Sun, 02 Mar 2003

A close look at EOR: Enhanced oil recovery

David M. Schiller Contributor Jakarta

Many people unfortunately believe that oil occurs as huge underground pools or lakes, trapped in massive subterranean caves. This misconception, however, is far from the truth.

Oil is actually found within rock. That's right! Hard rock -- typically sandstone or limestone -- occurring at depths of anywhere from several hundred to tens of thousands of feet below the Earth's surface.

The reason why rocks are capable of containing oil is because many are not 100 percent solid. Reservoir rocks, such as sandstone and limestone, are riddled with a network of microscopic holes, just like the tiny pores one finds between the individual grains of sand on a beach. For high quality reservoir rocks, these pores typically comprise from 10 percent to 35 percent of the rock volume.

Once an oil-bearing reservoir rock is penetrated by drilling, the oil will typically flow to the surface due to the natural pressure of the reservoir. This pressure is caused by the massive weight of the overlying hundreds or thousands of feet of rock and sediments, in addition to fluid pressures that build up over time due to compaction and rock/fluid interactions.

* The life of an oil field

The producing life of most oil fields can be divided into several distinct phases: an early primary phase, where the oil flows by itself to the surface through natural reservoir pressure; a later primary phase, where oil has to be artificially lifted by pumps once the natural pressure is depleted (usually by the bobbing ant-like pumps seen atop many oil fields); a secondary phase, where water can be injected into the reservoir in order to flush-out remaining oil; and a tertiary phase, where more expensive and unconventional techniques such as steam or polymer flooding can be used.

The secondary and tertiary recovery phases are either separately or together referred to as enhanced oil recovery (EOR), since they enhance the amount of oil that is ultimately recovered from an oil field.

Even with today's most advanced primary recovery techniques, we still leave behind about 40 percent to 50 percent of the original oil in place (OOIP). That's right! After the oil company can no longer pump any more oil to the surface, approximately half of the oil is still trapped in the ground.

The main reason for this is because the surface tension or adhesive force between the rock and the oil becomes so great, that oil sticks to the rock instead of flowing out of the reservoir. The force is similar to what makes drops of water cling to the outside of a water glass. The influence of this force, as well as other detrimental processes, gradually increase as oil is pumped out and reservoir pressures decline.

* Enhanced Oil Recovery

A major spotlight was thrown on EOR in Indonesia's oil and gas industry in a recent seminar on the subject. The seminar, sponsored by the Indonesian Oil & Gas Chronicle (IOG), LEMIGAS (the petroleum research division of MIGAS) and Debindo Multi- Expo, was attended by geoscientists and executives from Pertamina, multinational oil companies and commercial research institutes.

The goal of EOR is to squeeze out some of this remaining oil left behind in mature oil fields. The financial stakes are high, for -- according to LEMIGAS -- this amounts to approximately 60 billion barrels of proven but trapped Indonesian oil that cannot be produced through primary means. At US$25 a barrel, this amounts to over US$1.5 trillion dollars worth of oil that is left trapped inside Indonesian oil fields.

Realistically, only a fraction of this oil can ever be economically produced at current oil prices, even with the most advanced water-flood and EOR technologies. But because this amount is so enormous, even 5 percent to 10 percent increase can amount to millions of additional barrels of oil produced from a field.

So far, many different EOR technologies have been developed, but most operate on one or more of the following principles: to pump water, gas or some other fluid into the reservoir in order to more thoroughly "sweep" or displace trapped oil; to lower the thickness/viscosity of the oil so it flows more easily (steam- flooding; injection with CO2, N2 or natural gas); to chemically change the oil/rock interaction so the oil does not cling so easily to the rock (surfactant/polymer flooding).

Other even more esoteric technologies exist, such as microbial EOR, where oil-eating bacteria breakdown heavy hydrocarbons into lighter, more mobile phases; and vibroseismic impact (VSIT), pioneered in Russia, where mild earthquakes, i.e. seismic waves, are mechanically generated in order to loosen and mobilize trapped hydrocarbons.

Most Indonesian EOR activity thus far has involved the water flooding of several onshore fields in Sumatra and Kalimantan (e.g. Limau, Sago and Tanjung), in addition to several gas injection programs conducted by TOTALFINAELF in the Mahakam Delta area, East Kalimantan.

The most ambitious Indonesian EOR project to date is the Caltex-operated Duri Steam Flood Project, located in Riau. The project injects massive quantities of steam into the reservoir in order to free-up thick waxy oils that would otherwise be immobile. Duri is the second largest oil field in Southeast Asia, and the world's largest steam-flood operation, producing close to 300,000 barrels of oil per day.

The most important point to remember with EOR is that every oil field is unique, and that each must be carefully evaluated in order to develop the most economically feasible EOR strategy. Profit drives the petroleum industry, so implementation of the most cost-effective and profitable EOR strategy is essential in order to economically squeeze those hard to produce barrels out of Indonesia's maturing oil fields.

-- The writer is an American petroleum geologist who has worked for over fifteen years for various oil and service companies in the United States, the Middle East and Southeast Asia. He currently works as an Independent Consultant in Jakarta.