TECHNICAL FIELD

The present invention relates to the use of surfactant compounds in enhanced oil recovery processes, more particularly using liquid or preferably supercritical carbon dioxide.

TECHNICAL BACKGROUND

Hydrocarbons in an underground reservoir can be recovered or produced by means of one or more wells drilled in the reservoir. Before production begins, the formation (a porous medium) is saturated with hydrocarbons.

The initial recovery of hydrocarbons is generally carried out by techniques of "primary recovery, in which only the natural forces present in the reservoir are relied upon. In this primary recovery, only part of the hydrocarbons is ejected from the pores by the pressure of the formation. Typically, once the natural forces are exhausted and primary recovery is completed, there is still a large volume of hydrocarbons left in the reservoir, generally more than two thirds.

This phenomenon has been known for a long time and has led to the development of many techniques of enhanced oil recovery (EOR). Many of these techniques rely on the injection of a fluid into the underground reservoir (or subterranean formation) in order to produce an additional quantity of e.g. crude oil. The fluid used can be water, steam, carbon dioxide, natural gas, nitrogen, etc.

In particular, the injection of carbon dioxide, preferably in the supercritical state, provides a number of advantages. First, reservoir pressure is maintained. Second, oil viscosity is reduced: as carbon dioxide is miscible with oil, the oil expands and swells when put in contact with carbon dioxide. Third, oil displacement is improved because the interfacial tension between oil and water is reduced.

Furthermore, carbon dioxide EOR provides an opportunity for carbon dioxide storage or sequestration underground, which is advantageous since carbon dioxide is considered the primary contributor to the increase in the levels of greenhouse gases in the atmosphere, causing a concern about climate change.

One of the main challenges of carbon dioxide EOR is the early breakthrough of carbon dioxide due to its physical properties. The viscosity of carbon dioxide is low relative to the targeted oil, causing viscous fingering and low oil recovery. Also, the low density of carbon dioxide results in gravity override where carbon dioxide rises to the top parts of the porous medium without contacting the targeted oil.

Mitigation of these issues can be achieved by the addition of small amounts of surfactants to generate carbon dioxide / water emulsions (sometimes also referred to as "foams"). Emulsions have a relatively high viscosity, which makes it possible to prevent or limit viscous fingering and gravity override.

However, the selection of appropriate surfactants is difficult. Non-ionic surfactants tend not to work well at high temperature and high salinity conditions. Anionic surfactants generally cause adsorption issues on minerals. And cationic surfactants tend to have a low solubility in carbon dioxide.

The article entitled "Switchable Nonionic to Cationic Ethoxylated Amine Surfactants for CO2 Enhanced Oil Recovery in High-Temperature, High-Salinity Carbonate Reservoirs" by Chen et al., with the reference SPE-154222-PA (2014), as well as the article entitled "Mobility of Ethomeen C 12 and Carbon Dioxide (C02) Foam at High Temperature/High Salinity and in Carbonate Cores" by Cui et al., with the reference SPE-179726-PA (2016), both disclose the use of ethoxylated monoamine compounds for carbon dioxide EOR.

The article entitled "Switchable diamine surfactants for CO2 mobility control in enhanced oil recovery and sequestration" by Elhag et al., in Energy Procedia 63:7709-7716 (2014) discloses the use of ethoxylated diamine compounds for carbon dioxide EOR.

The PhD thesis entitled "Selection of Switchable Amine Surfactants for Stable C02-in-Water Foams for High Temperature CO2 Mobility Control' by Elhag, The University of Texas at Austin (2016), discloses the use of an alkyl di- tertiary amine for carbon dioxide EOR, wherein the amine compound comprises more than 22 carbon atoms.

However, these surfactants are not fully satisfactory. There is still a need for surfactants which provide higher efficiency in carbon dioxide EOR. SUMMARY OF THE INVENTION

It is a first object of the invention to provide a method of extracting hydrocarbons from a subterranean formation, comprising:

- injecting a surfactant composition into the subterranean formation, and

- collecting hydrocarbons displaced by the injected surfactant composition;

wherein the surfactant composition comprises at least one surfactant compound of formula (I):

wherein x is an integer from 0 to 3, Ri , R2, R3, R4 and each R are independently a hydrogen atom or an alkyl group, each A is an alkylene group, and the total number of carbon atoms in the surfactant compound of formula (I) is from 10 to 21 .

