Enhanced oil recovery projects are started with the general goal of improving oil recovery, but many times with no defined method of measuring the benefits of the process. In TIORCO's quarter century of implementing EOR projects, a number of case histories were discounted as marginal or failures until a detailed review of the performance was made and a different picture emerged. To assess the benefit of an EOR project, the measurement of oil recovery success or failure should begin with the production side of the system using real time measurements.


The goal of any EOR project is to increase the ultimate oil recovery. Starting with the production side where the revenue is generated, one needs to relate the water-oil ratio (WOR) with oil production. A graph used to monitor the production side changes is WOR versus cumulative oil production for a producing well, a group of wells, or for the entire field. Once a WOR trend is established, benefits of an EOR technology (i.e.: sweep improvement, ASP, CO2, etc.) are measured by a reduction of WOR. Most straight waterfloods establish a well defined WOR trend. The incremental oil recovery is the oil produced at a lower WOR than the conventional waterflood trend.

An example of the use of the WOR vs. cumulative oil recovery curve is the Sage Spring Creek Unit using in-depth gelled polymer to improve waterflood sweep efficiency. Prior to the EOR program, the field wide WOR was on a very well established trend. Shortly after starting the gelled polymer program from the injection side, the WOR broke at 1.0, declined to less than 0.7, and then returned to a similar pretreatment trend. The incremental oil is the oil produced from the time the WOR broke from 1.0 and again reached 1.0. In this example, the recovery was 800,000 STB at a process cost of $1.30/incremental BBL of oil.

Since most EOR processes are applied to injection wells, it is important to monitor injectivity changes during the program. The Hall Plot is an excellent tool; it is a graph of cumulative daily pressure versus cumulative injection. The slope of the curve is inversely proportional to the in-depth permeability of the water.

At Sage Spring Creek, where the goal was to divert injection into the unswept low permeability sections of the reservoir, an increase in Hall Slope showed a reduction in permeability to water and a diversion of water into unswept rock. Processes designed to reduce residual oil saturation, like ASP, should reduce the Hall Slope, showing an increase in water saturation and relative permeability to water. Since oil response to EOR processes takes time, the Hall Plot is useful early in the process to measure technology benefits.

The injection side is tied to the production side with the cumulative oil production versus cumulative water injection plot. The curve is a measure of waterflood efficiency. The goal of every flood mechanism is to maximize the oil produced per barrel of water injected. Ideally oil production should equal water injection until all oil is produced and water breaks through to the producers. This represents true piston-like displacement of oil and maximum sweep efficiency and would be a 45 degree angle line on the plot. Unfortunately, conventional floods show a drop off in the slope of the curve after water breakthrough.

TIORCO has been involved in over 40 sweep improvement projects in the Minnelusa formation of the Powder River Basin in Wyoming. A similar number of straight waterfloods provides an excellent comparison of results. The cumulative injection vs. cumulative oil recovery curves for the two distinct populations of projects show clearly the performance advantage of the EOR process.

The injected volume of water per barrel of oil produced is much less with the sweep improvement technology. At the same water injection (1.0 PV), the EOR projects had recovered 7% OOIP more oil than the conventional waterfloods.

When starting an EOR program, it is important to define the oil recovery expectations of the technology, prepare the graphs to measure the performance of the process, and monitor the results against expectations to maximize the benefits of the EOR investment.