Subsea Well Intervention

Future deepwater well intervention will reduce costs, boost output

Lightweight, autonomous, subsea well intervention systems offer a cost-effective step-change that will greatly improve subsea wells' production outlook, particularly in deeper waters. The key is use of low-cost industry vessels rather than more expensive drilling units to launch the systems.

Alan J. Dick, Subsea Group, Expro International Group PLC, Aberdeen, Scotland

As subsea oil production extends farther into the world's deepwater regions, so, too, does the cost of intervening in these wells increase. This cost increases in proportion to the water depths involved. A revolutionary, new, joint industry project (JIP) was launched recently by the Expro Group with the support of three major operators. Its purpose is to develop a system that will deliver cost-effective, subsea intervention operations in deep water, Fig. 1.

Fig. 1. The subsea intervention system, in testing.

SUBSEA COMPLETION TECHNOLOGY

Subsea completions are now a firmly established requirement of the offshore oil and gas production business. To date, around 2,300 subsea wells have been completed, utilizing this technique (as sourced from the Quest Subsea Database). With forecasts indicating that a further 2,000-plus wells will be commissioned in the next five years, the subsea well population is growing significantly.

Subsea completion technology has proved its economic value in the development of hydrocarbon reserves in deeper water, and in remote areas far from established offshore infrastructure. Reducing the initial capital cost of developing reserves using subsea technology has proved attractive in the exploitation of small- and medium-sized offshore accumulations in all parts of the world.

This economic benefit, combined with the technological advantages of subsea completions, has prompted extensive use of the technique in Brazil, Norway and the UK. It is also proving to be the method of choice in key deepwater provinces, such as West Africa and the Gulf of Mexico.

The majority of subsea Christmas trees in use are of the first-generation, dual-bore (or vertical) type. Such a tree requires that the well be drilled and cased to total depth (TD). The completion string must also be installed prior to removal of the blowout preventer (BOP) and installation of the Christmas tree.

To achieve this, dedicated dual-riser and pressure control equipment is required to ensure safe installation of these systems. The dual-bore moniker refers to the fact that the tree is penetrated by two offset vertical bores. These bores allow access to the production tubing, and to the A or production annulus. Well fluid is excluded from the environment by two valves in each of the vertical bores.

In recent years, significant numbers of wells have been completed using horizontal, subsea Christmas trees. This type has a single vertical bore, allowing access to the well's production bore, with a side exit for fluid output. The wellbore is isolated from the environment by using one or two plugs set (usually with wireline) in the tubing hanger and/or in the tree cap.

The horizontal tree system is more compact than the dual-bore version. It allows for larger completion sizes (the completion is run through the drilling BOP and hung-off in the tree), and does not require dedicated running or intervention equipment. Since the drilling BOP can be installed on top of the horizontal Christmas tree, all completion, installation and intervention work can be conducted using this BOP as the primary pressure control and containment system.

SUBSEA INTERVENTION ECONOMICS

The economics of subsea development have been, and remain, compelling. Lower capital costs and shorter development times to first oil have provided significant benefits to the net present value (NPV) of deepwater and small-to medium-sized offshore reserves.

In addition, larger bore completions made possible by recent advances in subsea tree technology have enabled operators to optimize the locations, numbers and configurations of development wells drilled in these reservoirs. Operating expenditures for such wells, if intervention operations are required, are very high and risk-prone (both technically and commercially). Operators wisely attempt to limit the number of interventions in subsea wells to an absolute minimum.

However, evidence demonstrates that subsea developments will be abandoned with significantly lower recovery factors than those offshore assets developed using fixed structures with surface or “dry” trees. This may be due partly to the less attractive properties of the reservoirs and fluids developed using subsea technology. It may also be due to additional back pressures introduced into the system by the subsea architecture.

There is a strong argument that there is an additional reduction in recovery factors caused by the inability to intervene in the wells on a regular basis, compared to when dry tree developments are undertaken. Intervention covers a wide range of well operations, from tubing recovery and renewal requiring a rig, right through to simple mechanical reconfigurations of the well using slick wireline (plugs, sliding sleeves, etc.).

