Revolutionary expandable tubulars and connectors will change the basic design and construction of oil and gas wells. With these innovations, operators can reach reservoirs that are economically inaccessible with conventional technology.

Traditional drilling operations use progressively smaller casing diameters that telescope down to the reservoir objective.
This limits the number of casing strings that can be deployed and restricts the wellbore cross-sectional area available for fluid flow.
With expandable-tubular technology, the operator runs casing into the wellbore normally, then expands it in situ by hydraulically pushing or pulling a specially designed mandrel, or pig, through the pipe. The result is a minimal reduction in internal casing diameter from the previous string to the newly expanded string. Thus, expandable-tubular technology greatly increases the number of casing diameters available.
This conservation of wellbore size allows operators to drill deeper wells, extend the length of deviated wells and reach deepwater objectives previously thought impractical or impossible.
The ability of expanded-tubular systems to be deployed downhole permits operators to control problems associated with particular formations, including lost circulation, salt, water flow, subsidence and high temperature. By expanding tubulars inside existing casing, old wellbores can be repaired, depleted zones isolated and burst or collapse properties of existing casing enhanced.

Development of the expandable connection
Shell E&P Technology Co. began working with Grant Prideco in 1998 in the continuing development of expanded tubular technology.
At that time, 31/2in. coiled tubing and 4in. rigid tubing with welded joints had been expanded successfully in above-ground tests, proving the viability of the tubular expansion process.
However, they realized they needed to develop a process to expand jointed tubulars for practical oilfield applications. They also knew a critical factor in the success of such a process would be the availability of connections that could maintain mechanical and hydraulic integrity before, during and after expansion. Grant Prideco had the oilfield connection experience and resources to quickly transform such connections from concept to reality.
As Grant Prideco was beginning development of expandable connections, Enventure Global Technology was being created to develop, market and provide expandable tubular services. Enventure is an equal partnership between Shell Technology Ventures Inc. and Halliburton Energy Services. Realizing the importance of connections to expandable tubular systems, Enventure proposed a cooperative development relationship with Grant Prideco in which each company contributes to the engineering effort and development cost.
The cooperative relationship enabled the two companies to gain mutual benefit and reduce the project's total cost by eliminating duplication of effort, simplifying processes and capitalizing on their combined expertise. This successful business relationship allows engineers and scientists to work toward common goals, and the co-operation between the two companies has opened many opportunities for learning and quickly developing new technologies.

Unique design challenges
A traditional casing or tubing connection has two functions.
One is the mechanical or structural function that connects the two joints of pipe together. The connection system must resist the loads of tension, compression and bending imposed during the tubular string's functional life. The other is a pressure-containment function providing a fluid-tight seal to contain the internal and external pressure to which the tubular is subjected.
With expandable tubulars, the connections must maintain mechanical integrity and pressure integrity before, during and after the expansion process. The connection profile for expandable tubulars also required special consideration of the design challenges that engineers had to overcome to accomplish these goals.
Connection profile. With conventional connections, wall thickness is usually greater across the connection than it is across the remainder of the pipe, which can cause problems during the pipe-expansion process. If tubular ID is smaller across the connection because of the greater wall thickness, this may hinder or completely block passage of the pig during expansion. Or if tubular OD is larger across the connection because of the greater wall thickness, the outer profile of the expanded tubular may not match the inner profile of the tubular in which it is being expanded. This profile mismatch could compromise the integrity of the seal generated by the expansion. Finally, because the pressure required to expand pipe is a function of the pipe's wall thickness, care must be taken to ensure the increased pressure required to expand the pipe across the connection does not exceed the burst pressure of the remainder of the pipe.
Mechanical integrity
When connection threads are machined in pipe, the wall thickness and the effective stress-carrying cross-sectional area of the pipe are reduced across the connection. Connection efficiency is the ratio of the connection's tensile strength to the pipe body's tensile strength.
Thus, one of the goals in designing a connection is to have connection efficiency as close to unity as possible. To offset the reduction in connection efficiency caused by the machined threads, wall thickness is often increased across conventional connections. To expand pipe downhole, the pig must deform the pipe into its plastic region. The hoop (tangential) stresses applied to the pipe must therefore be greater than the yield strength of the pipe. When pressure is applied to create the required hoop stress, a longitudinal tensile stress about equal to half the hoop stress is created in the pipe. Thus, connection efficiency
for expandable tubulars must be at least equal to about 0.5, or about 0.55 with a 10% safety factor.
Pressure integrity.
Conventional oilfield-country tubular goods (OCTG) connections, including API and proprietary connections, rely on one or more of three sealing mechanism types: thread seal, non-resilient seal ring or metal-to-metal seal. All three seal types depend on bearing pressure. Bearing pressure results from an interference fit between the thread flanks, roots or crest in combination with thread compound, generating a thread seal. Clearances between the thread elements
are sealed by the entrapment of solids in
the thread compound. For API modified thread compound, the solids include a combination of powdered graphite, lead powder and zinc dust.

