New aqueous fracturing fluid composition and fracturing method implementing the fluid

The invention also relates to a method for fracturing unconventional oil or gas fields, according to which:

- fracturing fluid is prepared as described above,

- fluid is injected under pressure to create gaps arranged perpendicular to the production well.

Optionally, after forming the gap, at least one oxidizing compound and / or at least one surfactant is injected into the bed. Preferably, at least one high concentration surfactant will be injected.

Injection of the surfactant in high concentration makes it possible to eliminate the viscosity produced by the polymer by inhibiting hydrophobic interactions between the chains, while injection of the oxidizing compound destroys the polymer. In both cases, the injection allows the fluid viscosity to be restored to near water viscosity.

For surfactant injection, after fracturing, the concentration is at least 500 ppm, more preferably at least 1000 ppm, even more preferably at least 2000 ppm.

As an oxidizing compound, Javel (i.e. bleaching) products, persulphates, permanganates or perchlorates can be mentioned.

The chemical nature of surfactants is not critical. They can be anionic, nonionic, amphoteric, zwitterionic and / or cationic. Preferably, such surfactants according to the invention have anionic charges.

preferably, the surfactants used are selected from anionic surfactants and their zwitterions selected from the group consisting of alkyl sulfates, alkyl ether, aryloalkilosiarczany, aryloalkiloeterosiarczany, arylalkyl sulfonates, aryloalkilofosforany, aryloalkilofosfoniany, alkyl ether, alkyl ether, alkiloeterofosfoniany, aryloalkiloeterosulfoniany, aryloalkiloeterofosforany, aryloalkiloeterofosfoniany, aryloalkilokarboksylany, alkyl, alkyl phosphates, alkylphosphonates, of alkyl, alkyl ether, aryloalkiloeterokarboksylany, alkyl polyethers, arylalkyl polyethers ...

By "alkyl chain" is meant a chain of from 6 to 24 carbon atoms, branched or not, with or without many moieties, which may optionally contain one or more heteroatoms (O, N, S). An arylalkyl chain is a chain of 6 to 24 carbon atoms, branched or not, containing one or more aromatic rings, which may optionally contain one or more heteroatoms (O, N, S).

Surfactants commonly used for reasons of cost, stability and availability are sulfonates or sulfates, present in the form of alkali or ammonium metal salts.

The invention and its benefits will be better demonstrated based on the following embodiments, with reference to the attached figures.

Fig. 1 is a graph comparing the viscosity in seawater of various polymers according to the state of the art compared to the polymer present in the fluid according to the invention depending on the shear rate at 20 ° C.

Fig. 2 is a graph comparing the viscosity in seawater of various polymers according to the state of the art compared to the polymer present in the fluid according to the invention depending on the shear rate at 50 ° C.

Fig. 3 is a graph comparing the viscosity in seawater of different guar gums compared to the polymer present in the fluid according to the invention depending on the shear rate.

Fig. 4 is a graph comparing the viscosity of guar gum in seawater compared to the polymer present in the fluid according to the invention depending on the shear rate and salinity.

EXAMPLE 1:

Polymer produced by gel polymerization:

In a beaker, x mol% of the hydrophobic cationic derivative acrylamide, y mol% acrylic acid, with mol% acrylamide was dissolved in water to obtain a concentration of active substance of 30%. The additives described above can then preferably be added to improve the dissolution of the monomers. The sum of x + y + z is 100. The solution is then mixed, cooled and neutralized by the addition of soda lye. The solution is then introduced into a Dewar vessel and then degassed under a stream of nitrogen to remove oxygen.

Polymerization is initiated using a red / ox pair. The temperature rises adiabatically.

The resulting gel is left for 3 hours in a Dewar vessel. It is then mixed and dried in the oven overnight. A white powder is obtained, which is mixed again.

1 / Preparation of an aqueous polymer solution

The dry polymer extract must be examined to determine the percentage of active substance: weigh the glass cup thoroughly to the nearest 0.001 g. Save mass as Mc. Weigh 10 g of powder in this glass dish and record the total weight Mc + P. Place the glass cup for 2 hours in the oven at 120 ° C. After 2 hours at 120 ° C, leave the cup in the desiccator until it cools down. Weigh the cup with the mass of the resulting solid and record as Mc + ps. The dry matter percentage X is obtained from the formula:

X = [(Mc + ps-Mc) / (Mc + P-Mc)] * 100, expressed in%

The polymer solution is prepared according to the following general procedure:

Preparation of a 10 g / l polymer stock solution (product according to the invention or guar gum powder)

Weigh 200- (2 / (X / 100)) g of brine representative of the water used for injection in the field where the fracturing is carried out in a 400 ml beaker. Using a mechanical stirrer, mix the solution at 500 rpm. 2 / (X / 100) g of dry polymer in powder form slowly add to the wall of the shaker formed by stirring at room temperature. Leave the solution stirring for 2 hours. Dilution of the stock solution to obtain a diluted solution with a polymer concentration of Y%.

Y * 100 g stock solution up to 10 g / l is drawn by syringe and poured into a 400 ml beaker. 100-Y g of previously prepared brine is added to the beaker. The solution is then mixed with a magnetic stirrer at 250 rpm for 20 minutes.

2 / Rheological assessment of polymers

Guar gum samples ECOPOL ™ 500 and GW 27 were tested. These products correspond to guar gums used in fracturing fields in the United States.

The polymers present in the fluid according to the invention are numbered from 1 to 4 and are described in the following table. For comparison, the non-associative anionic polymer from very high molar mass was also evaluated: polymer 5 and non-associative amphoteric polymer described in the prior art in WO 02/084075: polymer 6.

