Stable emulsion drilling fluids

1. A method of extending a wellbore through a subterranean formation, the method comprising:

providing a stable emulsion drilling fluid comprising:

an aqueous liquid;

a biopolymer;

an emulsifier;

solid particulates; and

an oil;

wherein a continuous phase of the stable emulsion drilling fluid comprises the aqueous liquid and a dispersed phase of the stable emulsion drilling fluid comprises the oil, and wherein the stable emulsion drilling fluid is capable of remaining in quiescent storage at approximately 100° F. and atmospheric pressure with less than about 5% by volume phase separation for about 24 hours or longer;

circulating the stable emulsion drilling fluid to a drilling rig;

monitoring a density of the stable emulsion drilling fluid;

determining an additional volume of oil to add to the stable emulsion drilling fluid such that a target density of the stable emulsion drilling fluid is reached, wherein the target density is about 1% to about 30% lower than the density of the stable emulsion drilling fluid;

adding the additional volume of oil to the stable emulsion drilling fluid to produce a reduced density stable emulsion drilling fluid; and

circulating the reduced density stable emulsion drilling fluid though a drill string and a bottom hole assembly.

2. The method of claim 1 wherein the aqueous liquid comprises a saturated brine.

3. The method of claim 1 wherein the biopolymer is selected from the group consisting of xanthan gum, welan gum, guar gum, hydroxyethyl cellulose, hydroxy-propyl guar, carboxy-methyl-hydroxy-propyl guar, starches and modified polysaccharides, partially-hydrolyzed polyacrylamide, diutan gum, scleroglucan, carboxy-methyl-cellulose, polyanionic cellulose, and combinations thereof.

4. The method of claim 1 wherein the emulsifier is selected from the group consisting of fatty amines, ethoxylated nonylphenols, fatty acids, fatty acid esters, and combinations thereof.

5. The method of claim 1 wherein the solid particulates comprise a solid selected from the group consisting of sepiolite, attapulgite, calcium bentonite, sodium bentonite, calcium montmorillonite, organoclays, and combinations thereof and wherein the solid particles have a mean particle size in a point ranging from 1 micron to 200 microns.

6. The method of claim 1 wherein the oil is selected from the group consisting of alkanes, alkenes, alkynes, aromatics, a tall oil, crude oil, light cycle oil, synthetic ester oils, diesel, and combinations thereof.

7. The method of claim 1 wherein the target density is about 1% to about 30% lower than the density of the stable emulsion drilling fluid.

8. The method of claim 1 wherein the wellbore is extended through a salt dome, and wherein the aqueous liquid comprises a saturated brine, wherein the biopolymer is present in an amount ranging from about 0.5 wt. % to about 0.75 wt. % based on a total weight of the stable emulsion drilling fluid, wherein the surfactant comprises a fatty ester, wherein the surfactant is present in an amount ranging from about 1 wt. % to about 5 wt. % based on a total weight of the stable emulsion drilling fluid, wherein the solid particulates comprise sepiolite, wherein the sepiolite is present in an amount ranging from about 5 wt. % to about 10 wt. % based on a total weight of the stable emulsion drilling fluid, and wherein the oil is present in an amount ranging from about 1 wt. % to about 40 wt. % based on a total weight of the stable emulsion drilling fluid.

9. A method of extending a wellbore through a subterranean formation, the method comprising:

providing a stable emulsion drilling fluid comprising:

an aqueous liquid;

a biopolymer;

an emulsifier;

solid particulates; and

an oil;

wherein a continuous phase of the stable emulsion drilling fluid comprises the aqueous liquid and a dispersed phase of the stable emulsion drilling fluid comprises the oil, and wherein the stable emulsion drilling fluid is capable of remaining in quiescent storage at approximately 100° F. and atmospheric pressure with less than about 5% by volume phase separation for about 24 hours or longer;

circulating the stable emulsion drilling fluid though a drill string and a bottom hole assembly;

monitoring a density of the stable emulsion drilling fluid;

determining an additional volume of oil to add to the stable emulsion drilling fluid such that a target density of the stable emulsion drilling fluid is reached;

adding the additional volume of oil to the stable emulsion drilling fluid to produce a reduced density stable emulsion drilling fluid; and

circulating the reduced density stable emulsion drilling fluid through the drill string and the bottom hole assembly.

10. The method of claim 9 wherein the aqueous liquid comprises a saturated brine.

11. The method of claim 9 wherein the biopolymer comprises xanthan gum.

12. The method of claim 9 wherein the emulsifier comprises a fatty acid.

13. The method of claim 12 wherein the emulsifier has a Griffin's HLB (hydrophilic-lipophilic balance) of about 9 or greater.

14. The method of claim 9 wherein the solid particulates comprise sepiolite.

15. The method of claim 14 wherein the solid particulates have a mean particle size from 100 micron to 200 microns.

16. The method of claim 9 wherein the oil comprises diesel.

17. The method of claim 9 wherein the target density is about 8 lb/gal to about 12 lb/gal.

18. The method of claim 9 wherein the wellbore is extended through a salt dome, and wherein the aqueous liquid comprises a saturated brine, wherein the biopolymer is present in an amount ranging from about 0.5 wt. % to about 0.75 wt. % based on a total weight of the stable emulsion drilling fluid, wherein the surfactant comprises a fatty ester, wherein the surfactant is present in an amount ranging from about 1 wt. % to about 5 wt. % based on a total weight of the stable emulsion drilling fluid, wherein the solid particulates comprise sepiolite, wherein the sepiolite is present in an amount ranging from about 5 wt. % to about 10 wt. % based on a total weight of the stable emulsion drilling fluid, and wherein the oil is present in an amount ranging from about 1 wt. % to about 40 wt. % based on a total weight of the stable emulsion drilling fluid.

19. The method of claim 1 wherein the oil comprises diesel.