ENHANCED OIL RECOVERY – DEFINITION AND DECLINE CURVES

Enhanced oil recovery (EOR) is oil recovery by the injection of materials not normally present in the reservoir. This definition covers all modes of oil recovery processes (drive, push-pull, and well treatments) and most oil recovery agents.

The definition does not restrict EOR to a particular phase (primary, secondary, or tertiary) in the

producing life of a reservoir. Primary recovery is oil recovery by natural drive mechanisms: solution gas, water influx, and gas cap drives, or gravity drainage.

Secondary recovery refers to techniques, such as gas or water injection, whose purpose is mainly to raise or maintain reservoir pressure. Tertiary recovery is any technique applied after secondary recovery. Nearly all EOR processes have been at least field tested as secondary displacements. Many thermal methods are commercial in both primary and secondary modes.

We are interested in EOR because of the amount of oil to which it is potentially applicable. This EOR target oil is the amount unrecoverable by conventional means. A large body of statistics shows that conventional ultimate oil recovery (the percentage of the original oil in place at the time for which further conventional recovery becomes uneconomic) is about 35%.

THE NEED FOR EOR:

Enhanced oil recovery is one of the technologies needed to maintain reserves.

Reserves:

Reserves are petroleum (crude and condensate) recoverable from known reservoirs under prevailing economics and technology. They are given by the following material balance equation:

There are actually several categories of reservoirs (proven, etc.) which distinctions are very important to economic evaluation. Clearly, reserves can change with time because the last two terms on the right do change with time. It is in the best interests of producers to maintain reserves constant with time, or even to have them increase.

INCREMETAL OIL:

Definition

A universal technical measure of the success of an EOR project is the amount of incremental oil recovered. The next picture defines incremental oil. Imagine a field, reservoir, or well whose oil rate is declining as from A to B. At B, an EOR project is initiated and, if successful, the rate should show a deviation from the projected decline at some time after B. Incremental oil is the difference between what was actually recovered, B to D, and what would have been recovered had the process not been initiated, B to C. Since areas under rate-time curves are amounts, this is the shaded region shown below.

As simple as the concept in the picture above is, EOR is difficult to determine in practice. There are several reasons for this.

1. Combined (comingled) production from EOR and nonEOR wells. Such production makes it difficult to allocate the EOR-produced oil to the EOR project. Comingling occurs when, as is usually the case, the EOR project is phased into a field undergoing other types of recovery.
2. Oil from other sources. Usually the EOR project has experienced substantial well cleanup or other improvements before startup. The oil produced as a result of such treatment is not easily differentiated from the EOR oil.
3. Inaccurate estimate of hypothetical decline. The curve from B to C  must be accurately estimated. But since it did not occur, there is no way of assessing this accuracy.

Ways to infer incremental oil recovery from production data range from highly sophisticated numerical models to graphical procedures. One of the latter, based on decine curve analysis.

OIL RECOVERY THROUGH DECLINE CURVES

Decline curve analysis can be applied to virtually any hydrocarbon production operation. The objective is to derive relations between oil rate and time, and then between cumulative production and rate.

that graphic schematically illustrates a set of data (points) which begin an exponential decline at the ninth point where,

by definition t = 0. The solid line represents the fit of the decline curve model to the data points. qi is the rate given by the model at t=0, not necessarily the measured rate at this point.

 The decline curve rate of descent depends on a number of factors, including changes in demand, government regulations, and engineering methods.

Enhanced oil recovery (EOR), or tertiary recovery, facilitates the extraction of more oil from each well than traditional recovery methods. EOR can increase the oil yield by 10 to 20 percent. It incorporates the use of chemicals, gases, microbes, or steam to force oil out of the surrounding foundation to enhance recovery. Although the process adds to the cost of production, the United States Department of Energy has estimated that the use of EOR could lead to the production of an additional 240 billion barrels of oil, which would decrease the steepness of the decline curve.