US3425492A - Oil production by steam drive - Google Patents

Description

3,425,492 OIL PRODUCTION BY STEAM DRIVE Ralph E. Gilchrist, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Jan. 10, 1966, Ser. No. 519,422 US. Cl. 166-272 7 Claims lnt. Cl. E21b 43/24, 43/20 ABSTRACT OF THE DISCLOSURE Oil is produced from a permeable oil stratum penetrated by an injection well and at least one olfset production well by first injecting steam into the stratum thru the injection well to form a steam bubble around this well, injecting solely a gas, other than steam, thru the injection welldirectly behind the steam to drive the steam bubble and oil toward the production we ll(s), and recovering produced oil from the production well(s). Water may be injected following the gas injection and steam injection may be repeated.

This invention relates to a process for producing oil from an oil-bearing stratum by steam drive.

Oil is produced from an oil reservoir or stratum by injecting steam thru an injection well penetrating the stratum and driving the steam to one or more offset production wells. Because of the expense involved in steam injection projects (fuel, water treating, etc.), a number of innovations have been utilized in an attempt to reduce the amount of steam required in the production process. One such method of decreasing the steam injection time and quantity of injected steam, aptly called heat salvage, comprises injecting steam for about of the life of the production project and following steam injection with a chasing slug of water for the remaining /3 of the production time. In practice, the cold water is heated by the hot rock in the stratum and this heat stored in the reservoir by the steam is reclaimed in the hot water, a portion of which may be converted to steam. There still exists the problem of reducing the steam injection period and the quantity of steam required in the production process in order to effect substantial savings in the cost of production of each barrel of oil.

This invention is concerned with a modified steam injection process which reduces both the steam injection period and the quantity of steam required in any given steam injection project.

Accordingly, it is an object of the invention to provide an improved steam injection process for the production of oil from an oil stratum. Another object is to provide a process or method of steam drive thru an oil stratum which reduces the loss and dissipation of heat from the injected steam. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises injecting a slug of steam into the stratum from which oil is to be produced, utilizing an injection well penetrating the stratum and one or more offset wells as production wells, and driving the resulting steam bubble thru the stratum toward the production well(s) with a slug of gas (other than steam), also injected thru the injection well, which displaces oil from the stratum and drives same into the production well(s). The gas injection step of the process may then be followed by a water injection step, the water forcing the gas and the steam slugs thrus the stratum to the production well(s).

Gases have a much lower thermal conductivity and specific heat than water, which is illustrated in the table below.

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Patented Feb. 4, 1969 Substance: Heat conductivity Water, 20 C 0.00143 Air, 0 C 0.0000568 CO 0 C 0.0000307 CH 78 C 0.0000647 N 78 C 0.0000524 The above gases, as well as flue gas, natural gas, and similar gases, are utilized as the gas slug in the process. The use of gas serves a two-fold purpose. First, because of the lower heat conductivity of various gases, the reservoir rock releases its heat at a slower rate and thus prolongs the collapse of the steam slug (by condensation) for a longer time; second, with the prolonged life of the steam slug, less steam is required than when the water is injected directly behind the steam slug.

From another point of view, gases have a much lower heat capacity or specific heat than water so that filling the unoccupied pore space with a gas requires only a small fraction of the heat (supplied by the rock) to raise the temperature of the gas to ambient temperature as compared with the amount of heat required to raise the temperature of water (under the same circumstances) to ambient temperature. In addition to the higher specific heat of water, the heat of vaporization of water is also to be taken into consideration and this is extremely high. Another factor which has an important bearing on the heat absorption of the gas as compared to the water is the vastly smaller mass of gas (based on its weight) in the available pore space as compared to water. Thus, injection of a suitable gas directly behind the steam slug is more effective in pushing the steam bubble and heated zone thru the stratum than water is.

In a steam driveat high pressure, it is advantageous to utilize natural gas which is available at the locus of the project at high pressure, thereby minimizing the compression costs. However, if compression is necessary, the heat of compression is in the compressed gas and this heat is effective when the gas in injected into the stratum to chase the steam. Compression of other gases also raises the temperature of the compressed gas and therefore the compression cost is not lost since most of the heat is imparted to the reservoir or stratum.

Carbon dioxide is particularly effective as a chasing gas in view of its beneficial effect on reservoir liquids. Liquid CO at pressure greater than the critical pressure (1073 p.s.i.a.) and below the critical temperature (88 F.) is preferably injected behind the steam slug when CO is to be utilized. As the injected liquid CO is heated to reservoir temperature, it vaporizes, expands, and fills the reservoir space previously occupied by steam. This produces a beneficial effect on the production of oil.

