Miscible displacement of oil by CO2 injection is one of the most successful enhanced oil recovery (EOR) processes and has been widely implemented in fields around the world since the early 1980s. The advantage of CO2 compared to the other gases is its high extraction power and dissolution rate. As a result, CO2 can develop the miscibility front in the light and medium gravity crude oils at relatively low pressures.

A comprehensive set of experimental studies were conducted using bottomhole oil samples (BHS) and stock tank oil to investigate the viability of miscible CO2 flood in a North African field. The objectives of the study were:

  • To measure physical and thermodynamic properties of the oil and CO2 mixtures
  • To investigate minimum miscibility pressure and minimum miscibility concentration.

This paper explains the technical approach that was followed to combine laboratory experiments and simulation studies in order to improve quality of the data and tuning of the equation of state. The study started with standard PVT tests (constant composition expansion, differential vaporization, separator tests and viscosity tests) to measure the physical and thermodynamic properties of the reservoir oil. To characterize CO2/oil interaction the study continued with swelling tests. Miscibility of oil and CO2 at reservoir conditions was investigated by visual techniques and the results were verified by slim-tube analysis.

The data from PVT analysis were used to develop three equations of state (EOS) for the reservoir oil from very early stages of the study. The EOS model was then used to design the CO2/oil interaction experiments and was updated once tests were completed.

Simulation of the slim-tube tests were done in order to: (1) verify that simulated FC and MC MMP lies in the range of measured values in laboratory; (2) select the best EOS for conceptual simulation model; (3) calibrate conceptual model for slim-tube test; and (4) understand combined condensing/vaporizing mechanism for a given oil and estimate thermodynamic residual oil at different pressures. Detailed explanations of vaporizing and condensing drives were given in order to allocate them in combined drive along slim-tube.

For conceptual model preparation special attention was given to establish reference interfacial tension and immiscible base case. Further improvements for experimental set up were suggested based on the simulation.


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