Field of the invention

The invention relates to an analytical method, in particular to a method for determining the weight- average molecular weight (M _w ) of a water-soluble high molecular weight polymer.

Background

Improving recovery of conventional oil reservoirs has become imperative to support the global oil output since an increasing number of oil fields are maturing and classic targets of exploitation are becoming scarce. The technologies designed to raise recovery factors are collectively known as enhanced oil recovery (EOR) technologies.

Water-flooding is the oldest method used to increase oil recovery. In water-flooding, water is injected to support pressure of the reservoir and to sweep or displace oil from the reservoir and push it towards a production well. However, in the conditions of the reservoirs, water exhibits almost always a mobility (ratio between specific resistance to flow and fluid viscosity) higher that the oil phase. Therefore, water has a tendency to get into the paths of least resistance offered by the reservoir instead of pushing the oil phase. To solve this problem, a viscosity-enhancing high-molecular-weight polymer is added to the water to decrease the mobility ratio between water and oil, and in turn to improve macroscopic sweep efficiency. The primary purpose of adding polymer is to increase the viscosity of the pushing water.

Acrylamide polymers are the most widely used synthetic polymers for application in polymer flooding. This is due in part because of cost and availability issues and because of their favorable chemical robustness and biological stability. Polyacrylamide (PAM) is the simplest and most basic form of acrylamide polymers. Because pure polyacrylamide is neutral (not charged), it will adsorb by hydrogen bonding on negatively charged rock surfaces such as sands and sandstone pore surfaces. For these reasons, copolymers of acrylamide and sodium acrylate (called partially hydrolyzed polyacrylamide, HP AM) are more often favored for use in polymer flooding. This polymer is negatively charged. As a consequence, adsorption on negatively charged rock surfaces is lowered compared to a neutral polyacrylamide because of the electrostatic repulsion between negative charges. Copolymers containing 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) monomers and acrylamide monomers have been suggested to form acrylamide polymers for use in polymer waterflooding of high-temperature (e.g. 93 °C) and high-salinity reservoirs where the AMPS copolymer' s performance and stability will be somewhat better than comparable HP AM. Other synthetic polymers, such as copolymers of N-vinylpyrrolidone and acrylamide, along with ter-polymers of N-vinylpyrrolidone, acrylamide, and sodium acrylate, have been reported to be candidate polymers for use in polymer floods. Biopolymers, such as xanthan, scleroglucan or schizophyllan can also be used for polymer waterflooding.

The choice of the polymer depends on many parameters such as the reservoir characteristics (temperature, salinity, rock permeability, formation heterogeneity, maximum allowable injection pressure), the composition and properties of the oil contained in the reservoir. When all other factors are equal (such as polymer type and the brine solution into which the polymer is dissolved), as the molecular weight of the polymer increases, the polymer's viscosity enhancing ability increases when dissolved in a given brine.

Because polymers used in polymer waterflooding are polydispersed in molecular weight, polymer molecular weight distribution is an important factor relating to how a given polymer will function during a polymer flood. Unfortunately, good molecular weight distribution of such high molecular weight polymers data are not readily and widely available for the polymers that are normally used in polymer flooding, because the determination of a polymer's molecular weight distribution is relatively expensive and time consuming.

The characterization of the molecular weight averages and size distribution of a polydispersed polymer is generally accomplished using a Size Exclusion Chromatography (SEC) column coupled with a molecular weight sensitive detector, such as a light scattering photometer, in particular a Multi Angle Light Scattering (MALS) detector.

SEC separates molecules based on their size by filtration through a gel. The gel consists of spherical beads containing pores of a specific size distribution. Separation occurs when molecules of different sizes are included or excluded from the pores within the matrix. Small molecules diffuse into the pores and their flow through the column is retarded according to their size, while large molecules do not enter the pores and are eluted in the column's void volume. Consequently, molecules separate based on their size as they pass through the column and are eluted in order of decreasing molecular weight.

Coupling a SEC column to a concentration-sensitive detector, such as a Differential Refractive Index detector (DRI), allows the determination of the concentration of the polymer at any time, which is needed for calculating the molar mass of the polymer. MALS allows the determination of the absolute molar mass and the average size of particles in solution by detecting how they scatter light. The expression "multi angle" refers to the detection of scattered light at different discrete angles as measured by a single detector moved over a range of particular angles or by an array of detectors fixed at specific angular locations. MALS measurements work by calculating the amount of scattered light at each angle detected. This process overcomes the problems associated with low angle detectors (typically there is around ten times the noise at an angle of 11° or below compared to 90°) and allows a reliable and accurate measure of the light scattered. The higher the number of detectors, the better the accuracy of the experiment. The amount of light scattered is then related to the molar mass.

