Yield Stress Measurement Of Gelling Oils

Abstract

Abstract An instrument has been developed which permits the reliable measurement at yield stress in gelling crude oils. The improvement of the present apparatus over the standard tubular instruments appears to be related to a lack of formation of fissures in the gel on cooling and also to the fact that a direct measurement o yield stress is obtained. Implications for pipeline start-up are described which suggest that the shear stress must be exceeded locally in all parts of the line. The temperature dependence of yield stress is illustrated for both a refined oil preparation and a crude oil. For the particular curing temperatures employed, the yield stress does not appear to be sensitive to duration of annealing, duration of cooling or the rate of application of stress. Introduction Many crude oils throughout the world display properties which can be rheologically described as being thixotropic(1,6,8,10,17,20). Such oils are equivalently characterized as being "high-pour-point" oils. Using a standardized test, the pour point is that experimentally measured temperature at which an oil shows a tendency to gel or to not pour while it is being cooled. The word "high" refers to situations in which this temperature is higher than the ambient conditions which can be encountered as the oil is moved to the refinery. Consequently, its interpretation differs from one geographical region to another. The undesirable gelling behaviour of a thixotropic oil can manifest itself as the oil is being cooled while at rest below a certain temperature, or when the cooled oil is permitted to remain static for a prolonged period. Often, sufficiently low temperatures can be expected to occur during handling. The implications for storage or pipeline transportation of this type of oil are beyond those normally considered in the design of facilities (1,4,6,7,10,15-19). Difficulties in starting due to the gelling of a thixotropic oil can arise in a pipeline once flow has terminated either as the result of a breakdown or a refitting of the line. With the temperature of the oil at a suitably low level and if the oil has had sufficient time to set, the pump head required to initiate flow can be considerably higher than that necessary for the normal operation of the line. The necessary head which can be related to the shear strength of the oil in its gelled condition is affected by the previous shear and thermal history of the oil as well as the temperature present in the pipeline. The lower the temperature of the oil, the greater the head necessary to fracture the gel. Therefore, provision to accommodate restarting of the line must be incorporated into the design of any facilities transporting such an oil. There are many alternate approaches to the problem, which can be considered either separately or in combination with one another depending on the economics associated with them. Those most often included are:heat tracing of the line to maintain an elevated temperature, which can certainly be most useful over short distances,