<td>products</td><td>acrylamide (% mol)</td><td>acrylate sodium (% moth)</td><td>Akryloamidoterbutylo sodium sulfonate (% mol)</td><td>Chloride diallyl (DADMAC) (% mol)</td><td>monomer associative type C12H25 (mol%)</td><td>mw (million g / mol)</td>

<td>Polymer 1</td><td>85</td><td>14.8</td><td>0</td><td>0</td><td>0.2</td><td>6-8</td>

<td>Polymer 2</td><td>85</td><td>0</td><td>14.8</td><td>0</td><td>0.2</td><td>4-7</td>

<td>Polymer 3</td><td>85</td><td>4.8</td><td>10</td><td>0</td><td>0.2</td><td>7-10</td>

<td>Polymer 4</td><td>80</td><td>4.8</td><td>15</td><td>0</td><td>0.2</td><td>6-8</td>

<td>Polymer 5</td><td>75</td><td>25</td><td>0</td><td>0</td><td>0</td><td>18-20</td>

<td>Polymer 6</td><td>80</td><td>18</td><td>0</td><td>2</td><td>0</td><td>6-8</td>

Rheological properties were determined using a Malvern Bohlin Gemini rheometer with a cone / plate geometry inclined at an angle of 2 ° and a diameter of 6 cm. A Peltier heating and cooling system was used to allow measurements at various temperatures. The test conditions are described in each of figures 1 to 4.

The viscosity depending on shear at various temperatures is given in figures 1 to

4. The target polymer concentration is 2000 ppm.

Comments: It was found that at 2000 ppm polymer and at low shear, the polymers used in the present invention exhibit a clearly higher viscosity than the guar gum tested, both at 20 ° C and 50 ° C. This is especially visible for shear less than 10 s^-1, which is a desirable feature for keeping proppant in suspension. It should be noted that above 50 s^-1 the viscosity for all tested products is low, which ensures good pumping properties. The use of a non-associative polymer, as well as the use of a non-associative amphoteric polymer, is also not preferred.

However, despite the high salinity, it should be noted that the associative product used in the invention is 100 to 1000 more viscous in the shear range suitable for suspending proppant material. The viscosity of the guar gums tested is almost identical.

The polymer used in the invention has a higher viscosity than the guar gum tested, regardless of the salinity of the brine used. Regardless of the degree of salinity of the water used to prepare the stimulation fluid, the polymer used in the invention therefore has a greater ability to suspend and hold the proppant, especially at low shear.

Even at high salinity, the viscosity of the polymer used in the invention remains significantly higher than for other polymers tested.

EXAMPLE 2

Sand sedimentation test

Solutions of 230 g polymer at a concentration of 3000 ppm in water simulating sea water were prepared according to the procedure described above. 20 g of sand used in fracturing projects were added to each solution. The solutions were mixed with a mechanical stirrer at 400 rpm for 5 minutes. Each solution was then poured into a 250 ml tube with a scale and the stopwatch was started at the same time. The times corresponding to the total falling of the sand are given in the table below

<td>products</td><td>Sedimentation time 20 g sand</td>

<td>ECOPOL 500</td><td>Less than 2 minutes</td>

<td>Polymer 1</td><td>15 minutes</td>

<td>Polymer 2</td><td>Twenty minutes</td>

<td>Polymer 4</td><td>25 minutes</td>

<td>Polymer 6</td><td>Less than 2 minutes</td>

Sand settling times are much higher for the products according to the invention compared to guar gum. The products according to the invention have a much higher sand-holding capacity than guar gum.

Viscosity degradation by adding surfactant after fracturing

At the production stage, after adding sand, the viscosity of the polymer must be reduced to facilitate the placement of sand in the gaps. In general, the addition of an oxidant can lead to polymer destruction and restore fluid viscosity to near water.

To demonstrate the effect of surfactants on polymer solutions in the presence of sand, the same sedimentation procedure was used. A solution of surfactant (sodium dodecyl sulfate (SDS)) at a concentration of 10% (5 g or 2000 ppm) was added with stirring for 30 seconds before pouring into the tube. The times corresponding to the total falling of the sand are given in the table below:

<td>products</td><td>Sedimentation time 20g sand</td>

<td>ECOPOL 500</td><td>Less than 2 minutes</td>

<td>Polymer 1 + SDS</td><td>Less than 2 minutes</td>

<td>Polymer 2 + SDS</td><td>Less than 2 minutes</td>

<td>Polymer 4 + SDS</td><td>Less than 2 minutes</td>

<td>Polymer 6 + SDS</td><td>Less than 2 minutes</td>

Comparing this table with previous results, it is found that the addition of a sufficient amount of SDS after fracturing allows a radical reduction of sand settling time. It is therefore advantageous to use the polymer of the present invention because of its sand holding properties, as well as the ease of reducing the viscosity of the posteriori solution by adding a surfactant after fracturing.

EXAMPLE 3: Measurement of the fracturing fluid viscosity after addition of surfactant and before injection

Polymer 1 was dissolved in water with 30 g / L NaCl to give a final concentration of 5000 ppm. Identical solutions were prepared on the basis of polymers 3, 4 and 6.

The viscosity of each of the polymer solutions was measured using a viscometer

Brookfield LVT at 20 ° C.

Then 350 ppm SDS was added to each polymer solution.

The viscosity of each of the polymer solutions also containing SDS was measured using a Brookfield LVT viscometer at 20 ° C.

After addition of SDS, the viscosity of solutions containing polymers 1, 3 and 4 increases by

250, 230 and 280%. However, no significant change in the viscosity of the prior art polymer-containing solution 6 was observed.

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