Because ML wells that have higher TAML designations are more complex, they are more costly and their configurations are more flexible. As they do with ML geometry, engineers choose a TAML level junction based primarily on reservoir characteristics, costs and function.
The most frequent applications of ML wells are in reservoirs that require sand control and in heavy oil reservoirs. In the case of sand control, the reason is often slot management on offshore platforms. More reservoir can be accessed from a single casing string to surface. Relatively lower-cost sand control methods for multilaterals include slotted liners or stand-alone screens that act as filters to prevent formation sand entering the wellbore. These are typically Level 5 wells.
In heavy oil applications, the goal is to access more reservoir from a single parent bore. The amount of reservoir accessed from a single surface casing can approach 100,000 ft. These are typically Level 1, 2 or 3 wells.
Like many wells, multilaterals may require some form of artificial lift to supplement reservoir energy to bring produced fluids to the surface. Because most junctions do not contain pressure seals across the annulus, laterals are rarely good candidates for injection wells that operate at elevated pressures. Pump-type systems are applicable when formation fluid is produced naturally to a depth that is above the junction and shallow enough for a pump to lift economic volumes of liquids to the surface.
Multilateral wells are suited to offshore and subsea operations. Offshore platforms are often slot limited; they are designed to accommodate a finite number of casing strings and wellheads at the surface. Should an operator discover the need for more wells than are allotted in the original platform design, installing laterals that branch from the existing main bore is significantly less expensive than acquiring another platform. In addition, the capital and operating expenses of offshore operations are such that the cost of MLs and junctions is small by comparison.
In many areas, commingled production—in which formation fluids from separate zones are mixed downhole and allowed to flow as a single stream to the surface—is not allowed or is impossible because of pressure differences between zones. Only TAML Level 5 or Level 6 junctions provide the pressure integrity required to prevent commingling of flow from the lateral and the main bore; TAML Levels 2 through 6 junctions may be used if the well includes a dual upper completion to keep the flow streams separated.
Handling the Pressure
Junctions can fail as a consequence of high drawdown pressures, and engineers must understand the scale of those pressures when choosing a junction. Because drawdown pressures are exerted directly on the formation in TAML Levels 1, 2 and 3 junctions and on a cement sheath in TAML Level 4 junctions, these junctions should be used only in low-drawdown environments. TAML Levels 5 and 6 junctions are separated from the formation by casing and cement and able to withstand significant drawdown pressure.
The ability to reenter the lateral for well intervention operations is another ML design consideration. Because it is a directionally drilled section that has no junction, the lower lateral is almost always easily accessed using standard intervention methods. Operators must make an economics-based decision during the well planning stage to include junctions that allow through-tubing access, junctions that can be adopted to allow access after installation or junctions through which no access is possible.
The decision to deploy lateral junctions that allow fullbore or restricted access is a function of the overall well design. Engineers usually opt for fullbore access if a packer is to be placed below the junction or if an artifi-cial lift system must be located near the lower lateral. In addition, based on their knowledge of the reservoir, operators may require fullbore access to perform perforating, gravel packing, cleanout and other remedial operations. Fullbore access can be adapted to all TAML level junctions but must be specified before installation; some commercially available junctions allow no access or only restricted access to the lateral and cannot be adapted after installation.
Costs and Benefits
The decision to use an ML system and what type to use are the result of cost-benefit analyses. In general, the less complex junctions present operators with lower risks and costs. But risk mitigation and cost savings must be balanced against individual well and field development expectations. In low-value reservoirs, a simple openhole lateral that has no reentry capability may increase ultimate reserve recovery or accelerate production while having little impact on overall drilling and completion costs. In high-value deepwater plays, installing a hydraulically sealed TAML Level 5 or Level 6 junction can drive total well costs into millions of dollars and still be a good investment because it may save drilling another well or preserve a well slot on an existing production platform.