Claims
- 1. A method of drilling a well below a body of water in which a drill bit is rotated at the end of a drill string, comprising:
(a) injecting into the well at a depth below the water surface a liquid having a lower density than a density of a drilling mud producing a mixture of drilling mud and low-density liquid in the well; (b) withdrawing the mixture of drilling mud and low-density liquid from an upper end of the well; (c) separating at least a portion of low-density liquid from the mixture of drilling mud and low-density liquid, thereby producing a drilling mud depleted of low-density liquid; (d) returning at least a portion of the separated low-density liquid to the depth below the water surface; and (e) returning at least a portion of the drilling mud depleted of low-density liquid to an upper end of the drill string.
- 2. The method of claim 1, wherein the low-density liquid is immiscible with the drilling mud.
- 3. The method of claim 2, wherein the drilling fluid is water-based and the low-density liquid is at least one of oil-based, synthetic and non-aqueous liquid.
- 4. The method of claim 2, wherein the low-density liquid comprises density-reducing particulate material.
- 5. The method of claim 1, wherein the low-density liquid is miscible with the drilling mud.
- 6. The method of claim 1, wherein separating comprises:
at least one of mechanical separation, gravity separation and centrifugal separation.
- 7. The method of claim 1, further comprising:
controlling a rate of the liquid injecting so that a bottom-hole pressure in the well is below a fracture pressure of an earth formation and above a pore pressure of the formation.
- 8. The method of claim 1, further comprising:
controlling a rate of the liquid injecting into the lower end of riser so the cuttings within the riser pipe have an upward velocity in excess of the settling rate of the cuttings in the riser pipe.
- 9. The method of claim 1, wherein the low-density liquid comprises density-reducing particulate material.
- 10. The method of claim 1, further comprising:
ceasing the injection of the low-density fluid into the well at a depth below the water surface to switch from dual-gradient drilling to conventional drilling.
- 11. The method of claim 1, wherein substantially all of the separated low-density liquid is returned to the depth below the water surface, and substantially all of the separated drilling mud is returned to the upper end of the drill string in a closed system.
- 12. The method of claim 1, wherein the depth below the water surface is between the drill string and the wellbore at a position below a wellhead.
- 13. The method of claim 1, wherein the depth below the water surface is at a lower end of a riser pipe that extends from a drilling vessel on the surface of the ocean downwardly to wellhead equipment on the sea floor.
- 14. The method of claim 1, wherein the low-density liquid is injected via a parasite string into an annular space between the drill string and a casing's inner wall at a position below a wellhead.
- 15. The method of claim 1, wherein the low-density liquid is injected into a lower end of a riser pipe that extends from a drilling vessel on the surface of the body of water downwardly to wellhead equipment on the floor of the body of water.
- 16. The method of claim 9, wherein the particulate material comprises low-density glass beads.
- 17. The method of claim 9, wherein the particulate material comprises low-density micro spheres.
- 18. A method of treating a drilling fluid used in drilling a wellbore in a earth formation below a body of water in which a drill string extends from a water-surface drilling facility into the wellbore and the drilling fluid passes through the drill string and flows from the drill string into the wellbore whereby cuttings resulting from the drilling becomes entrained in the drilling fluid and the drilling fluid with the entrained cuttings returns to the surface of the body of water by means of a return flow system, comprising
(a) injecting into the return flow system at a depth below the surface of the body of water a liquid having a density lower than a density of the drilling fluid, thereby producing in a return flow system a mixture of drilling fluid and a low-density liquid; (b) withdrawing the mixture of drilling fluid and low-density liquid from an upper end of the return flow system; (c) separating at least a portion of the low-density liquid from the mixture of drilling fluid and low-density liquid, thereby producing a drilling fluid depleted of low-density liquid; (d) returning at least a portion of the separated low-density liquid to the return flow system to the depth below the water surface; and (e) returning at least a portion of the drilling fluid depleted of low-density liquid to the drill string.
- 19. The method of claim 18 in which the return flow system comprises a first annular space between the drill string and the wall of the wellbore, and a second annular space between the drill string and the inner wall of a casing positioned in the wellbore, and a third annular space between the drill string and a riser extending between the cased wellbore and the surface of the body of water, wherein the return of the separated low-density liquid of step (d) is to the annular space at the lower end of the third annular space.
- 20. The method of claim 18 in which the return flow system comprises a first annular space between the drill string and the wall of the wellbore, a second annular space between the drill string and the inner wall of a casing positioned in the wellbore, and a third annular space between the drill string and a riser extending between the cased wellbore and the surface of the body of water, wherein the return of the separated low-density liquid of step (d) is to the second annular space.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority benefit from U.S. provisional application No. 60/271,304 filed on Feb. 23, 2001.
Provisional Applications (1)
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Number |
Date |
Country |
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60271304 |
Feb 2001 |
US |