How to keep old oil wells bubbling

Gasoline prices rose less than expected this year and have started their normal end-of-summer drop two weeks ahead of schedule.

But this good news for drivers and bad news for the oil industry can't last, according to oil industry planners and government energy experts. Industry forecasters say that demand for oil will pick up again when the economy begins to recover.

Meanwhile, plunging profits have cut into oil drilling programs throughout the United States. The Hughes Tool Company reports that the number of active rotary drilling rigs has dropped from 4,142 for Aug. 17, 1981, to 2,645 currently. Much of the cutback has affected high-cost drilling operations in search of deep reservoirs in difficult terrain such as Oklahoma's Anadarko Basin.

While oil companies wait for an economic upturn to make such drilling attractive again, they're seeking out more cost-effective methods to meet current demand.

The result: Instead of glamorous but expensive deep drilling for gas and oil, the industry is spending its money on operations that bring a quicker return on smaller investments, most notably ''workover'' efforts designed to get the last drop of petroleum out of older fields.

But along with these operations, the industry is continuing its investment in advanced research, a sign it knows that underground oil and natural gas supplies are limited. At Texas A&M University, for instance, electrochemist John Bockris is pushing ahead with an industry-backed drive to turn water into fuel by splitting molecules to release hydrogen gas.

Such long-range attempts to provide new fuel sources may one day pay off. For the present, however, traditional methods still play an important role in supplying energy for a high-technology world. Typical of such methods are those in use in the Hastings Field 20 miles south of Houston, which has produced 500 million barrels of oil since pumping started in 1934. Hastings is a prime example of a successful ''workover'' operation.

A pair of hot pipes at each valve-bristling wellhead confirms that crude oil is rushing up from a mile underground. Key to this continuing production, says Amoco Oil Company district engineer Jean Hinn, is an elaborate program to pull even more oil out of the venerable field. Under this program, each of the field's present 282 wells is constantly monitored by computer and by systematic inspection. If a well's output begins to decline, Ms. Hinn contracts a privately operated ''workover rig'' to anchor its tall derrick over the well, ''trip out'' more than a mile of old tubing, do any necessary down-hole repair work, and then ''trip in'' new tubing. Last year, Ms. Hinn's workover program cost Amoco and its parent Standard Oil Company of Indiana $7.6 million. This included 393 workovers at an average cost of nearly $20,000 each.

Ms. Hinn and her team of engineers are fortunate in sitting over a highly productive field with 37 separate oil-reservoir sandstones. This means that when a well loses production in one zone, the engineers can have the workover crew pump cement down to plug that zone and ''reperforate'' to start production from a different layer of sandstone.

Hastings is productive because its underlying ''unconsolidated Frio sandstones'' are so porous that there is a r%latively large amount of room for oil storage and movement. A sample of Hastings sandstone dredged from deep underground crumbles easily in my fingers. This characteristic helps oil flow from tiny pores in the rock into each well. But it also means that sand particles quickly fill wells and wear out pipes and equipment.

A second Hastings characteristic is the ''water drive,'' whereby abundant underground water forces oil into each well. But this also presents problems. After many years of production, the water-oil mix has changed. Hastings now pumps up 280,000 barrels of water daily to recover the 16,000 barrels of oil mixed with the water. As well, the water is extremely salty and corrosive. Special water injection wells are needed to pump this unwanted water back underground into separate zones where it can't contaminate fresh-water supplies.

Despite such problems, says Ms. Hinn, ''Amoco is convinced that workover is the best money you can spend.''

The value of reworking old well becomes clear when you consider that the last new well drilled here in the Hastings Field cost $600,000 - or enough to pay for 30 workovers.