According to some embodiments, the total number of carbon atoms in the surfactant compound of formula (I) is from 12 to 20, preferably from 15 to 20, and more preferably from 17 to 19.

According to some embodiments, x is 0.

According to some embodiments, each R is a hydrogen atom.

According to some embodiments, each A comprises from 1 to 5 carbon atoms, preferably from 2 to 4 carbon atoms, and more preferably comprises 3 carbon atoms.

According to some embodiments, at least one of Ri , R2, R3 and R4 is a hydrogen atom.

According to some embodiments, at least one of Ri , R2, R3 and R4 is an alkyl group comprising from 8 to 16 carbon atoms, preferably from 10 to 15 carbon atoms, and more preferably from 12 to 14 carbon atoms.

According to some embodiments, Ri , R2, R3 and R4 are independently selected from a hydrogen atom and linear alkyl groups.

According to some embodiments, at least one, and preferably two, of Ri , R2, R3 and R4 is/are a methyl group. According to some embodiments, x is 0, A comprises 3 carbon atoms, Ri is an alkyl group comprising from 6 to 16 carbon atoms, R2 is a hydrogen atom, R3 is a methyl group and R4 is a methyl group.

According to some embodiments, Ri is an alkyl group comprising at least 8 carbon atoms, preferably from 10 to 16 carbon atoms, more preferably from 12 to 14 carbon atoms, and most preferably 12 carbon atoms.

According to some embodiments, the surfactant composition comprises a single surfactant compound of formula (I).

According to some embodiments, the single surfactant compound of formula (I) is selected from N ¹ -dodecyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine, N ¹ - dodecyl-N ¹ ,N ³ ,N ³ -trimethylpropane-1 ,3-diamine, N ¹ -(2,2-diethyloctyl)-N ³ ,N ³ - dimethylpropane-1 ,3-diamine, N ¹ -octyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine, N ¹ - decyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine and N ¹ -tetradecyl-N ³ ,N ³ - dimethylpropane-1 ,3-diamine.

According to some embodiments, the surfactant composition comprises a plurality of surfactant compounds of formula (I).

According to some embodiments, the surfactant composition comprises a plurality of compounds of formula (I), wherein x is 0, A comprises 3 carbon atoms, Ri is a linear alkyl group ranging from 8 to 16 carbon atoms, or from 12 to 14 carbon atoms, R2 is a hydrogen atom, R3 is a methyl group and R4 is a methyl group.

According to some embodiments, the surfactant composition comprises at least one additional surfactant which is not according to formula (I), preferably selected from cationic and/or nonionic surfactants.

According to some embodiments, the concentration of surfactant compound(s) of formula (I) in the surfactant composition is from 500 to 50,000 ppm, preferably from 1 ,000 to 20,000 ppm (w/v).

According to some embodiments, the surfactant composition is an aqueous solution.

According to some embodiments, the aqueous solution is a buffered aqueous solution.

According to some embodiments, the surfactant composition comprises liquid or supercritical carbon dioxide.

According to some embodiments, at least one step of injecting liquid or supercritical carbon dioxide into the subterranean formation.

According to some embodiments, the method comprises alternating steps of injecting an aqueous solution and of injecting liquid or supercritical carbon dioxide into the subterranean formation. According to some embodiments, the injecting step(s) are carried out via at least one injection well, and the step(s) of collecting hydrocarbons are carried out via at least one production well.

The invention also relates to a composition comprising liquid or supercritical carbon dioxide and at least one surfactant compound of formula (I):

wherein x is an integer from 0 to 3, Ri , R2, R3, R4 and each R are independently a hydrogen atom or an alkyl group, each A is an alkylene group, and the total number of carbon atoms in the surfactant compound of formula (I) is from 10 to 21 .

