Page 98 - Fundamentals of Enhanced Oil and Gas Recovery

86 Ramin Moghadasi et al.

modification to the other parameter (i.e., σ) in order to raise the value of the capillary
number. Indeed, CO 2 injection would practically decrease IFT value and conse-
quently increase N ca . Ultimately, it results in the mobilization of trapped oil droplets
within the porous media. This means higher microscopic efficiency.

3.2.2.2.13 Blowdown Effects
After CO 2 injection terminates, pressure depletion into the reservoir will occur natu-
rally. As a result, dissolved CO 2 will be liberated. This process acts just as the solution
gas drive, which is responsible for oil recovery. This process is called the blowdown
recovery effect. This is highly dependent to the compressibility of oil and CO 2 at res-
ervoir condition as well as the amount of the CO 2 dissolved in oil [18].

3.2.2.2.14 Injectivity Increase
A mixture of water and CO 2 could create an acidic solution that can react with the
carbonated portion of the rock matrix. Such reactions result in rock dissolution, thus
increasing the permeability of the rock. As this acidic solution contacts the reservoir
oil, it may lead to asphaltene precipitations, which is highly dependent on oil proper-
ties. Some oils have a high potential for asphaltene precipitation occurrence as they
make contact with such acidic solutions [52].

3.2.2.3 Immiscible Flooding Field Cases
In most of the field case studies, miscible flooding has been actually implemented.
However, as heavy oil resources are increasing in number, the use of immiscible
flooding has also been attracting more global attention. The first field CO 2 immiscible
project was carried out in 1949. Although at that time oil recovery was lower than
that of measured in laboratory, it was proved that CO 2 immiscible flooding could
improve oil recovery to an acceptable level. As examples, two field projects with
detailed information are reviewed here.

3.2.2.3.1 Lick Creek Field, United States [53]
This field is located in southern Arkansas and was discovered in 1957. It is reported
that production rate peaked at 1900 BPD and declined at 230 BPD in 1960 and
1976, respectively. Total oil recovery till the start of immiscible CO 2 flood in 1976
was 4.486 million bbl, equivalent to 28.3% OOIP.
The CO 2 was sourced from 65 miles away, transported through pipelines in super-
critical state. As its production history showed, the field responded positively to the
CO 2 project. Production increased from 8000 to 28000 bbl/month. Till 1990, CO 2
injection contributed to cumulative production of about 11% of OOIP. It has been
reported that the relatively low tertiary recovery was due to the occurrence of CO 2
channeling in the high permeability sands [53].