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Glass fiber based thermoset composites handle high temperatures and extreme pressures yet are easily drilled out in downhole applications.
|The total market for composites in the oil and gas industry has increased by a factor of 10 in just the past five years.|
In most US and many overseas oil reservoirs the easy crude oil was produced long ago. Today, production gains rely on secondary and increasingly on tertiary enhanced oil recovery (EOR) techniques. Only about 10% of the oil in a reservoir is typically produced during primary recovery, where natural formation pressure or gravity force oil to the surface. Secondary recovery—injecting gas or water into the formation to drive oil to the well bore—has been used for decades to produce 20% to 40% of the original oil in the reservoir.
Highly competitive E&P companies are increasingly applying advanced EOR techniques to produce an additional 30% to 60% of the reservoir’s original crude. These advanced technologies are benefiting from new, lightweight, high-strength composite materials to replace heavier metal components for critical downhole applications.
One key to making advanced recovery more cost-effective and successful is the accurate placement of durable and easily removed downhole seals. Such seals, which serve to section off segments of tubular goods up to 6 in. in diameter, assure that high-pressure liquids, gases, or chemicals are injected where desired in the formation. They must be designed to operate at pressures up to 10,000 psi and temperatures of up to 400°F (204°C), often under corrosive conditions.
About 95% of seals used today are metal: steel, brass, or bronze. While metals have been used for years, are strong, and perform well, they pose two problems for E&P companies. Metal seals can weigh 160 lbs or more each, causing significant logistical and production challenges. Second, they are difficult to drill out after a recovery operation, often requiring a special drill bit and several hours of drilling to remove.
New composite seals
In the past few years, a new generation of non-metallic seals made from composites of polymer resins reinforced with fiber (such as glass) have been developed and proven in EOR applications. Compared with conventional metal seals, the composite seals are lightweight, extremely strong and durable, and easy to set in a pipe string and drill out in minutes without special equipment. Composite seals can be pre-engineered to downhole requirements by varying reinforcement fiber content and characteristics and adjusting the properties of the polymer matrix. Two general types of polymers are used: thermoplastics, like PTFE (polytetrafluoroethelene), which soften as temperature increases, and thermoset composites, like epoxies, which maintain strength properties at elevated temperatures.
Thermoplastics are subject to creep—also known as cold flow and compression set—because they cannot be cross-linked into a rigid structure like a thermoset plastic. Too much creep compromises a seal and leads to failure. The excellent creep resistance of thermoset composites means that even after exposure to high-pressure/high-temperature (HP/HT) conditions, it returns to its original shape without compromising the seal properties.
Advanced thermoset composite seal
Norplex-Micarta of Postville, Iowa, has developed a unique thermoset composite (NP571) that combines continuous woven glass fibers with an IEEE Class H epoxy resin. The composite has a tight molecular structure that functions well in the acidic, HP/HT environments often found downhole in EOR operations. The tough, non-brittle material has a Tg (glass transition temperature) of greater than 410°F (210°C). Tg is the critical temperature at which a material changes from being rigid to being more rubbery and flexible.
Metal seals are four times as heavy as composite seals fabricated from NP571 of the same strength. Therefore, a 160-lb metal seal can be replaced by a 40-lb composite seal.
Lighter composite seals are easily handled on site and readily loaded by hand. Material cost on a volume basis is equal to or slightly higher than that of metal. But lower transportation costs, ease of handling, and quick drill out capabilities tend to offset the initial higher cost of composites.
Thermoset composite seals also outperform thermoplastic composites in demanding EOR applications.
Thermoset seals are about three times as strong as thermoplastic seals, which lose strength as temperature increases. Excellent creep resistance makes NP571 a good alternative to PTFE approaches.
Composite seals are fabricated off site.
The driller or contractor makes a molding for the required seals. Then, glass reinforcement and thermoset resin are laid up on the mold to a thickness that meets the seal strength requirements. Seals can be fabricated in thicknesses ranging from wafer-thin 0.031 in. to more than 4 in. NP571 composite materials are used to make seals across this range of thickness.
In offshore and onshore vertical drilling operations composite seals are run down inside the pipe like conventional metal seals and secured at appropriate pre-determined depths with trip mechanisms. Composite seals also can be inserted when pulling or running drill pipe in the well bore.
Composite seals offer many advantages because they can handle HP/HT conditions and provide superior thermal insulation and corrosion resistance, while displaying high mechanical strength.
Evolution of a composite
NP571 started as a product for the power generation industry, which relies on HT thermoset laminates, pre-pregs, and specialty molded shapes that can endure extreme heat and mechanical conditions. Many of the physical properties that are attractive in power generation applications also are of interest for downhole EOR applications, including resilience at elevated temperatures; tensile, compressive, shear, and flexural strength; and creep resistance.
Norplex-Micarta is an experienced partner to oil and gas industry customers, where high-performance thermoset composites are used for gasketing in wellheads and pipelines as well as for pipeline supports and various components for upstream and downstream applications.
In response to a request from an oil industry customer for a composite seal with certain properties, the company worked with suppliers to satisfy the customer’s specific application requirements. Field tests provided a proof of concept. Soon, a new market for advanced composite seals had developed, primarily with a few pioneering exploration companies looking for better, more effective and efficient ways to do things.
The total market for composites in the oil and gas industry has increased by a factor of 10 in just the past five years. In the special niche for seals, composite seals are beginning to gain market share since their introduction about three years ago.
The demand for composite seals is growing rapidly, particularly for offshore and remote onshore applications where lightweight seals lower transportation costs and reduce onsite handling problems.
This trend is expected to continue to accelerate as the advantages of thermoset composites become better known in the industry.