The Importance of Asphaltene Deposition Measurements in the Design and Operation of Subsea Pipelines

Abstract

Abstract Shell Exploration & Production Company has been operating fields producing asphaltenic fluids in the Gulf of Mexico (GOM) for several years and had previously been unable to accurately predict the extent and location of asphaltene deposition. As a result, well bores and subsea flowlines have in some cases been plugged whereas in other cases it is likely that inhibitor injections have been unnecessarily employed. In this study, a flow-through high-pressure deposition cell was used to measure the deposition rate of asphaltenes from a problematic field in the GOM under realistic conditions of pressure, temperature and composition. The impacts of shear (corresponding to the production rate in the field), residence time of the fluid in the cell, pressure, and chemical injection on the asphaltene deposition rate were investigated. Finally, the importance of these measurements for field production is discussed. Introduction As production from conventional onshore and shallow off-shore fields decline, deepwater production will continue to increase in importance. Large pressure and temperature drops often encountered in deepwater production systems increase the risk of asphaltene precipitation and deposition. Commingling of incompatible fluids and gas lift may also destabilize the system. The large capital and operating costs associated with prevention and remediation of deposits has created the need for improved methods to measure and model for optimization of system design and operations while still ensuring minimized risk of deposition issues. The techniques applied for prevention of asphaltene deposition in the field include retaining the operating pressure above the detected onset pressure, increasing the production rate, decreasing the residence time of the fluid in pipeline and chemical injection. In the absence of lab-scale measurement of asphaltene deposition, however, the operators may not be able to apply such techniques properly and deposition may happen unexpectedly. In other cases, despite a system incurring asphaltene precipitation under operating conditions of temperature and pressure, asphaltene deposition may not happen due to other factors such as shear, kinetics, and physiochemical characteristics of asphaltenes and pipeline surface. The high-pressure deposition cell, designed based on the Taylor-Couette flow principles, is a tool for generating organic solids deposits under a wide variety of operating conditions. This equipment in its batch mode (closed system) has been used to measure the deposition rate of waxes and asphaltenes from live fluids under turbulent flow conditions (Zougari et al., 2005, 2006; Akbarzadeh and Zougari, 2008; Akbarzadeh et al., 2008a, 2008b). However, the experimental measurements have been inconclusive for asphaltene deposition from crudes with low asphaltene content. For these fluids, the amount of deposit obtained from a batch experiment, with 150 cm3 fluid in the cell, is often very small (less than 15 mg). This results in a large relative uncertainty in the measured data, and therefore interpretation is difficult. The other problem with deposition in a batch system is depletion. A decrease in the amount of depositing asphaltenes in the cell over a short period of time (typically two hours) will yield averaged deposition rates that are not representative of those in the field.