Abstract
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:
1) different single fractured reservoirs of different inclination angles, 2)
different permeability configurations of layered reservoirs, and 3) the
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.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
8 March 2007
- Meeting paper published:
12 June 2007
- Revised manuscript received:
11 June 2008
- Manuscript approved:
23 December 2009
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