Concentric tube well servicing technology is employed in numerous applications where pressure and fluid containment must be controlled at the surface. By design, the primary mechanism for maintaining pressure and fluid containment in these types of concentric-tube services is the purpose-built well control stack working in combination with fluid flow control devices in the surface treatment piping. As such, concentric tube well servicing systems have a unique advantage in that they are not bound by use of balance-weight fluids to maintain pressure and flow control as employed in conventional jointed-pipe rig systems.
However, when service conditions dictate that well pressure be at balance at the surface, well control practices which focus on fluid hydrostatics must be employed. Although based on the processes used in conventional jointed-pipe rig systems, coiled tubing well control practices performed concentric to existing well tubulars are more complex and require greater attention in design and implementation.
This paper provides an overview of the recommended coiled tubing well control stack and flow control device configurations for defined pressure control conditions and offers practical approaches which address the various concerns unique to coiled tubing well control applications.
Coiled tubing applications have been widely used to conduct various through-tubing operations in the oil and gas industry1. If surface pressure is present within the wellbore during coiled tubing operations, an underbalanced pressure condition exists, requiring implementation of well control practices which affect the required type of seal needed to safely contain the pressure. In conventional jointed-tubing rig systems, the primary means of well control is obtained through hydrostatic balance of well pressure using weighted fluids. In the event that hydrostatic pressure balance is lost, then well control is regained through contingency operations conducted using the surface well control stack components and a detailed fluids pumping program where weighted fluids are circulated within the wellbore to reestablish the hydrostatic pressure balance at the exposed formation(s).
In contrast, the primary means of well control used in coiled tubing operations is through the function of the surface well control stack. The continuous-length coiled tubing string is deployed into and retrieved from the wellbore through a specially-equipped well control stack which is designed to provide various pressure sealing functions for containment of surface well pressure. As a result, coiled tubing operations can be safely performed in live well conditions. The performance requirements for coiled tubing well control stacks are defined in industry standards, such as API RP 5C72, stipulating that the stack components incorporate functions directed to maintain pressure isolation.
In addition, a secondary means of well control must be incorporated into coiled tubing operations as a contingency should the primary function of the well control stack fail. To be consistent with industry-established well control practices, coiled tubing operations must provide a means for regaining well control through the pumping of kill-weight fluids to create the desired hydrostatic pressure balance within the wellbore.
Each coiled tubing operation performed in an underbalanced pressure condition should be reviewed to identify all potential hazards which may compromise well control operations. This paper addresses various issues which should be considered in the proper planning of well control operations using coiled tubing services. The issues related to the candidate well include wellbore type, completion design, pressure balance at the given temperature, desired kill-weight fluid rheology and concerns for formation damage resulting from the kill operation. The issues related to the coiled tubing equipment include the pressure rating of the well control stack assembly and surface piping with regard to the maximum anticipated wellbore surface pressure, condition of the coiled tubing string, surface tankage volume available onsite, the ability to acquire, blend and pump the prescribed kill-weight fluid and the frictional pressure losses expected when circulating kill fluids within the system.