GB2385341A

GB2385341A - Side window cutting using a drill collar

Info

Publication number: GB2385341A
Application number: GB0203641A
Authority: GB
Inventors: Paul Anthony Mcclure; Stephen Webster
Original assignee: Target Well Control Ltd
Current assignee: Target Well Control Ltd
Priority date: 2002-02-15
Filing date: 2002-02-15
Publication date: 2003-08-20
Anticipated expiration: 2022-02-15
Also published as: AU2003207319A1; GB0203641D0; GB2385341B; WO2003069114A1; CA2476459A1; US20060016603A1; EP1474589A1

Abstract

A method for drilling out from a casing string (3) involves the steps of introducing a reaming assembly into the borehole and fixing the assembly in position. A drillable collar (7) is attached between the reaming assembly and the casing (3). A drilling assembly is then introduced into the casing (3) and positioned so as to drill through the drillable collar (7) at a predetermined angle into the formation surrounding the borehole. The drillable collar (7) may be made from aluminium.

Description

1 CASING REAMING ASSEMBLY

3 The present invention relates to a method for 4 drilling out of a casing string which has been run 5 and fixed in place with the use of a sacrificial 6 motor assembly.

8 It is known to drill out sections of a casing string 9 in order to drill side-tracked holes that run off 10 the casing string. Typically, this is achieved 11 using a reaming assembly connected directly to the 12 bottom of a casing. These components are then run 13 to the bottom of a borehole and cemented in place.

15 In order to create a hole running off the main 16 borehole, a whipstock, which is a cylindrical 17 triangular metal device, is positioned at a point 18 near the bottom of the casing along with a milling 19 assembly. The whipstock is oriented with a giro 20 device to the desired direction of drilling and

1 fixed in place. The milling assembly is then 2 mechanically detached from the whipstock and 3 activated so as to mill a section of the casing to 4 create a window at and around the whipstock. After 5 this has been done the milling assembly is pulled 6 out of the casing. Depending on the condition of 7 the window created by the milling assembly, a second 8 run of the milling assembly may be required. After 9 this, a steerable drilling assembly is run inside 10 the casing and on passing over the whipstock is 11 guided to point in a set direction, thereafter, 12 drilling is initiated and continues to the 13 prescribed target.

15 Typically the distance from the surface to the 16 borehole is around 10,000 to 11,000 feet, this means 17 that it takes around 24 hours to complete one round 18 trip to and from the surface for any of these 19 components. Clearly, any reduction in the number of - if. t; 20 round trips required would produce significant i 21 savings of rig time and therefore cost.

23 In accordance with a first aspect of the present 24 invention there is provided a method for drilling 25 out from a casing string in a borehole, the method 26 comprising the steps of: 27 introducing a reaming assembly into the borehole to 28 a predetermined location; fixing the reaming 29 assembly in position, wherein a drillable collar is 30 attached between the reaming assembly and a casing; 31 introducing a drilling assembly into the casing; and 32 positioning the drilling assembly so as to drill

1 through the drillable collar at a predetermined 2 angle into the formation surrounding the borehole.

4 Preferably, the point at which the drilling assembly 5 enters the formation surrounding the borehole is at 6 least in part determined by the thickness of the 7 drillable collar through which the drilling assembly 8 drills.

10 Preferably, the drilling assembly is a bent housing 11 motor assembly with a PDC drill bit.

13 Preferably, the reaming assembly is fixed in place 14 using cement.

16 In accordance with a second aspect of the invention À 17 there is provided an apparatus for drilling out from 18 a casing string in a borehole, the apparatus 19 comprising a drillable collar attached between a .. 20 reaming assembly and a casing string.

22 Preferably, the drillable collar is provided with a 23 borehole extending from the reaming assembly to the 24 casing string.

26 Preferably, the drillable collar is made from 27 aluminium.

29 Aluminium is preferred because it is drillable and 30 sufficiently robust to retain its integrity in the 31 conditions found in boreholes. Other materials with

1 similar properties may be used in place of 2 aluminium.

4 The present invention will now be described by way 5 of example only with reference to the accompanying 6 drawings of which: 7 Fig. 1 shows a casing reaming assembly in accordance 8 with the present invention; 9 Fig. 2 is a cross sectional view of a drillable 10 collar; and ll Fig. 3 shows the effect of a change in width of the 12 collar.

14 Referring to Fig. 1, the assembly 1 is shown with 15 respect to the base of an unstable subsea formation 16 15. The assembly 1 comprises a casing 3 which in 17 this example is of 9 5/3 inch diameter connected to 18 a float collar 5 which is in turn connected to a 19 drillable collar 7. The drillable collar 7 is 20 connected to a reaming assembly 10 which consists of 21 a mud motor 9 having, on its end, a 12% inch bit 11.

23 Figure 1 also shows a side-tracked hole 13, which is 24 created by the use of a drilling assembly (not 25 shown) capable of drilling through the drillable 26 collar 7. In this example a borehole is drilled to 27 a casing point to provide a 121K inch diameter hole.

