During the past 10 years, thousands of multilateral wells have been drilled worldwide, while scores of "intelligent" wells have been monitored and controlled downhole. It is only natural that these two technologies should meet and start a family of "intelligent multilaterals."

The ability to both access and control production from multiple reservoirs from a single parent wellbore can have a significant effect on the economics of hydrocarbon recovery. Higher production rates can be obtained through the installation of intelligent artificial-lift equipment (gas lift valves) by switching producing intervals as zones begin to deplete, or by optimizing inflow from multiple points along a horizontal section. By turning off the offending zones, oil production can continue long after water or gas production ordinarily would have exceeded the facility's handling capacity, thus increasing ultimate recovery. The ability to choke flow allows production from different zones to be commingled, also increasing ultimate recovery.
Impact of multilateral systems
Multilaterals have the potential of being a revolutionary technology that could revamp the way we do business in the oil field. Multilateral wells have a marked impact in several important areas, including development plans, production capabilities, facility capabilities, current and future technology available and planning. Ignoring one or more areas could have a large effect on the overall financial success of the project. These areas can be grouped into three main categories:
• well cost reduction;
• reservoir value enhancement; and
• project or facilities cost reduction.
In reducing well costs, the idea is to make one multilateral well cost less than two wells. This is especially true in expensive wells, such as extended-reach and deep or hostile-environment wells. It is also important to take into account surface considerations such as blowout preventers (BOPs), production equipment and available slots or surface facilities. It has been shown that worldwide the cost of a multilateral well is 1.2 to 1.8 times the cost of a single well.
Facilitating more complete drainage with multilateral wells can enhance reservoir value. This can be done by mitigating reservoir heterogeneity problems, improving sweep efficiencies and delaying water or gas breakthrough with lower draw down and better drainage patterns, and enhancing production in tight formations or of viscous fluids by increasing reservoir exposure and connecting natural fractures. Depletion can be accelerated because having more wellbore branches exposed to the reservoir results in higher production rates. There is also the ability to add secondary (risky, deeper, shallower) exploration targets as an add-on to infill drilling projects.
But perhaps the most significant benefit can be found in project cost reductions. Smaller facilities or platforms can be employed to access the same or more reservoir targets, thereby reducing the environmental impact. In addition, fewer wells, wellheads and flow lines are needed to access multiple reservoir targets in subsea developments. This enables economic development of satellite or marginal fields that otherwise would fall below the economic threshold.
Classification
To understand the way multilateral technologies and intelligent completions can be combined, one must first appreciate the basic components of each. Multilateral systems are typically categorized in terms of mechanical capabilities of the junction. Intelligent wells are usually categorized by the level of sensors for monitoring and the level of precision in the control, from simple on-off valves to infinitely variable chokes.
The industry has adopted a six-level system of classification for multilateral wells known as TAML (Technology Advancement for Multi-Laterals) based on the capabilities of the multilateral junction. Because the first three levels do not provide for any flow isolation at the junction, they do not lend themselves to the control aspect of intelligent multilaterals. Therefore, we will focus only on TAML level 4, 5 and 6 junctions to address the practical considerations in choosing one for an intelligent well application.
Level 4
The Level 4 junction is created by cementing a liner to the primary casing string. The cement acts as a flow barrier, but is not considered to be a true hydraulic seal, so the inside of the junction will be exposed to flow from both the mainbore and lateral producing zones. The Level 4 system is ideally suited to commingled flow of low-pressure oil reservoirs such as heavy oil fields, where the junction is placed in the zone in a horizontal mainbore. Another application is in areas where the junction can be located in a shale section or some other natural permeability barrier to minimize unwanted flow at the junction.
In a Level 4 junction, where non-commingled flow is desired, flow from different laterals is segregated by means of a dual completion. One tubing string extends below the junction and is set into a packer, while the second string opens immediately below a dual packer that is set above the junction. By incorporating independent intelligent well monitoring and control into each tubing string, it is possible to monitor and control the production from either leg independently. An alternative is to run a single string that includes a tubing window joint that locates precisely in the multilateral window joint. With this approach, re-entry access to the lateral is possible through tubing.
With more than 200 Level 4 multilaterals installed worldwide, the Level 4 multilateral junction is anticipated to continue as the "workhorse" for intelligent multilateral applications for several reasons:
• A proven track record from around the world has shown this to be a very stable junction and flow barrier in many applications.
