Extreme Materials for Oilfield Applications

The oil and gas industry is working in ever more extreme conditions and failure is very expensive to fix in some of these locations: the replacement of a faulty seal or a broken pipe can cost much more than the time spent on proper research and specification, besides the environmental impact. Applied Market Information (AMI) and MERL joined forces in organising an international industry conference in London, MERL Oilfield Engineering with Polymers 2010, to bring together polymer experts and oilfield operators to look at the new demands on materials.

Dr. Philip Abrams has been working with the operator ExxonMobil for three decades and has studied many aspects of elastomer use in the oilfield. Historic work can be useful today and is often forgotten, like the paper by J.D. Burley in 1977 on "Packer seal system developments for deep sour gas wells". Studies have been conducted on high temperature seal failure, which has lead to dynamic seal design using materials such as PEEK and PTFE rings, combined with elastomers in different shapes. The elastomers were screened for factors such as resistance to rapid gas decompression (RGD) and wellbore fluids. Quality control of suppliers is proving to be a critical factor - if a material or seal is substituted without agreement it can have severe consequences. The future wish list includes improved strength and toughness of fluoroelastomers (FKM) and perfluoroelastomers (FFKM), combined with better metal bonding, and elastomers for continuous service above 260C.

A consortium of MERL, Clwyd Compounders, Precision Polymer Engineering and Baker Hughes has undertaken research on new elastomer compounds. With oil resources diminishing, it is increasingly common practice to undertake enhanced oil recovery procedures including injecting fluid into the well, for example water, steam, hydrogen sulfide and supercritical carbon dioxide. The latter is known to have a solvating effect, which can cause material swelling of seals and liners and affect performance. The consortium developed and tested new elastomer compounds for resistance to these fluids, for use both in oil recovery and in carbon capture and storage applications. Several elastomer types: hydrogenated nitrile rubber (HNBR), FKM and FFKM and many different compounds, were screened. The conditions included steam to 265C and 250bar, and supercritical carbon dioxide to 110C and 410bar. The best materials underwent long term fluid exposure and Aflas 80 was selected for testing in a packer in a down-hole tool.

Combining elastomers in seals may be one way to enhance performance. James Walker is examining this using materials at different points in the seal cross-section: a core of high temperature RGD -resistant elastomer and a surface material with low temperature sealing performance. One of the materials under test is HNBR with different levels of acrylonitrile (ACN): this work is being patented. Lanxess Deutschland has been developing HNBR for seals in carbon dioxide high pressure service: it found that high acrylonitrile content plus a high level of diatomaceous earth filler provided good barrier properties. The vulcanisates are flexible to -15C and further tests are being carried out, for example immersion in liquid, critical and supercritical carbon dioxide.

The Techlam division of Hutchinson is responsible for designing flexible joint systems to connect rigid risers to floating structures with a lifetime of at least 20 years without maintenance. In this application, elastomer layers are bonded to metal inserts that are subjected to high levels of compression and repeated oscillatory movements. The creep behaviour of the rubber is very important, and HNBR has been found to be much better than NBR in tests. Hutchinson has developed its own "pancake" specimen method for testing materials in this application area.

There are FKM curing innovations: Daikin Industries has tested a new crosslinking system for FKM with the aim of having 30C higher heat resistant than bisphenol cured FKM and 60C higher heat resistance than peroxide cured. Curing time is longer and the new material appears to have similar mechanical properties to the bisphenol FKM.

In terms of perfluoroelastomers, Solvay Polymers has developed a new fluorinated monomer technology (perfluoromethoxy vinyl ethers) to extend the low temperature capability of FFKM to -30C, while retaining sealing properties. The new material, Tecnoflon PFR LT, has good compression set and fluid resistance. DuPont de Nemours has developed FFKM grades for RGD and low temperature sealing, working to standards such as Norsok M710 V2, for conditions where HNBR and FKM are not sufficient, for example where hydrogen sulfide levels reach 50%. Around 200 formulations were tested and 2 were suitable for further investigation.

Steel pipes account for more than 90% of oil and gas pipelines in the network of the China National Petroleum Corporation. However, corrosion occurs frequently from exposure to chemical factors such as hydrogen sulfide and chloride ions: this is getting worse with the increase in well depth, temperature and pressure. One solution is to use more chemical-resistant materials such as composite pipes, which are also lower in life-cycle costs. To date, over 17,000km of composite pipes have been installed and this is growing at a rate of 2,000km per year. Around 10,500 km of GRP pipe and 2,000km of PVC/CPVC lined GRP pipe are in use in water injection, polymer flooding, and oil and water transmission. Reinforced thermoplastic pipe (RTP) is used in similar applications and also in methanol injection and water disposal. There have been problems, for example, a thermoplastic lined GRP pipe tested in a condensate pipeline at 40C and 6.5MPa had several leak points within 24 hours.

Glass-reinforced epoxy pipe is used for water injection and is manufactured by companies such as Ameron: typically a 12m steel mandrel is covered with glass roving and passed through an epoxy bath. There are a variety of connections including mechanical key-lock, coil-lock and threaded, flanged and adhesive bonded taper. The design conditions are 26 and 40bar at 93C. These pipes have been used in a 135km installation in injection wells in Libya: the pipes were joined before trenching, which makes working conditions better and reduces the size of trench needed. The average assembly rate was 60 joints per day.

Swagelining has worked with Subsea7 on the effectiveness of lined pipelines for subsea water injection. A section of PE80 lined steel pipe was retrieved by the operator from a water injection line during modifications after 13 years of service in the North Sea. The liner had retained its mechanical properties when compared with the original PE80 and a pull out test confirmed joint integrity up to the yield point of the polymer. The carbon steel only showed superficial corrosion confirming the effectiveness of the coating for long term subsea water injection service.

