The effects of rheometer response characteristics on yield stress test results of waxy crude oil

Yielding characteristics of waxy crude oil play a dominating role in the shutdown and restart of pipelines. At present, yield stress of waxy crude oil is mainly determined by the measured rheological curves according to a certain criteria. In this paper, rheological experiments were conducted on two stress-control rheometers of different types. It is shown that in the initial stages of testing, different rheometers have different actual response characteristics (shear rate) under the same shear loading condition, and the rheological curves vary greatly. Then, yielding response characteristics of waxy crude oil in two rheometers were analyzed. It is revealed that the yield stress and yield strain vary greatly. That is to say the yield characteristics of waxy crude oil are strongly associated with the actual response characteristics of rheometer. To determine the yield stress of waxy crude oil, therefore, it is necessary to take into consideration the actual loading condition of rheometers in the process of testing. Based on the concept of ultimate material strength in solid mechanics, the maximum value of elastic stress in the viscoelastic-thixotropic model for waxy crude oil is defined as the yield stress. This newly proposed yield stress determination method takes into consideration the actual loading process of rheometers in the process of testing, so it is more definite in physical significance.