US3537526A

US3537526A - Method of recovering hydrocarbons from a hydrocarbon-containing subsurface formation

Info

Publication number: US3537526A
Application number: US768668A
Authority: US
Inventors: Jan Offeringa
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1967-11-15
Filing date: 1968-10-18
Publication date: 1970-11-03
Anticipated expiration: 1987-11-03
Also published as: NL154577B; DE1808672B2; DE1808672A1; NL6715499A

Description

United States Patent [50] Field ofSearch 166/245, 272, 273. 274, 269, 303

l 56] References Cited UNITED STATES PATENTS 2,885,002 5/1959 Jenks 166/245 3,042,114 7/1962 Willman 166/272 3,253,652 5/1966 Conna11y,eta1 166/245 3.421.583 1/1969 Koons 166/272X 3.353.598 11/1967 Smith 166/245 3.456.730 7/1969 Langem'. 166/272X Primary Eraminer-Stephen J. Novosad Attorneys-George G. Pritzker and J. H. Mc Carthy ABSTRACT: A process of improving oil recovery using hot water injecting means wherein small quantities of hot water can be used by making up the required balance by use of connate water or cold water injected into the formation through a different injection well.

. I2 o u 12 4 57 i Patented Nov. 3, 1970 3,537,526

Sheet 1 of 2 FIG. I

INVENTOR:

JAN OFFERINGA WWW HIS AGENT Patented Nov. 3, 1970 Sheet 3 012 FIG. 4

FIG. 6

m T N E V m YJAN OFFERINGA METHOD OF RECOVERING HYDROCARBONS FROM A HYDROCARBON-CONTAINING SUBSURFACE FORMATION BACKGROUND OF THE INVENTION The present invention relates to a method of recovering hydrocarbons from a hydrocarbon-containing subsurface formation. In particular, the present method relates to the recovery of hydrocarbons from a subsurface formation which cannot economically be produced therefrom under their internal energy. This may be due, for instance, to a low level of internal energy and/or a high value of the viscosity of the hydrocarbons and/or a low permeability of the pore space of the formation.

Various methods have already been proposed to solve this problem, a number of them applying thermal energy to the formation to lower the viscosity of the hydrocarbons, thereby reducing their resistance to flow through the pore space ofthe formation, and increasing the production rate of the hydrocarbons to a value at which the recovery thereof from the subsurface formation becomes economically attractive.

One of these thermal recovery methods is the hot aqueous fluid drive (such as steam flood and hot water flood). Use is made herein of at least one well penetrating into the hydrocarbon-containing formation, which well is suitable for injecting hot aqueous fluid thereinto, and at least one other well penetrating into this formation, which well is suitable for producing fluids from the pore space of the formation, The hot fluid having a temperature which is higher than the formation temperature is injected via the first well into the pore space of the formation, thereby heating the hydrocarbons contained in the formation and subsequently displacing the heated hydrocarbons towards the second well via which they are recovered.

A disadvantage of this method is the enormous amount of heat which has to be injected into the formation before any hydrocarbon can be recovered therefrom A further disadvantage is the thermal energy which is lost in the formation at the end of the production method by the large amount of hot fluid which has displaced the hydrocarbons from the formation, but remains itself in the formation pore space without any chance of recovering the thermal energy therefrom.

An object of the present invention is a thermalrecovery method for recovering hydrocarbons from a hydrocarboncontaining formation by applying an amount of hot fluid which is only a fraction of the hot fluid consumption as required in the prior art hot fluid floods.

A further object of the invention is a thermal recovery method in which hydrocarbon production starts within a short time after the'injection of the hot fluid has been initiated. Such a quick response requires in fact smaller investments than the known hot fluid floods, in which hydrocarbons are not produced until a large amount of heat has been introduced into the pore space of the hydrocarbon-containing formation. If hot water is used, a volume of water has to be injected which is about equal to the pore volume, that is, the volume of the pore space of the formation from which the hydrocarbons are to be removed and extending between the injection well and the production well.