According to some embodiments, the composition is in the form of a liquid or supercritical carbon dioxide / water emulsion.

According to some embodiments, the total number of carbon atoms in the surfactant compound of formula (I) is from 12 to 20, preferably from 15 to 20, and more preferably from 17 to 19.

According to some embodiments, x is 0.

According to some embodiments, each R is a hydrogen atom.

According to some embodiments, each A comprises from 1 to 5 carbon atoms, preferably from 2 to 4 carbon atoms, and more preferably comprises 3 carbon atoms.

According to some embodiments, at least one of Ri , R2, R3 and R4 is a hydrogen atom.

According to some embodiments, at least one of Ri , R2, R3 and R4 is an alkyl group comprising from 10 to 16 carbon atoms, preferably from 1 1 to 15 carbon atoms, and more preferably from 12 to 14 carbon atoms.

According to some embodiments, Ri , R2, R3 and R4 are selected from a hydrogen atom and linear alkyl groups.

According to some embodiments, at least one, and preferably two, of Ri , R2, R3 and R4 is/are a methyl group.

According to some embodiments, x is 0, A comprises 3 carbon atoms, Ri is an alkyl group comprising from 6 to 16 carbon atoms, R2 is a hydrogen atom, R3 is a methyl group and R4 is a methyl group. According to some embodiments, Ri is an alkyl group comprising at least 8 carbon atoms, preferably from 10 to 16 carbon atoms, more preferably from 12 to 14 carbon atoms, and most preferably 12 carbon atoms.

According to some embodiments, the surfactant composition comprises a single surfactant compound of formula (I).

According to some embodiments, the single surfactant compound of formula (I) is selected from N ¹ -dodecyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine, N ¹ - dodecyl-N ¹ ,N ³ ,N ³ -trimethylpropane-1 ,3-diamine, N ¹ -(2,2-diethyloctyl)-N ³ ,N ³ - dimethylpropane-1 ,3-diamine, N ¹ -octyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine, N ¹ - decyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine and N ¹ -tetradecyl-N ³ ,N ³ - dimethylpropane-1 ,3-diamine.

According to some embodiments, the surfactant composition comprises a plurality of surfactant compounds of formula (I).

According to some embodiments, the surfactant composition comprises a plurality of compounds of formula (I), wherein x is 0, A comprises 3 carbon atoms, Ri is a linear alkyl group ranging from 8 to 16 carbon atoms, preferably from 12 to 14 carbon atoms, R2 is a hydrogen atom, R3 is a methyl group and R4 is a methyl group.

According to some embodiments, the surfactant composition comprises at least one additional surfactant which is not according to formula (I), preferably selected from cationic and/or nonionic surfactants.

According to some embodiments, the concentration of surfactant compound(s) of formula (I) in the surfactant composition is from 500 to 50,000 ppm, preferably from 1 ,000 to 20,000 ppm (w/v).

The present invention makes it possible to overcome the drawbacks of the prior art. In particular, the invention provides surfactant compounds which are suitable for carbon dioxide EOR.

Some important requirements for a surfactant useful in carbon dioxide EOR are the following:

- Good chemical stability.

- Good thermal stability, desirably up to a temperature of at least 90°C, or 100°C, or even 1 10°C.

- Low adsorption on minerals present in the subterranean formation, and in particular carbonate minerals.

- High solubility in carbon dioxide, including at high temperature of more than 100°C. - High solubility in water, especially at high temperature of more than 100°C, especially in a wide range of pH of 3-7, and especially at a high salinity of e.g. more than 200,000 ppm.

- A satisfactory partitioning coefficient between water and carbon dioxide.

The surfactant compounds of the invention meet some and advantageously all of these requirements.

In some embodiments, the surfactant compounds of the invention make it possible to more effectively generate carbon dioxide / water emulsions (also referred to as "foams") than prior art surfactants, especially at high temperature and high salinity, thereby achieving a larger and/or quicker increase in apparent viscosity.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the rise in apparent viscosity (on the Y-axis, in cP) achieved when various surfactant compositions are co-injected with carbon dioxide in a slim tube experiment. The injected volume is on the X-axis, expressed in pore volumes. For more details, see example 1 below.

