Keynote lecture at 6th International Conference on Petroleum Phase Behavior and Fouling, Amsterdam, June 2005
What has been achieved in understanding of waxy crude behaviour the last four decades, and what are the future needs?
Hans Petter Rønningsen, Statoil
Abstract
Waxy crude oils have been produced and transported in pipelines for decades. All the time, the problems of wax accumulation and gelling of the oil, have been obstacles to efficient utilization of these resources. In parallel with production, there has been a more or less continuous effort in the oil industry, in cooperation with academia and research institutes, to describe, analyze and understand the properties and behaviour of these complex fluids. Nevertheless, we are still struggling to characterize the fluids and to model their behaviour. We are now faced with even more challenging and extreme conditions than in earlier times, i.e. subsea developments in deep and cold waters, long-distance transport of unprocessed fluids, onshore production and transport in extremely cold environments etc.. These circumstances make it even more important to be able to understand and handle such fluids. Is today’s research extensive enough in order to meet the challenges, and are we doing the right things?
Concerning wax deposition, hardly any new fundamental ideas concerning mechanisms of wax accumulation on pipe walls, have appeared the last three to four decades. Although Bott and Gudmundson already in 1976-77 investigated the effects of shear and time, we are still, basically, using the same simple transport equation (Fick’s law) to model wax deposition, just like in the design of Forties pipeline system, Trans Alaska pipeline and others decades ago. Improvements have occurred in analytical techniques (e.g. live fluid analysis), thermodynamic modelling etc., and new data have been produced on ageing, multiphase effects etc. Some new ideas at Tulsa University on incorporation of kinetic effects in single phase systems seem promising, but have not been tested against field data yet. Wax deposition modelling in multiphase systems is still highly uncertain. On the effects of an ever-present water phase, we know effectively nothing and are not able to account for it in modelling. So, with the challenging field developments facing us, have the right things been addressed, and enough of it, in order to understand the deposition process? Perhaps the complexity of these processes prevents us from doing much better, but I think the lack of understanding of the fundamental growth process of paraffin layers on solid surfaces is one major obstacle, together with lack of good field data for testing of models against the ‘real world’. New monitoring techniques, like pressure pulse tests and tracer injection, are promising in regard to getting deposit thickness data from relatively long field pipelines.
Concerning gelled oil problems, the situation is much the same. A ‘golden decade’ in the late 60’s and early 70’s produced a wealth of good research work on gelled oils, topped with the 1971 J. Pet. Inst. Conference on waxy crudes with a series of excellent papers on various aspects of gelled oil behaviour. Time-dependent non-Newtonian rheology, pilot-scale tests describing e.g. auto-destruction effects, field tests, heat transfer, additive testing etc. were covered. A firm base for further development was actually in place, but although much good work on non-Newtonian rheology, dissolved gas and multiphase effects has been produced since then, we are still, most often, using the simple wall shear stress-pressure drop relation for laminar flow, in order to analyse potential restart problems with yield stress fluids. Multiphase systems are still, essentially, beyond our capability. If possible, the formation and shear-degradation of an oil gel is even more complex than the paraffin deposition process. But again, it is relevant to ask whether the right things have been done the last decades in this area, given the good starting point around 1970.
This may sound somewhat negative and pessimistic, but when trying to look a little bit above all the ‘details’ that have been produced and all the ‘small’ pieces of knowledge that have been put in place the last three to four decades, it is difficult to see any major achievements, and in fact hard to foresee any real breakthrough the next few years. This keynote lecture will elaborate a little bit on the history, the current status and the future.