EOR Screening Criteria Revisited - Part 1: Introduction to Screening Criteria and Enhanced Recovery Field Projects

Summary

Screening criteria have been proposed for all enhanced oil recovery (EOR) methods. Data from EOR projects around the world have been examined and the optimum reservoir/oil characteristics for successful projects have been noted. The oil gravity ranges of the oils of current EOR methods have been compiled and the results are presented graphically. The proposed screening criteria are based on both field results and oil recovery mechanisms. The current state of the art for all methods is presented briefly, and relationships between them are described. Steamflooding is still the dominant EOR method. All chemical flooding has been declining, but polymers and gels are being used successfully for sweep improvement and water shutoff. Only CO₂ flooding activity has increased continuously.

Introduction

Oil-production from EOR projects continues to supply an increasing percentage of the world's oil. About 3% of the worldwide production now comes from EOR. Even though EOR production in the U.S. appeared to peak in 1992, Fig. 1 shows that the EOR percentage of the U.S. production is larger than ever, because conventional oil production in the U.S. has continued to fall. Therefore, the importance of choosing the "best" recovery method becomes increasingly important to petroleum engineers.

About 100 years ago, oil producers injected gas to restore pressure to their dying oil wells.¹ Because air was cheaper than gas, air was often injected to increase production from the older fields. For many years, operators had the choice of air or gas, and sometimes they injected both into the same reservoir.² Naturally, there were safety and other problems with air. However, not until about 1928 did natural gas become the injectant of choice for pressure maintenance.³ Water injection was legalized in Pennsylvania in 1921 (it was done secretly before that).⁴

The choice of injectants has widened considerably since those early days, but the petroleum engineer still must choose an injection fluid and an overall process to try to recover the maximum amount of oil from the reservoir while still making a profit. Screening criteria have evolved through the years to help the petroleum engineer make these decisions.^5-15 Some of the early work in this field was done by Geffen^5,6 before there was much field experience with most EOR methods. Many of his criteria have stood the test of time. Perhaps the best known, and most widely used, screening criteria appeared in the 1976 and 1984 Natl. Petroleum Council (NPC) reports.^7,8 We comment in Ref. 16 on some of the predictions based on these criteria. Ref. 9 is one paper that we are "revisiting." Although we retain the format of some of the tables in Ref. 9, all have been revised. We are basing our criteria in this paper on the results of much more field and laboratory information that has become available. Additional information (especially on the use of gelled polymers for water shutoff) is given in Ref. 17, the original version of this paper.

In recent years, computer technology has improved the application of screening criteria through the use of artificial intelligence techniques, but the value of these programs depends on the accuracy of the input data used.^11-14 In this paper, we present screening criteria based on a combination of the reservoir and oil characteristics of successful projects plus our understanding of the optimum conditions needed for good oil displacement by the different EOR fluids. One goal is to provide realistic parameters that can be used in the newer computer-assisted tools for reservoir management.

EOR/Improved Oil Recovery (IOR)/Advanced Secondary Recovery (ASR)/Reservoir Management.

In the past few years, the term IOR has been used increasingly instead of the traditional EOR, or the more restrictive "tertiary recovery." Most petroleum engineers understand the meaning of all the words and phrases, but our technical communications are improved if we use the terms with their intended technical meanings. We certainly endorse the wider use of IOR, but we cling to the technical meanings of EOR and tertiary recovery. Successful enhanced recovery projects are being conducted as tertiary, secondary, and even enhanced primary operations. The terms should continue to be used with their evolved historic meanings. Tertiary should not be used as a synonym for EOR because some EOR methods work quite well as either secondary or tertiary projects (e.g., CO₂ flooding), while others, such as steam- or polymer flooding, are most effective as enhanced secondary operations. To us, EOR simply means that something other than plain water or brine is being injected into the reservoir. We use the terms "enhanced secondary" or tertiary when necessary for clarity. Others may use the phrase ASR^18-22 for EOR in the secondary mode. We are convinced that engineers should consider this improved (enhanced or advanced) secondary option much more often in the future.

Classification of EOR Methods.

