Asphaltenes Risk Assessment and Mitigation – Designing Appropriate Laboratory Test Protocols

Abstract

Methods currently used to evaluate laboratory performance of asphaltenes inhibitors are non-optimal because the conditions used are so far from those prevailing in the field, leading to incorrect assessment of dose rates or even selection of chemicals that may not be beneficial at all. We present a dynamic flow test method for asphaltenes risk assessment and inhibitor qualification that uses field-representative temperature, pressure and fluid dynamics to enable successful correlation with field behaviour. This paper discusses the most commonly used laboratory test methods for asphaltenes testing and proposes a new dynamic flow method that offers a significant improvement over other widely-used techniques. Reconditioned dead crude oil is co-injected with n-heptane through a steel capillary and an inline filter. Differential pressures are recorded to monitor the extent of asphaltenes precipitation and deposition. We highlight key parameters that should be optimised to ensure that chemical performance is tested against the actual functionality required in the field and under conditions that are as representative as practicable. We present a case study describing the use of the dynamic flow test equipment to assess asphaltenes deposition risk and to qualify asphaltenes inhibitors for field application. We demonstrate that the method is able to rank chemicals for performance at inhibiting deposition under flowing conditions and at more field-representative temperature and pressure, with much lower percentages of n-heptane than required for conventional dispersancy testing. We discuss the effect of critical parameters affecting the extent of asphaltenes deposition. Fluid dynamics are recognised to play a key role in asphaltenes deposition in the field, not least, because at higher wall velocities the erosive force acting on field deposits is high enough to limit further growth and steady state can be reached. Flowing tests were conducted under a number of fluid-dynamic regimes in which asphaltenic crude oil was destabilised by addition of n-heptane. The effects of wall shear stress, wall velocity, residence time, and other factors were evaluated upon asphaltenes deposition in a steel capillary and upon bulk precipitation by subsequent filtration. The results obtained from laboratory tests correlate well with field observations and demonstrate that flow regimes in laboratory tests can approach those occurring in the field. This paper presents the development of a new laboratory test method utilising dead crude both for asphaltenes risk assessment and inhibitor qualification that offers significantly improved correlation with field behaviour over conventional dispersancy testing, yet remains much more cost effective than labour-intensive autoclave testing utilising live fluids. When considering asphaltenes risk analysis the approach also allows for deposition vs. precipitation to be examined under field realistic conditions, and we demonstrate how this can be of significant importance when, for example, introducing gas lift into asphaltenic crudes.