Modeling Requirements For Numerical Simulation Of CO2 Recovery Processes
Authors: Todd M.R. 
Published: December 2007

There have already been some successes in industry laboratories in simulating CO2 phase and displacement behaviors for linear core floods using compositional simulators. There have also been some limited successes in simulating unstable miscible displacements as observed in physical model experiments and field trials using modified black oil models. However, it seems generally accepted that both the rigorous compositional approach and the modified black oil approach have severe limitations. Regardless of the limitations, because it is widely believed that the success of specific projects will depend strongly on the interaction of the projects will depend strongly on the interaction of the process, field characteristics and operator controlled process, field characteristics and operator controlled parameters, simulators will in all likelihood be used to parameters, simulators will in all likelihood be used to aid in design and evaluation.

In this paper, we describe several aspects of CO2, flooding and note how these relate to the requirements of a numerical reservoir simulator. Next, capabilities and limitations of currently available modeling techniques are discussed. Finally, assumptions are made as to the salient features that must be included in a viable CO2, flooding model. This leads to a suite of simulator specifications.

Introduction

When considering a candidate reservoir for CO2 flooding, the first question historically asked is whether or not CO2 is "miscible" with the in-place crude at reservoir temperature and obtainable reservoir pressures. However, for CO2 flooding, multiple contact or pressures. However, for CO2 flooding, multiple contact or dynamic miscibility is only one of the recovery mechanisms and enhancement, which should be considered. Others include:*oil swelling with CO2,*extraction or vaporization of oil into the CO2 rich phase,*reduction of oil viscosity for improved mobility ratio,*modification of relative permeabilities and/or reduction*in residual oil saturation due to a reduction in CO2-oil interfacial tension.

While features such as these, as well as less favorable aspects such as CO2 loss to the aqueous phase and water blockage of oil from contact with CO2 are subjects for study in the laboratory, other characteristics such as unstable frontal advance due to viscous fingering and gravity tonguing are less suitable for laboratory investigation. In addition, scale up to reservoir conditions involves questions concerning the reservoir characterization as well as questions concerning pattern development and operating alternatives. Examples of these field performance considerations include: performance considerations include:* Oil recovery performance,*CO2 required per barrel of oil recovered,*Vertical and areal conformance,*Efficacy of mobility control alternatives,*Effect of well configuration and pattern balance,*Effect of injection and production well strategies,*Effect of pressure level and gradients,*Effects of stratification and dip,*Effect of process alternatives such as:- continuous CO2 injection;- WAG (water alternate with gas);- CO2 slug followed by flue gas;- reservoir blowdown.