For wide acceptance of expandables to occur, they must be reliable, strong, cost-effective and integrate into drilling operations. A new approach overcomes existing limitations and promises to satisfy these requirements.
Conventional expansion
Conventional tubular expansion uses cold-working processes to permanently deform low-alloy steel. A mandrel
is forced through standard casing, which has been modified slightly to accept the cold-work processes. High mud-pump pressure is applied internally to provide the expansion force. Expanding commonly available casings was a logical starting approach for early development, but many technical drawbacks exist.
The forces required to deform casings are high. Full expansion is difficult in complex downhole environments such as eccentrically loaded, deviated or stuck conditions. Also, abrasion or other surface damage as shallow as .012 in. or less creates stress risers as metallurgy is later altered through plastic regions regardless of the effects of further stresses externally. Maintaining the mechanical and pressure integrity of an entire assembly of such material while it is being subjected to compound expansion stresses - particularly through the coupled intervals - is a notable but unreliable engineering accomplishment.
An unwanted byproduct of deformation is longitudinal shrinkage, which is unavoidable in all conventional expansion systems. If the longitudinal material "feeding" requirement is not almost perfectly supplied, further significant stress works against expansion reliability and integrity.
To mitigate risks of deployment failure, the well bore must be thoroughly conditioned and control maintained throughout cementing and expansion operations. It can be expensive or impossible for the driller to provide essentially ideal conditions in problem zones downhole.
Solid expandables are increasingly being used as cladded casing extensions in order to help prevent premature loss of telescoped-casing inside diameter (ID). These extensions have costs including repairing the problem downhole, risk of expansion failure, and the costs of the expandable products and services themselves. For most drilling operations, these combined costs and risks are not acceptable.
Some strength properties resulting from plasticized expansion are also not acceptable. Often, substantially lowered pressure ratings result from expansion. In order to provide more acceptable post-expansion properties, one potential solution is to substantially increase the beginning wall thickness. This can increase the initiating force requirements and exacerbate local shrinkage issues, particularly in the threaded sections.
Expandable technologies must create significant value by mitigating or eliminating many conventional casing and stabilization-related steps and their costs. These steps ultimately represent more than 50% of all investment for drilling wells. The most direct approach is to deliver the expandable in a manner more seamless with the actual drilling process while simultaneously integrating some former casing-job cost items into the expandable system itself.
Strain-energized expansion
A new approach uses opposite, elastic-phase processes to reliably expand high-strength pipe. The new technology, called CFEX, is under development by Confluent Systems through its affiliate, Dynamic Tubulars Corp., and is slated for field trials in 2007.
The new expandable is constructed from compressible cells and other types of strain-energized members formed into a tubular with a naturally oversized outside diameter. The device is temporarily compressed during manufacture and held by removable bonds and integral wrappings. Once placed into the well, the temporary bonds are removed by electric, mechanical and chemical means. The result is a strain-energized tubular having natural dimensions larger than its nominal size. Where needed, high amounts of conventional hydraulic and mechanical force compound the tubular's outward energy. The tubular also has high-pressure sealing capability.
Since residual strain-energy is exerted against the formation, there is no "spring-back" effect to create voids. This provides a foundation for high-pressure wellbore sealing. The tubular's structure is also adjustable during expansion, making the device highly compliant to irregular wellbore surfaces. Compounded expansion forces can actually reshape local geology. The new concept does not substantially sacrifice strength properties as sealing geometry is obtained.
One casing design (Figure 1) is constructed from high-deflection members. The expansion capability of this design is more than 200%. Generally, an expansion ratio of 135% is required for an expandable to be integrated into drilling operations, allowing for adequate wall thickness and annular space. The technology's diametric capabilities as a drilling tubular are practically unlimited, ranging from less than 3 in. to greater than 28 in.
Though unnecessary for most applications, numerous elastic members can be arranged to form a device with considerable thickness, as shown by Figure 1. It can be constructed from very high-yield materials. Performance specifications are limited only by the quality and quantity of material. For example, for large-diameter well design, 16-in. casing can be provided with 1.5-in. or greater wall-thickness, 250-ksi+ material construction, 135%+ expansion capability and no loss of standard 16-in. ID. Similarly, a 2-in. or greater thickness expandable can be implemented into conventional 16-in. casing programs. Towards more typical sizes, 95¼8-in. casing using high-yield materials and 1-in. or greater wall is also in reach of the technology.
The new expandable can be integrated into drilling operations. Because the new tubular needs only certain regions of elastic function in order to properly become opened, drilling stresses do not automatically destroy the material's expansion integrity. One method of integrating expandables into the actual drilling process is depicted in Figure 2.
Connecting sections
Emphasis on the use of elastic structures and high wall-thickness provides opportunities to incorporate optimal elements from many different connection types. Because so much engagement material is available from high wall thickness, the connections are designed completely non-upset. The flush-coupled arrangement also maximizes working and circulating clearances downhole. Connecting tube segments uses familiar elements from threaded, quick-couple and high-pressure sealing designs.
Connection integrity is further improved due to the elimination of the shrinkage problem because the new technology provides for complete control over longitudinal behavior. Control over the unpredictable longitudinal "feeding" problem also provides opportunities to advance monodiameter system development since the previous reliance on forming complex, "connecting" overlaps to integrate separate casing assemblies is also now simplified and able to be exact.
Current results
This new technology is now in testing and field demonstration phases, with development supported by the US Department of Energy-NETL Microhole Technology Program. Modeling shows the system functioning readily within purely elastic regions. Physical testing of components consistently confirms conservative analytical predictions for basic mechanical functions. Pressure testing criteria for thick-walled expandable test-specimen, equivalent 95¼8-in. casing is 10,000 psi. The column tensile and buckling properties of the system range between ratings given by premium casing and those for heavy-wall drilling tubulars.
Preliminary testing conclusions are that the new expandable properties correlate well with all levels of modeling rigor. These design properties approximate 75% to 90% of the values of equivalent solid tubes. This indicates that the new, tunable expandable technology can exceed the performance of conventional, non-expanded tubulars by supplying necessary amounts of pre-expanded material and better structural functionality throughout.

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