Assessment of Asphaltene Deposition Risk in an EOR Intervention Through CO2 Injection

Abstract

Enhanced Oil Recovery through CO 2 injection allows to reach a twofold valuable objective, increasing the amount of crude oil extracted from an oil field while mitigating the contribution of emissions to global warming. Such an intervention is planned in an oil production field in Sicily (Italy) by employing a CO 2 stream already available from an oil refinery located near to the field. The oil is currently produced through a pump artificial lift and fluidized by a fluidizing stream injected at the bottom of the well, due to its characteristics: it is an heavy, asphaltene-rich oil. Just these characteristics worried about the possibility that injected CO 2 could destabilize the asphaltene fraction, leading to the formation a of a sludge or a solid phase, that would plug the formation pores and, ultimately, decrease injectivity and productivity. In order to assess this risk, the eni in-house-developed asphaltene deposition model was employed to investigate various conditions close to the expected operating conditions. At the same time, a lab activity was undertaken, in order to have an experimental check at least on a narrow range of conditions. Results of these activities showed that no deposition happens in the expected range of operating conditions. A description of both simulation and experimental activities is reported, in order to show how it is possible to assess the risk of asphaltene deposition before operations potentially dangerous to the field productivity. Case studyThe field object of this study lies in south-east Sicily and has been discovered in the early '80. Heavy oil mineralization (API gravity around 10°) has been detected both into a lower dolomites formation (300 cp viscosity at reservoir conditions) and into limestones of the overlying formation (40 cp viscosity at reservoir conditions). The reservoir depth is more than 3200 mssl, corresponding to an initial pressure and temperature of 326.5 bara (@3200 m) and 102 °C, respectively. The lowermost formation has been put in production in September 1990 with the drilling of five wells, one of which has been sidetracked in 2003. The uppermost formation has never been put in production. In order to optimize the investment, a study has been performed to evaluate carbon-dioxide injection scenarios for the field (both formations), which could be implemented. The reservoir is characterized by the following geological features: complex and interacting processes of initial facies deposition, progressive faulting/fracturing and several diagenetic phases of secondary porosity creation; oil migration through the fracture system only. The relevant conceptual model can be summarized by the existence of two main flow components: one vertical (fracture corridors) and one layer-driven (pseudo-matrix system, i.e. an interaction between macro-meso porous system and diffuse fractures). In order to improve the oil recovery factor, CO 2 injection was proposed, as it is possible to have a CO 2 stream from a refinery plant located near the field. The best CO 2 injecti...

Re-equilibrium of Asphaltenes by Repressurizing after Precipitation in Natural Depletion and CO2 Enhanced Oil Recovery Schemes

“…This survey reveals that the CO 2 -induced asphaltene risk has been studied from many aspects, such as simulation, core flood test, compatibility test, etc. Some reports concluded no influence; , however, the majority of studies pointed out risks caused by CO 2 injection. One of the severer cases observed concrete deterioration of porosity and permeability with accelerated asphaltene deposition in the pore throat, and another severe case lost oil recovery as a result of plugging of a high asphaltene content in the core …”