Thus, a SEC column coupled to a MALS detector and a Differential Refractive Index detector allows the determination of the weight-average molecular weight of a polydispersed polymer.

However, high molecular weight polyacrylamides are very sensitive to mechanical degradation. When the elongational component of the flow is high enough, flexible polymer coils in dilute solution experiences a sudden coil-stretched transition. As the chains start to extend, hydrodynamic tension exerted through the friction of the solvent on the stretched chains is increased, and can lead to chain rupture. This tension scales with the square of the length of the stretched polymer chain. In general, biopolymers are present in solution as double or triple helix conformation similar to rigid rods. The length of these rigid rods is far smaller than the length of a flexible coil after stretching. It explains why flexible polymers are more prone to degradation compared to biopolymers.

Elongational flow occurs each time the fluid experiences an acceleration, for example, during the passage through a contraction or through the succession of contractions enlargements of a porous medium. Hence, in a SEC experiment, polymer is subjected to elongation in the porous material of the chromatography column, pre-column filters or frits or in the upstream or downstream tubings and fittings. As a consequence, high molecular weight polymer chains can be degraded during the analysis. By using a technique of molecular weight distribution characterization by elongation flow, Odell et al. (Odell, Jeffrey A., et al. "Degradation of polymer solutions in extensional flows." Macromolecules 23.12 (1990): 3092-3103) studied degradation of Polymer Solutions when passing through a SEC column. They also reported results on other polymers from other authors. They concluded that the highly elongational character of the pore flow in the SEC column is responsible for stretching and breaking of polymer chains in half. The use of SEC would be limited to the characterization of chains smaller than approximately 8.10 ⁶ g/mol. A.M. Striegel et al. ("An SEC/MALS study of alternan degradation during size-exclusion chromatographic analysis", Anal Bioanal Chem (2009) 394: 1887-1893) provide unambiguous evidence that the polysaccharide alternan (an ultrahigh-molar-mass polymer) in DMSO at 50°C actually degrades during SEC analysis using 20μιη particle size columns (which are among the largest-particle-size columns available for analytical work in organic solvents). The results of their experiments show that the weight-average molar mass (M _w ) of alternan, obtained using the online MALS and DRI detectors, decreases significantly (~ 20%) as a function of increased flow rate, from 4.4xl0 ⁷ g/mol at 0.2 mL/min to 3.5xl0 ⁷ g/mol at 1.0 mL/min. Furthermore, they show that working at a lower flow rate (i.e. below 0.2 mL/min) does not eliminate the small amount of degradation which may be occurring at 0.2 mL/min. These findings demonstrate the extreme fragility of high molecular weight polymers during SEC analysis even when using the largest- particle-size column and also show that even at very low flow rates, it is not possible to prevent the degradation the polymer during the SEC analysis.

Surprisingly, and contrary to the teaching of A.M. Striegel et al., the inventors of the instant patent application have found that when using a SEC column connected to a MALS and DRI detectors with flow rates of water mobile phase below 0.20 mL/min, the degradation of the polymer does decrease until a level not detectable as the flow rate decreases. It is therefore possible to improve the accuracy of a SEC-MALS method for determining the weight-average molecular weight of a water-soluble high molecular weight and highly flexible polymer by working with a mobile phase flow rate below 0.20 mL/min. .

Summary of the invention

The object of the present invention is a method for determining the weight- average molecular weight of a water-soluble high molecular weight polymer, said method comprising the following steps:

a) Providing a Size Exclusion Chromatography column connected to a Multi Angle

Light Scattering detector and a Differential Refractive Index detector; b) Providing an aqueous solution as mobile phase of said Size Exclusion Chromatography column;

c) Injecting said polymer into said mobile phase of said Size Exclusion Chromatography column;

d) Measuring the weight-average molecular weight of said polymer with the Multi Angle Laser Light Scattering detector and the Differential Refractive Index detector ; wherein the flow rate of said mobile phase is lower than 0.20 mL/min, in particular is from 0.05 mL/min to 0.20 mL/min, more particularly from 0.10 mL/min to 0.15 mL/min.

