Method and apparatus for varying the density of drilling fluids in deep water oil drilling applications

• US 20020108782A1
• Filed: 02/15/2001
• Published: 08/15/2002
• Est. Priority Date: 02/15/2001
• Status: Active Grant

• Current Assignee: Dual Gradient Systems, LLC
• Original Assignee: Luc D.E. Boer
• Inventors: Boer, Luc de
• Granted Patent: US 6,536,540 B2
• Time in Patent Office: Days
• US Class Current: 175/7
• CPC Class Codes: E21B 21/001 specially adapted for underwater drilling dual gradient drilling E21B21/082
• CPC Class Codes: E21B 21/08 Controlling or monitoring pressure or flow of drilling fluid, e.g. automatic filling of boreholes, automatic control of bottom pressure valve arrangements therefor E21B21/10
• CPC Class Codes: E21B 21/085 Underbalanced techniques, i.e. where borehole fluid pressure is below formation pressure

Abstract

A method and apparatus for controlling drilling mud density at or near the seabed of wells in deep water and ultra deep-water applications combines a base fluid of lesser density than the mud required at the wellhead to produce a diluted mud in the riser. By combining the appropriate quantities of drilling mud with base fluid, a riser mud density at or near the density of seawater may be achieved. No additional hardware is required below the surface. The riser charging lines are used to inject the low-density base fluid at or near the BOP stack on the seabed. The cuttings are brought to the surface with the diluted mud and separated in the usual manner. The diluted mud is then passed through a centrifuge system to separate the heavier drilling mud from the lighter base fluid.

Claims

1. A method for varying the density of the fluid in a subsea riser of a drilling system comprising the steps of:

a. Introducing a first fluid a first density into a drill tube, such fluid being released from the drill tube and into the riser;

b. Introducing a second fluid having a second density into the riser for producing a combination fluid having a density that is defined by a predetermined ratio of the first fluid and the second fluid, such fluid rising to the surface; and

c. Separating the first fluid from the second fluid at the surface.

2. The method of claim 1, wherein there is a riser charging line associated with the subsea riser and the second fluid is introduced into the riser via the riser charging line.

3. The method of claim 1, wherein there is a blowout preventer system associated with the drilling system through which the drill tube and riser pass, the blowout preventer system being positioned at the seabed and wherein the second fluid is introduced into the riser above the blowout preventer system.

4. The method of claim 1, wherein the second density is lower than the first density.

5. The method of claim 1, wherein the second density is lower than the density of seawater.

6. The method of claim 1, wherein the second density is lower than 8.6 PPG.

7. The method of claim 6, wherein the second density is 6.5 PPG.

8. The method of claim 4, wherein the second density is lower than the density of seawater and the first density is higher than the density of seawater.

9. The method of claim 8, wherein the second density is less than 8.6 PPG and the first density is greater than 8.6 PPG.

10. The method of claim 9, wherein the second density is 6.5 PPG and the first density is 12.5 PPG.

11. The method of claim 1, wherein the first fluid is introduced into the drill tube at a first flow rate and the second fluid is introduced into the riser at a second flow rate.

12. The method of claim 12, wherein the first flow rate is slower than the second flow rate.

13. The method of claim 12, wherein the density of the combination fluid is determined by the combined densities of the first fluid and the second fluid and the first and second flow rates.

14. The method of claim 13, wherein the density of the combination fluid is defined by the formula:

Mr=(F_Mi×

Mi)+(F_Mb×

Mb)/(F_Mi+F_Mb),

where;

F_Mi=flow rate F_i of the first fluid,

F_Mb=flow rate F_b of the second fluid,

Mi=first density,

Mb=second density, and

Mr=density of combination fluid.

15. The method of claim 14, wherein:

Mi=12.5 PPG,

Mb=6.5 PPG,

F_Mi=800 gpm, and

F_Mb=1500 gpm,

16. The method of claim 15, wherein the flow rate F_r of the combination fluid is the combined flow rate F_i of the first fluid and F_b of the second fluid, specifically F_r=F_i+F_b.

18. The apparatus of claim 17, wherein said first density is greater than said second density.

19. The apparatus of claim 17, further including a separator at the platform for separating the drilling fluid from the additional fluid.

20. The apparatus of claim 19, wherein the separator comprises a centrifuge system.

21. The apparatus of claim 17, further including a pump for pumping the drill fluid into the drill tube at a first rate of flow and a pump for pumping the additional fluid into the charging lines at a second rate of flow.

22. The apparatus of claim 21, wherein the first rate of flow is slower than the second rate of flow.

17. An apparatus for generating a riser fluid in a subsea well having a density different from the drilling fluid, comprising:

a. A drilling platform;

b. A wellhead on the seabed;

c. A drilling string connecting the platform to the wellhead and including a drillstem, drill tube and casing for defining the riser;

d. Charging lines associated with the riser;

e. A source of drilling fluid of a first density for introducing said drilling fluid into the drill tube; and

f. A source of additional fluid of a second density for introducing said additional fluid into the charging lines, whereby the first fluid and the second fluid are combined in the riser.