Figure 2 shows the final apparent viscosity (on the Y-axis, in cP) achieved when various surfactant compositions are co-injected with carbon dioxide in a slim tube experiment. The temperature applied (in °C) is on the X- axis. For more details, see example 2 below.

DESCRIPTION OF EMBODIMENTS

The invention will now be described in more detail without limitation in the following description.

Surfactant compounds of formula (I)

The invention relies on the use of at least one surfactant compound of formula (I):

in carbon dioxide EOR. In this formula, x is an integer from 0 to 3, Ri , R2, R3, R4 and each R are independently a hydrogen atom or an alkyl group, each A is an alkylene group, and wherein the total number of carbon atoms in the surfactant compound of formula (I) is from 10 to 21 .

Each alkyl group in the compound can be linear or branched.

Each alkylene group A can be linear or branched and is preferably linear. The alkyl and alkylene groups are non-substituted. Therefore, the alkyl groups are of the generic formula -Cnhbn+i , where n is an integer, and the alkylene groups A have the formula -Cnhbn-, where n is an integer.

According to some embodiments, the total number of carbon atoms is 1 1 , or 12, or 13, or 14, or 15, or 16, or 17, or 18, or 19, or 20, or 21 . Preferred ranges of carbon atoms are from 15 to 20, preferably from 16 to 19, and more preferably from 17 to 19.

According to some embodiments x is 0, or 1 , or 2, or 3. Preferably x is from 0 to 2, or from 0 to 1 . Most preferably x is 0, so that the compound of formula (I) is a diamine.

If x is not 0, preferably each R in formula (I) is a hydrogen atom.

If x is not 0, the various groups A can be identical or different. They are preferably identical.

In some embodiments, each group A (or the group A if x=0) may comprise 1 carbon atom, or 2 carbon atoms, or 3 carbon atoms, or 4 carbon atoms, or 5 carbon atoms, or 6 carbon atoms. Number of carbon atoms of 1 to 5 and 2 to 4 are preferred. More preferably, A is -C3H6-. Most preferably, x=0 and

In some embodiments, at least one of Ri , R2, R3 and R4 is a hydrogen atom. Preferably, only one among Ri , R2, R3 and R4 is a hydrogen atom, and the other three are alkyl groups. In such a case, when x=0, the compound is a diamine compound comprising both a secondary amine function and a tertiary amine function.

The alkyl groups among Ri , R2, R3 and R ₄ can be linear and/or branched.

According to some preferred embodiments, one (and only one) of the alkyl groups among Ri , R2, R3 and R4 is branched. According to other preferred embodiments, all the alkyl groups among Ri , R2, R3 and R4 are linear.

Preferably, one and only one among Ri , R2, R3 and R4 is a hydrogen atom. Therefore, in some preferred embodiments, one and only one of Ri , R2,

R3 and R4 is a hydrogen atom and one and only one of Ri , R2, R3 and R4 is a branched alkyl group. In other preferred embodiments, one and only one of Ri ,

R2, R3 and R4 is a hydrogen atom and the other three of Ri , R2, R3 and R4 are linear alkyl groups. Preferably, one (and only one) of Ri , R2, R3 and R4 is an alkyl group having a relatively long carbon chain, i.e. comprises at least 6 carbon atoms. The long chain alkyl group preferably comprises at least 7, or at least 8, or at least 9, or at least 10, or at least 1 1 , or at least 12 carbon atoms. Preferred numbers of carbon atoms for this group may range from 8 to 16, or from 10 to 16, or from 1 1 to 15, or from 12 to 14.

Alternatively, two of Ri , R2, R3 and R4 are alkyl groups having a relatively long carbon chain (i.e. containing at least 6 carbon atoms, possibly at least 7 carbon atoms or at least 8 carbon atoms). In this case, the long chain alkyl groups are preferably geminal, i.e. they can be Ri and R2, or R3 and R4.

