We present results studying the IOR potential for CO2 injection in the naturally-fractured Haft Kel field, Iran, based on detailed compositional simulations of the matrix-fracture system. Oil recoveries from CO2 injection consistently range from 80–90% for reservoir pressures 1400 psia and higher (i.e. at and above current reservoir pressure of 1500–1800 psia). This compares with 15–25% recoveries reported for gas cap expansion and/or injection of hydrocarbon gas.

Fundamentally-different recovery mechanisms develop above and below 2000 psia, the pressure where CO2 density equals the reservoir oil density. At lower pressures, CO2 is less dense than reservoir oil and traditional gas-oil gravity segregation results, with a highly efficient process driven by gravity, compositional effects and IFT gradients that cause capillary-induced oil flow. At pressures greater than 2000 psia, CO2 density is greater than reservoir oil density, resulting in an unusual gravity drainage mechanism whereby CO2 enters the bottom of a matrix block and pushes oil out the sides and top of the matrix block.

The effect of several key parameters has been studied in detail – matrix permeability, matrix block size, matrix-matrix capillary continuity (stacked blocks), and the use of mixtures of CO2 and hydrocarbon gas. One of the key results is how the rate of recovery differs for HC and CO2 injection, and how it varies for CO2 injection for different model parameters.

IOR results are affected by grid sensitivity. Grid effects have been quantified and compared for different model parameters. Final IOR assessment is made using models where sufficient grid refinement is used to minimize grid sensitivity.

CO2 injection in fractured reservoirs in the Middle East appears to have great potential, even at relatively low pressures. Large sources of CO2 would need to be available from gas-driven power plants and reservoirs producing CO2 in significant quantities.


When an oil-saturated matrix block is surrounded by gas in the fracture, oil will drain from the matrix as a result of density difference between the gas and the oil. However, drainage can occur only if the matrix block height is greater than the capillary entry height.

This content is only available via PDF.
You can access this article if you purchase or spend a download.