Understanding of Asphaltene Deposition in the Production Tubing and Reservoir Rock While Flowing at Bottom-Hole Pressure Below Asphaltene Onset Pressure, AOP in the Magwa-Marrat Field

Abstract

Asphaltene deposition in reservoir rock is very difficult to remediate. If precipitated, asphaltenes could be trapped in the formation pores, the particles can deposit and plug the porous media reducing permeability. However, it has been hypothesized that precipitated asphaltene could entrain back into the liquid phase if the shear rate is high enough before it is deposited, adsorbed and anchored to the rock. This work intends to evaluate the role of rate in the asphaltene deposition tendency for the asphaltenic Magwa-Marrat reservoir fluid. Precisely, the purpose of this work is to study the effect of production rates and operating pressures on asphaltene deposition in the production tubing and reservoir rock at lab level running Coreflooding tests and at field level producing a well at different rates. This work provides insights into field observations of a trial well producing at a bottom hole flowing pressure below AOP. Several multi rate tests and pressure transient analysis were performed to understand asphaltene deposition in the reservoir near wellbore region and away from the well. Asphaltene deposition in the production tubing was also assessed by means of friction coefficient calculations to better understand the deposition mechanism, especially the roles played by shear rate and pressure. Coreflooding experiments at different flow rates below and above AOP were run after proper characterization of the cores and reservoir fluids. As expected, the laboratory Coreflooding results demonstrated that there were no changes in the cores’ flow capacity whether at low or at high velocities when the pore pressure was kept above AOP. However, when the pore pressure was brought below AOP, Coreflooding tests showed that the higher the velocity, the lower the permeability impairment. This concludes that fluid velocity is an important factor in the asphaltene deposition mechanism. Field tests were also conducted, and the field observations were fully consistent with laboratory results. In the case of asphaltenic crude oils, industry standards recommend depleting the reservoir to pressures no lower than AOP. Based on results of this study, and alternative approach is proposed; basically, depending on the rock-fluid properties and their interaction, it is possible to deplete the reservoir pressure significantly below AOP. Asphaltene deposition is nowadays an area of research and this study has brought some uniqueness to this subject. 1) The laboratory tests were designed together with field tests to confirm the validity of conclusions; 2) It demonstrates that a reservoir can be operated at pressures below AOP and wells produced at higher production rates as a result of operating at higher drawdowns. Altogether, the proposed approach in this paper to mitigate asphaltene deposition maximizes production offtake to the full potential of the wells while optimizing ultimate recovery; 3) Results from these field and laboratory tests have been used for field development planning that would increase the net present value of the project by a) depleting the reservoir pressure below AOP, which increases recovery factor, b) delaying water injection which minimizes CAPEX, and c) decreasing well interventions that minimizes OPEX.