Influences of Different Modes of Reservoir Heterogeneity on Performance and Oil Recovery of Carbon Dioxide Miscible Flooding
Authors: Shedid S.A. 
Published: March 2010

Reservoir heterogeneity represents one of the most dominant factors affecting the performance of CO2 miscible flooding and its expected oil recovery. The main goal of this study is to investigate the influence of different modes of reservoir heterogeneity on oil recovery by supercritical CO2 miscible flooding. The investigated heterogeneity modes include:different single fractured reservoirs of different inclination angles,different permeability configurations of layered reservoirs, andthe sequence of permeability distributions in composite reservoirs.

Complete reservoir rock and oil compositional analyses were performed. The minimum miscibility pressure (MMP) of oil-CO2 was mathematically calculated using several empirical correlations and determined experimentally using slim tube tests. The core flood tests were achieved using actual fluids injected through 12 actual reservoir rock samples. Of these, four samples were of different fracturing angles as single fractured reservoirs, four samples were of different permeability configurations as layered rocks and four samples represented composite reservoirs. The slug size of supercritical CO2 was optimized to be 0.15 PV, injected and chased by actual reservoir brine through these different simulated modes of reservoir heterogeneity.

The results indicated that all different modes of reservoir rock heterogeneity have a crucial influence on oil recovery by CO2 miscible flooding in carbonate oil reservoirs. Of note, unfractured reservoirs produced higher oil recovery by CO2 miscible flooding than single fractured ones. An oil reservoir with a 30 degree inclination angle of single fracture produced the highest oil recovery, whereas, fractured rocks with a 45 degree fracture produced the minimum oil recovery in this category. The rock permeability sequences of medium-low-high (MLH) mode for composite reservoirs and medium-high-low (MHL) distribution mode for layered reservoirs are highly recommended for CO2 miscible flooding.

The results have proven the suitability of the CO2 application for layered and composite heterogeneous carbonate reservoirs, however, it does not recommend this EOR process for single fractured reservoirs. The results have also shown a real impact on oil recovery of the reservoir heterogeneity mode prevailing in the reservoir under development by this EOR process.

Introduction

The major applications of active carbon dioxide flooding projects were found in the U.S. Permian Basin, located in West Texas and Eastern New Mexico. Other projects were also active in Colorado, Louisiana, Mississippi, Oklahoma, Utah, Wyoming, and in Canada, Turkey (Bati Raman), Trinidad and Hungary (Budafa). The number of CO2 projects have increased in the U.S. and worldwide over the last number of years(1–3).

Reservoir characterization has been considered one of the most important components of reservoir development because it provides a better description and distinguishes essential features of reservoir heterogeneities affecting fluid flow in pay formations. It has been proven that a better reservoir description and/or characterization reduces the amount of oil left in oil reservoirs. The main purpose of reservoir characterization is to outline and specifically integrate different forms of heterogeneities of the reservoir oil. Forgotson(4) defined reservoir characterization as the quantitative description of the physical and chemical properties of the porous medium and its contained fluids.