Viscosifier for oil well fluids

Abstract

The present invention comprises a viscosifter for oil well fluids, said viscosifier comprising a cross-linked micro- or nano-fibrillated cellulose (MFC).

Claims

1. A viscosifier for oil well fluids, said viscosifier comprising a cross-linked Micro Fibrillar Cellulose (MFC) microfibers or Micro Fibrillar Cellulose (MFC) nanofibers, wherein the MCF is cationic, anionic or non-ionic and selected from the group consisting of TEMPO mediated MFC, Enzymatic assisted MFC, mechanically produced MFC and Carboxymethylated MFC; the MFC has a length in the range of 1-100 m and an average diameter in the range of 5-100 nm; and wherein the cross-linking is a chemical cross-linking, the cross-linking agent is selected from the group consisting of formaldehyde, difunctional aldehydes, dichloroacetic acid, polyepoxides, urea, sodium trimetaphosphate, sodium tripolyphosphate, epichlorophydrin, phosphoryl chloride, glyoxal (OCHCHO), and ammonium zirconium (IV) carbonate.

2. The viscosifier according to claim 1, wherein the difunctional aldehydes is glutaraldehyde.

3. The viscosifier according to claim 1, wherein the MFC is in form of an aqueous dispersion.

4. The viscosifier according to claim 1, wherein the MFC is in form of a non-aqueous dispersion.

5. An oil-well fluid comprising a dispersion including a viscosifier according to claim 1.

6. The oil-well fluid according to claim 5, wherein the dispersion is an aqueous dispersion.

7. The oil-well fluid according to claim 6, wherein the viscosifier is present in an amount 1-50 g/l, or in an amount of 1-30 g/l, or in an amount 5-15 g/l.

8. The oil-well fluid according to claim 5, wherein the oil-well fluid additionally comprises a proppant and the concentration of the cross-linked MFC in the fluid is from 0.1-2.5 wt %.

9. The oil-well fluid according to claim 8, wherein the proppant is sand or a ceramic material.

10. An oil-well cementing slurry comprising a dispersion including a viscosifier according to claim 1.

11. The oil-well cementing slurry according to claim 10, wherein the viscosifier is present in an amount 1-50 g/l, or in an amount of 1-30 g/l, or in an amount 5-15 g/l.

Description

(1) The invention will be described below with reference to the accompanying figures in which:

(2) FIG. 1 shows the viscosity of CM-MFC fiber solution at a shear rate of 10 s.sup.1 with different types and concentrations of Tyzor crosslinker (see example 1.1);

(3) FIG. 2 shows the shear viscosity of CM-MFC fiber solution as function of shear rate (see example 1.1);

(4) FIG. 3 shows the shear viscosity of ME-MFC fiber solution as function of shear rate (see example 1.2);

(5) FIG. 4 shows the viscosity of ME-MFC fiber solution at a shear rate of 10 s.sup.1 with different types and concentrations of Tyzor crosslinker (see example 1.2);

(6) FIG. 5 shows the dynamic rheology of ME-MFC with Tyzor 212 before and after heat aging for 3 h at 150 C. (see example 3);

(7) FIG. 6 shows the dynamic rheology of MFC measured at 20 C. and a pH of 9 for three different fibers at the same conditions of pH, temperature and ionic concentration (see example 3);

(8) FIG. 7 shows the shear viscosity of TEMPO-MFC aqueous solution as function of shear rate with and without BPDA (see example 4):

(9) FIG. 8 shows the dynamic rheology of TEMPO-MFC measured at 20 C. (see example 5);

(10) FIG. 9 shows the shear rheology of TEMPO-MFC solution treated with cellulose enzyme as function of shear rate at different time interval measured at 50 C. (see example 6.1):

(11) FIG. 10 shows the effect of adding an enzyme and/or a co-enzyme on the degradation of TEMPO-MFC (see example 6.1); and

(12) FIG. 11 shows the effect of enzyme and co-enzyme on the degradation of EN-MFC (see example 6.2).

(13) The MFC materials used in the examples below were produced in the laboratory as described in the literature as follows.

(14) TEMPO mediated MFC (TEMPO-MFC) was produced according to the publication of Saito et al. (Saito, T. Nishiyama, Y. Putaux. J. L. Vignon M. and Isogai. A. (2006). Biomacromolecules, 7(6): 1687-1691).

