GB2297702A

GB2297702A - A method of recovering drilling mud

Info

Publication number: GB2297702A
Application number: GB9605071A
Authority: GB
Inventors: David Wood; Tom Johnston
Original assignee: Mud Rentals Ltd
Current assignee: Mud Rentals Ltd
Priority date: 1995-09-21
Filing date: 1996-03-11
Publication date: 1996-08-14
Anticipated expiration: 2016-03-11
Also published as: NO963959D0; NO312655B1; GB2297702B; GB9519248D0; GB9605071D0; NO963959L

Abstract

A method of recovering drilling mud from a mixture of drilling mud and drill cuttings is described. The mixture is applied to the inner surface of an annular filter screen (5) which is then rotated such that the drilling mud is able to pass through apertures in the annular filter screen (5) but the drill cuttings, being too large, cannot pass through the apertures. The screen may be conical or frusto-conical and a linear motion may additionally be applied to the screen.

Description

"A Method of Recovering Drilling Mud" The present invention relates to a method of mud recovery particularly but not exclusively to the recovery of oil based muds adhered to the surface of drilling cuttings.

Oil based muds are used to enhance drilling performance, increase drilling rates, extend drill bit life and stabilise reactive formations whilst drilling for hydrocarbons. The oil based muds bring the drilling cuttings produced by the drilling action to the surface of the well.

The drilling cuttings are generally discarded into the sea. However, the drill cuttings are normally coated with oil based muds which are detrimental to the environment. The drill cuttings may have increased in weight by up to 25%, due to the adherence of oil based mud to them. Also, disposal of these oil coated drill cuttings is restricted by government regulations and further, particularly in the case of the new synthetic oil based muds which are typically expensive, the financial cost of replacing the lost drilling mud is also an important issue.

Conventionally, oil based muds are separated from drill cuttings in two stages, by using primary separation equipment and then secondary separation.

The primary separation equipment is based on linear motion or rotary motion shale shakers which may operate with a separation force per unit mass of up to 8g, with the standard equipment operating with a separation force per unit mass of between 4g and 5g, where "g" is acceleration due to gravity.

The secondary equipment, such as hydrocyclones and decanting centrifuges are used to further treat the oil based mud after the primary separation equipment. The secondary equipment is used to enhance the recovery and disposal of the finer solid particles still present in the mud after the primary separation stage.

Other existing methods for the partial removal of oil based mud from drill cuttings include washing the cuttings with base oil or seawater and surfactant followed by drying the cuttings again using vibrating screens or decanting centrifuges. Typically, these systems are able to reduce the level of oil retained to between 0.07 kg and 0.1 kg of oil per kilogram of oil based mud coated drilling cuttings.

All these systems can be classed as high attrition systems in that they produce a relatively high degree of fine drill cuttings at the end of the process when compared to the start of the process. The disadvantage of this outcome is that the fine drill cuttings will collectively have a larger surface area which reduces the efficiency of the process. Further, the fine drill cuttings produced in this way are not as biodegradable as a whole drill cutting.

Other systems are available where the drilling cuttings are ground down and slurrified and then reinjected into the well.

According to the present invention there is provided a method of recovering drilling mud from a mixture of drilling mud and drill cuttings the method comprising applying the mixture to the inner surface of an annular filter screen, rotating the annular filter screen, the annular screen having a plurality of apertures, the apertures being of a size such that the drilling mud can pass through the apertures but the drill cuttings are substantially prevented from passing through the apertures.

Preferably, the annular screen is formed in a conical or frusto-conical shape. Typically, the mixture is applied to one end of the conical annular screen and most preferably the end with the smaller diameter.

Typically, a linear motion is further applied to the annular screen. Typically the linear motion is applied substantially along the longitudinal axis of rotation of the annular screen.

Preferably, the drill cuttings exit out of the other end of the conical annular screen, the other end having the largest diameter.

Typically, the angle that the conical annular screen extends from the longitudinal axis of rotation is between 100 and 1100.

Preferably, the force per unit mass produced by the rotation of the conical annular screen ranges from 10g to 200g.

Preferably the force per unit mass produced by the linear motion is up to 5g and typically the amplitude of the linear motion is up to 10mm.

Typically, the filter screen is constructed from a wedge wire screen, but alternatively may be constructed from any other suitable filter media such as woven steel wire mesh or woven plastic mesh or perforated plate.

Preferably, the drilling mud is an oil based drilling mud.

Typically the oil based drilling mud and drill cuttings are gravity fed onto the conical annular screen.

Alternatively, the oil based drilling mud and drilling cuttings may be mechanically fed onto the conical annular screen by, for instance, a mechanical screw feed system or a pump.