SUMMARY OF THE INVENTION According to the invention, a method of recovering hydrocarbons from a hydrocarbon-containing subsurface formation into which at least one well suitable for injecting fluids into the pore space of the formation and at least one well suitable for producing fluids from the pore space of the formation penetrate, is characterized by a hot channel in the boundary between the hydrocarbon-containing part and the watercontaining part of the formation and extending between an injection well and a production well and serving for transport of hydrocarbons to the production well, comprising the creation ofa hot channel by:

a. the injection of hot aqueous fluid into the formation pore space via at least one injection well;

b. the inflow of cold water into the formation pore space simultaneously with the injection of hot aqueous fluid but at a location different from the location where the hot aqueous fluid is being injected, and

. the'withdrawal of fluid from the formation pore space via at least one production well, the total volume of fluid which is being withdrawn per unit of time from the formation pore space being at least equal to the sum of the total volume of hot aqueous fluid which is being injected per unit of time into the formation pore space and the total volume of cold water which is flowing into the formation pore space per unit of time; and further producing hydrocarbons from the formation after the creation of the hot channel by:

. the injection of hot aqueous fluid into the formation pore space via the injection well,

2. the inflow of cold water into the formation pore space at a location different from the location where the hot aque ous fluid is being injected, the injection of hot aqueous fluid taking place over at least one period which falls within the period over which the inflow of cold water takes place; and

3. the withdrawal offluid from the formation pore space via the production well, the total volume of fluid which is being withdrawn per unit of time from the formation pore space during the period(s) in which hot aqueous fluid and cold water are simultaneously flowing into the formation being at least equal to the sum of the total volume of hot aqueous fluid which is being injected per unit of time into the formation pore space and the total volume of cold water which is flowing into the formation pore space per unit of time.

The hot aqueous fluid may be constituted either by hot water, or by steam, or by a mixture of these two fluids. lf hot water as well as steam are applied as hot aqueous fluids, these fluids may be injected simultaneously or alternately. If desired, one of these fluids may be injected continuously and the other fluid may be injected periodically. The injection of hot aqueous fluid may be stopped even before the recovery of the hydrocarbons is finished. Then the total volume of fluid which is withdrawn per unit of time from the formation pore space is at least equal to the total amount of cold water which is flowing into the formation pore space per unit oftime.

In the present specification and claims, the volume of that part of the hot aqueous fluid constituted by steam, is to be understood to be the volume of water used for the generation of this steam.

For the purposes of the present specification and claims, the expression hot water is to be understood as meaning a liquid medium consisting as to at least percent of water and having a temperature on entering the formation pore space which is at least 10 C. higher than the original formation temperature, the latter being the temperature of the formation just before the injection of the hot water is started.

For the purposes ofthe present specification and claims, the expression cold water is to be understood as meaning a liquid medium consisting as to at least 90 percent of water and having a temperature which is at most 5C. above the original formation temperature.

The inflowing cold water may consist partly of cold water which is injected into the formation pore space via at least one injection well and partly ofcold water which is supplied from a water-saturated part of the formation pore space.

In an alternative method, the inflowing cold water may consist solely of cold water which is injected into the formation pore space via at least one injection well.

In another alternative method, the inflowing cold water may consist solely of cold water which is supplied from a watersaturated part of the formation pore space.

The invention is based on the application of a hot aqueous fluid drive and a cold water drive which for at least the greater part thereof take place simultaneously. wherein the cold water is either injected into the formation and/or formed by edge water which is present in a water-saturated part of the formation. The amount of .hot aqueous fluid required to obtain an increased production of hydrocarbons will then be less than the amount required when injecting hot aqueous fluid exclusivel Ari important feature of the present invention is the forma tion of a hot channel at the lower boundary of the hydrocarbon-containing part of the .formation and extending between an injection and a production well. Thus, if a water-saturated layer is present in the formation, the hot channel will lie between the oil-saturated part of the formation pore space and the cold water-saturated part of the formation pore space. Thus, the oil heated by the hot channel will be easily transported to the production well. If hot water is applied as a hot aqueous fluid, the hot water forming the hot channel will always be pressed against the oil-saturated part of the formation pore space, since it rides on the cold water in this pore space (due to the difference in specific gravity between hot water and cold water). The cold water volume as present in that part of the pore space of the formation from which oil is being produced is continuously increased to make up for the removal of oil from the formation. Thus the interface between the oil and the cold water in the pore space will continuously rise, and the hot channel lying in this interface will move in an upward direction. If desired, the level at which'the hot water is injected via the injection well and the level at which the fluids are removed from the formation via the production well, may be displaced in an upward direction to match the upward displacement of the hot channel.