(15) Enzymatic assisted MFC (EN-MFC) was produced according to the publication of Henriksson et al. European polymer journal (2007), 43: 3434-3441 (An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers) and M. Pkk et al. Biomacromolecules, 2007, 8 (6), pp 1934-1941. Enzymatic Hydrolysis Combined with Mechanical Shearing and High-Pressure Homogenization for Nanoscale Cellulose Fibrils and Strong Gels.

(16) Mechanically produced MFC (ME-MFC) was produced as described by Turbak A. et al. (1983) Microfibrillated cellulose: a new cellulose product: properties, uses, and commercial potential. J Appl Polym Sci Appl Polym Symp 37:815-827. ME-MFC can also be produced by one of the following methods: homogenization, microfluidization, microgrinding, and cryocrushing. Further information about these methods can be found in paper of Spence et al. in Cellulose (2011) 18:1097-1111, A comparative study of energy consumption and physical properties of microfibrillated cellulose produced by different processing methods.

(17) Carboxymethylated MFC (CM-MFC) was produced according to the method set out in The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes Wgberg L, Decher G, Norgen M, Lindstrm T, Ankerfors M, Axns K Langmuir (2008) 24(3), 784-795.

(18) The rheological properties of the various fibrillated cellulose with crosslinking agents were investigated in the laboratory in fresh water, sea water, and in brine solutions at different pH level, and at different temperatures from room temperature up to 175 C.

(19) The equipment used to measure the various properties included a mass balance, a constant speed mixer up to 12000 rpm, a pH meter, a Fann 35 rheometer, a Physica Rheometer MCR-Anton Paar with Couette geometry CC27, and a heat aging oven (up to 260 C. at pressure of 100-1000 psi).

(20) As mentioned above microfibrillated cellulose can be produced with one of the following methods and the resulting MFC can have slightly different properties.

(21) Mechanically produced MFC: just mechanical shearing is used for the defibrillation. The surface charge of the fibril is quite small and similar to the original fiber.

(22) Chemically assisted process; chemicals such as TEMPO are used to lower the energy consumption and make the defibrillation easier when compared to the pure mechanical method. Such chemical treatments introduce a negative charge on the surface of the fibril which in turn might affect the crosslinking reaction.

(23) Enzymatic assisted process; enzymes such as cellulase are used to shorten the length of the fiber and make it easier to defibrillate. The surface charge is similar to the original fiber but might change slightly.

(24) A combination of some or all of the above methods is also possible and may be beneficial in certain circumstances. Also crosslinked MFC can be used in viscosifying oil well fluids solely or combined with any commercially available viscosifiers such as guar gum, modified guar gum, starch and starch derivatives, cellulose and cellulose derivatives, xanthen gum, synthetic copolymers such as polyacrylamide and its derivatives, acrylates and its derivatives, viscoelastic surfactant or any clay minerals such as bentonite, sepiolite or attapulgite.

(25) The concentration of a well-defibrillated MFC aqueous dispersion is normally below 50 g/l due to the high viscosity of the dispersion. In the examples below dispersions with concentrations of 10-30 g/l were diluted with distilled water and mixed in a Warring blender before adding the crosslinking agent. The pH of the dispersion was adjusted sometime before or after the addition of the crosslinking agent. The viscosity of the dispersion with and without crosslinker was measured at room and elevated temperatures. In some examples a salt such as potassium chloride (KCl) was added to the dispersions since it can be a main component in the fracturing fluid to minimize the shale hydration.

EXAMPLE 1: TYZOR PRODUCTS AS A CROSSLINKER

(26) Organometallic zirconium-complexes such as Tyzor products have been used in fracturing fluid to crosslink guar gum. The crosslinking reaction depends on many parameters such as the type and the concentration of the polymer, the type and the concentration of the crosslinking agent, particularly the ligands attached to the metal ion, temperature, pH and the ionic strength. Also the ratio of ligands to metal was observed to have a significant impact on the crosslinker efficacy. The following examples illustrate the use of Tyzor 212 and Tyzor 215 to crosslink various types of MFC.

EXAMPLE 1.1: CROSSLINKING REACTIONS OF CM-MFC WITH TYZOR-212 AND TYZOR-215

(27) To a 100 ml solution of CM-MFC with the concentration of 0.4 wt % the following amounts of Tyzor 212 and 215 are added.