• The junction can be built without loss in hole size, enabling the use of conventional diameter tubulars. Thus, there is plenty of room for intelligent well components.
• The Level 4 junction works well with standard completion techniques like case-and-perforate, gravel packing and hydraulic fracturing, as well as the new expandable screens.
For a Level 4 junction to be successful, it is critical that there is competent cement at the junction. Extra measures to ensure this are easily justified by the negative ramifications of an unstable junction.
Level 5
In a Level 5 multilateral, the junction area is isolated from flow by the completion. Single packers are located in the lateral leg and in the mainbore below the junction. The well is produced through dual tubing strings, with a dual packer above the junction. Alternatively, the strings can be merged into a single larger string above the window area using a single-string packer above the junction for pressure isolation. The casing junction can be a cemented junction (Level 4), a mechanically attached non-cemented liner (Level 3), or a drop liner (Level 2) that is stung into the lateral tubing string. Most applications have been built on a Level 4 junction. Although the junction is temporarily exposed to flow during well construction, the use of packers or polished bore receptacles eliminates this exposure once the completion is installed.
The number of Level 5 multilateral installations to date is estimated to be between 35 and 50. These more complex completions generally have required more installation time; however, newer systems are decreasing the installation time dramatically. Because the junction is completely protected from pressure, the likelihood of long-term junction integrity is improved relative to a Level 4 junction. The Level 5 also provides more options for flow control on each leg because the tubing strings extend into each leg, thereby creating more isolation.
Level 6
The Level 6 multilateral takes isolation to the next level - the casing string itself provides the isolation. Currently two techniques exist for accomplishing this. The first involves collapsing a dual-leg casing joint so that it will fit in the well. This joint is run as part of the production casing string on the mainbore. The joint is positioned in an under-reamed section of the well and then expanded to full size downhole, either by exerting pressure or by driving a mandrel through it. The second technique is to use a large-diameter tubular and build in two smaller seal surfaces such that two standard casing strings can be hung off side by side. This latter technique results in considerably smaller casing sizes compared to the expansion technique.
Level 6 multilateral installations to date are estimated at fewer than 20 worldwide. Depending on the system used, this multilateral junction can produce the largest final flow diameters since packers are no longer required for pressure isolation at the junction. However, independent control and isolation of the laterals will still dictate the use of packers. Expandable systems deliver a somewhat reduced casing pressure rating compared to standard casing, so this must be taken into account when designing the well. Combining this larger junction area with a smart well approach gives the most room for packaging sensors and actuators.
Intelligent completions
An intelligent completion allows the operator to obtain real-time or near real-time data and operate downhole flow-control devices remotely to reconfigure the well architecture without well intervention. The degree of complexity of these systems depends on the application and reflects the degree of both data and control resolution required. The type of system installed can define the degrees of resolution:
• On/Off (Open/Close) hydraulic system without position feedback confirming valve position and non-integrated sensing (permanent downhole gauges, flowmeters or fiber optics). This system allows the operator to produce independent intervals without choking capabilities; the zones are either produced or shut in. The same pressure and temperature information can be obtained as in the fully integrated case, but using that information for reservoir optimization can be limited by the flow-control devices.
• Incremented hydraulic system without position feedback confirming valve position and non-integrated sensing. This system allows a greater degree of resolution by positioning the flow-control devices at various positions between full open and full close. The lack of positive position feedback via electrical communication requires the operator to assume position based on feedback of the hydraulic system. The monitoring of hydraulic flow can be particularly difficult in subsea wells producing through an open-loop subsea control module where the hydraulic control fluid is vented to the ocean.
• Fully integrated electro-hydraulic or all-electric system with positive position feedback and sensing functionality integrated with flow control devices. Such a system provides the highest degree of resolution. The acquisition of real-time reservoir data is controlled by the same device that controls actuation of the flow-control device. Flow rate at the sand face can be determined through the use of flowmeters. Alternatively, the information obtained from positive valve position, reservoir pressure, reservoir temperature and basic fluid composition can be used to calculate flow rate. If implemented correctly, full redundancy can greatly enhance the reliability of the intelligent completion equipment.
There are about 100 intelligent systems installed worldwide, scattered from Brazil to Norway to Malaysia to the Gulf of Mexico. The majority of these are hydraulic systems with non-integrated pressure and temperature data.
