The invention relates to a process for increasing the yield of crude oil reservoirs ex¬ ploited ^" by subterranean burning or by steam flood¬ ing.

It is known from exploitation experiences with crude oil reservoirs, that efficiency of known oil displacement processes is limited, because the fluid applied to displace and drive out oil does not flood the whole volume of the reservoir- It was concluded that, while microscopic displacement coefficient is close to 1, the oil yield is not satisfactory, because the coefficient of volume flooding is small.

The value of the volume flooding coefficient is small because the fluid used for displacement and for driving out can not flood a part of the oil bearing-rock formation due to its micro- and macroscopic heterogeneity; furthermore, being displaced and displacing fluids of different mobility, parts of the reservoir with considerable oil content remain un¬ touched. This unfavourable process in the reservoir is usually

referred as "fingering" or "disjoining".

Disadvantage of exploitation processes working by a mixing drive-out gas or by an other partially or non-mixing gas is that flooding of the volume is very poor. The large difference in the viscosity of the gas and the oil results in a pro¬ nounced "finger formation" and, on the other hand, the gas pressed into heterogeneous reservoirs passes through parts of good permeability and reaches well fast without providing a suitable yield of oil.

The process of subterraneon burning and the process of flooding by overheated steam has the same disadvantage because air and over- heated steam pressed in behaves similarly to that previously described.

To avoid above mentioned effects different processes were suggested in the prior art. A general feature of these processes is that displacing fluid is forced to low-permeability parts of the reservoir by descreasing the permeability of those parts fluid penetrates easily.

Pressing in gas and water alternatively or simultaneously is a generally applied method for increasing efficiency of volume flooding CCaudle, B.H., Dyes, A. B., AIME 1958, 213-81]. In the three-phase flooding zoneV formed this way, permeabi¬ lity for gas decreases, resulting for some reservoires

in an increased efficiency of volume flooding.

According to U.S. Patent 3,096,821 an increased efficiency of volume flooding is achieved by pressing water and an oil-mixable gaseous fluid in the reservoir simultaneously.

U.S. Patent 3,599,715 relates to a process wherein the mobility of the displacing gas is decreased by adding foam forming agents to the gas.

U.S. Patent 3,342,256 describes a process wherein the efficiency of oil production using carbon dioxide is increased by forming foam in situ in the reservoir.

A combined process for crude oil produc¬ tion is described in Hungarian Patent 177,703 wherein efficiency of gas flooding was improved by pressing in foam forming materials and a disperse system alternatively.

U.S. Patent 3,866,680 describes a process wherein the undesired effect of hetero- geneity can be eliminated by injecting an emulsion.

The fast "break-through" of a mixible fluid, according to U.S. Patent 4,129,182 can be avoided by applying previously a solution, e.g. a polimer solution, to decrease its permeability. Hungarian Patent 173,636 describes a process, wherein the efficiency of an underground burning process is increased by pressing in metal additives and water.

Industrial practice demonstrates that neither of the known processes offer a satisfactory solution. A further disadvantage of these processes is that their application requires expensive foam forming detergents, polimers, dispersing agents, consequently these processes are uneconomical.

Application of the known processes is hindered by the fact that oil-bearing ^' rock and the fluid in the stratum may change the composition of the pressed in compounds, resulting in a flooding process out of control.

The aim of the present invention is to increase the efficiency of volume flooding effec¬ tively for oil reservoirs exploited by underground burning or by using overheated steam.

It has been recognized that the aim required can be achieved by introducing an aqueous salt solution into the reservoir and then producing salt-crystals in the pores of the oil-bearing rock formation by total or partial elimination of the sol¬ vent or by decreasing of solubility of the salt.

In the aqueous solution one or more inorganic salt, e.g. sodium chloride potassium chloride, sodium sulphate, etc., or a double salt, e.g. potassium aluminium sulphate is applied. The salt can be formed in situ in the reservoir as well.

The application of a sodium chloride as

a salt in an aqueous solution is considered especially preferable.

Salt-crystals in the pores of the oil- -bearing rock are produced by eliminating the 5 solvent totally or partially applying known methods, for example evaporation, dissolution, etc. Another possible way is decrease the solubility of the salt to the desired extent by known methods.

In the highly permeable parts of the ^~ -o heterogeneous oil-bearing rock exerting undesirable effect on volume flooding, salt-crystals hinder or cease streams.

Optionally salt-crystals precipitated are partially or completely resolved by pressing 15 additional solvent in the reservoir and, optionally, in an other part of the reservoir salt-crystals are formed again. This way permeability of different parts of the oil-bearing rocks can be controlled in accordance with the existing conditions. 20 According to our process an increased yield of oil reservoirs exploited by subterraneon burning or by steam flooding is achieved by intro¬ ducing an aqueous salt solution into the reservoir, and forming salt-crystals in the pores of the oil- 25 -bearing rock by eliminating solvent partially or totally; or by decreasing the solubility of the salt in another way and optionally by resolving the salt-crystals separated partially or totally by

pressing into the reservoir additional solvent, and- optionally forming salt-crystals in an other part of the reservoir.

The process according to the present in- vention can be generally applied for increasing the yield of oil-reservoirs exploited in different way, i.e. by gas driving, subterraneon burning or overheated steam flooding, and moreover even such oil-resources can be exploited which can not be mobilized by known methods.