(28) Test 1: Tyzor-212 (0.1%) at 93.3 C. (200 F.), pH=8.75

(29) Test 2: Tyzor-212 (0.1%) at 149 C. (300 F.), pH=8.75

(30) Test 3: Tyzor-215 (0.2%) at 23.9 C. (75 CF), pH=9.07

(31) Test 4: Tyzor-215 (0.1%) at 149 C. (300 F.), pH=8.82

(32) Test 5: Tyzor-215 (0.2%) at 149 C. (300 F.), pH=9.07

(33) Test 6: Tyzor-215 (0.3%) at 149 C. (300 F.), pH=9.26

(34) Sample preparation: A solution of CM-MFC with a concentration of 0.4 wt % was prepared. The crosslinking agent, Tyzor-212 (0.1 ml) or Tyzor-215 (0.1 ml, 0.2 ml or 0.3 ml) was then added to 100 ml CM-MFC solution. The pH was in the range of 8.75 to 9.30. The mixture was mixed in Warring blender for 2 min at speed of 2000 rpm.

(35) The mixture was loaded into HPHT heat aging cell, and heated to 93.3 C. (200 F.) or 149 C. (300 F.). After 3 hours, the sample was cooled down, and the viscosity vs. shear rate was recorded at room temperature.

(36) The increase of the viscosity of the fiber fluid with the addition of Tyzor-212 or Tyzor 215 at elevated temperatures as shown in FIGS. 1 and 2 indicates a crosslinking reaction took place at elevated temperature. Such enhancement of viscosity was more significant at 149 C. (300 F.) than 93.3 C. (200 F.).

(37) FIG. 1 shows the viscosity of CM-MFC fiber solution at a shear rate of 10 s.sup.1 with different types and concentrations of Tyzor crosslinker and at different aging temperatures. Referring to FIG. 2, the shear viscosity of CM-MFC fiber solution as function of shear rate is shown. The open diamond is the CM-MFC solution without any heat aging, the open circle is the data for test 2 after heat aging and open triangle is the data for test 4 after heat aging.

EXAMPLE 1.2: CROSSLINKING REACTIONS OF ME-MFC WITH TYZOR-212 AND TYZOR-215

(38) A substantially similar effect of crosslinking was observed with ME-MFC as shown in FIGS. 3 and 4 as was obtained with CM-MFC with Tyzor-212 and Tyzor-215 as shown in FIGS. 1 and 2. The viscosity at a low shear rate range was doubled with the crosslink compare to the viscosity of the fiber solution without crosslinking agent. At similar concentration of fiber and crosslinking agent the viscosity of CM-MFC is significantly higher compare to the viscosity of ME-MFC.

(39) A solution of ME-MFC with a concentration of 0.4 wt % was used in the following tests

(40) Test 7: Tyzor-212 (0.2%) at 149 C. (300 F.), heat aged for 3 hours

(41) Test 8: Tyzor-215 (0.2%) at 25 C. (77 F.), heat aged for 3 hours

(42) Test 9: Tyzor-215 (0.2%) at 149 C. (300 F.), heat aged for 1 hour

(43) Test 10: Tyzor-215 (0.2%) at 149 C. (300 F.), heat aged for 3 hours

(44) The pH for test 7-10 was adjusted between 9.4 and 9.6.

(45) Referring to FIG. 3, the shear viscosity of ME-MFC fiber solution as function of shear rate is shown. The open diamond is the ME-MFC solution without any heat aging, the open circle is the data for test 7 after heat aging and open triangle is the data for test 10 after heat aging. FIG. 4 shows the viscosity of ME-MFC fiber solution at a shear rate of 10 s.sup.1 with different types and concentrations of Tyzor crosslinker.

EXAMPLE 2: CROSSLINKING REACTIONS OF DIFFERENT FORMS OF MFC WITH TYZOR-212 AND TYZOR-215 IN PRESENCE OF KCL SALT

(46) This example shows the effect of KCl salt as an additive used in fracturing fluid. Before crosslinking it was observed that the addition of KCl salt reduces the viscosity of most of the cellulose fibers. This can be related to adsorption of the K ions on the negative sites on the surface of the fiber and slightly disrupting the fibers interaction. This was proved by the zeta potential measurement in the presence and absence of KCl where the zeta potential was dimensioned in presence of KCl. The effect of KCl in reducing the viscosity was much less for the non-charged MFC such as EN- and ME-MFC compared to the charged MFC such as CM- and TEMPO-MFC.

