(51e) Experimental and Theoretical Investigation of Paraffinic Oil Gel Breaking Mechanism

Highly paraffinic (or waxy) crude oil can cause significant problems such as blockage of a pipeline due to the precipitation and deposition of the wax components during the production and transportation of crude oil. When the crude oil stops flowing in a pipeline during a long shut-in and cools down below a gelation temperature (i.e. pour point temperature) due to the heat loss to the surroundings, the solubility of wax further decreases causing more wax molecules to precipitate out of liquid phase to form a gel structure. Once the wax-oil gel is formed, the pipeline flow can not be started with the steady state operating pressure and may require significantly higher pressures to restart the flow. In this research, we have investigated the gel breaking mechanism using a model pipeline (gelometer), a controlled stress rheometer and a cross-polarized microscope at various temperatures and cooling rates. This study has revealed that the controlled stress rheometer can successfully predict the required gel breaking pressure of a gelled pipeline if the cooling rate is low and there is a gel breaking at the pipe wall (adhesive failure). Furthermore, we have experimentally shown that there exists a delineation point between cohesive and adhesive failures when the measured gel strength is plotted as a function of cooling rate. Using the controlled stress rheometer experiments and the cross-polarized microscope, this study has also investigated the possible reasons why there exists a delineation point between cohesive and adhesive failures. Based on the results of model pipeline experiments and compressibility of the wax-oil system, a theoretical model has been developed that can explain the gel breaking process in pipelines. The gel breaking model incorporates the pressure propagation phenomenon and can predict the required time to break the gel. The required time as well as the required pressure to restart the pipeline flow obtained from the theoretical model and experimental data are in good agreement. These results can be utilized to predict the restart time and the restart pressure in field pipelines. This presentation will describe the restart model based on gel compressibility and pressure propagation, summarize the operating procedures of gel restart model pipeline, discuss the experimental results, and show the agreement between experimental and theoretical results.