Preferably, the other groups among Ri , R2, R3 and R4 are hydrogen atoms or short chain alkyl groups, i.e. alkyl groups comprising 1 to 3 carbon atoms, preferably 1 to 2 carbon atoms, and most preferably a single carbon atom (i.e. methyl groups).

In one preferred embodiment, one among Ri , R2, R3 and R4 is a hydrogen atom, one among Ri , R2, R3 and R4 is a long chain alkyl group as defined above, and the other two among Ri , R2, R3 and R4 are short chain alkyl groups as defined above, and more preferably methyl groups.

In another preferred embodiment, two among Ri , R2, R3 and R4 are long chain alkyl groups as defined above, and the other two among Ri , R2, R3 and R4 are short chain alkyl groups as defined above, and more preferably methyl groups.

One preferred subgroup of compounds useful in the invention are those of formula (II):

(II) wherein A, Ri , R2, R3 and R4 are as defined above. Exampl preferred compounds of formula (II) are those listed in the table below:

Compound No. A Ri R2 Rs R ₄

1 CsHe octyl hydrogen methyl methyl

2 CsHe nonyl hydrogen methyl methyl

3 CsHe decyl hydrogen methyl methyl

4 CsHe undecyl hydrogen methyl methyl Compound No. A Ri R ₂ Rs R ₄

5 CsHe dodecyl hydrogen methyl methyl

6 CsHe tridecyl hydrogen methyl methyl

7 CsHe tetradecyl hydrogen methyl methyl

8 CsHe pentadecyl hydrogen methyl methyl

9 CsHe hexadecyl hydrogen methyl methyl

10 CsHe 2,2-diethyloctyl hydrogen methyl methyl

1 1 CsHe octyl methyl methyl methyl

12 CsHe nonyl methyl methyl methyl

13 CsHe decyl methyl methyl methyl

14 CsHe undecyl methyl methyl methyl

15 CsHe dodecyl methyl methyl methyl

16 CsHe tridecyl methyl methyl methyl

17 CsHe tetradecyl methyl methyl methyl

18 CsHe pentadecyl methyl methyl methyl

19 CsHe 2,2-diethyloctyl methyl methyl methyl

EOR process

According to the invention, a surfactant composition is used in the context of an EOR process, in which hydrocarbons in gaseous and/or liquid phase are recovered from a subterranean formation. The subterranean formation may in particular be a carbonated reservoir. Water within the subterranean formation may have a salinity of 0 to 200 or even 250 g/L, preferably of 100 to 200 or 250 g/L, and more preferably of 150 to 200 or 250 g/L. Salinity is defined herein as the total concentration of dissolved inorganic salts in water, including e.g. NaCI, CaCl2, MgC and any other inorganic salts.

The temperature within the subterranean formation may range from 25 to 140°C, preferably from 80 to 140°C and more preferably from 100 to 120°C.

The permeability of at least a portion of the subterranean formation may range from 5 to 2000 md, preferably from 10 to 1000 md and more preferably from 100 to 1000 md, as estimated by well log.

The process may comprise injecting an aqueous solution (such as water or brine) and/or injecting carbon dioxide in the liquid state or preferably in the supercritical state into the subterranean formation. Preferably, said injection is performed via one or several injecting wells, while hydrocarbon collection is performed via one or more production wells. Preferably both an aqueous solution and carbon dioxide are injected into the subterranean formation. In particular, separate steps or alternating steps of aqueous solution injection and carbon dioxide injection can be provided. Alternatively, the aqueous solution and carbon dioxide can be injected simultaneously, be it via different injection wells or via the same injection well(s). In the latter case, they can be injected via distinct pipelines within a same injection well. Alternatively, the aqueous solution and the carbon dioxide can be premixed and injected as one composition via the same pipeline, although this is generally not preferred due to the high pressure drop generated by the carbon dioxide / water emulsion in the well(s).

