Hans Petter Rønningsen

Isothermal start-up of pipeline transporting waxy crude oil ☆

Abstract This paper is concerned with the start-up problem in pipeline transportation of gelled waxy crude oil after a period of shutdown. The analysis presented is based on a three-yield-stress model, which has been experimentally verified for waxy crude oil. Three possibilities of the start-up process are discussed according to the applied pressure relative to the complex yielding behaviour of the oil, which is represented by three characteristic yield stresses – an elastic-limit, a static and a dynamic yield stress. The physical model of the start-up assumes that the gelled oil is to be displaced by introducing another fluid under constant pump pressure and that the displacing fluid displays time-independent yield stress behaviour. Using rheological property data for a gelled crude oil from the North Sea and a time-dependent Bingham style equation, the flow after a successful start-up is simulated by computing changes in the oil flow rate with time, and the clearing time as function of the applied pressure and characteristics of the displacing fluid. Both laminar and turbulent flows of the two fluids are considered in the model. The results indicate that the yield stress and the time-dependent rheology of the gelled oil play an important role in determining the oil flow rate after start-up, and that the start-up computer model is highly sensitive to the rheological behaviour of the gelled oil. An appropriate method for measuring the rheological properties of waxy crude oil for this purpose is described.

Rheological behaviour of gelled, waxy North Sea crude oils

Abstract The design of subsea pipelines carrying waxy crude oil, especially in Northern and arctic areas, is largely controlled by start-up pressure drop requirements. The correct measurement of rheological parameters associated with start-up and line-clearing of gelled crude oil pipelines, is thus important for proper design of pumping capacity and pipeline dimensions. In Part 1, the yielding behaviour of some waxy North Sea crude oils was studied by means of different controlled stress experiments, using a controlled stress rheometer (CSR) with cone-and-plate geometry and a model pipeline. Strong dependence of yield stress on cooling rate and stress loading rate was observed. Reasonably good estimates of yield stress, as compared to model pipeline results, were obtained with the CSR. CSR measurements are thus considered to be a valuable supplement to model pipeline experiments, particularly for comparative purposes, such as flow improver screening. In Part 2, the time-dependent rheological properties of two waxy oils were studied under controlled stress and controlled shear rate conditions, mainly in the cone-and-plate geometry. Two different approaches for modelling of viscosity decay at constant shear stress, based on a first-order rate equation and the Bingham rheological equation of state respectively, reproduced experiments reasonably well. Equilibrium properties obtained in different instruments (controlled stress/ controlled shear) and geometries (cone-and-plate and concentric cylinder) agreed within about 15% for shear rates between 14 and 700 s−1. At constant temperature, the broken-down apparent viscosity recovered moderately at rest, whereas the breakdown of static yield stress was essentially irreversible. Slight temperature cycling (±1°C) induced rebuilding of a weak structure, but did not restore the original gel strength completely.

Viscosity of ‘live’ water-in-crude-oil emulsions: experimental work and validation of correlations

Abstract A method for measuring emulsion viscosity under pressurized (1–100 bar) and temperature-controlled flow conditions is described. It makes use of a flow simulator shaped as a hollow wheel. As the wheel is rotating, the liquid inside will have a relative motion in the opposite direction of the rotation. The torque acting on the wheel shaft is measured and transformed, via a calibration model, into the viscosity of the fluid. The method has been applied to water-in-oil emulsions with several live North Sea crude oils with saturation pressures up to 100 bar and water cuts in the range 0–90%. The method is shown to be a useful way of obtaining estimates of emulsion viscosity for live oil systems in a relatively small scale apparatus. Even though the crude oils studied were rather different and had very different viscosities, the relative viscosities as a function of water cut up to at least 60% showed small variation and might be represented by a general ‘master curve’. The relative viscosities obtained experimentally have been compared to a temperature-dependent Richardson-type correlation [Ronningsen, H.P., 1995. Correlations for predicting viscosity of W/O-emulsions based on North Sea crude oils. Proc. SPE Int. Symp. Oil Field Chem., Houston, TX, USA, SPE 28968], and three other correlations proposed by Mooney [J. Colloid Sci. 6 (1951) 162], Pal and Rhodes [J. Rheol. 337 (1989) 1021] (P&R) and Pal [J. Colloid Interface Sci. 231 (2000) 168], respectively. Overall, when being tuned to measured data, the P&R correlation gives the best match, closely followed by the Mooney correlation, but for low and medium water cuts, the Ronningsen correlation is comparable. This correlation does not require any measurements for tuning. Although it may be slightly conservative at low to medium water cuts, and somewhat optimistic at very high water cuts, for practical purposes in design of flow lines, it provides a reasonably accurate first estimate of the effective liquid phase viscosity. The recent correlation of Pal [J. Colloid Interface Sci. 231 (2000) 168] was found to be extremely sensitive to the choice of tuning point and, thus, to be rather inaccurate either at low water cuts or high water cuts.

Ageing of interfacially active components and its effect on emulsion stability as studied by means of high voltage dielectric spectroscopy measurements

Interfacially active components (asphaltenes and resins) have been extracted from North Sea crude oils and aged at 50°C. Model water-in-oil emulsions have been stabilized by these compounds in different amounts and ratios. The stability of these emulsions has been established by means of critical electric fields measured by time domain dielectric spectroscopy in the presence of an external electric field. It is found that the stability depends mainly on the amount of asphaltenes, the degree of aging of asphaltenes and resins and the ratio between asphaltenes and resins.

Water-in-crude oil emulsions from the Norwegian continental shelf 10. Ageing of the interfacially active components and the influence on the emulsion stability

Abstract Interfacially active fractions have been separated from North Sea crude oils and have been exposed to ageing under normal atmospheric and ultraviolet conditions. Fourier transform-infrared spectra reveal that the carbonyl peak grows markedly on account of the CC mode. At the same time the spectral region between 900 and 700 cm−1 reveals that a condensation process takes place upon ageing. The interfacial activity increases in all fractions as the ageing proceeds. For two of the crude oils this is accompanied by an increase in the water/oil emulsion stability.

Water-in-crude oil emulsions from the Norwegian continental shelf 11. Ageing of crude oils and its influence on the emulsion stability

Abstract The stability of water-in-oil emulsions based on some North Sea crude oils has been studied with particular emphasis on the effects of ageing, i.e. exposure of the crude oils to air (and light). It is clearly demonstrated that the interfacial tension of a crude oil towards formation water decreases substantially as a result of ageing. This seems to be caused by the formation of various oxidation products, mainly carbonyl compounds. In addition, the water-in-oil emulsions in general become more stable. However, in some cases a reversal of this trend has been observed, indicating that oxidation products still with high interfacial activity but having less beneficial film properties are formed in the later stages of the ageing process.