Abstract

Many experimental and analytical investigations of the CO2 performance at reservoir conditions have been performed world-wide in the recent years. However integrating many of those findings into reservoir simulation models still remains an open issue.

In this manuscript we address only the reservoir aspects of the CO2 injection to Enhance Oil Recovery (EOR) leaving behind transport, metal corrosion and economic issues related to CO2 injection. CO2 is an efficient agent to increase oil recovery, especially, if conditions of oil displacement are miscible. However, CO2 "gas like" low viscosity at reservoir conditions may cause severe viscous fingering and displacement front instability resulting in low sweep efficiency. CO2 may boost precipitation of waxes and asphaltenes, thus reducing productivity or completely plugging the wells. CO2, being a chemically active component at reservoir conditions, may provoke chemical reactions and mineralogical changes in situ resulting in increased permeability, loose of sealing properties of the cap rock.

Based on the laboratory investigations we aim at building a consistent model to evaluate CO2 injection performance at reservoir conditions. The laboratory studies were conducted in such a way that a single or several natural phenomena were isolated and individually studied. The experimental results allowed us to gain a better knowledge of the process mechanisms. Doing history matching simulations of the experiments we evaluate critical parameters, validate and calibrate numerical models. The complexity of the simulation model is gradually built up from simple experiments to full scale simulation. The mechanisms addressed investigated are:

  • Viscous fingering;

  • Multiphase compositional effects;

  • Flow in naturally fractured porous media (diffusive fluxes and gravity segregation);

  • CO2 dissolution in aqueous phase;

  • Chemical interaction between rock and CO2;

  • CO2 mobility control agents.

Special attention in this paper is paid to the viscous effects during CO2 injection at above critical reservoir conditions. The laboratory experiments were performed with different oils. Pure CO2 and CO2 -rich oleic phases were injected to investigate gravitational and viscous effects in the porous medium. It was shown that using relative permeabilities dependant on rate and interfacial tension can improve the accuracy of CO2 injection simulation. Water Alternating Gas (WAG) injection was shown to be an effective way for remediation of viscous fingering.

Conclusions on various CO2 injection phenomena and ability of reservoir modelling tools to handle those phenomena are drawn.

Introduction

Environmental aspects are playing now more important role in the evaluation of field development strategies. Sequestration of greenhouse gases and developments in the environmentally sensitive offshore areas present new technology drivers and challenges in the same time.

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