Application of Geologic Studies to Enhanced Oil Recovery (EOR)

Charles W. Spencer

Enhanced oil recovery (EOR) methods are currently yielding about 6% of the annual oil production in the United States. EOR methods are those techniques that will recover that portion of the original oil-in-place (OOIP) not recovered by primary and secondary (usually water-flood) methods. Approximately 30-35% of the OOIP can be recovered by primary and secondary methods, but recoveries range from less than 10% to nearly 80% of the OOIP. The recovery factor is highly dependent on stratigraphy, lithology, and pore and permeability characteristics and their distribution. Other parameters affecting recovery are oil characteristics, well spacing, natural fractures, reservoir temperature, and pressure.

In 1984, the National Petroleum Council estimated that the presently available EOR technology could recover about 15 billion bbl of oil from already discovered fields. Advanced EOR technology applied to existing fields and EOR oil from undiscovered accumulations will considerably increase this oil volume. Most EOR projects are very costly; therefore, oil prices have a significant impact on the volume of oil to be recovered by EOR at any given time.

The importance of pre-EOR and syn-EOR geologic studies has become more appreciated in recent years. The expense of good geologic studies is nominal compared with the potential benefit. Reservoir heterogeneity is one of the most critical problems in most EOR projects. This heterogeneity can be caused by highly variable environments of deposition, facies changes, diagenesis, natural fractures, and sedimentary structures. Petrographic studies are needed to better understand pore and permeability characteristics, rock-EOR fluid interaction, and rock wettability. Natural fracture studies are needed in many reservoirs in order to better understand and prevent premature EOR fluid breakthrough. Detailed log and core studies are needed to better understand reservoir stratigraphy and the effect of sedimentary structures on reservoir-fluid flow.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988