In one embodiment, the Size Exclusion Chromatography column is further connected to a Differential Refractometer. In one embodiment, the polymer has a weight- average molecular weight higher than 10 ⁶ g/mol, in particular higher than 5.10 ⁶ g/mol, in particular higher than 7.5.10 ⁶ , more particularly higher than 10.10 ⁶ g/mol.

In one embodiment, the polymer is a polyacrylamide or a partially hydrolyzed polyacrylamide, a polyoxyethylene, a poly(styrenesulfonic acid), a copolymer of acrylamide and acrylamidomethylpropanesulfonate, or of acrylic acid, acrylamide and acrylamidomethylpropanesulfonate and N-vinylpyrrolidone or of acrylamide, acrylamidomethylpropanesulfonate and N-vinylpyrrolidone.

In one embodiment, the mobile phase of the Size Exclusion Chromatography column contains a buffering agent leading to a pH higher than 6, in particular a pH of 8. In one embodiment, the buffering agent is 4-(2-hydroxyethyl)-l-piperazineethanesulfonic acid, 2-amino-2-(hydroxymethyl)- 1,3 -propanediol or N,N-Bis(2-hydroxyethyl)glycine.

In one embodiment, the salinity of the mobile phase is from 18 g/L to 26 g/L.

In one embodiment, the Size Exclusion Chromatography column is packed with particles having a size above 30 μιη. In one particular embodiment, the Size Exclusion Chromatography column is packed with a bed of spheres of polyhydroxymethacrylate of 35 μιη diameter, with dimensions of 8.0 x 300 mm.

In one embodiment, the Size Exclusion Chromatography column is composed of a set of two Size Exclusion Chromatography columns connected in series, the first column being packed with a bed of Spheres of polyhydroxymethacrylate of 13 μηι and the second column being packed with a bed of Spheres of polyhydroxymethacrylate of 35 μιη, each bed having a dimension of 8.0 x 300 mm. Brief description of the figures

Figure 1 shows the variation of the area of the pic of the MALS signal with respect to the flow rate of the polymer.

Figure 2 shows the variation of molar mass of the polymer with a polymer flow rate of 0.1 mL/min.

Figure 3 shows the variation of molar mass of the polymer with a polymer flow rate of 0.15 mL/min.

Figure 4 shows the variation of molar mass of the polymer with a polymer flow rate of 0.25 mL/min.

Figure 5 shows the variation of molar mass of the polymer with a polymer flow rate of 0.5 mL/min.

Figure 6 shows the variation of molar mass of Polyacrylamide (PAM) with a polymer flow rate of 0.1 mL/min.

Detailed description of the invention The method of the invention is used for determining the weight- average molecular weight and the associated molecular weight distribution of a water-soluble high molecular weight polymer. It can be applied to any water-soluble high molecular weight polymer susceptible to mechanical degradation during SEC analysis, in particular synthetic flexible polymers (i.e. very long molecules sparsely branched and un-crosslinked)). Mechanical degradation refers to the chemical process in which the activation energy of polymer chain scission is supplied by the mechanical action on the polymer chain and bond rupture occurs, thereby leading to shorter polymer chains. Longer molecules are more susceptible to mechanical degradation, accompanying more rapid degradation.

According to the invention, high-molecular weight polymers are polymers having a weight- average molecular weight equal to or higher than 10 ^s g/mol. The polymer may have a weight- average molecular weight higher than 10 ⁶ g/mol, in particular higher than 5.10 ⁶ g/mol, in particular higher than 7.5.10 ⁶ , more particularly higher than 10.10 ⁵ g/mol.

The weight-average molecular weight of a polymer (M _w ) is defined as follows: M _w = (∑NiMi ² )/(∑NiMi) where Mi is the molecular weight of a chain of the polymer and Ni is the number of chains of that molecular weight. The weight- average molecular weight M _w takes into account the molecular weight of a chain in determining contributions to the molecular weight average. M _w can be determined by using a MALS and DRI detectors, as described below.

The polymer may be a monodisperse polymer or a polydisperse polymer.

The polydispersity index of the polymer is defined as the ratio of the weight- average molecular weight of the polymer and the number average molecular weight of the polymer (I _p = M _w M _n ). It is higher than 1 and is generally from 1 to 10.

According to the invention, high-molecular weight polymers are chosen among water-soluble polymers, i.e. polymers which are able to dissolve completely in water. The method of the invention applies in particular to high-molecular weight polymers used in EOR, in particular in waterflooding or conformance treatments, but also to polymers used in other fields, such as textile, pharmaceutical, automotive, etc.

