Abstract

Despite a successful history of CO2-EOR onshore, its application has been limited offshore. There are fundamental differences between offshore and onshore environments. Offshore, reservoirs are typically bigger and are characterised by larger well spacing. There are also other important differences in terms of prevailing reservoir conditions. This study compares the performance of CO2 flooding between these environments and investigates how the combinations of these differences affect the performance of CO2-EOR offshore compared to onshore. Two synthetic box models with different dimensions, each representing a relevant environment were constructed and the CO2 flooding process were simulated. Onshore model represents a United States CO2 flooded reservoir, while offshore model represents a North Sea reservoir. The offshore model is thicker and has larger well spacing. The same injection and production rate in terms of pore volume and the same flood strategy were applied in both models. Identical flooding strategy ensures that both models experience the same average pressure evolution during simulations and the injected CO2 volumes remain comparable. This also makes the results comparable between them. The results show that, larger well spacing offshore favourably affects the pressure evolution of the system. The higher injection pressures offshore coupled with high CO2 compressibility, makes CO2 and reservoir fluid densities become comparable offshore, which reduces the gravity segregation of CO2 and increases CO2 and reservoir fluids compositional interactions. The same rate of depletion in a larger well spacing environment translates to higher fluid velocities inside the reservoir, which makes the flow less affected by gravity. Offshore, the model thickness is comparable with well spacing which retards the gravity segregation of CO2.

The inclusion of other differences mainly reservoir temperature and formation permeability characteristics of offshore environment adversely affects the CO2 flood performance offshore. This poor performance however can be compensated by operating the flood at higher pressures. The results show that the CO2 flood offshore is less gravity dominated and WAG does not bring the same benefit offshore as it does onshore. Numerous simulations confirmed that there is always a correlation between EOR and CCS performance of the system. Scale differences can bring improvements for CO2 application offshore characterised by thicker pays and larger well spacing. However this improvement may not be able to compensate for the adverse effects of the prevailing reservoir conditions. Given other considerations regarding CO2 application offshore (e.g. production problems) the in-situ reservoir conditions may make the EOR performance of the CO2 flood so poor that further motivations like CCS will be required to bring CO2 injection offshore.

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