This invention relates to methods of forming and re- entering multilateral wells in open hole, and to apparatus useful in such methods.

There has been a general trend in oilfield work towards the use of multilateral wells. The majority of multilateral wells are cased throughout. The use of open hole multilaterals is known and is advantageous in suitable formations for reasons of low cost, but the adoption of such techniques has been limited.

One reason for this is that it has been difficult to effect controlled re-entry. Re-entry can be desirable for example to insert a packer to close off a well producing water, or to permit workover

such as fracturing on a desired one of multiple laterals.

One object of the present invention is to provide an improved method of forming open hole multilateral wells in a manner which facilitates controlled re- entry.

The present invention, in one aspect, accordingly provides a method of installing a permanent non- obstructive and orientation reference in an open hole allowing drilling of a lateral and re-entry in said lateral or in the main borehole, said method comprising the following steps: positioning a locating assembly at a pre-determined section of the open hole, below the location of the desired lateral; and securing the positioning of the locating assembly in the open hole by pumping cement or injecting resin into the annular space comprised between the outside wall of said locating assembly and the open hole wall.

In particular, the locating assembly is positioned at a position immediately below the location of the desired lateral and comprises locating means providing a positive axial and radial location for engagement by a complementary tool, and the locating assembly being formed to provide a through passage having a predetermined drift, and the method of the invention further comprises the following steps: releasably attaching a whipstock assembly to the locating assembly;

the whipstock assembly comprising a whipstock mounted on a complementary tool engageable with said locating means; running a drilling assembly in hole to be engaged and deflected by the whipstock and further running the drilling assembly to form the lateral; pulling the drilling assembly out of hole; and removing the whipstock assembly.

The method may include, after releasably attaching the whipstock assembly, adjusting the azimuth of the whipstock to a desired direction; and for this purpose the whipstock assembly may include a lockable rotary joint between the whipstock and the complementary tool.

Preferably, the whipstock assembly is retained for subsequent use, with the orientation of the whipstock with respect to the complementary tool fixed.

The method of the present invention may comprise the further step of re-entering the main borehole, which may be done by running a tubular having an outer diameter less than said drift. Alternatively, before running the tubular a guide assembly may be positioned on said locating means, the guide assembly including a complementary tool and a length of casing sufficient to span the entry to the lateral.

The method may also comprise the further step of re- entering the lateral, which may be done by re- running the whipstock assembly on the locating assembly, and using the whipstock to guide an assembly into the lateral.

From another aspect, the invention provides a device for use in the foregoing method, comprising a locating assembly able to be positioned at a pre- determined position of an open hole, below the location of a desired lateral; and a tool allowing pumping of cement or injection of resin into the annular space comprised between the outside wall of said locating assembly and the open hole wall.

In particular, the locating assembly comprises securing means for securing the assembly in the open hole, and locating means providing a positive axial and radial location for engagement by a complementary tool, and the locating assembly being formed to provide a through passage having a predetermined drift.

For example, the securing means is an expandable casing packer secured on a length of casing defining said drift, and the locating means comprises a mule shoe.

Further, the present invention provides a whipstock assembly for use in the foregoing method, the whipstock assembly comprising a whipstock mounted on

a complementary tool engageable with said locating means.

For example, the complementary tool may suitably be a spring loaded key assembly.

The whipstock assembly, in one form, may include a lockable rotary joint between the key assembly and the whipstock.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 illustrates the drilling of an open hole using a drilling assembly; Figs. 2A to 2X illustrate a method of forming and re-entering multilateral wells in an open hole according to a first embodiment of the invention; Figs. 3A to 3C are detailed views of the complementary tool according to said first embodiment of the invention ; Figs. 4 to 13 are schematic side views of part of a multilateral well, showing successive stages in forming one sidetrack and in re-entry procedures according to a second embodiment of the invention; and Fig. 14 is a schematic cross-sectional side view of a spring loaded key device according to said second embodiment of the'invention.

Referring to Fig. 1, the procedure of the present invention starts with a pre-drilled borehole, which has been cased with a casing 1 and cemented at 2.

The borehole is extended by drilling with a drillstring 3 and bit 4 to form an open hole 5. The drilling assembly 6 is then withdrawn.

Figs. 2A to 2X illustrate a first embodiment providing a method of drilling a lateral well from a borehole according to the invention.

Referring to Fig. 2A a locating assembly 10, made of one or more joints of tubular, is shown held at the surface by slips 20, blocks or similar devices. The locating assembly 10 includes an orientation device 11 in the form of a mule shoe sleeve, which will provide depth and orientation control. It further includes, at its lower end, a float shoe 12 or a basket cementing shoe as the type B basket cementing shoe commercialised by the company Industrial Rubber Inc..