The process according to our invention can be easily realized, requires unexpensive and everywhere available materials only.

Considering the displacement of the oil the salt-solution introduced into the oil-bearing rock has a favourable effect on the partition of gases, between the water and ^; the oil phase, especially on the partition of gases of good solubility in water, e.g. carbon dioxide. The following examples illustrate our process without delimiting our scope of claims.

Example 1

Characteristic parameters of a porous sandstone are: porosity, 0.21 (extreme values are

0.17 and 0.23)} permeability, 0.060 u (extreme values are 0.01 - 0.1 um ); bed-temperature, 64 C; starting bed-pressure: 100 bar; dissolved gas-oil

ratio; 5 πr3/irr3j volume-change coefficient: 1.225; the density of the oil in the reservoir at 20 C, 819 kg/nr; and starting relative water content: 0.3. Reservoir was exploited during primery depletion to a pressure of 36 bar. The production (Er,-,) was O.38 on effect of dissolved gas and water flooding.

After primary depletion reservoir was further exploited by a secondary recovery method using natural gas containing 80 % carbon dioxide.

Two neighboring units on an oil field having identical parameters and being exploited by 5-point flooding were considered suitable for making comparison between the traditional carbon dioxide method -and our invented method.

Both of the 5-point units were of a

17500 πr 3 pore volume and contained a 10 000 πr3 oil at start. In the course of primary exploitation

38 000 πr oil was produced ( ~^ _ = 0.38). Exploita- tion of unit No. 1 by the traditional carbon dioxide method was as follows:

Bed pressure of the oil-bearing forma¬ tion was increased to 100 bar by introducing a natural gas containing carbon dioxide through the injection well while at producing wells a controlled oil production was carried out. Pressing in a natural gas containing carbon dioxide in an amount of 0.36 pore volume (at 100 bar and 64 °C) the extra

oil production was 0.03, reservoir was subjected then to a cyclic water-gas introduction at 100 bar. The gas was a natural gas containing 80 % carbon dioxide and the water was fresh water. - As a result of pressing in gas in an amount of 0.2 pore volume, oil yield increased further by a value of 0.12. At the end of the secon¬ dary exploitation 0.3 pore volume water was pressed in again and an increase of 0.03 in oil yield was observed. Secondary production increased oil yield

(E _R12 ) by 0.18 i.e. the total oil yield (E-n ) obtain¬ ed was 0.56. In unit No. 1. exploited by a traditional method applying a secondary production using carbon dioxide the process of the present invention can be applied as a method for a tertiary exploitation.

In the first step of the process, accord¬ ing to our invention 0.3 pore volume natural gas containing carbon dioxide and 0.3 pore volume concentrated salt solution is pressed in the reservoir through the injection well of the unit in doses of max. 0.05 pore volume, alternately, and finally a dose of 0.15 pore volume carbon dioxide is intro¬ duced into the reservoir.

As a result of above measures oil yield increased by 0.12. On subsequent injection of 0.3 pore volume fresh water, the oil yield increased further by 0.08. In the tertiary oil production applying the invented process En-,, = 0.2 excess oil

was exploited, i.e. final oil yield amounted to - = 0.75.

Example 2 According to the process of the present invention a secondary exploitation is carried out in unit No. 2.

The first step of the process, i.e. rais¬ ing bed-pressure to 100 bar by natural gas containing 80 % carbon dioxide, while a controlled oil-produc¬ tion is carried out, was identical with that applied in unit No. 1.

The amount of produced excess oil was 0.03 upon pressing in 0.36 pore volume of gas. Further on 0.3 pore volume concentrated salt solution and 0.3 pore volume carbon dioxide containing natural gas was pressed in the reservoir in doses of about 0.05 pore volume or in smaller doses and, finally, 0.15 pore volume gas was introduced into the bed. The increase in oil yield was 0.24.

In the last step of the process 0.3 pore volume fresh water was pressed in and an excess oil yield of 0.12 was obtained.

As a result of the secondary exploitation the excess oil amounted to -E- -i = 0.39 and the total oil yield increased to E-p = 0.77. Latter result ex¬ ceeds the oil yield obtained by the traditional method of secondary production by 0.21.

Example 3

A depleted oil-reservoir containing heavy oil is exploited by a wet-combustion process. The parameters of the reservoir are: porosity, 0.28; per- p meability, 1 /um ; bed-pressure, 20 bar; bed-tempera¬ ture, 40 °C; oil density at 20 °C, 895 kg/m ³ ; oil viscosity, 50 m Pa.s.

The primary method of exploitation was depletion and water flooding. The primary oil yield was ET _} , = 0.22; the portion of residual oil was 0.52.

The relative air consumption in the wet-

-combustion process applied for secondary production was 230 πr 3/m3 pore volume, while water-air ratio was

0.002 m ³ /m3. A _s a final oil yield E _R2 = 0.52 could be achieved. The yield of excess oil was Er,, _p = 0.30.

When, in accordance of the present inven¬ tion, in the wet-combustion process a concentrated salt solution was used in spite of fresh water and same water to air ratio of 0.002 πr/m is applied,

3 3 the relative air demand was 190 πr/πr pore volume, the final oil yield (Ξ-op) ^as 0.66 and the excess oil yield ^was °«44.

Examples clearly demonstrate that using the process of the present invention, a considerable amount of excess oil can be produced.