Synthetic polymers include polyacrylamide (PAM) or partially hydrolyzed polyacrylamide (HPAM), polyoxyethylene, poly(styrenesulfonic acid), copolymers of acrylamide and acrylamidomethylpropane-isulfonate, or of acrylic acid, acrylamide and acrylamidomethylpropanesulfonate and N-vinylpyrrolidone or of acrylamide, acrylamidomethylpropanesulfonate and N vinylpyrrolidonea, etc.

The method of the invention comprises the following steps:

a) Providing a Size Exclusion Chromatography column connected to a Multi Angle Light Scattering detector and a Differential Refractive Index detector; b) Providing aqueous buffer solution as mobile phase of said Size Exclusion Chromatography column;

c) Injecting said solution of polymer into said mobile phase of said Size Exclusion Chromatography column;

d) Measuring the weight-average molecular weight of said polymer with the Multi Angle Laser Light Scattering detector and the Differential Refractive Index detector;

wherein the flow rate of said mobile phase is lower than 0.20 mL/min, in particular is from 0.05 mL/min to 0.20 mL/min (the value 0.20 mL/min being excluded), more particularly from 0.10 mL/min to 0.15 mL/min. The Size Exclusion Chromatography (SEC) column is able to separate the polymer molecules based on their size by filtration through a gel. The gel consists of spherical beads containing pores of a specific size distribution.

In one embodiment, the size of the spherical beads is from 13 μιη to 35 μηι. In one embodiment, the Size Exclusion Chromatography column is packed with a bed of spheres of polyhydroxymethacrylate of 35 μιη, with dimensions of 8.0 x 300 mm (maximum pore size 3 μπι).

In another embodiment, the method is performed with a set of two Size Exclusion Chromatography columns connected in series. The first column (considering the direction of flow) is packed with a bed of Spheres of polyhydroxymethacrylate of 13 μιη. The second column is packed with a bed of Spheres of polyhydroxymethacrylate of 35 μιη. Each column has a dimension of 8.0 x 300 mm (maximum pore size 3 μιη).

Furthermore, the SEC column is connected to a Multi Angle Light Scattering (MALS) detector in order to measure the weight- average molecular weight of the polymer. The MALS detector is provided with a data acquisition software for collection and treatment of the data.

For instance, a 8 x 300 mm SEC column OHpak SB-807 HQ (particle size: 35 μιη, pore size: max 3 μιη) marketed by SHODEX may be used.

The Size Exclusion Chromatography column is connected to a Differential Refractometer to determine the concentration needed to calculate the weight molecular mass. The mobile phase of the SEC column is an aqueous solution, e.g. a salt solution buffered at pH = 8.

The polymer sample is injected into the SEC column at a suitable concentration into the mobile phase, typically from 0.01 to 0.02 g/L (the concentration can be measured by Taylor's dosage).

In one embodiment, the mobile phase of the Size Exclusion Chromatography column contains a buffering agent in order to prevent precipitation of divalent cations that may be present in the polymer solution. The buffer is preferably chosen among buffers leading to a pH higher than 6, in particular a pH of 8. The buffering agent may be, disodium phosphate or 4-(2-hydroxyethyl)-l- piperazineethanesulfonic acid (HEPES), 2-amino-2-(hydroxymethyl)-l,3-propanediol (TRIS) or N,N-Bis(2-hydroxyethyl)glycine (BICINE).

HEPES is particularly advantageous for samples of PAM or HP AM containing a few amounts of calcium (Ca ²⁺ ). This buffer does not cause precipitation of phosphate calcium in the chromatographic system, contrary to a phosphate buffer.

Preferably, the salinity of the mobile phase is from 18 g/L to 26 g/L in order to lock the polymer into a coil conformation.

The method of the invention allows accurate determination of the weight-average molecular weight of a water-soluble high molecular weight polymer with minimum degradation of the polymer, in a fast and reproducible way. For instance, a complete analysis can be run in 400 min. The method of the invention also allows the determination of the distribution curve of the molar mass of a polymer, the polydispersity index (Ip), the concentration of polymer in a sample, as well as the average size of said polymer.