Intelligent multilaterals
There are currently several combination designs of intelligent wells with multilateral junctions either possible or in various stages of construction. These include economical re-entry wells, wells with above-junction control and wells with the intelligent control systems deployed through the junction to the productive zones. These systems incorporate different levels of complexity and cost, but this range provides flexibility for a wide range of applications.
Level 4 junction construction with lateral re-entry capabilities. Level 4 intelligent multilateral construction offers an effective yet economical method of merging intelligent well and multilateral technologies. The simplistic nature of this technique allows for the installation of conventional completion equipment in both the parent bore below the junction and the upper lateral extending from the window junction. Fluid loss and well-control issues are addressed through conventional means. The completion design criteria and field implementation are very similar to those of a single wellbore completion installation.
With the completion equipment in place, the Smart MLT lateral re-entry system is installed adjacent to the window junction (Figure 1.). The lower end of the assembly is sealed into a packer bore below the junction while the assembly is oriented into position. This system provides for mechanical isolation between the upper and lower completions, through-tubing re-entry capabilities into either lateral, simultaneous flow control and monitoring from either lateral, and no hydraulic wet connects. By placing the control valve manifold in the parent bore, larger control valves can be utilized, which have greater flow-through area. With intelligent multilateral technology, remedial through-tubing isolation and whipstock devices can be deployed for lateral access without well intervention.
Level 5 junction construction with above-junction flow control. The next level incorporates flow and pressure segregation at the junction, but with the monitoring and control capabilities in the main bore above the junction (Figure 2). This system is a unique blend of existing multilateral and intelligent completion technology. The advantages of this type of system include ease of deployment because the controls are run after the multilateral junction is constructed. This keeps everything centralized and compact. The disadvantage is that it is not possible to control multiple zones in the mainbore or the lateral.
Junction construction with in-zone flow control. Figure 3 illustrates a system in which controls for multiple zones are deployed through the junction to the zones themselves. This allows for control and monitoring of multiple zones on both sides of the junction, truly multiplying the effect of both the multilateral and the intelligent completion. This system, currently in development, can offer some truly unique capabilities to reservoir development. Intelligent multilaterals can also be implemented in existing wells to access previously by-passed reserves. In this scenario, the existing completion is isolated using through-tubing knock-out or isolation devices. A seal bore packer and latch assembly is installed at a predetermined depth just below the proposed window location. This assembly will remain in the wellbore and become part of the final completion. In a retrofitted multilateral application, a window-milling tool is used to create uniform window geometry. The milling tool is oriented into the latch device, and the window is milled. Next, the milling tool is removed, and a drilling whipstock is deployed and latched into position. The upper lateral can then be drilled to its targeted depth, cased and cemented.
Applications
Intelligent multilateral technology could be an ideal solution for various scenarios.
Elevated workover costs and lost production for subsea wells can often turn a project that has marginal economic viability into one that has none. By utilizing intelligent multilaterals and sectioning the reservoir(s) into managed nodes, an operator can eliminate the need for remediation and deferred production.
Use of intelligent completion equipment in remote locations like the jungle or unmanned platforms allows operators to produce from multiple production intervals and to control production from many miles away.
In large, mature fields with extensive injection schemes, the use of intelligent completion equipment allows the operator to maximize sweep efficiency by controlling injection into specific areas of the reservoir. Multilateral technology can increase the total fluid injected because greater portions of the injection zone are contacted by the wellbore.
The ultimate goal for these systems is a fully automated reservoir. The system would monitor downhole pressure, temperature and fluid flow characteristics and automatically adjust itself to respond to conditions. This system of the future could include downhole factories that would separate fluid streams into their desirable and undesirable components and re-inject the unwanted fluids into the formations.
Improved asset value would be achieved through accelerated production and improved ultimate recovery while mitigating risks, such as early breakthrough of unwanted fluids. Combining multilateral and intelligent completion technologies also provides for a potential reduction in capital expenditures through a reduction in the number of wells needed to exploit the reservoir(s).
The technology is available, and the applications are identified. The next step is to implement the tools and begin reaping the benefits. Rather than achieving incremental increases in efficiency by tweaking current processes, truly significant improvements can result from step changes in thinking and approach. As horizontal drilling and 3-D seismic technologies were new paradigms in their infancy, so do intelligent multilaterals have the potential to revolutionize our industry today. It is just a matter of quantifying the risks and evaluating the rewards.

Editor's note: This article is based on a paper prepared for presentation at the 2002 Offshore Technology Conference held in Houston, Texas.

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