Carbon dioxide / water emulsions which are either generated in situ or premade are preferably characterized by a carbon dioxide / water volume fraction ratio of more than 1 .

In the invention, at least one surfactant compound of formula (I) is added to at least of the above streams of aqueous solution and/or carbon dioxide, so as to make a surfactant composition, prior to injection. The injection of the surfactant composition may be performed at a pressure of from 72.9 to 300 bar, preferably from 100 to 250 bar.

Therefore, use is made of a surfactant composition which comprises an aqueous solution, or carbon dioxide, or a mixture of aqueous solution and carbon dioxide, and which further comprises at least one surfactant compound of formula (I).

According to some embodiments, the surfactant composition comprises a single surfactant compound of formula (I).

According to other embodiments, the surfactant composition comprises a plurality of (i.e. at least two) surfactant compounds of formula (I). In particular, the surfactant composition may comprise a statistical distribution of compounds of formula (I), as can be obtained for instance starting from a natural oil. It has been found that mixtures of surfactant compounds of formula (I) may provide better performances in EOR than single compound formulations, due to different individual physicochemical properties of the compounds.

In particular, in some of these embodiments, the surfactant composition comprises a plurality of surfactant compounds of formula (II). In preferred variants, A, R2, R3 and R4 are the same for the plurality of surfactant compounds, and Ri is a different alkyl group. In more preferred variants, A is C3H6, R2 is H, R3 and R4 are methyl groups in the various surfactant compounds of formula (II), while Ri is a different alkyl group, such as in particular an alkyl group (preferably a linear alkyl group) comprising 8 to 16 carbon atoms or comprising 12 to 14 carbon atoms.

The amount of surfactant compound(s) of formula (I) in the surfactant composition is preferably from 500 to 50,000 ppm, and more preferably from 1 ,000 to 20,000 ppm (w/v).

The surfactant composition may also comprise one or more additives. Such additives may include additional surfactants (not according to formula (I)), salts, sacrificial agents, mobility control polymers, pH adjustment agents, solvents and mixtures thereof.

Additional surfactants may notably include cationic and/or nonionic surfactants, and for instance ammonium cationic surfactants.

According to some embodiments, the surfactant composition is a buffered aqueous solution, which makes it possible to more precisely control the physicochemical properties of the surfactant compounds. The pH of the surfactant composition is thus preferably from 4 to 8, more preferably from 5 to 7 and even more preferably from 5.5 to 6.5.

According to some embodiments, the surfactant composition is a brine solution, having a salinity of from 70 to 300 g/L, preferably from 120 to 220 g/L.

In some embodiments, at least part of the surfactant compound(s) of formula (I) are recovered in the stream of collected hydrocarbons. This part of surfactant compounds can advantageously be separated from the hydrocarbons so as to be recycled and reused.

In addition to carbon dioxide EOR, the above surfactant compounds of formula (I) can also be used in other EOR processes, such as chemical EOR processes (such as Surfactant Flooding, Surfactant and Polymer Flooding, Alkaline-Surfactant-Polymer Flooding), gas EOR processes (using e.g. N2, natural gas or CO2) and thermal processes (such as Steam Flooding).

Furthermore, the above surfactant compounds of formula (I) can also be useful additives for transporting collected hydrocarbons, as they can provide an anti-agglomerate function. Accordingly, the invention also relates to a method of extracting hydrocarbons from a subterranean formation, comprising:

- injecting a surfactant composition as described above into the subterranean formation,

- collecting hydrocarbons displaced by the injected surfactant composition, and

- transporting the collected hydrocarbons containing said surfactant composition. Preparation of compounds of formula (I)

Compounds of formula (I), in particular those for which x=0, may be synthesized by reducing compounds having the same formula, except that one of the alkyl groups is replaced by a corresponding acyl group which therefore forms an amide bond with the neighboring nitrogen atom.

By way of example, the preferred compound N ¹ -dodecyl-N ³ ,N ³ - dimethylpropane-1 ,3-diamine can be reduced from dodecylamidopropyl dimethylamine according to the following reaction scheme:

A similar reduction reaction can also be performed starting from a complex mixture, such as cocamidopropyl dimethylamine (which is a mixture of amide compounds, predominantly having a C8-C16 alkyl chain).