Instead of hot water, steam may be applied as the hot aqueous fluid for forming the hot channel near the lower boundary of the oil-containing part of the formation, if there is a marked difference between the injectivity of the oil-containing zone and the water-saturated zone of the formation. Such is the case in tar sand formations having a lower zone with a mobile water saturation. The steam on being injected for forming the channel will pass to the production well at a level between the oil-saturated part of the formation and the water-saturated part of the formation.

The main advantage of the method according to the invention as compared to the known hot aqueous fluid drive is the reduction in the amount of thermal energy which is required for injection into the formation to obtain an increased production of oil therefrom. Firstly, the amount of hot aqueous fluid injected during the present method in the period before an increased production occurs in the production well is remarkably less than the amount of hot fluid which is injected over the similar period when applying a hot aqueous fluid drive.

Secondly, the amount of thermal energy which is injected into the formation after response in the production well is less than in the known hot aqueous fluid drive when producing equal amounts of hydrocarbon, since in this latter drive the pore volume from which the oil has been removed is filled with hot fluid, whereas in the method according to the invention this volume is filled up with cold water, the latter being either edge water and/or water which is injected into the formation via at least one injection well.

This efficient use of the thermal energy which is available for the production of oil from the formation is made possible by the application of the hot channel principle, whereby the thermal energy is applied only in those zones where this energy should be available for reducing the viscosity of the oil. The hot channel is created by the application of a difference between the injection rate of the hot aqueous fluid and the production rate of the fluids in the production well. Hereby, the lateral distribution of the hot aqueous fluid after injection into the formation is hampered The injected hot aqueous fluid is displaced straight forwardly towards the nearest production well, which results in an early breakthrough of the hot fluid in the production well. Consequently, there is a quick response in the production well to the injection of hot fluid into the injection well, which is very advantageous as compared to the known hot fluid floods, since these latter methods require the injection of large amounts of heat into the formation before there is any response in the production well in the form of oil production.

The invention will now be described with reference to some examples, which are illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawings shows a perspective view of part of a hydrocarbon-containing formation together with the cap and base rock thereof and the equipment installed on the surface for carrying out the method according to the invention;

FIGS. 2, 3 and 4 show sections of the hydrocarbon-contain ing formation taken in the direction of arrows Il-Il during various stages of the method according to the present invention; and,

FIGS. 5, 6 and 7 show examples of well patterns, suitable to be applied in carrying out the method according to the present invention.

The hydrocarbon-containing formation 1 as shown in FIG. l of the drawing is enclosed in a vertical sense by a caprock 2 and a base rock 3 which are substantially impermeable to the fluids contained in the pore space of the formation 1. The lower part of the pore space of this formation 1 is substantially filled with formation water, whereas the upper part of this pore space is substantially filled with hydrocarbons. The interface between these two fluids is schematically indicated by the plane 4, of which the intersection with the sides of the formation part as shown in the drawing is indicated by the dash-dot line 5. The relationship between the viscosity of the hydrocarbons in the formation pore space and the permeability of the pore space of the formation 1 is such that the hydrocarbons cannot easily flow within this pore space under the influence of pressure differences existing between various parts of the formation. The formation water as present in the lower part of the formation 1, however, has a much higher mobility than the hydrocarbons and can flow easily "'mough this pore space under the influence of pressure different-es. In the example as shown, two wells have been arranged in the caprock 2 and the formation 1. The well 6 is provided with the means by which this well is suitable for use as an injection well. A heater 7 for heating water is provided in communication with the hot water outlet thereof with the inlet of the injection well 6. The well 8 is provided with the equipment which allows this well to be used as a production well. Pumping equipment 9 has been mounted on the well 8 suitable for actuating pumping means (not shown) arranged in the well 8. The other means and equipment of the wells 6 and 8 (such as casing, cementing layers, liners, injection and production tubing strings, wellheads) have not been described in detail, since they are known per se.