Most well operations involve the use of wireline of some type, or possibly the use of coiled tubing. Such operations include well surveying (pressure/ temperature, production logging, thermal neutron) and mechanical well reconfigurations or repairs (plugs, sliding side doors, reperforating, zone isolations) performed on either slick or electric line.

The key issue in the development of such intervention technology is removal of the necessity to use a high-cost semisubmersible rig or drillship, as well as the need to new-build a vessel to deploy the technology. The ability to use a wider spectrum of existing vessels would result in significantly decreased costs, due to lower vessel day rates. The trade-off against these savings is the limited heavy-duty functionality of such an intervention system.

SCOPE OF THE JIP

The JIP was set up in early 2004, to investigate the feasibility of developing a lightweight (wireline), subsea, well intervention system, suitable for deployment from a wide range of (low-cost) surface vessels, Fig. 2. The intention is that the combined intervention and deployment system will provide a new standard in the provision of cost-effective well monitoring and remedial services. The system will also serve as a platform for the deployment of new, high-value well intervention services.

Fig. 2. The entire subsea intervention system can be deployed from a wide variety of relatively low-cost surface vessels.

The development program has been organized into a series of projects, each forming part of the phased development/ engineering of the system. These are being run as individual projects, with Expro managing and engineering each phase. The basis of the overall JIP was an initial concept developed by Expro, based around the perceived technical requirements for a deepwater intervention system.

Phase I (Project 1)

• Technical feasibility
• Initial commercial viability
• Scope and operational envelope
• Operator functionality
•  Functional and technical specification 
•  Initial HAZOP/ HAZID and reliability analysis 
• Concept refinement
• Intellectual property aspects

Phase II (Project 2)

• Detail design and engineering
• Detail HAZOP/ HAZID and reliability analysis
•  Subcontractor identification and scope

Phase III (Project 3.0)

• Construction and testing

Phase III+ (Project 3.1)

• Operations

Phase I (Project 1) was undertaken with three major operators of subsea assets. At the time this article was written, this phase had completed. In addition, a detailed workscope had been identified, together with costings and timings, to be undertaken during Phase II.

It is likely that Phase I partners will participate in the next phase, but there is also the possibility of other operators joining at this stage. It is expected that the second phase will take about 14 months to complete. Consequently, it is likely that the first system will be available during first-half 2007.

First-phase results have been extremely encouraging, and the JIP participants are confident that the proposed system will deliver a significant, positive change to subsea well intervention practices. What follows is a more detailed look at the system being proposed.

SYSTEM DESCRIPTION

The subsea intervention system has been designed to meet the intervention requirements of a large number of existing (a.k.a. legacy) wells, as well as meeting the intervention needs of the operator community. Fig. 1 shows the intervention system installed on a subsea Christmas tree. In general, the system can be divided into four sub-systems or packages.

Well control package. This package provides the ability to close the well, so that the upper package can be removed, if a system malfunction or emergency occurs. It provides two testable barriers, and can (through use of the appropriate crossover) be used on any make or type of Christmas tree. The top of this package has a standard connector allowing a BOP, lower marine riser package or high-pressure riser to be connected to the Christmas tree from a surface vessel.

Tool carrier package. The system will be able to deliver a full suite of well intervention tools in one run, without recovery to surface of the intervention package during operations. The tool carrier package contains up to eight wireline tools for use during well intervention.

The tools are held around the periphery of the carrier on hydraulically activated clamps. When a tool is selected, the hydraulic ram presents the tool onto the well's center-line, where it is made up onto the running tool/ well conveyance tractor in the above package. The hydraulic clamp is then released, and the tool can be run into the well.

The different wireline tools can be interchanged onto the downhole conveyance tool during a single run of the intervention system. The package will include well maintenance and reconfiguration tools, plus well/ reservoir measurement and monitoring tools.