Table 1 lists more than 20 EOR methods that experienced intensive laboratory and, in most cases, significant field testing. The methods use about 15 different substances (or specific mixtures) that must be purchased and injected into the reservoir, always at costs somewhat greater than for the injection of water. The economics of EOR are discussed more later, but experience shows that the best profits come only from those methods where several barrels of fluid (liquid or gas at reservoir pressure) can be injected per barrel of incremental oil produced.^23,24 This limits the main methods to either water (including heated, as steam, or as a dilute chemical solution) or one of the inexpensive gases. For some methods (e.g., micellar/polymer) there have been some technical successes but relatively few economic successes. These methods are included in our screening criteria because they are still being studied and applied in the field. If oil prices rise significantly, there is hope that these methods might become more profitable.

We provide screening criteria for the eight methods that are either the most important or still have some promise. These eight methods are shown in in Table 1, along with the number of the table in Ref. 16 for those methods that are examined in detail. These "current" EOR or IOR methods include the three gas (nitrogen, hydrocarbon, CO₂), three water [micellar/polymer plus alkaline/surfactant/polymer (ASP); polymer flooding; gel treatments] and the three thermal/mechanical (combustion, steam, surface mining) methods.

A convenient way to show these methods is to arrange them by oil gravity as shown in Fig. 2. This "at-a-glance" display also provides approximate oil gravity ranges for the field projects now under way. The size of the type in Fig. 2 is intended to show the relative importance of each of the EOR methods in terms of current incremental oil production.

EOR/Improved Oil Recovery (IOR)/Advanced Secondary Recovery (ASR)/Reservoir Management.

In the past few years, the term IOR has been used increasingly instead of the traditional EOR, or the more restrictive "tertiary recovery." Most petroleum engineers understand the meaning of all the words and phrases, but our technical communications are improved if we use the terms with their intended technical meanings. We certainly endorse the wider use of IOR, but we cling to the technical meanings of EOR and tertiary recovery. Successful enhanced recovery projects are being conducted as tertiary, secondary, and even enhanced primary operations. The terms should continue to be used with their evolved historic meanings. Tertiary should not be used as a synonym for EOR because some EOR methods work quite well as either secondary or tertiary projects (e.g., CO₂ flooding), while others, such as steam- or polymer flooding, are most effective as enhanced secondary operations. To us, EOR simply means that something other than plain water or brine is being injected into the reservoir. We use the terms "enhanced secondary" or tertiary when necessary for clarity. Others may use the phrase ASR^18-22 for EOR in the secondary mode. We are convinced that engineers should consider this improved (enhanced or advanced) secondary option much more often in the future.

Classification of EOR Methods.

Table 1 lists more than 20 EOR methods that experienced intensive laboratory and, in most cases, significant field testing. The methods use about 15 different substances (or specific mixtures) that must be purchased and injected into the reservoir, always at costs somewhat greater than for the injection of water. The economics of EOR are discussed more later, but experience shows that the best profits come only from those methods where several barrels of fluid (liquid or gas at reservoir pressure) can be injected per barrel of incremental oil produced.^23,24 This limits the main methods to either water (including heated, as steam, or as a dilute chemical solution) or one of the inexpensive gases. For some methods (e.g., micellar/polymer) there have been some technical successes but relatively few economic successes. These methods are included in our screening criteria because they are still being studied and applied in the field. If oil prices rise significantly, there is hope that these methods might become more profitable.

We provide screening criteria for the eight methods that are either the most important or still have some promise. These eight methods are shown in in Table 1, along with the number of the table in Ref. 16 for those methods that are examined in detail. These "current" EOR or IOR methods include the three gas (nitrogen, hydrocarbon, CO₂), three water [micellar/polymer plus alkaline/surfactant/polymer (ASP); polymer flooding; gel treatments] and the three thermal/mechanical (combustion, steam, surface mining) methods.

A convenient way to show these methods is to arrange them by oil gravity as shown in Fig. 2. This "at-a-glance" display also provides approximate oil gravity ranges for the field projects now under way. The size of the type in Fig. 2 is intended to show the relative importance of each of the EOR methods in terms of current incremental oil production.