Dynamic Mitigation Strategy to Flow Assurance Challenges Associated with CO2 EOR in an Onshore Abu Dhabi Field

Abstract

Abstract A CO2-WAG (water-alternate-gas) injection EOR pilot is currently ongoing in a carbonate reservoir, in an Onshore Abu Dhabi field, as part of the company's strategy for Carbon Capture, Utilization and Storage. The pilot is an important phase to de-risk challenges associated with CO2 injection prior to full field implementation. This paper will focus on the mitigation of subsurface flow assurance challenges to handle corrosion, asphaltene and inorganic scaling risks in production wells. As the conditions change during the CO2 pilot duration from producing dry oil to producing oil with miscible gas along with water, the flow assurance mitigation strategy must change to address the different challenges at each stage. Based on extensive lab studies and modeling work confirming the risk of corrosion, asphaltene and scale, it was decided to equip producer wells with downhole chemical injection facilities. Inhibitors for different applications were lab qualified as part of the chemical program to be injected, upon demand, during the pilot life cycle. A comprehensive surveillance program of gauge cutter runs, logging and fluid sampling was put in place to monitor pilot performance and changes in fluid properties and conditions that warrant mitigation by chemicals. The plan was to inject corrosion inhibitor at the start of the pilot and switch to a corrosion/asphaltene combo chemical with CO2 break-through, as it tends to destabilize asphaltene and lead to deposition. Once water cut increases, asphaltene deposition starts to diminish and the risk of inorganic scale increases. At that stage the strategy is to switch back to corrosion inhibitor injection with scale inhibitor squeeze application into the reservoir to protect from scaling. The pilot has been operational for 5 years. Water cut increased in producers from 0% to > 10% with instances of inorganic scale deposition in the lower part of the tubing. This was handled through acid washes due to favorable economics in comparison to inhibitor injection. CO2 levels at the producers increased from 6 mol% to 13-20 mol%, confirming CO2 break-through. With an effective corrosion inhibitor injection, the time-lapse corrosion logs confirmed that the tubing was in good condition. No asphaltene deposition experienced up to date hence no asphaltene inhibitor injection was administered. This paper describes steps required to assess different flow assurance challenges during a CO2-WAG pilot and outlines the mitigation strategy including a robust surveillance program during pilot execution. This mitigation strategy should be dynamic, responding to the changes in challenges posed at each phase considering the most cost-effective solution. For scale and asphaltene deposition the frequency of occurrence should be considered to decide on whether to inject inhibitors or apply solvent soaks.