An example of a method of recovering drilling mud in accordance with the invention will now be described, with reference to the accompanying drawing, in which: Fig. 1 is a side sectional view of a vibrating centrifuge for use with the method of the present invention.

Fig. 1 shows a vibrating centrifuge 1 for use with the present invention, consisting of an outer body 3, a conical screen 5 having a small radius end 6 and a large radius end 8, a drive shaft 7 for rotating the conical screen 5 and a feed tube 9.

The conical screen 5 is rotated by the drive shaft 7, with a centrifugal force acting on the conical screen 5 of between 10g and 200g.

A linear motion is applied along the longitudinal axis of the drive shaft 7 with a force per unit mass of up to 5g and an amplitude of up to 10mm. As the conical screen 5 is directly coupled to the drive shaft 7, this linear motion is imparted onto the conical screen 5.

The angle of the conical screen 5 is critical to the efficiency of the process and can range from 10 degrees to 110 degrees depending on the efficiency required.

A mixture of drilling cuttings and oil based drilling mud is conveyed into the input port 11, falls down the feed tube 9 and is guided onto the small radius end 6 of the conical screen 5 by a feed tube guide 13.

The vibrating centrifuge 1 separates the drilling mud from the drilling cuttings by the combination of the centrifugal force supplied by the rotating conical screen 5, the linear motion imparted on the conical screen 5 and the angle of the conical screen 5.

As the mixture of drilling mud and drilling cuttings are conveyed onto the rotating conical screen 5, the centrifugal force forces the drilling mud to migrate through apertures in the conical screen 5. However, the apertures are of a size such that the drilling cuttings are too large to migrate through the apertures in the conical screen 5, and hence are retained on the inside surface 15 of the conical screen 5.

The linear motion, which is produced by the drive assembly of the vibrating centrifuge 1, conveys the retained drilling cuttings towards the large radius end 8 of the conical screen 5. Because of the conical form of the screen 5, as the drilling cuttings are conveyed towards the large radius end 8 of the conical screen 5, the force per unit mass acting on the drilling cuttings increases and so further removing any remaining residual oil based drilling mud from the drilling cuttings.

The recovered drilling mud flows off the outside surface 17 of the conical screen 5 and exits the outer body 3 through recovered mud exit pipe 19. After the drilling cuttings have been conveyed along the length of the conical screen 5 and passed through the large radius end 8, the drilling cuttings exit the outer body 3 through dry drilling cutting exit ports 21, 23.

The level of oil retained on the drilling cuttings after the cuttings have been ejected from the vibration centrifuge is reduced to between 0.015 kg and 0.04 kg of oil per kilogram of drilling cuttings.

The oil based mud recovered is suitable for processing directly by the secondary equipment after which it can be returned directly into the active mud system to be re-circulated.

Claims

1. A method of recovering drilling mud from a mixture of drilling mud and drill cuttings, the method comprising applying the mixture to the inner surface of an annular filter screen, rotating the annular filter screen, the annular screen having a plurality of apertures, the apertures being of a size such that the drilling mud can pass through the apertures but the drill cuttings are substantially prevented from passing through the apertures.

2. A method according to claim 1 wherein, the annular screen is formed in a conical shape.

3. A method according to claim 1 or claim 2 wherein, the annular screen is formed in a frusto-conical shape.

4. A method according to either claim 2 or claim 3 wherein, the mixture is applied to the end of the conical annular screen with the smaller diameter.

5. A method according to claim 4 wherein, the drill cuttings exit out of the other end of the conical annular screen, the other end having the largest diameter.

6. A method according to any of the preceding claims wherein, a linear motion is further applied to the annular screen.

7. A method according to claim 6 wherein, the linear motion is applied substantially along the longitudinal axis of rotation of the annular screen.

8. A method according to any of claims 2 to 7 wherein, the angle that the conical annular screen extends from the longitudinal axis of rotation is between 100 and 1100.

9. A method according to any of claims 2 to 8 wherein, the force per unit mass produced by the rotation of the conical annular screen ranges from 10g to 200g.

10. A method according to any of claims 6 to 9 wherein, the force per unit mass produced by the linear motion is up to 5g.

11. A method according to any of claims 6 to 10 wherein, the amplitude of the linear motion is up to 10mm.

12. A method according to any of the preceding claims wherein, the filter screen is constructed from a wedge wire screen.

13. A method according to any of claims 2 to 12 wherein, the drilling mud and drill cuttings are gravity fed onto the conical annular screen.

14. A method according to any of claims 2 to 12 wherein, the drilling mud and drilling cuttings are mechanically fed onto the conical annular screen.

15. A method according to any of the preceding claims wherein, the drilling mud is an oil based drilling mud.

16. A method substantially as hereinbefore described with reference to the accompanying drawings.