Research On The Yielding Properties Of Gelled Crude Oil

The yielding properties of Qinghai gelled waxy crude oil was studied in detail based on the rheology measurement technology of crude oils, by applying the RS75 rheometer in this study.First,effect of factors, such as cooling rates during static cooling, high-speed shear temperatures and shear rates during dynamic cooling on the properties of waxy crude oil at the initial stage of yielding process was studied by creep/recovery experiments. Results show that both the instantaneous elastic strain and the total strain caused by creep will increase with the decrease of cooling rate. When temperature of high shearing is higher than wax appearance temperature, there is no influence on the gel structure of waxy crude oil; When the high shear temperature varies between wax appearance temperature and gel point, the gel structure becomes harder with the raise of shear temperature; While around the gel point, the high shear rate will makes the gel structure weaker. Furthermore, the higher the shear rates during dynamic cooling is,the weaker the gel structure while the larger the strain in creep is.Second, effect of temperatures, cooling rates during static cooling and shear history as external factors on the yielding properties of gelled crude oil was investigated. Results show that with the increase of cooling rate or decrease of measurement temperature, both the yield stress and yield time increase, but the yield strain decreases. When the ending temperature of shear was 4℃higher than abnormal point, it has little influence on the gel structure. Round the abnormal point, the gel structure became stronger. When the ending temperature of shear was 4℃lower than abnormal point, the gel structure became weaker as the reduction of ending temperature of shear. Isothermal pre-shearing would not affect on the gel structure of waxy crude oil with extreme low shear rate or short shear time, but it will make the waxy crude oil show double yielding behaviors with fairly low shear rate or short shear time. If the shear rate is high enough or shear time is long enough, the waxy crude oil will yield immediately as soon as applied of stress.Third, the yielding properties of waxy crude oil under different measuring method were studied. And it show that for the shear stress loading in a stepwise way, with the increase rate of shear stress or decrease of time duration in each step, the yield stress increases and the yielding time decreases. For the shear rate loading in a linear way, if the increase rate was small, the shear stress will appear fluctuation phenomena with the increase of shear rate, as the rate of shear rate increases, the fluctuation phenomena will decrease gradually and until disappear.It is considered that two boundary values, static state yield stress and instantaneous yield stress can be used to describe the structural regime of gelled crude oil. If the applied stress was lower than static state yield stress, the structure of gelled crude oil cannot be destroyed no matter how long the applying time is; if the applied stress is higher than instantaneous yield stress, the structure yields immediately and the oil sample begins to flow; while the applied stress varies between the two yield stress, the structure will yield if the time is long enough, but the yielding time will increase with the increase of the applied stress.At last, a precise and convenience method is obtained to describe the thixotropic properties of waxy crude oil under different shear rates by improvement of Houska model with combination of experiment data and rheological theoretical derivation.