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Energy Systems Division
Geothermal Materials Group
HT resistant elastomeric materials in drilling tools
Elastomeric materials are used in pump bearings in geothermal environments since, unlike metallic bearings, they can be lubricated with water, possess high resilience and stiffness absorbing impact- or shock-loads without permanent deformation, have low friction and excellent corrosion resistance, they can also readily restore the original dimensions distorted by passing mineral particles. For geothermal applications (e.g. submerged pumps) elastomeric materials must possess a combination of chemical and thermal stability. BNL in collaboration with Sandia National Laboratory and with the support from DOE GTO conducted a study evaluating the degradation of six elastomeric polymers used for O-rings: EPDM, FEPM, type I- and II-FKM, and FSR in five different geothermal environments at 300oC: 1) non-aerated steam/cooling cycles, 2) aerated steam/cooling cycles, 3) water-based drilling fluid, 4) CO2-rich geo-brine, and 5) heat-cool water quenching cycles. The reliability of the O-rings depended on the elastomeric polymer and the exposure environment.
Elastomers with hydrocarbons in their back bone chain (EPDM and FEPM) showed a good resistance to chemically aggressive fluids but poor performance in dry heat, the elastomers with a combination of fluorocarbon and hydrocarbon bonds with α hydrogen with respect to fluorocarbons (FKM, types I and II) performed poorly in all but high heat environments; the fluorosilicone-based elastomer (FSR) degraded in all tests. Degradation of EPDM and FEPM went through chain scission and oxygen incorporation with formation of carboxylate ions; FKM elastomers degraded with the formation of carboxylate ions and hydrofluoric acid with further reactions leading to calcium permeation into the elastomer body; FSR elastomer degraded through the breakage of silicon-carbon and hydrocarbon bonds, formation of silanols and their condensation creating new Si-O-Si linkages.
The FFKM, assembled by fluorocarbon backbone chains and lacking hydrocarbon chains, displayed outstanding compatibility with all the environments. Its only drawback was the precipitation of silica- and silicate-scales on O-ring surfaces in geo-brine. Consequently, performance of the mineral-additive free FFKM nanocomposite, prepared by incorporating highly crosslinked nano-sized fluoropolymer particles fabricated by using the advanced branching and pseudo-living polymerization technology into FFKM matrix, was evaluated for its ability to mitigate silica-scaling, minimize the rate of thermochemical and heat-initiated oxidation at 300oC in the five geothermal environments. The tensile strength and ultimate elongation of the mineral-free nanocomposite O-ring were equal to, and even higher than those of the mineral additive-filled composite O-rings.
The magnitude of silica- and silicate-scaling on O-ring surfaces was governed by: 1) The oxidation reactions of the –CH2- groups; 2) the incorporation of oxidation derivatives, such as C=C, C=O, and COO- groups; and, 3) the water uptake by the mineral additive and the degraded polymer. Among the tested environments, the geo-brine fluid caused the highest precipitation of these scales. The water uptake by O-rings was due to both their oxidative degradation and the presence of mineral additives. In contrast, mineral-free FFKM nanocomposite did not absorb any water in all environments, demonstrated a low susceptibility of -CH2- groups, the minimal rate of oxidation, and the limited scaling. The high rate of scale deposition strongly affected mechanical behaviors of the O-rings.
In the thermal shock tests, the degradation of the O-ring was promoted by the sensitivity of mineral additives to the reaction with CO2 (gas) leading to the dry carbonation. EPDM underwent very high oxidation, allowing water to permeate through it. The mineral additive-free FFKM nanocomposite displayed the best performance in maintaining the integrity of O-ring in all test environments demonstrating a high potential not only for O-rings use, but also for gaskets, seals, and packers.