Modified cellulose nanocrystals and their use in drilling fluids

1. A drilling fluid comprising an aqueous suspension of clay particles, sodium hydroxide for pH control, lignite as a deflocculant, polyanionic cellulose as a solid control agent, a finely-ground calcium montmorillonite clay/silica mixture as a viscosifier, and a composition comprising 0.01% to 5% of said drilling fluid by mass; wherein:

(a) said composition comprises nanocellulose covalently bonded to first and second polymers;

(b) said nanocellulose comprises cellulose particles 1 μm or less in length, and 100 nm or less in diameter;

(c) said first polymer is negatively charged at pH 7.5-10; and

(d) said second polymer is positively charged at pH 7.5-10.

2. The drilling fluid of claim 1 , wherein at pH 7.5-10:

(a) the clay particles have both positively-charged surfaces and negatively-charged surfaces:

(b) said first polymer will tend to associate with the positively-charged surfaces of the clay particles;

(c) said second polymer will tend to associate with the negatively-charged surfaces of the clay particles; and

(d) the clay particles in suspension with said composition will aggregate with one another less than identical clay particles would otherwise aggregate with one another under otherwise identical conditions in an otherwise identical aqueous suspension, except that the otherwise identical suspension lacks said composition.

3. The drilling fluid of claim 1 , wherein said cellulose particles are selected from the group consisting of unmodified nanocellulose, carboxylated nanocellulose, and cationic nanocellulose.

4. The drilling fluid of claim 1 , wherein said cellulose particles comprise carboxylated cellulose nanocrystals.

5. The drilling fluid of claim 1 , wherein:

said first polymer is selected from the group consisting of 2-Acrylamido-2-methyl-1-propanesulfonic acid, poly(sodium 4-styrenesulfonate, acrylic acid), N,N-dimethyl acrylamide, poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid), poly(acrylamide-co-sodium 4-styrenesulfonate), poly(acrylamide-co-acrylic acid-co-2-acrylamido-2-methyl-1-propanesulfonic acid); poly(acrylamide-co-acrylic acid-co-sodium 4-styrenesulfonate); poly(acrylamide-co-N,N-dimethyl acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid); and poly(acrylamide-co-N,N-dimethyl acrylamide-co-sodium 4-styrenesulfonate); and

said second polymer is selected from the group consisting of acrylamide, dimethyl diallyl ammonium chloride, (3-(methacryloylamino) propyl) trimethyl ammonium chloride, (2-(methacryloyloxy) ethyl) trimethyl ammonium chloride, vinylbenzyl trimethyl ammonium chloride, vinyl-3-ethylimidazolium bromide, N-vinylpyridine, and N-vinylpyrrolidone.

6. The drilling fluid of claim 5 ; wherein the ratio of said first polymer to said cellulose is between 10% and 80% by mass; and wherein the ratio of said second polymer to said cellulose is between 10% and 80% by mass.

7. The drilling fluid of claim 1 , wherein said composition is evenly distributed throughout said drilling fluid; wherein said cellulose particles associate with both the negatively-charged surfaces of the clay particles, and with the positively-charged surfaces of said clay particles via one or more of the following mechanisms: van der Waals forces, hydrogen bonding, and electrostatic attraction; and wherein at least one of the following properties of the drilling fluid is improved in a subterranean formation or a porous soil bed, as compared to the properties of an otherwise identical drilling fluid under otherwise identical conditions, except that the otherwise identical drilling fluid lacks said composition: salt resistance, viscosity, yield stress, stability, and fluid loss.

8. The drilling fluid of claim 1 , wherein said clay particles comprise one or more clays selected from the group consisting of kaolinite, illite, chlorite, vermiculite, smectite, sodium bentonite, calcium bentonite, and attapulgite.

9. The drilling fluid of claim 1 , wherein said clay particles comprise sodium bentonite.