(47) Table 1, below, shows the composition of MFC dispersions containing 2 wt % KCl salt and 0.2 wt % Tyzor 212 as a crosslinking agent.

(48) TABLE-US-00001 TABLE 1 Tyzor Distilled Mixing in Fiber KCl 212 water warring blender Material g g g g speed 2000 rpm ME-MFC 1.6 8 0.8 400 3 min EN-MFC 1.6 8 0.8 400 3 min CM-MFC 1.6 8 0.8 400 3 min TEMPO-MFC 1.6 8 0.8 400 3 min

(49) Table 2, below, lists the viscosity measurements of MFC dispersions before (BHA) and after (AHA) heat aging. The heat aging was for 3 hours at 150C.

(50) Crosslinking of various types of MFC was also tested in sea water and a similar trend was observed as non-modified MFC showed significant increase in viscosity after heat aging whereas for modified MFC the increase was insignificant. The increase in viscosity for non-modified MFC indicates that the crosslinking reaction took place in sea water.

(51) TABLE-US-00002 TABLE 2 Shear viscosity in mPa .Math. s Temperature shear rate of shear rate of Material C. pH 20 s.sup.1 100 s.sup.1 ME-MFC BHA 8.9 162 60 AHA at 150 C. 8.9 300 98 EN-MFC BHA 9.2 62 23 AHA at 150 C. 8.8 106 36 CM-MFC BHA 9.2 9 5 AHA at 150 C. 9.3 62 22 TEMPO-MFC BHA 8.9 80 26 AHA at 150 C. 8.8 258 87

(52) As shown in Table 2 the viscosity after heat aging was increased to a value which is double the value before the heat aging or even more, indicating a crosslinking reaction.

EXAMPLE 3: DYNAMIC RHEOLOGY

(53) Most MFC type materials exhibit some viscoelastic properties even at a very low concentration of 0.1 wt %. This is related to the fiber entanglement, hydrogen bonding and other electrostatic interactions. The use of crosslinkers in many cases leads to an enhancement of the strength of the internal structural network. Dynamic or oscillatory rheology is a known method to study the viscoelastic properties of materials in suspension, emulsion, solution or gel forms.

(54) As demonstrated in this example, both the moduli G and G were increased after crosslinking of ME-MFC using Tyzor 212 at 150 C. In a dynamic viscosity measurement the elastic (storage) modulus (G) is the ability of the material to store energy and the viscous (loss) modulus (G) is the ability of material to dissipate energy. It is clear to see in the FIG. 5 that the linear viscoelastic region (LVR) of the crosslinked material is longer than the one without crosslinking which means that with crosslinking the network is stronger than without crosslinking. Also the magnitude of the G and the yield point for the crosslinked MFC dispersion is larger than the G of the non-crosslinked MFC.

(55) Referring to FIG. 5, this shows the dynamic rheology of ME-MFC with Tyzor 212 before and after heat aging for 3 h at 150 C. Rheology was measured at 20 C. and the pH was 8.9. The line shown by the empty circle (o) represents G before heat aging and the line shown by the empty diamond () is for G before heat aging. The line shown by the solid circle (.circle-solid.) is for G after heat aging and the line shown by the solid diamond (.diamond-solid.) is for G after heat aging.

(56) Referring now to FIG. 6, this shows the dynamic rheology of MFC measured at 20 C. and a pH of 9 for three different fibers at the same conditions of pH, temperature and ionic concentration. In this case, the line shown by the empty circle (o) represents G and solid circle (.circle-solid.) is for G for ME-MFC. The line shown by the empty diamond () is for G and solid diamond (.diamond-solid.) is for G for EN-MFC. The line shown by the empty triangle () is for G and solid triangle (.box-tangle-solidup.) is for G for TEMPO-MFC.

(57) The curve in FIG. 6 shows that the internal network of the different fibers at similar solid concentration exhibit different rheological properties such as the storage modulus (G) and the yield point.

