Identification of Fluid and System Parameters that Drive Asphaltene Deposition in Ageing Fields
Authors: Grutters M. , Punnapala S. , Cristea Z. , Kherroubi D. 
Published: November 2024

Abstract
Predicting where and when asphaltene deposition will occur remains a big challenge, particularly with changes to produced fluids over the field life such as changes to GOR and water cut. Small changes in fluid properties or changes in flow dynamics can alter the plugging frequency of a well. Changes in fluid properties affect thermodynamics and, therefore, change the precipitation tendency. Moreover, changes in fluid properties can alter the interaction of asphaltene aggregates with steel surfaces and, therefore, the deposition tendency. Deposition tendency is also influenced by changes in flow dynamics, such as variations in fluid velocities and flow regimes. There is a common industry perception that deposition becomes less severe or even ceases in the presence of water. The reason for this is not entirely understood, but the presence of water might induce chemical changes in asphaltenes as well as modifying tubular wettability, both of which can influence asphaltene to surface adhesion. In our previous publication (SPE-213844) we investigated the potential causes behind the disappearance of asphaltene deposition in an onshore carbonate oilfield over a period of twenty years. A deposition severity score was calculated from the number of asphaltene related interventions, such as wireline inspection, analysis of field deposits and chemical soaks. This deposition severity score was correlated against water cut. This empirical relationship showed that deposition severity decreases with water cut and stops above 40%. The review confirmed that there was no evidence that the decrease in deposition occurred because the precipitation onset moved inside the formation, as result of changes to reservoir pressure or mixing with injection gas. In this paper the same dataset was used to evaluate if changes to asphaltene deposition severity can be explained by changes in flow conditions. This was evaluated by first correlating changes in deposition severity to changes in the volume of oil produced. Secondly, changes in deposition severity were evaluated for transitions in flow conditions (pressure, temperature, flow rate and flow pattern). The evaluation was done by calculations with historic well testing in a steady state well simulator as well as in a dynamic multiphase flow simulator. The simulations clearly indicated that changes in asphaltene deposition severity are not related to the amount of oil produced, lower asphaltene deposition over a period of time is not the result of less oil produced for that period of time. Steady state wellbore simulations showed a drop in Productivity Index (PI) after water breakthrough, then stabilizes for a period of time followed by a further drop in PI. These results might imply that the presence of water initially exacerbates the asphaltene build-up but then decreases as a result of changing hydrodynamics. Dynamic multiphase flow simulations revealed that the depths at which historical deposits have been recorded coincide with flow regime. No deposits have been recorded at depths below PSAT, where the flow regime changes to bubble flow with reduced contact between oil and tubular and increased oil film wall shear stress. The section of the well where deposits occur (from 3,000’ to 8.600’) is in annular flow, from low to high water cut. However, the oil film wall shear stress increases with water cut which could possibly contribute to the reduced deposition severity. The results of this new study indicate that changes in flow dynamics, resulting from increasing water cut affect asphaltene deposition and appear aligned with results of other laboratory and modeling studies.