Petroleum

V

Exploration

In order to find crude oil underground, geologists must search for a sedimentary basin in which shales rich in organic material have been buried for a sufficiently long time for petroleum to have formed. The petroleum must also have had an opportunity to migrate into porous traps that are capable of holding large amounts of fluid. The occurrence of crude oil in Earth’s crust is limited both by these conditions, which must be met simultaneously, and by the time span of tens of millions to a hundred million years required for the oil’s formation. See also Geology.

Petroleum geologists and geophysicists have many tools at their disposal to assist in identifying potential areas for drilling. Thus, surface mapping of outcrops of sedimentary beds makes possible the interpretation of subsurface features, which can then be supplemented with information obtained by drilling into the crust and retrieving cores or samples of the rock layers encountered. In addition, increasingly sophisticated seismic techniques—the reflection and refraction of sound waves propagated through Earth—reveal details of the structure and interrelationship of various layers in the subsurface. Ultimately, however, the only way to prove that oil is present in the subsurface is to drill a well. In fact, most of the oil provinces in the world have initially been identified by the presence of surface seeps, and most of the actual reservoirs have been discovered by so-called wildcatters who relied perhaps as much on intuition as on science. (The term wildcatter comes from West Texas, where in the early 1920s drilling crews encountered many wildcats as they cleared locations for exploratory wells. Shot wildcats were hung on the oil derricks, and the wells became known as wildcat wells.)

An oil field, once found, may comprise more than one reservoir—that is, more than one single, continuous, bounded accumulation of oil. Several reservoirs may be stacked one above the other, isolated by intervening shales and impervious rock strata. Such reservoirs may vary in size from a few tens of hectares to tens of square kilometers, and from a few meters in thickness to several hundred or more. Most of the oil that has been discovered and exploited in the world has been found in a relatively few large reservoirs. In the United States, for example, 60 of approximately 10,000 oil fields have accounted for half of the productive capacity and reserves.

VI

Primary Production

Most oil wells in the United States are drilled by the rotary method that was first described in a British patent in 1844 assigned to R. Beart. In rotary drilling, the drill string, a series of connected pipes, is supported by a derrick. The string is rotated by being coupled to the rotating table on the derrick floor. The drill bit at the end of the string is generally designed with three cone-shaped wheels tipped with hardened teeth. Drill cuttings are lifted continually to the surface by a circulating-fluid system driven by a pump.

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Trapped crude oil is under pressure; were it not trapped by impermeable rock it would have continued to migrate upward, because of the pressure differential caused by its buoyancy, until it escaped at the surface of Earth. When a well bore is drilled into this pressured accumulation of oil, the oil expands into the low-pressure sink created by the well bore in communication with Earth’s surface. As the well fills up with fluid, however, a back pressure is exerted on the reservoir, and the flow of additional fluid into the well bore would soon stop, were no other conditions involved. Most crude oils, however, contain a significant amount of natural gas in solution, and this gas is kept in solution by the high pressure in the reservoir. The gas comes out of solution when the low pressure in the well bore is encountered, and the gas, once liberated, immediately begins to expand. This expansion, together with the dilution of the column of oil by the less dense gas, results in the propulsion of oil up to Earth’s surface.

Nevertheless, as fluid withdrawal continues from the reservoir, the pressure within the reservoir gradually decreases, and the amount of gas in solution decreases. As a result, the flow rate of fluid into the well bore decreases, and less gas is liberated. The fluid may not reach the surface, so a pump (artificial lift) must be installed in the well bore to continue producing the crude oil.

Eventually, the flow rate of the crude oil becomes so small, and the cost of lifting the oil to the surface becomes so great, that the well costs more to operate than the revenues that can be gained from selling the crude oil (after discounting the price for operating costs, taxes, insurance, and return on capital). The well’s economic limit has then been reached and it is abandoned.

VII

Enhanced Oil Recovery

In primary production, no extraneous energy is added to the reservoir other than that required for lifting fluids from the producing wells. Most reservoirs are developed by numerous wells; and as primary production approaches its economic limit, perhaps only a few percent and no more than about 25 percent of the crude oil has been withdrawn from a given reservoir.

The oil industry has developed methods for supplementing the production of crude oil that can be obtained mostly by taking advantage of the natural reservoir energy. These supplementary methods, collectively known as enhanced oil recovery technology, can increase the recovery of crude oil, but only at the additional cost of supplying extraneous energy to the reservoir. In this way, the recovery of crude oil has been increased to an overall average of 33 percent of the original oil. Two successful supplementary methods are in use at this time: water injection and steam injection.

A

Water Injection

In a completely developed oil field, the wells may be drilled anywhere from 60 to 600 m (200 to 2,000 ft) from one another, depending on the nature of the reservoir. If water is pumped into alternate wells in such a field, the pressure in the reservoir as a whole can be maintained or even increased. In this way the rate of production of the crude oil also can be increased; in addition, the water physically displaces the oil, thus increasing the recovery efficiency. In some reservoirs with a high degree of uniformity and little clay content, water flooding may increase the recovery efficiency to as much as 60 percent or more of the original oil in place. Water flooding was first introduced in the Pennsylvania oil fields, more or less accidentally, in the late 19th century, and it has since spread throughout the world.

B

Steam Injection

Steam injection is used in reservoirs that contain very viscous oils, those that are thick and flow slowly. The steam not only provides a source of energy to displace the oil, but also causes a marked reduction in viscosity (by raising the temperature of the reservoir), so that the crude oil flows faster under any given pressure differential. This scheme has been used extensively in California and in the state of Zulia in Venezuela, where large reservoirs contain viscous oil. This technology is also being used to recover some of the vast accumulations of viscous crude oil, known as bitumen, along the Athabasca River in north central Alberta, Canada, and along the Orinoco River in eastern Venezuela.

Bitumen must be converted into a form of petroleum known as synthetic crude and is considered an unconventional crude oil. In Canada the steam technique most often used to extract bitumen is called steam-assisted gravity drainage (SAGD). This procedure requires a significant amount of energy and consequently is considered cost-efficient only when the price of conventional crude oil is high. See also Tar Sand; Synthetic Fuels.

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