28 This drilling assembly is then removed from the 29 borehole. Thereafter a reaming assembly also known 30 as a sacrificial reaming assembly is constructed.

31 This consists of a reaming assembly as shown having 32 a mud motor with a 12% inch drill bit connected to

1 its end. This reaming assembly is connected to the 2 drillable collar 7. In this example, as shown in 3 Fig.2, the drillable collar is made from aluminium 4 and has an outer diameter of 9 inch and an inner 5 diameter of 3 inches. Therefore the annular 6 thickness of the drill collar is 31K inches.

8 The collar 7 is connected to a float collar 5, which 9 is in turn connected to the casing 3. The reaming 10 assembly 10 and casing 3 are then run into the bore 11 hole until the bottom of the casing 3 and the drill 12 collar 7 are set in an appropriate formation.

14 Thereafter cement is added in order to complete the 15 casing and reaming assembly in place and to land the 16 cement plug in the float collar. In this example, 17 the collar is 40 feet in length.

19 In order to create a side tracked hole a steerable . 20 drilling assembly capable of fitting inside the ... 21 casing is run into the hole. In this example the 22 drill assembly consists of a 1.5 degree bent housing 23 steerable motor assembly having a 63 inch outside 24 diameter and a PDC drill bit having an 8 inch 25 diameter. This drilling implement is controllable 26 from the surface in a known manner. This steerable 27 drill assembly is used to drill through the drill 28 collar and is aligned remotely so as to drill in a 29 predetermined direction towards a target position.

31 The use of a drillable collar as described above 32 allows the passage of drilling fluid from the casing

1 string to the reaming motor which provides 2 hydraulic and mechanical power to and from the motor 3 for reaming operations.

5 In addition, the bore 8 contained in the collar 6 provides a passage to allow the addition of cement 7 in order to cement the casing string in place.

8 Furthermore, the collar 7 serves as a hard plug to 9 enable the steerable motor assembly of the second 10 drill to deviate in the direction of the next well 11 path into new formation.

13 Whilst the above example shows an aluminium collar, 14 it will be appreciated that a drill collar may be 15 made of any other material that is drillable and 16 mechanically robust enough to perform the functions 17 of the collar described in the conditions typically 18 found in a bore hole.

j 20 Referring now to figure 2, this shows the variation 21 in exit point of the drill that may be achieved 22 simply by varying the annual thickness of the collar 23 7. 25 The sideways displacement can be calculated by using 26 the following equation.

28 Sideways displacement = hole diameter + collar diameter 29 2 30 Therefore, if the collar diameter alone is 31 increased, the sideways displacement will be 32 increased.

1 Flqure 2 shows the exit point from the drillable 2 collar where an 8 inch collar 7 is used in 3 comparison to the exit point where a 9 inch collar 4 is used. This clearly shows that the exit point for 5 the 9k inch collar is displaced.

7 Whilst the above examples show a borehole of 12 If 8 inches with a drillable collar and casing sized 9 appropriately. It will be appreciated that the 10 present invention is not restricted to boreholes and 11 casings of these dimensions and that the above is an 12 illustrative example of the general concept set out 13 in the statements of invention and claims.

15 The present invention contains a number of 16 advantages with respect to the prior art. In

17 particular: it reduces the number of round trips 18 from the surface required, therefore reducing the 19 amount of rig time required; a whipstock is not . ; 20 required, these cost around ú100000 to buy and take 21 a significant time to orient; and the cost and time 22 of milling a window is removed from the process.

<'-I 24 Improvements and modifications may be incorporated - - 25 herein without deviating from the scope of the 26 invention.

Claims

1 CLAIMS

3 1. A method for drilling out from a casing string 4 in a borehole, the method comprising the steps of: 5 introducing a reaming assembly into the borehole to 6 a predetermined location; fixing the reaming 7 assembly in position, wherein a drillable collar is 8 attached between the reaming assembly and a casing; 9 introducing a drilling assembly into the casing; and 10 positioning the drilling assembly so as to drill 11 through the drillable collar at a predetermined 12 angle into the formation surrounding the borehole.

14

2. A method as claimed in claim 1 wherein the 15 point at which the drilling assembly enters the 16 formation surrounding the borehole is at least in 17 part determined by the thickness of the drillable 18 collar through which the drilling assembly drills.

20

3. A method as claimed in claim 1 or claim 2 21 wherein the drilling assembly is a bent housing 22 motor assembly with a PDC drill bit.

24

4. A method as claimed in claim 1 wherein the 25 reaming assembly is fixed in place using cement.

27

5. Apparatus for drilling out from a casing string 28 in a borehole, the apparatus comprising a drillable 29 collar attached between a reaming assembly and a 30 casing string.

1

6. Apparatus as claimed in claim 5 wherein the 2 drillable collar is provided with a borehole 3 extending from the reaming assembly to the casing 4 string.

6

7. Apparatus as claimed in claim 5 or claim 6 7 wherein the drillable collar is made from aluminium.