Seal-ring seal
The seal-ring seal is generated by bearing pressure from the total entrapment of a non-resilient ring, usually composed of Teflon and fiberglass, within the thread elements.
Bearing pressure results from seal-ring overfill, which is seal-ring volume in excess of the entrapped-space volume. Finally, bearing pressure between the mating metal surfaces generates metal seals. Bearing pressure results from an interference fit between the pin seal surface and the box seal surface. For all three conventional seal types, the bearing pressure, and therefore the sealing mechanism, is crippled by the expansion process.
To achieve pressure integrity across conventional connections, threads on the boxes and pins are usually designed to mate as tightly as possible. However, such tight mating can cause problems with expandable connectors.
The threads on one joint may be expanding through one profile (e.g., convex, where threads are spreading) while the threads on the mating joint are expanding through a different profile (e.g., concave, where threads are compressing), potentially damaging the threads and destroying any hydraulic seal that may have been present.

New expandable connection design
Finite-element analysis (FEA) was used to model the expansion process and simulate various designs. FEA helped minimize the time, effort and expense needed to arrive at a suitable design.
Testing was conducted in a laboratory (Figure 1), at a surface-test facility (Figure 2), and in a test well to confirm the FEA results.
The resulting connection is flush/flush - tubular wall thickness does not change across the connection. Its special threads are designed to remain undamaged during expansion of up to 23% of pipe OD. The expandable connection uses completely new sealing technology, placing the connection's sealing mechanism at the back of the box for internal sealing, and positioning it at the back of the pin for increased external sealing capabilities. The threads and sealing mechanism allow the connection to float during the expansion process, then seal pressure after expansion.
Expandable tubulars are run pin up. With this running convention, as the pig is propagated up the wellbore, the pin threads are expanded out against the box threads to maintain optimum thread contact during the expansion process. If the expandable tubulars are run box up, the pig will initially tend to expand the box threads away from the pin threads with some loss of connection integrity.
Tables 1, 2 and 3 display data from tests on 7 5/8-in.-OD connections. Listed are results concerning collapse, burst and tensile strength for various tested connections. All connections were expanded 14% (from 7.625in. to 8.528in. OD).

Manufacturing processes
The nature of expandable connections requires precision manufacturing and rigorous quality control.
Pipe selection is critical due to the thinness of the box and pin noses required by the flush/flush connection profile. Engineers found OCTG could be used as expandable tubulars with the new connections if the pipe meets certain materials specifications and is selected in accordance with tight tolerances for ovality, wall-thickness variations and mechanical properties.
The new connections are produced at Grant Prideco's manufacturing facilities, which are ISO 9000 certified and API Q1 compliant. Expandable connection quality is assured through every step of a premium manufacturing process.
Quality checks include:
l incoming material inspection;
l 100% inspection of thread-form tooling;
l statistical sampling and inspection of seal rings;
l strict gauge-calibration procedures at the start of each setup;
l first/last-article high-magnification verification of critical thread-form and seal elements;
l 100% inspection of connections during threading; and
l visual inspection and anchor-pattern measurements of surface finish treatment.
Traceability of all calibration procedures, gauge setup, first-article verification and inspection records are maintained.

Field tests
In mid-November 1999, a 985ft expandable liner with the new expandable connections was deployed in an offshore Louisiana well to a total depth of 13,131ft (Figure 3).
After expansion, the liner ended up 946ft long. Enventure co-ordinated the efforts of the installation team, which included members from Enventure, Grant Prideco and other service companies. As part of the expandable-tubular job design, preparation and installation process, Grant Prideco field service representatives conducted a final inspection at the rig site, field-repaired any connections damaged during transportation, and helped supervise the make-up and running of the tubulars.
The expandable system was used to case off a high-pressure zone so that a weaker, depleted zone could be drilled with lighter mud. For this drilling to be successful, a tight seal was needed at the liner top, and the expanded liner had to withstand collapsing under a 2,500psi differential load.
Seal integrity and liner strength were checked after the expandable-tubular installation.
The system was positive-pressure-tested at 3,540psi for 30 minutes, then was subjected to a 2,165psi negative pressure differential.

Outlook for expandable connections
Today's expandable tubular solutions - openhole liners, cased-hole liners and liner hangers - represent the tip of the iceberg. Restricted primarily to drilling, expandable tubular applications soon will be moving into production operations.
As expandable tubular technology develops, the expandable connection technology will also develop.
Post-expansion testing of expandable connections to industry-standard connection evaluation formats such as API RP5C5 or ISO 13679 will be required. Eventually, monobore wells will become a reality. n

Bibliography
l 1. Filippov, A., et al: "Expandable Tubular Solutions." Paper SPE 56500 presented at the 1999 SPE Annual Technical Conference and Exhibition, Houston. Oct. 3-6, 1999.
l 2. Haut, R.C., and Sharif, Q.: "Meeting Economic Challenges of Deepwater Drilling With Expandable-Tubular Technology." Paper presented at the 1999 Deep Offshore Technology International Conference and Exhibition, Stavanger. Oct. 19-21 1999.