PVDF is used as a 3-layer pressure barrier in flexible pipes and problems have occurred when the barrier pulls out of the end fittings. The fittings were re-designed to prevent the problem, and the cracking that occurred in the PVDF during testing was investigated by Statoil. The company compared crack growth in theory and in operational experience on PVDF materials used in risers by Technip, Wellstream and NKT. The crack growth due to mechanical stresses was different to that caused by thermal cycling.

One of the pipe manufacturers, Technip - Flexi France, has been running its own material testing programme. A typical flow line is complex including several armour layers, an anti-collapse sheath, a pressure vault and an inner tube. The polymer layer can be polyethylene up to 65C, crosslinked PE up to 90C, polyamides to about 100C and PVDF to about 130C maximum bore temperature. A low plasticiser (2.5%) grade of PVDF (Coflon XD) was developed for good ductile behaviour, processability, and lower permeation properties without losing the benefits of the virgin PVDF. Bureau Veritas reviewed the Material File and found it in compliance with API 17J. Sample pipe has been tested for dynamic performance to over 1 million cycles.

Ticona has tested its polyphenylene sulfide (PPS) material for oil and gas. It has a high continuous use temperature, low permeation, good creep resistance, excellent hydrolysis and chemical resistance, dimensional stability and is inherently flame resistant (UL94 V-0). It is suitable for use as a barrier liner in a multilayer tubular combined with a structural layer and tie layer, and is in use in Europe.

In 2006 Evonik Industries entered the offshore flexible pipe market with polyamide 12 (PA12) and carried out extensive tests in conjunction with Wellstream International to meet performance standards API 17J and ISO 13628-2. The PA12 showed improvements compared to PA11 in creep, ductility, thermal expansion, methanol compatibility and hydrolysis resistance. Since that time other groups have published studies, however there appear to be differences in the grade of PA tested and the water type. Evonik Degussa has carried out further testing including accelerated ageing (using the Arrhenius model for lifetime prediction) to further characterise the PA12 behaviour.

PEEK is a high performance plastic that has been used in oilfield applications for 20 years. Victrex and Western Falcon have been examining its use as a liner in oilfield tubular goods where PE and PA are no longer sufficient. A tight fit process is used to place a PEEK liner inside a host steel pipe avoiding the use of grout or adhesives. The liner shows high resistance to fluid and gas permeation. It offers a smooth bore, which reduces energy consumption and rod wear on rod pumped wells. It was installed in 2008 in a rod pumped well in California with "medium" level chemical aggression.

Aramid accounts for a high proportion of all high performance fibres in the oil and gas industry according to Teijin Aramid. With yarn, some of the critical properties are the instant breaking strength under load (BS), the long-term breaking load with a static load, and creep. The creep rate of aramids is relatively insensitive to temperature: in fact Technora shows a decreased creep rate with higher temperature. Cable may need to be protected in use from factors such as wear, chemicals and UV-radiation using a jacket.

Schlumberger is working on advanced composite materials: it is looking to enable service up to 200-250C. Currently a commercial epoxy casing and pipe is specified up to 100C, but in reality tests found a 30% drop in tensile strength at 100C, and high performance tubular can be rated to 175C with strength reduction factored in. The resin being studied is bismaleimide (BMI), which is used in aerospace. It is difficult to mould as it foams during heating and is solid at room temperature: the answer was to melt it at 116C under vacuum, heat to 177C and then cool. Schlumberger has developed a protocol to manufacture BMI filament-wound tubes. The target applications of this new composite are high temperature/high pressure (HPHT) such as observation well casing sections and logging tool housings. The long-term objective is to have an impermeable coating and limit water ingress - one material being tested is a flexible glass layer.

Greene Tweed is working on ultrahigh performance polymers for HPHT conditions. The company has seen operating conditions as high as 42,000psi (290 MPa) and 230C, and 35,000psi (240 MPa) and 260C. In terms of suitable elastomers, at 200C there are several potential FKMs including tetrafluoroethylene-propylene copolymers (FEPM), however at temperatures above 260C there are only a few FFKM compounds that will perform adequately. At the other extreme, some rigs are now in the arctic and operating at severe low temperatures, so Greene Tweed has developed a test FFKM compound with a glass transition temperature (Tg) of -30C. It has also developed a new high temperature plastic known as LHT, with a chemical structure involving both cardo and biphenyl linkages, which raises the Tg above 260C and up to 279C if the material is crosslinked. LHT appears to have better hot/wet performance and acid resistance than high temperature thermosets like polyimide.

Power is needed in harsh environments in the oilfield including down-hole for working equipment. The cables are manufactured using high performance fluoropolymers from companies such as DuPont International. Data logging down hole cables are now commonly 4-5,000m long: a metal armour protects the cable and the fluoropolymer insulation provides the chemical resistance to hydrogen sulfide and carbon dioxide under HPHT conditions. DuPont has recently been working on cable insulation for submersible pumps. The demand is great in regions like Russia with thousands of active wells requiring artificial lift for oil extraction. Traditionally EPDM with a lead jacket has been used in the bottom of the well - the lead prevents the rubber from absorbing gases, which could lead to RGD. However this is a thick and heavy cable and the lead is vulnerable to failure under manipulation. DuPont has been testing perfluoroalkoxy (PFA) and fluorinated ethylene-propylene (FEP) as lightweight insulation, including RGD resistance, electrical treeing, chemical resistance and environmental stress cracking (ESC).

There are several new materials coming into the market that needed to undergo appropriate testing to NORSOK, API and other relevant standards. The MERL Oilfield Engineering with Polymers conference organised with AMI provides a regular forum for discussions of advanced, applied research. The next London event is scheduled for autumn 2012.

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