Low fluid loss drilling fluid compositions comprising diblock copolymers
The invention relates to a selectively hydrogenated isoprene-styrene diblock copolymer having a polystyrene content of at least 40 wt. %, which when added to an oil-based mud composition, substantially reduces fluid loss. The polystyrene in the diblock has a true molecular weight greater than 60 kg/mole and the total apparent molecular weight of the diblock copolymer is at least 160 kg/mole. The drilling mud that contains about 0.5 pounds per barrel to about 10 pounds per barrel of a selectively hydrogenated isoprene-styrene diblock copolymer and a weighting agent, resulting in a drilling fluid composition exhibiting good suspension of the weighting agent and improved fluid loss performance.
1. A drilling fluid comprising an oil-based mud and a selectively hydrogenated isoprene-styrene diblock copolymer having at least a 40 wt. % polystyrene content and having an apparent total molecular weight of at least 160 kg/mole, wherein the fluid loss property of the drilling fluid is reduced.
2. The drilling fluid composition according to claim 1 in which the selectively hydrogenated isoprene-styrene diblock is added at a concentration of 0.5-10 pounds per barrel (ppb).
3. The drilling fluid composition according to claim 1 in which the hydrogenated isoprene-styrene diblock is added at a concentration of 1-6 pounds per barrel (ppb).
4. The drilling fluid composition according to claim 1 in which the hydrogenated isoprene-styrene diblock is added at a concentration of 2-4 pounds per barrel (ppb).
5. The drilling fluid composition according to claim 1 in which the hydrogenated isoprene-styrene diblock has a polystyrene content of at least 40 wt. % and the polystyrene block true molecular weight is at least 60 kg/mole.
6. The drilling fluid composition according to claim 1 , wherein said oil-based mud includes an oil selected from synthetic low toxicity oil, mineral oil, diesel oil, an internal-olefin oil, or an alpha-olefinic oil.
7. The drilling fluid composition according to claim 6 , further including a weighting agent.
8. The drilling fluid composition according to claim 7 also comprising an organophillic clay.
9. The drilling fluid composition according to claim 8 in which the concentration of the organophilic clay is in the range of 3-4 ppb.
10. The drilling fluid composition of claim 6 , wherein said oil-based mud further includes brine, lime, and emulsifier.
11. A selectively hydrogenated isoprene-styrene diblock copolymer having at least a 40 wt. % polystyrene content and having an apparent total molecular weight of at least 160 kg/mol.
12. The diblock copolymer of claim 11 wherein the true molecular weight of said polystyrene is at least 60 kg/mole.
13. The diblock copolymer of claim 12 wherein said true molecular weight of said polystyrene is 60 to 140 kg/mole.
14. The diblock copolymer of claim 13 wherein said true molecular weight of said polystyrene is 60 to 110 kg/mole.
15. The diblock copolymer of claim 14 wherein said true molecular weight of said polystyrene is 65 to 105 kg/mole.
16. The diblock copolymer of claim 11 wherein said apparent total molecular weight is 160 to 360 kg/mole.
17. The diblock copolymer of claim 16 wherein said apparent total molecular weight is 160 to 340 kg/mole.
18. The diblock copolymer of claim 17 wherein said apparent total molecular weight is 200 to 320 kg/mole.
19. The diblock copolymer of claim 11 which has been selectively hydrogenated wherein at least 90 mol % of the isoprene double bonds have been hydrogenated and less than 10 mol % of the styrene double bonds have been hydrogenated.