Titel Zu den Tribosystemen von metallischen Dichtungen an Ölfeldrohren für Gasbohrungen: Theoretische-mathematische Einblicke und deren Einfluss auf kritische Ölfeldaktivitäten.
Autor Oscar Grijalva Meza, Javier Holzmann Berdasco, Joachim Oppelt, Samuel Zulkhifly Sinaga
Infos zum Autor MSc. Oscar Grijalva Meza
Wissenschaftlicher Mitarbeiter (OCTG Full Scale Testing - Premium Connections)

Institut für Erdöl-und Erdgastechnik, TU Clausthal
Agricolastraße 10, Raum H108
38678 Clausthal

Tel. +49-5323-72-5016
Inhalt Zusammenfassung
In modern oilfield operations, environmental protection play a critical role during the design of oil and gas wells. For the last, the concept of “Well Integrity” has become the standard in the industry and is guaranteed through the fluid tightness of tubulars used to isolate the producing wellbore from its environment. To achieve this, the use of Premium Threaded Connections, as known in the Industry, is recommended. Premium Connections consist of a specific, non-standard thread profile and a metallic seal in both pin nose and box seal areas. Fluid tightness is achieved when those 3 metallic seals abut and generate a certain amount of “contact pressure” while screwing both connection elements. In most applications a thread dope is used not only to lubricate the mating tubular elements, but also to serve as a “second seal” hindering the leakage of produced fluids. However, high make up torque levels generated while screwing the tubular elements can lead to the development of galling and wear phenomena along the metallic seal surface. Currently there are no standard values defining the exact amount of make-up torque necessary to achieve a minimum of gas sealability (contact pressure) which doesn´t damage the metallic seal (galling onset). By understanding the metallic seal surface and the lubricating layer as a tribosystem, it is possible to determine, or at least understand the tribological relationships impacting the performance of such specialty connectors. The present work discusses the impact that the coefficient of friction (CoF) has not only on the make-up torque needed to screw oilfield tubulars together, but also according to the characteristics of each tribosystem in function of the type of lubricant used. Wear phenomenon affecting the metallic seal surface in Premium Connections is also handled on this work and the corresponding non-Hertzian expressions dictating the development of contact pressure in elastic materials (e.g steel) are also derived in the context of wear. Galling prevention, as a specific manifestation of adhesive wear, is the most critical parameter considered during the design of a gas well using Premium connections with metallic seals; the modification of the lubrication regime existing at the contact area and the consequent surface damage after this has motivated several developments concerning phosphatizing technologies: the impact of such technologies in the development of oilfield activities in environmentally critical zones (e.g. North Sea) will be briefly discussed from the tribological point of view, putting emphasis on wear behavior and its impact in connection tightness for gas service.

Environmental protection plays a central role in current Oil & Gas operations. For this, Premium Connections for casing tubulars installed in gas wells account for total isolation of the produced hydrocarbons from the reservoir to the surface, thus hindering possible leakage paths and hence pollution or catastrophic incidents. Premium Connections feature metallic seals which get energized during the screwing of the connection and the use of oil-based thread dopes contributes to both lubrication and galling prevention while doing this. Concerning this, galling resistance and a uniform contact pressure distribution account for the integrity of the connection; however both factors are paradoxically not well understood yet. This paper aims to throw light to the relevance of the elements comprising the tribosystem of Premium casing connections in the context of current critical E&P operations while, at the same time, the mechanical preconditions impacting their corresponding triboelements are analyzed from a mathematical and empirical perspective. For this, the relationship between the coefficient of friction, applied make up torque and the Non-Hertzian distribution of contact stresses at the metallic seal will be explained, as well as the impact of differences in their lubricating tribosystem will be assessed. The results of this introductory work will help future research work defining a quantitative ″galling window″ for Premium Connections, understanding it in terms of ponderated values for contact pressure giving a minimum of gas tightness and maximum sealing performance before the onset of adhesive wear (galling).
Datum 2017