An important factor in the overall success and cost of an offshore well intervention depends significantly on the choice of the equipment including downhole tools and surface units. Using a conventional workover rig for well intervention is expensive and may not be appropriate in many circumstances. Additionally, intervention of a subsea well can further increase the costs due to the requirements of a floating vessel and subsea systems to access the wellbore.
This paper systematically categorizes downhole applications that are performed during well intervention. It reviews techniques such as Wireline, Coiled Tubing (CT) and Hydraulic Workover (HWO) that do not require use of a conventional workover rig and have capability of performing downhole applications in live (under pressure) wells. Included are guidelines for selection of these rigless techniques for the various downhole applications.
The rigless techniques are further reviewed in relation to subsea well intervention. The emphasis is placed on selection of a floating vessel along with subsea systems to connect to the well. The paper includes a discussion on market trends that are directed towards either reducing intervention costs or minimizing its frequency.
Well intervention is defined as remedial operations that are performed on producing wells with the intention of restoring or increasing production. A well may require intervention due to flow restrictions, changes in reservoir characteristics, sand production, mechanical failure, or to access additional hydrocarbon pay zones. Downhole applications that are performed during well interventions include well surveillance and diagnostics, implementation of reservoir management techniques, completion repair and reentry drilling to reach new producing intervals. This paper defines and categorizes these downhole applications for use in determining guidelines for selection of rigless techniques.
The choice of techniques and equipment to perform the downhole applications determines the mechanical and economic success of a well intervention job. A conventional workover rig comprising of a derrick, rotary table and heavy machinery can be used to perform well intervention. However mobilizing an offshore rig and its associated support operations is expensive. Furthermore, using a rig generally requires killing the well (i.e. displacement of fluids in the wellbore to counteract the downhole well pressure) and creates the risk of damaging the reservoir. Well intervention methods that do not require a rig and have the capability of being performed on live wells (without killing the well) are the focus of the discussion herein. These rigless techniques include Wireline, Coiled Tubing (CT) and Hydraulic Workover (HWO). The paper discusses the equipment and characteristics of these rigless techniques and provides guidelines for the selection of these techniques for various downhole applications.
Downhole there is little difference in the tools that are deployed for dry (platform) wells versus subsea wells to remedy flow problems. But subsea well intervention in deepwater is typically much more expensive than dry well intervention due to the high day rates of floating vessels and the equipment costs of the subsea systems that are required to access the wellbore. The selection of the floating vessel and the subsea system is therefore critical.
The paper identifies various types of floating vessels along with subsea systems that can provide subsea well intervention.