference from conventional drilling allows components of the primary barrier envelope to be used without having to activate secondary well control equipment, as the MPD system is designed to handle influxes to a maximum size. Most commonly in MPD operations, this can be achieved by closing an MPD choke, while circulation is maintained. The advent of dual gradient drilling offers further variation in dynamic well control practices.
MPD is a deviation from traditional drilling practices and the technique is relatively new to industry. MPD systems are designed to detect and control influxes of a certain size, however, many MPD operations revert quickly to conventional shut in procedures when confronted with an influx regardless of the size. Effectively, all influxes are treated as kicks.
Inconsistent definitions and misunderstandings of MPD systems' equipment and capabilities amplify the problem of determining when dynamic operations are appropriate. The switch from influx management with the MPD system to conventional well control is governed procedurally through the definition of operational decision points based on influx indicators and surface backpressure during drilling. Quantification of manageable influx size is not standard. The onus is on the operator to define these decision points such that influxes are safely managed by the MPD system, while the rig handles kicks. Further, the use of MPD equipment can also be used to limit the resultant size of an influx that has become a kick, demonstrating that MPD equipment can ultimately enhance drilling safety, in most cases.
In this paper, differentiation between appropriate MPD operations using the primary and secondary barrier envelopes is proposed. The work explores the key factors that must be considered in determining when the appropriate initial response to any influx is dynamic or conventional. A common understanding throughout industry of the safety related aspects of MPD may lead to the use of such systems on every rig, defining the new normal.