Investigation and Downhole Mitigation of Asphaltene Deposition at Electrical Submersible Pumps – A Mature Offshore North Sea Oilfield Case Study

Abstract

Abstract The objective of this paper is to demonstrate the problematic of asphaltene deposition during oil production via ESPs in Mittelplate oilfield. Furthermore, different measures implemented for prevention and removal of asphaltene precipitations are presented and summarized. Production impairment due to asphaltene deposition has been observed in two wells in Mittelplate oil field. First indications of asphaltene accumulations were noticed at the production well M-W1. The well experienced severe asphaltene-plugging issues resulting in rapid production decline. Damages caused by asphaltene precipitation in the ESP have been observed during tear down analysis. The well was regularly treated with solvent washouts to disperse asphaltene depositions. Despite high frequency of solvent washouts, the well productivity could not be restored. Later on, the well was shut in and recompleted into a water injection well according to the field development strategy to support the pressure in the reservoir. At the production well M-W2, a strong production decline was observed as well. ESP production behavior and laboratory analysis - determination of colloidal instability index (CII) based on saturate, aromatic, resin and asphaltene (SARA) analyses – confirmed high likelihood of downhole asphaltene accumulations. As a first attempt, the well was treated via solvent washout to remove asphaltene accumulations from the ESP. Following that, it has been decided to inject an appropriate asphaltene inhibitor through the chemical injection line (CIL) to mitigate further precipitation of asphaltenes within the ESP. Two candidate asphaltene inhibitors have been selected based on their effectiveness with respect to asphaltene precipitation in Mittelplate crude oil. Field tests started at the well to identify the most effective inhibitor at downhole conditions by optimizing the inhibitor dosage and respectively the asphaltene treatment. Preliminary results of field test application at the well M-W2 showed that continuous asphaltene inhibitor injection through the CIL is very effective under downhole conditions. Furthermore, surveillance procedures, such as monitoring of bottom hole flowing pressure, well production rate and inhibitor injection rate, are in place to understand and evaluate the effectiveness of the treatment. This paper presents the operational challenges and experiences related to asphaltene deposition during the oil production via ESP system in a mature North Sea oilfield. The early results of field test applications with different types of asphaltene inhibitors are presented. Lessons learned from field test applications and operational experiences have been summarized. In general, there are few similar field case studies available in the literature and therefore, the publication of the results would provide valuable information for other fields facing similar problems regarding asphaltene precipitation.