The most simple embodiment of the invention omits the water injection step and utilizes the injected gas solely to drive the steam slug and heat zone thru the stratum toward the production well(s). In this operation, the steam generators are shut down after a considerable period of steam injection to provide a steam bubble occupying a substantial section of the stratum within the production pattern and gas is injected so as to drive the steam bubble thru the reservoir into the production wells. It is also feasible to inject a sizable slug of gas and then inject a second steam slug to form a second steam bubble and follow this second steam injection with gas injection.

The injection of gas in accordance with the instant invention tends to drive the injected steam and heated zone farther into the productive oil zone so as to avoid steam condensation around the injection well, resulting in the reduction of pressure at this point and causing the steam bubble to be pulled back toward the injection well.

Air is an excellent gas for the gas injection step following steam injection. Air is the cheapest and most readily available gas and it is effective in driving the steam bubble and heat zone thru the reservoir. In some cases the injection of air into the hot stratum causes oxidation of the residual in-place oil behind the steam bubble. The steam drive often leaves 8 to 10 percent of the oil as residue and hot air in contact with this residual oil causes oxidation thereof with production of additional heat and expansion of the gas slug by formation of combustion gases and expansion due to the heating provided by the combustion period. This in situ combustion drive behind the steam bubble aids materially in the displacing of oil from the stratum.

The steam drive process of the invention is effectively applied to a strat-um which is hot from a previous in situ combustion drive and which leaves substantial concentrations of oil in the reservoir. By injecting either steam or water into the hot stratum a steam bubble is established which can readily be driven thru the stratum as described above, using gas drive alone or gas drive behind the steam, followed by water drive of the gas.

The advantages of the invention are in the conservation of heat in the reservoir or stratum because of the low dissipation of heat from the rock to the gas as contrasted to a substantially greater dissipation of heat from the rock to water when a water slug is injected immediately behind a steam slug. Further, it is possible to use a smaller steam slug because of the considerably longer cooling-off period (of the reservoir) than when water is injected directly behind the steam slug.

Certain gases offer other advantages. CO for example, swells the residual oil and is miscible with the forward steam slug and the rearward water sl-ug. Methane or natural gas also tends to swell the residual oil displaced by the steam.

The amount of the steam slug to be injected is in the range of 0.5 to several pore volumes of the sweep pattern and the temperature of the steam is in the range of about 300 to 750 F. The size of the injected steam slug for multiple injections, each followed by gas slugs, is in the lower portion of the range. The injected gas slug may be smaller than the steam slug because of the non-condensing nature of the gas. A gas slug in the range of 0.1 to 1 or more pore volumes may be utilized.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A steam drive process for producing oil from a permeable oil stratum penetrated by an injection well and as least one ofiset production well, which process comprises, in combination, the steps of:

(a) first depositing steam as the principal driving medium in said stratum around said injection well for an extended period of time to form a steam bubble occupying a section of said stratum in the well pattern;

(b) then, injecting a following slug solely of gas other than steam through said injection well directly behind said steam so as to drive said steam bubble and condensate therefrom deeper into said stratum toward said production well to displace oil into same;

(c) then, injecting a second substantial slug of steam directly behind said gas slug of step (b);

((1) then, injecting a second slug of gas other than steam directly behind said steam of step (c); and

(e) producing oil from said production well.

2. The process of claim 1 wherein said slug of gas is combustion gas.

3. The process of claim 1 wherein said slug of gas is C0 4. The process of claim 1 wherein said slug of gas comprises natural gas components.

5. The process of claim 1 wherein said slug of gas is arr.

6. The process of claim 1 wherein said slug of gas is N 7. The process of claim 1 wherein said stratum is initially at an elevated temperature following an in situ combustion drive and the steam of step (a), at least, is formed in situ by injecting water into the hot stratum through said injection well.

References Cited UNITED STATES PATENTS 2,584,606 2/ 1952 Merriam et al 16611 2,813,583 11/1957 Marx et al. 16640 X 2,897,894 8/ 1959 Draper et a1 166-9 3,036,632 5/1962 Koch et a1. 166-11 3,042,114 7/1962 Willman 16640 X 3,294,167 12/1966 Vogel 166-11 FOREIGN PATENTS 511,768 8/1939 Great Britain.

STEPHEN I. NOVOSAD, Primary Examiner.

US. Cl. X.R. 166-273, 305