The method of the invention is particularly useful for EOR (Enhanced Oil Recovery). Indeed, the reported Mw of a commercial high molecular weight polymer is based on its intrinsic viscosity and can differ significantly from the real Mw of the polymer. The method of the invention is thus particularly useful for determining the real Mw of the polymer and in turn, adjusting the optimal operating conditions of polymer flooding in EOR. The method according to the invention is also particularly useful for monitoring the degradation of the polymer in polymer flooding by checking the Mw of a polymer in produced waters and comparing the value with the one of the injected polymer. Checking the Mw of a polymer in produced waters can be also used for adjusting the operating conditions of the water treatment of the produced waters.

Thanks to the method of the invention, a sample of injection water containing the polymer or a sample of produced water can be analyzed by direct injection of the sample into the mobile phase of the Size Exclusion Chromatography column.

The following examples provide another illustration of the invention but without restraining to the scope of the invention. Examples

Example 1 : Demonstration that below 0.20 mL/min, the degradation of HP AM does decrease until a level not detectable, as the flow rate decreases.

Polymer sample: HP AM.

The analytical set-up comprises in the direction of flow:

A filter (0.1 μιη) for retaining the impurities contained in the mobile phase ;

A pre-SEC column (packed with a bed of Spheres of polyhydroxymethacrylate of 35 μιη, the bed having a dimension of 8.0 x 300 mm) for retaining the impurities contained in the sample to be analyzed ;

- A first SEC column (packed with a bed of Spheres of polyhydroxymethacrylate of 13 μηι, the bed having a dimension of 8.0 x 300 mm) ;

A second SEC column (packed with a bed of Spheres of polyhydroxymethacrylate of 35 μιη, the bed having a dimension of 8.0 x 300 mm) ;

All SEC columns were thermostated to 30 °C ;

- a Multi Angle Light Scattering detector and a Differential Refractive Index detector.

MALS Detector: Wyatt DAWN HELEOS-II (cell FS,) RI detector Wyatt Optilab T-rEX

- Mobile phase: 0.1 M NaN0 ₃ - HEPES/HN0 ₃ pH 8 filtered through 0.1 μιη ;

Concentration of polymer in the mobile phase: 200 ppm ;

- Injection volume: 100 iL.

Experiments made without the SEC columns:

Polymer sample solutions were injected through the chromatographic system without any SEC- or pre-SEC columns (so as to evidence the degradation only caused by the analytical set-up between the injector and the detection cell of the Multi Angle Light Scattering detector) with increasing flow rates: 0.10 mL/min, 0.20 mL/min, 0.30 mL/min and 0.50 mL/min.

As can be seen from Figure 1, the intensity of the peak MALS strongly decreases with the flow rate of the polymer from a flow rate of 0.20 mL/min. This means that the molar mass of the sample decreases from flow rate of 0.20 mL/min and therefore that a degradation of the polymer in this apparatus occurs from a flow rate of 0.2 mL/min.

Therefore, whatever the columns used, the flow rate should be less than 0.2 mL/min in order to avoid mechanical degradation of a high molecular weight polymer.

Experiments made with the SEC columns: The same experiment was performed but with the SEC columns at various flow rates: 0.10 mL/min, 0.15 mL/min, 0.25 mL/min and 0.50 mL/min. Figure 2 (0.10 mL/min), Figure 3 (0.15 mL/min), Figure 4 (0.25 mL/min) and Figure 5 (0.50 mL/min) show the measured detectors' signals (full line) and molar mass (dotted line) as a function of the elution volume of the polymer. By comparing Figures 2, 3, 4 and 5 one to each other, it can be seen that the higher the flow rate, the higher the elution volume, thus the smaller the molecules. This confirms that the polymer is degraded in the apparatus, therefore also in the SEC columns. Therefore, this confirms that below a flow rate of mobile phase of 0.20 mL/min, the degradation of a high molecular weight polymer can still be lowered by decreasing the flow rate of the mobile phase. Flow rate below 0.10 mL/min were tested but are very difficult to interpret due to light scattering noise of actual detector. Flow rates around 0.10 mL/min allow a good measuring of average Mw, minimizing chromatographic degradation.

Example 2: Measurement of Mw according to the method of the invention

The weight-average molecular weight of commercial polymers was determined by using the analytical set-up described in Example 1.

The results are indicated in table 1 below. Figure 5 shows the variation of molar mass of Polyacrylamide (PAM) with a polymer flow rate of 0.10 mL/min.

Table 1

These results show that the reported Mw of a commercial high molecular weight polymer can differ significantly from the real Mw of the polymer.