Both wells have their lower ends in communication with the pore space of the formation 1. As schematically indicated in FIG. 1, this communication takes place near or in the plane 4 which forms the interface between the water-saturated pore space and the hydrocarbon-saturated pore space of the formation 1. Hot water is now injected via the injection well 6 into the pore space of the formation 1. This water has been heated in the heater or boiler 7, after being treated in one of the known manners for preventing scaling thereof in the heater 7 as well as in the conduits through which it passes to the formation 1 and in the pore space of the formation 1. The water is heated in the heater 7 to a temperature which is sufficiently high to remain above the original temperature of the formation ll, even if some cooling of the water takes place during the transport thereof through the conduits leading into the formation 1. It will be appreciated that the heat losses in these conduits can be reduced by suitable insulating means arranged around these conduits.

Simultaneously with the injection of hot water via the well 6 into the formation 1, fluid is withdrawn from the pore space of the formation 1 via the well 8 by means of the well pump 9. This creates a region of relatively low pressure around the inlet of the well 8, which results in an inflow 'of formation water into the well 8. Further, there is created a pressure difference between the region around the inlet of the well 8 and the region around the outlet'of the well 6. Since the potentials in these regions are lower and higher, respectively. then the original fluid pressure in the formation pore space measured at the level of the plane 4, the fluid from the region around the outlet of the well 6 passes preferably to the inlet of the well 8. As a result of instability (the hot water having a higher mobility than the cold water), the hot water injected into the formation 1 short-circuits the distance between the outlet of the well 6 and the inlet of the well 8, thereby forming a hot channel between these two wells. Since the specific gravity of the hot injection water is smaller than the specific gravity of the relatively cold formation water, but higher than the specific gravity of the hydrocarbons, the hot injection water will pass between the interface of the formation water and the hydrocarbons. The various stages of the lateral and radial extension of the hot water channel are indicated in FIG. 1 (see lines 10, 11 and 12 indicating the boundary of the section of the hot channel in the plane 4) and in FIGS. 2, 3 and 4, which are cross sections taken in the direction ofsection ll-ll in PK]. 1. FIG. 2 shows the situation where the hot channel has advanced to the position as indicated by the boundary in FlG. 1, whereas FIGS. 3 and 4 show the situation in the positions 11 and 12, respectively. The heat from the channel increases the temperature of the hydrocarbons thereabove, thus decreasing the viscosity thereof. The cross section of the zone of hydrocarbons having a reduced viscosity is schematically indicated in H0. 4 by the hatched area 13. The hydrocarbons in the hot zone 13 adjoining the hot channel 12 move towards the production well 8 under influence of the potential difference existing between the wells 6 and 8. The hydrocarbons are lifted in the well 8 by means of the well pump 9 together with the hot water from the hot water channel 12 and formation water. Suitable means (not shown) are used for separating the hydrocarbons from the produced fluid.

There is an unbalance between the injection rate of the hot water, which is passed into the formation pore space via the well 6, and the production rate of fluids from the formation pore space via the well 8. Due to this unbalance, the lateral extension of the hot channel is retained within small limits, and further formation water flows (as schematically indicated by arrows 14) towards the region below the hot water channel 12, which water encroachment fills up the voids which would otherwise he formed by the withdrawal of the hydrocarbons and the formation water via the well 8. lt will be appreciated that a desirable situation is that the fluids produced via the well 8 comprise a minimum of water.