EXAMPLE 4: USE OF 4,4-BIPHENYLDIBORONIC ACID ((BPDA) FROM SIGMA-ALDRICH AS CROSSLINKING AGENT

(58) ##STR00001##

(59) BPDA as shown in structure above is diboroic acid and can be used as a crosslinker. The advantage of this molecule compared to boric acid is its large volume size which makes it possible to crosslink dilute polymer solution.

(60) Solution 1 comprises 1.6 g TEMPO-MFC+66 g water+0.92 g KCl.

(61) Solution 2 comprises 1.6 g TEMPO-MFC+66 g water+0.92 g KCl+0.05 g BPDA.

(62) The viscosity of both of the solutions is measured at 40 C. and at pH 9.7.

(63) BPDA was used to crosslink the TEMPO-MFC in Solution 2. Referring to FIG. 7, the solid triangle curve is the solution 1 and the solid circle curve is solution 2. As can be seen in the figure, the viscosity of TEMPO-MFC at a shear rate of 1 s.sup.1 was increased from 205 mPa.Math.s to 1095 mPa.Math.s going from solution 1 to solution 2. Such an increase in viscosity of 500% is a clear evidence for the crosslinking reaction. At the high shear region around a shear rate of 100-1000 s.sup.1 the increase in viscosity was around 30%.

EXAMPLE 5: CHEMICAL CROSSLINKING WITH GLUTARALDEHYDE

(64) In this example glutaraldehyde is used to demonstrate the chemical crosslinkability of different types of MFC material that can be used in some oil field applications such as enhanced oil recovery (EOR) or water shut-off. The pH was adjusted to 4.5 using HCl solution. After mixing glutaraldehyde with the chosen MFC solution and adjusting the pH the fluid was subjected to heat for 1 hour at 150 C. The fluid was then cooled down to room temperature and slightly homogenized and loaded into the rheometer for viscosity measurement. The different fluid compositions are given in Table 3 and the viscosity results are shown in Table 4. The big increase in viscosity was observed with TEMPO-MFC followed by ME-MFC. CM-MFC showed the least change and higher glutaraldehyde can be tried.

(65) TABLE-US-00003 TABLE 3 glutaraldehyde Distilled Mixing in Fiber solution (50%) water warring blender Material g g g speed 2000 rpm ME-MFC 0.8 0.5 200 3 min CM-MFC 0.8 0.5 200 3 min TEMPO-MFC 0.8 0.5 200 3 min

(66) TABLE-US-00004 TABLE 4 Shear viscosity in mPa .Math. s Temperature shear rate of shear rate of Material C. pH 20 s.sup.1 100 s.sup.1 ME-MFC BHA 4.2 19 9 AHA at 150 C. 4.0 62 21 CM-MFC BHA 4.4 218 74 AHA at 150 C. 4.4 183 64 TEMPO-MFC BHA 4.6 173 72 AHA at 150 C. 3.5 1126 132

(67) Referring to FIG. 8, this shows the dynamic rheology of TEMPO-MFC measured at 20 C. The line shown by the empty diamond () is for G and the solid diamond (.diamond-solid.) is for G for TEMPO-MFC with glutaraldehyde before heat aging and the line shown by the empty circle (o) represent G and the solid circle (.circle-solid.) is for G for TEMPO-MFC with glutaraldehyde after heat aging. FIG. 8 shows a big increase in the storage modulus and shear stress at the network break down after crosslinking at high temperature. Such an increase indicates that the internal network is strengthened by the crosslinking and one expects that such hydrogel will be more thermally stable than non-crosslinked MFC.

EXAMPLE 6: ENZYMATIC DEGRADATION

(68) In certain applications, such as drilling or fracturing, the viscosifier should be removed after the treatment since such gellants tend to impair the productivity of oil or gas. The gellant is normally removed by chemical or physical means. Enzymatic degradation is a known technique to remove bio-degradable polymers such as starch, guar gum, and cellulose.

(69) Simple tests were conducted on TEMPO-MFC and EN-MFC using Novozyme 188 and Celluclast 1.5 L from Novozyme North America, USA.

(70) The shear rheology of MFC solutions treated with cellulase enzyme as a function of shear rate at different time interval were measured at 50 C.

EXAMPLE 6.1: TEMPO-MFC WITH ENZYME

(71) The following mixture was formulated and heated at 50 C. overnight. Samples were taken at different intervals.

