The prospect of less petroleum in tomorrow's world is increasingly turning the oil men to an unusual source for more crude: played-out wells. The current oil glut notwithstanding, a new wave of so-called enhanced oil recovery projects aimed at squeezing reservoirs dry are coming on line throughout parts of the Southeastern and Southwestern United States.
Such techniques have been used in the oil patch for at least two decades. But enough fields are now being plumbed for their leftover booty that enhanced recovery seems on the way to fulfilling its long-expected promise as a significant contributor to US oil reserves.
Moreover, a number of newer techniques that are either being explored or just beginnig to be used on a large scale for the first time should extend the life of many fields. ''Enhanced oil recovery (EOR) has generally been coming on pretty strong the past couple of years,'' says K. Terry Koonce, production operations manager at Exxon USA. ''Projects are being launched in west Texas literally every few months.''
The allure, of course, is the idea of sizable pools of crude lying somewhere other than beneath an Arabian desert. The National Petroleum Council, in a recent study, estimated that EOR technology could ultimately add as much as 14.5 billion barrels to US reserves. That represents 40 percent of current recoverable domestic oil reserves - nearly triple the amount used in the US last year. Currently, about 460,000 barrels a day are siphoned out of wells using such techniques - less than 6 percent of total US daily production but a 23 percent jump over two years ago.
Wringing these residues from their rocky hideaways, however, is no easy chore. Normally oil flows from wells under its own power, aided by the natural pressure in the formation and by the expansion of gas dissolved in the oil. But nature only pushes up 10 to 20 percent of the crude. Another 20 to 50 percent can be drawn up using ''secondary'' recovery methods, usually the injection of water into the reservoir to maintain the pressure and force oil into producing wells. But that still leaves 50 to 60 percent behind.
Enter enhanced recovery. With it, oil companies may get another 20 to 30 percent of the reluctant crude. EOR approaches vary, but the idea is the same: Inject a gas or liquid into the reservoir, usually over a number of years, that will flush the petroleum out of the tiny pores in the rocks, as well as mix with the crude and cause it to flow more freely. Among the most promising players:
* Thermal. The chief agent here is steam. Indeed, steam injection accounts for about 77 percent of all domestic enhanced oil recovery. It is also the most tested technique: It has been used, mainly in California, since the early 1960s. Steam and water are usually shot into the formation. The heat causes the oil to flow more freely. The condensing steam, too, produces a headwall of water that sweeps the oil toward the producing well. Main drawbacks: It usually works only on heavy oils and tars - and then generally in shallow fields. It also requires a lot of energy and expensive pumping equipment.
Another thermal technique involves injecting air into the reservoir to ignite some oil. Expanding gases from the flames, as well as steam and hot water, push the crude toward the well. But controlling an underground fire is tricky, and the method is not used widely.
* Gas. The use of gases is the fastest-growing area but still represents only a small part of total EOR production. The idea is to inject a gas into the reservoir - usually carbon dioxide (CO2), but also nitrogen or natural gas - that will mix with the oil, increase its volume, and sweep it toward the production well. The problem is coming up with enough cheap, available gas.
The hot area now is the Permian Basin in table-flat West Texas and eastern New Mexico, where more than a dozen big CO2 projects have been launched to revive old fields in the past year or so. The main impetus: The area lies close to big sources of carbon dioxide. Taps on two pipelines were recently turned on, funneling CO2 from Colorado. A third is being laid to pipe the gas in from New Mexico. Fields in Mississippi and Louisiana are also active targets of CO2 flooding. Other gases that don't mix with the oil (immiscible) are used to boost recovery, too, but their potential is considered limited.
* Chemical. A third approach is to squirt chemicals down the well that either act like a soap and flush the crude out of pores (surfactants) or thicken the ''drive water'' and sweep out more oil (polymers). Both, however, have their drawbacks. For one thing, chemicals aren't cheap. Of the two, polymer flooding is simpler, and its use has been growing more quickly. Surfactant flooding remains tricky: There's always uncertainty about how chemicals will react to various temperatures, pressures, oils, and brines.
Therein, though, lies the big-stakes risk involved with any EOR technique. Each field differs from the next. Petro engineers are never sure what will happen with their wizardry in the underground tombs. Only certain fields, moreover, are economic to plumb to begin with.
Still, as Mike Britton, an EOR expert with Conoco Inc., points out, companies have become more adept at applying the techniques and understanding the geology of reservoirs. Enough so, at least, that most experts project EOR production to peak at about 1 million barrels of oil a day by the early 1990s - even under existing economic conditions and with no technological breakthroughs.
''At peak, we might be looking at 15 percent to 20 percent of our production from enhanced recovery,'' says Dr. Joseph Taber, director of the Petroleum Recovery Research Center at the New Mexico Institute of Mining and Technology. ''It will be a very significant part of our production.''
Significant, yes, but not Saudi-size. ''EOR is going to make a substantial contribution, but it is not going to solve our oil supply problems,'' cautions Fred Stalkup, senior research adviser at the Atlantic Richfield Company.