If the supply ofedge water is insufficient to make up the difference between the volume of fluids which is withdrawn per unit of time via the production well 8 and the volume of hot water which is injected per unit oftime via the injection well 6, additional water may be injected via other wells (not shown) to make up the balance. Suitable well patterns for this purpose are shown in FIGS. 5 and 6. This additional water is cold, which means that it has a temperature which is at most 5C. higher than the original temperature of the formation. The outlets of the supply wells for injecting this cold water preferably debouch in the water-saturated zone of the formation 1. it will be appreciated that such extraneous water is treated at the surface before being injected into the formation, in so far as necessary to prevent plugging in the formation pore space and/or corrosion in the well.

Although multiple completion wells can be used for injecting hot fluid and cold water to different levels in the formation, the undesirable heat exchange which may occur between the hot fluid and cold water streams if these streams were passed through a common well through separate conduits, makes it desirable to use separate wells when a combined injection into the formation is required. The application of a cold water injection in the method according to the invention then requires a greater number of wells than is necessary if an edge water drive alone can be relied upon.

The removal of hydrocarbons from the formation and the simultaneous supply of hotwater and additional cold water (either edge water or extraneous water) to the formation part from which hydrocarbons are being recovered, results in a lifting of the plane 4 forming the boundary between the hydrocarbons and the water present in the pore space of the formation 1. If the distance between the rising plane 4 and the outlet and inlet of the wells 6 and 8, respectively, becomes too large, the outlet and inlet may be plugged and the casing of the wells may be perforated at a higher level, which level may approximately coincide with the level of the plane 4 in the neighborhood of the wells 6 and 8. It will be appreciated that there is no need to shift the injection level of the additional cold water, if such water is supplied to the formation pore space.

Although, in the example as shown in FIG. 1, use is made of a heater or boiler 7 in which the water during heating remains in the liquid state, other types of heating equipment may be used as well. Thus, there may be provided a boiler for generating steam (either wet or dry or superheated), which steam is injected together with cold water into the injection well 6. The resulting mixture is hot water which is injected into the formation 1. The feed water for the steam generation as well the cold water are pretreated to prevent scale deposition and/or corrosion.

After the hot channel 12 has been formed by the hot water bridging the distance between the wells 6 and t5, the injection of hot water may be replaced by an injection of a mixture of water and steam (such as wet steam) or dry steam or superheated steam. The steam condenses in the hot channel, and the resulting hot water flows towards the production well 8. This switching over from a hot water injection to a steam injection may take place either directly or gradually.

It will be appreciated that the hot water applied for forming the hot channel in the lower boundary of the oil-containing formation, may be substituted by steam when the injectivity of the oil-saturated layer differs greatly from the injectivity ofthe water-saturated layer underlying the oil-saturated layer. Under these circumstances, the steam \Vlll pass from the injection well to the production well at a le\ el between the oil-saturated layer and the watensaturated la er, and the hot channel will be formed in the same manner as described with reference to HO. 1.

PREFERRED EMBODlMENT OF THE lNVENTlON Reference is now made to FIGS. 5 and 6 of the drawing,

showing by way of example two well patterns which may be used in carrying out the present method. Both patterns include wells for the injection of additional cold water.

Each pair ofinjection and production wells 15 and 16 of the well patterns as shown in FIGS. 5 and 6 is connected by a broken line 17 indicating the hot channel extending in the formation between each pair of wells. The injection wells 15 are situated on a substantially straight line which is substantially parallel to the substantially straight line on which the production wells 16 are located. Injection wells 18 suitable for injecting additional cold water into the formation are provided in both well patterns. In the pattern as shown in FIG. 5, these additional wells 18 are located on the same line as the injection wells 15, in such a manner that between each pair of adjacent wells 15 there are arranged two additional wells 18. In the pattern as shown in FIG. 6, the additional wells 18 are arranged on a substantially straight line which is substantially parallel to the line on which the injection wells 15 are arranged. These two lines do not coincide. In an alternative pattern, the line on which the wells 18 are arranged may be located at the other side of the line on which the injection wells 15 are arranged. lf sufficiently great volumes of edge water are available for filling up the voids which are formed by the withdrawal of the hydrocarbons and the formation water. no additional water should be injected. The wells 18 can then be omitted.