(72) TEMPO-MFC (0.32 g MFC+80 g water+0.085 g Celluclast 1.5 L+0.085 g Novozyme 188 mix with magnetic stirrer for 5 min), pH was adjusted to 5 using IM HCl

(73) Table 5 shows the viscosity of the TEMPO-MFC with enzyme as function of time

(74) TABLE-US-00005 TABLE 5 Time Viscosity (hour) Shear rate 20 s.sup.1 Shear rate 100 s.sup.1 Oscillation 0 267.1 182.4 Structure 1 83.4 38.8 No structure 3 43.5 17.5 No structure 6 31.4 13.5 No structure 26 17.4 8.7 No structure

(75) FIG. 9 shows the shear rheology of TEMPO-MFC solution treated with cellulose enzyme as function of shear rate at different time interval measured at 50 C.

(76) FIG. 10 shows the effect of adding an enzyme and/or a co-enzyme on the degradation of TEMPO-MFC. Looking at the lines at 3 hours, the top line is for TEMPO-MFC without any enzyme. The second line down is for TEMPO-MFC with Novozyme alone. The third line down is for TEMPO-MFC with Celluclast 1.5 L alone and the lowest line is for TEMPO-MFC with both Celluclast and Novozyme 188.

EXAMPLE 6.2: EN-MFC WITH ENZYME

(77) Similar to the previous test the following mixture was prepared and tested.

(78) EN-MFC (0.56 g MFC+80 g water+0.17 g Celluclast 1.5 L+0.17 g Novozyme 188 mix with magnetic stirrer for 5 min), pH was adjusted to 5 using IN HCl.

(79) Table 6 shows the viscosity of the EN-MFC with enzyme as function of time

(80) TABLE-US-00006 TABLE 6 Time Viscosity (hour) Shear rate 20 s.sup.1 Shear rate 100 s.sup.1 Oscillation 0 87.9 38.0 Structure 1 15.8 6.7 Structure 3 2.9 1.9 No structure 6 1.1 1.1 No structure 26 0.76 0.91 No structure

(81) FIG. 11 shows the effect of enzyme and co-enzyme on the degradation of EN-MFC. Looking at the lines at 3 hours, the top line is for EN-MRC without any enzyme. The second line down is for EN-MFC with Novozyme 188 alone. The third line down is for EN-MFC with Celluclast 1.5 L alone and the lowest line is for EN-MFC with both Celluclast 1.5 L and Novozyme 188.

(82) Both Tables 5 and 6 and FIGS. 9 to 11 show that different forms of MFC can be easily removed by enzyme within a time of less than 3 hours. Also other chemical treatment such as acid hydrolysis or chlorate oxidation can be used.

(83) The results from the various examples show that different forms of MFC which are crosslinked with a range of different materials exhibit good shear thinning properties. It forms a network structure that can suspend the solid particles such as drilling cuts or weighting agents in case of drilling fluid or proppant in case of fracturing. It also showed good thermal stability and high tolerance to the salt concentrations which may be encountered. The addition of crosslinking agent increased the viscosity and enhanced the strength of the structural network.

(84) The examples show that MFC can be used as a viscosifier for drilling, stimulation and enhanced oil recovery applications particularly in a high temperature environment. A range of different MFC types can be physically and chemically crosslinked. The crosslinking enhances the viscosity of the MFC fluid which can be used in various oilfield applications such as stimulation (fracturing), drilling, water shut-off, and enhanced oil recovery (EOR). Such crosslinked MFC, either physical or chemical, can also be used to prevent or minimize the loss of drilling fluids or cementing slurry into weak formations that have a high permeability. Such application is known by the industry as loss circulation materials (LCM). LCM provides certain seal or plugging to such high permeability formations.

(85) Optionally, physically crosslinked MFC with a Zr complex (such as a Tyzor product, for example) can be used for stimulation and drilling. Alternatively, chemically crosslinked MFC can be used in water shut-off and EOR. The use of crosslinked MFC will reduce the overall cost, reduce the formation damage, and enhance the thermal stability of the product. It has also been shown that while KCl salt (which is often present) has a negative impact on the viscosity of some crosslinked and non-crosslinked MFC's it does not prevent the crosslinking and the crosslinked product still has a higher viscosity than the non-crosslinked MFC.