Wireline winch and tractor package. This package contains an in-line wireline winch attached to a well tractor that is housed within the pressurized lubricator section. The entire system is maintained at well pressure, thereby minimizing the risk of well fluid escaping to the surrounding environment.

The winch design (with the wireline spooled around the central wellbore) minimizes geometry problems that may occur during spooling. It also keeps the system's center-of-gravity close to the well's center-line. The well tractor is attached to the wireline and is used to push (or pull) the wireline tools in the well's more highly deviated sections. On the lower end of the tractor is a quick-connect/ release system for connecting to the wireline tools.

Subsea power pack. This is a stand-alone power pack to provide hydraulic and electrical power to the intervention system via a single, electrical umbilical from the surface.

The intervention system is targeted to applications in water depths of up to 10,000 ft (3,000 m), with a pressure rating of 10,000 psi. This will enable interventions on large-bore subsea wells. The combined stack of packages will stand roughly 45 ft high and weigh about 60 t, making it both compact and relatively lightweight.

The entire system can be deployed from readily available platform supply or anchor handling vessels, Fig. 2. These vessels must have more than 500 sq m of deck space, as well as dynamically positioned (DP), Class 2 capability, using a heave compensated A-frame or a high-capacity, heave-compensated crane. Such vessels are available in all major offshore producing regions. In addition, a work-class ROV is required for system hook-up.

THE COMMERCIAL CASE

Subsea wells represent a considerable investment, both in terms of the wells and the infrastructure associated with each well. The ability to manage these wells, and thus optimize the economics and recoverable reserves, is a fundamental requirement of any asset development plan. New technologies are under development that could assist this process, the so-called intelligent completion technology that can be activated remotely from surface. This technology is commercially unproven, and does not address the problem of current, installed subsea wells. The management of these wells provides a real, immediate challenge to the industry, a challenge that requires a realignment of thinking on subsea well intervention.

Projected financial benefits include:

• An increase in recoverable reserves via improved well management
• A subsequent increase in the NPV of subsea projects, because the initial CAPEX requirements remain unchanged
• The possibility of moving marginal reserves into viable projects
• Providing the catalyst for new, value-enhancing well management and remedial technologies that can be launched from a subsea intervention platform
• Releasing costly, high-specification drilling units back to drilling.

Furthermore, reduced risk exposure in health, safety and the environment is anticipated, since fewer personnel and vessels will be required.

CONCLUSIONS

Phase I of this project has already demonstrated that it is technically feasible to develop an autonomous, subsea well intervention system that can be deployed in a wide range of water depths (300 to 10,000 ft) from a low-cost, widely available monohull vessel. The proposed technology (and its deployment system) can deliver significant cost savings to lightweight well intervention projects, compared to current technologies. In turn, these technologies will allow more cost-effective, lightweight well interventions to be made. The technology will, through the increased well intervention frequency, deliver:

• Increased subsea well productivity in middle-to-late well life
• Higher economic recovery per well
• Improved well efficiency
• Improved reservoir recovery factors.

In addition, the system will deliver a low-cost method of intervening in deepwater wells at depths exceeding the capability of current open-water wireline solutions. The ability to use a low-cost, high-availability vessel (the so-called vessel of opportunity) will increase the opportunity for performing well maintenance and optimization work, and decrease the execution cycle time for subsea well interventions. Furthermore, costly fifth-generation and other high-capability rigs will be released back to well drilling and heavyweight workover operations.  

THE AUTHOR
	Alan J. Dick is group marketing manager, Subsea, for Expro International Group PLC in Aberdeen, Scotland. He joined the Expro Group in that position during August 2003. Prior to that, he was the Wells Technology broker for the Industry Technology Facilitator in Aberdeen. His earlier experience includes five years with Halliburton Logging Services and four years in the Well Technology Division of AkerKvaerner. Mr. Dick holds B.Eng, Electrical and Electronic Engineering, and MBA degrees from Aberdeen's Robert Gordon University. During 2002, he joined the board of SPE Aberdeen, for which he holds the position of treasurer and chairman-elect.