The application of the method according to the invention is not limited to well patterns of the type as shown in FIGS. 5 and The outlets of the injection wells for additional cold water are located at such a level that the flow of additional cold water meets only a small resistance, if any, on entering the water-saturated pore space of the formation from which the hydrocarbons are to be recovered. Preferably, these outlets of the wells for injecting additional water are at a level below the outlets for the injection of hot aqueous fluid.

Although it is assumed in the above description that the water-saturated part of the formation pore space is filled with original formation water 'at the moment that the method according to the invention is started, it will be clear that the method according to the invention may be applied with equal results to a formation in which the lower pore space part is wholly or partly filled with extraneous water, such as water which has been left in the formation by a previous waterflood, wherein a water tongue has been formed on the bottom ofthe hydrocarbon-containing formation, which water tongue has started from the injection well and crept forward towards the production well, thus forming a breakthrough between the wells. Such an abandoned field in which the lower part of the formation pore space is filled with injection water and the upper part of the formation pore space is saturated with hydrocarbons, which cannot be produced economically, can be treated with success by means of the method according to the present invention. The same treatment as described above with reference to the drawings may be applied.

It is also possible to recover oil from a subsurface oil-containing formation which has no water-saturated pore space. To this end, there is first injected an amount ofcold water into the injection well, which amount is sufficient to create a breakthrough of water in the production well. The moment at which breakthrough occurs may be controlled by the backpressure exerted on the production well. This backpressure may be higher, equal to, or lower than the original formation pressure. A backpressure which is lower than the original formation pressure will speed up the moment of breakthrough ofthe cold water in the production well.

After this breakthrough has occurred, the injection of hot aqueous fluid is initiated. If this is in a well other than the first injection well, the injection of cold water may continue, if necessary, at another rate. If, however, the injection of hot aqueous fluid is via the first well, then the injection of cold water is interrrupted. Cold water then flows into the formation either from the edge water storage, and/or is injected via a well other than the hot water injection well. The relationship between the injection rates of the cold water and the hot aqueous fluid and the production rate of the fluids withdrawn from the formation is the same as hereinbefore described. The total volume of hot aqueous fluid and cold water injected per unit of time is equal to or smaller than the volume of fluids produced per unit of time by pumping action. The volume of hot aqueous fluid which is injected into the formation per unit of time is under all circumstances smaller than the volume of fluid produced per unit of time from the production well by the action of the pumping means. The difference between the volume of fluid produced per unit of time and the volume of hot aqueous fluid injected per unit of time is, in case hot water is applied as hot aqueous fluid, fully made up by the inflow of cold water into the formation, which cold water is either supplied from the storage of edge water or from another formation communicating with the first formation, and/or injected into the formation via injection wells.

if the hot aqueous fluid as applied in the method according to the invention, consists wholly or partly of steam, the pore space of the formation p'art occupied by the hot channel will partly be filled with steam. After having penetrated into the channel over a certain distance, the steam will condense and the resulting water will fill the remaining part of the hot channel and flow towards the production well. At a certain steam injection rate, the equilibrium position of the condensation front will be reached after some time. After this period, the sum of the injection rate (expressed in volume per unit of time of water used for the generation of this steam) and the inflow rate of the cold water will equal the production rate of the fluids withdrawn from the formation. Before this period, when the condensation front is still advancing towards the production well, the production rate of the fluids withdrawn from the well will be greater than the sum of the injection rate of the steam and the inflow rate ofthe cold water.

It will further be appreciated that the injection of hot aqueous fluid need not be continued until the end of the production period of the formation. If desired, the injection of hot aqueous fluid may be stopped when part of the hydrocarbons which is producible from the formation pore space has been produced, and the remaining part of such hydrocarbons is then produced by withdrawing fluids from the production well at a rate which is equal to the rate at which cold water is flowing into the formation pore space.

In an alternative method, the hot fluid may be injected during more than one period falling within the period over which the inflow of cold water takes place. During the periods that the hot fluid and cold water flow simultaneously into the formation, the total volume offluid which is being withdrawn per unit of time from the formation is at least equal to the sum of the total volume of the hot aqueous fluid injected per unit of time and the total volume of the cold water flowing into the formation pore space per unit oftime.