The reduction reaction may be performed in the presence of sodium bis(2-methoxyethoxy)aluminumhydride in toluene. Other possible reducing agents include LiAIH ₄ and NaBH ₄ .

The amide starting compounds may be obtained by reacting the corresponding carboxylic acid and amine. For instance dodecylamidopropyl dimethylamine may be obtained by reacting the carboxylic acid of the following formula: with the diamine of the following formula: The amidation reaction may be e.g. performed in the presence of a coupling agent such as 2-(1 H-benzotriazol-1 -yl)-1 ,1 ,3,3-tetramethyluronium hexafluorophosphate, of a base such as triethylamine, and in a solvent such as dimethylformamide and/or tetrahydrofurane.

EXAMPLES

The following examples illustrate the invention without limiting it.

Example 1 - surfactants according and not according to the invention

In this example, experiments were conducted within a slim tube packed with sand. The tube length was 25 cm, the tube diameter was 1 cm. The packed sand had a total pore volume of 6.55 ml_ and a permeability of 16.8 darcy.

Various surfactant compositions were made by dissolving 0.2 wt.% of an individual surfactant compound in brine having a NaCI content of 220 g/L, buffered at pH=6 with a sodium acetate / acetic acid buffer.

Carbon dioxide and the surfactant brine composition were co-injected into the slim tube via two separate inlets, at a temperature of 25°C and at a pressure of 150 bar, with a total flow rate of 60 ft/day and a carbon dioxide fraction of

50%.

The pressure drop across the tube was measured and the apparent viscosity was calculated based on Darcy's law.

The following individual surfactant compounds were tested:

- A: no surfactant, pure water (control).

- B: nonyl phenol ethoxylate in brine (comparative).

- C: bis-(2-hydroxyethyl) coconut alkylamine, marketed by Akzo Nobel as Ethomeen® C12, in brine (comparative).

- D: dodecylamidopropyl dimethylamine, in brine (comparative).

- E: N ¹ -dodecyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine, in brine (invention).

In this example, compound D was synthesized from pure chemicals (lauric acid and propanediamine), and compound E was prepared from compound D, according to the process described above.

The results of the experiments are shown on Figure 1. Compound E according to the invention provides a quicker and higher rise in viscosity and is therefore deemed to be more effective than comparative surfactant compounds in an EOR process. It is believed that the benefit offered by compound E may be even greater at lower permeability and/or higher temperature, i.e. in conditions closer to those of some actual subterranean formations.

In addition to the above, it should be noted that amide compounds such as compound D are not stable at high temperature.

Example 2 - various surfactants according to the invention

In this example, similar experiments to those of example 1 were conducted in a slim tube. In this case, three different surfactant compositions according to the invention were used and tested at different temperatures. All surfactant compositions were made with 0.2 wt.% surfactant in brine having a NaCI content of 220 g/L, buffered at pH=6 with a sodium acetate / acetic acid buffer:

- Composition A: N ¹ -dodecyl-N ³ ,N ³ -dimethylpropane-1 ,3-diamine (pure compound E of example 1 ), in brine.

- Composition B: mixture of compounds obtained by reducing cocamidopropyl dimethylamine in brine. The mixture contains not only compound E of example 1 (alkyl chain in C12) but more generally similar compounds having alkyl chains of various lengths (mainly C8- C16 and more particularly C12-C14). This composition was purified by passing in a silica chromatography column to remove organic solvents and by-products in the reducing reaction.

- Composition C: same as composition B, except that no purification step was performed.

The results of the experiments are shown on Figure 2. The data corresponds to the stabilized apparent viscosity after the transient regime (plateaued apparent viscosity) as a function of temperature.

The first observation is that the performance of the surfactant compositions of the invention does not decrease at high temperature, and in some cases even improves at high temperature.

The second observation is that mixtures of compounds according to the invention tend to be more efficient than single compounds.