I claim:

1. Method of recovering hydrocarbons from a hydrocarboncontaining subsurface formation, into which at least one well suitable for injecting fluids into the pore space of this formation and at least one well suitable for producing fluids from the pore space of this formation penetrate, characterized by a hot channel in the boundary between the hydrocarbon-containing part and the water-containing part of the formation and extending between an injection well and a production well and serving for transport of hydrocarbons to the production well, comprising the creation ofsaid hot channel by:

a. injecting hot aqueous fluid into the formation pore space via at least one injection well;

b. inflowing cold water into the formation pore space simultaneously with the injection of hot aqueous fluid but at a location different from the location where the hot aqueous fluid is being injected;

0. withdrawing fluid from the formation pore space via at least one production well, the total volume of fluid which is being withdrawn per unit of time from the formation pore space being at least equal to. the sum of the total volume of hot aqueous fluid which is being injected per unit of time into the formation pore space and the total volume of cold water which is flowing into the formation pore space per unit oftime; and

further producing hydrocarbons from the formation after the creation of the hot channel by:

l. injecting hot aqueous fluid into the formation pore space via the injection well;

2. inflowing cold water into the formation pore space at a location different from the location where the hot aqueous fluid is being injected, the injection of hot aqueous fluid taking place over at least one period which falls within the period over which the inflow of cold water takes place; and 3. withdrawing fluid from the formation pore space via the production well, the total volume of fluid which is being withdrawn per unit of time from the formation pore space during the period(s) in which hot aqueous fluid and cold water are simultaneously flowing into the formation being at least equal to the sum of the total volume of hot aqueous fluid which is being injected per unit of time into the formation pore space and the total volume of cold water which is flowing into the formation pore space per unit of time.

2. Method according to claim 1 wherein the inflowing cold water consists partly of cold water as hereinbefore defined which is injected into the formation pore space via at least one injection well, and partly of cold water as hereinbefore defined which is supplied from a water-saturated part of the formation pore space.

3. Method according to claim 2 wherein the hydrocarboncontaining formation has the lower part of the pore space thereof substantially saturated by water, the well or wells for injecting the cold water being in communication with the water-saturated part of the pore space.

4. Method according to claim 2 wherein more than one injection well for the hot aqueous fluid, more than one injection well for the cold water and more than one production well is used, the injection wells for the hot aqueous fluid, the injection wells for the cold water and the production wells being located on three substantially parallel lines, respectively, the line on which the cold water injections wells are located and the line on which the hot aqueous fluid injection wells are located being arranged at the same side of the line on which the production wells are located.

5. Method according to claim 4 wherein the wells for injecting cold water are placed between the lines forming the shortest distances between the injection wells for the hot aqueous fluid and the production wells.

6. Method according to claim 1 wherein the inflowing cold water consists of cold water as hereinbefore defined which is injected into the formation pore space via at least one injection well.

7. Method according to claim 1 wherein the inflowing cold water consists of cold water as hereinbefore defined which is supplied from a water-saturated part of the formation pore space.

8. Method according to claim 1 wherein the hydrocarboncontaining formation has the lower part of the pore space thereof substantially saturated by water and the higher part of the pore space thereof substantially saturated by hydrocarbons, the well or wells for injecting the hot aqueous fluid being in communication with the pore space of the formation near the interface existing between the water and the hydrocarbons present in the pore space.

9. Method according to claim 1 wherein the injection of the hot aqueous fluid comprises a period of injecting hot water followed by a period in which this hot water is at least partly replaced by steam.

10. Method according to claim I wherein the injection point of the hot aqueous fluid which is injected via the injection well and the drainage point of the fluids being withdrawn via the production well are shifted to a higher level at least once.

11. Method according to claim I wherein more than one injection well and more than one production well is used, the injection wells being located on a substantially straight line which is substantially parallel to a substantially straight line on which the production wells are located.

12. Method according to claim 11 wherein the wells for the injection of cold water are arranged on the substantially straight line on which the injection wells for the injection of hot aqueous fluid are arranged.