A cementing string 30 is shown in Figs. 1B, 1C and 1D. This cementing string 30 is made of joints connected together with tool joints 33,35. It includes, from its lower end, a terminal part 31, a centraliser 32, able to centralise the cementing string 30 within the locating assembly 10 and an inflatable packer 34 such as the inflatable packer named Single Set (TM) commercialised by the company TAM International North Sea Limited. According to the method of the invention, the cementing string 30

is positioned into the locating assembly 10. Once the terminal part 31 of the cementing string 30 is inserted into a receptacle 13 of the float shoe 12, the packer 34 is inflated as shown in Fig. 2D. The inflated packer 34 provides a holding force that will permit to use the cementing string 30 as a running tool of the locating assembly 10. In addition, the packer 34 also serves as a barrier avoiding contamination of the assembly 10 by cement or debris.

As shown in Figs. 2E, 2F and 2G, the slips 20 are taken out and the locating assembly 10, suspended by the cementing string 30, is lowered into a pre- determined section of the open hole 5 immediately below the location of the desired lateral, where they are rotated along the borehole axis to provide appropriate orientation of the locating assembly 10.

Referring now to Figs. 2H and 21, cement or resin is pumped through the cementing string 30 to the shoe 12 and displaced upward within the annular space located between the outside wall of the locating assembly 10 and the open hole 5 wall. The cement sets securing the positioning of the assembly. It is noted that, preferably, the above pre-determined section of the borehole is under-reamed prior to the positioning of the assembly, to improve the secured fixing of the assembly downhole.

Then, as illustrated in Figs. 13J, 13K and 13L, the packer 34 is deflated, the cementing string 30 is

detached from the assembly and withdrawn leaving the installation complete, suitably orientated and secured in the open hole 5.

Referring now to Fig. 2M, the whipstock assembly 40 is run into the wellbore 5, inside the locating assembly 10, using a starting mill 50. The whipstock assembly 40 is shown in the left side of Fig. 2M. It includes a whipstock 41 mounted on a complementary tool 42 such as a double-grip whipstock anchor commercialised by TIW (TM). The complementary tool 42 is detailed in Figs. 3A, 3B and 3C. It comprises an orientation reference 43, anchoring elements 44 and two mandrel members 45, 46. The lower end of the first mandrel member 45 is located inside the second mandrel member 46 and the two mandrel members 45,46 are maintained one relative to each other by pins 47. When running down the well (Fig. 3A), the anchoring elements 44 are in a retracted position. In a set position (Fig. 3B), the lower end of the first mandrel member 45 is pushed down into the first mandrel member 46, the pins 47 are sheared and the anchoring elements 44 expands externally. In a released position (Fig.

4C), the first mandrel member 45 is partially withdrawn out the second mandrel member 46 and the anchoring elements 44 are back in their retracted position.

As shown in Figs. 2N and 20, the whipstock assembly 40 is releasably attached to the locating assembly 10. To this effect, the complementary tool 42 seats

in the orienting sleeve 41 and the anchoring elements 44 expands externally to anchor the whipstock assembly 40 within the locating assembly 10.

Then, the whipstock pin 51 attaching the whipstock assembly 40 to the starting mill 50 is sheared (Fig.

2P), the starting mill is rotated in order to clean the formation wall (Fig. 2Q) and said starting mill 50 is withdrawn from the wellbore 5.

Referring now to Fig. 2R, a drilling assembly 60 is run in the borehole 518. The inclined wedge of the whipstock 41 forces the drill bit 61 to start drilling, through the formation, in the desired lateral direction away from the main borehole axis.

After drilling, the drilling assembly 60 is pulled out from the lateral 7 (Fig. 2S).

As shown in Figs. 2T, 2U and 2V, the whipstock assembly 40 is then released from the location assembly 10 and withdrawn from the borehole.

Therefore, according to the invention, it is possible to re-enter the main hole 5 after having milled the lower section of the cemented locating assembly 10 as shown in Figs. 2W and 2X.

Alternatively, it is possible to re-enter the lateral 7 after having releasably attached the whipstock assembly 40 and complementary tool 42 to deflect a bottom hole assembly into the lateral 7.

Figs. 4 to 14 illustrate a second embodiment providing a method of drilling a lateral well from a borehole according to the invention.

As shown in Fig. 4A, a locating device 120 in the form of a mule shoe 122 attached to a short length of casing 124 is run in hole. The casing section 124 is provided with an expandable casing packer (ECP) 126 on its outer surface. The assembly 120 is run in hole with a string 114 incorporating the usual equipment (not shown) required to set and inflate the packer.

When the mule shoe 122 is in the desired position (Fig. 4B), the ECP 126 is inflated to engage the open hole 5 (Fig. 4C), and the string 114 is then pulled out of hole. Correct positioning of the locating device 120 may be assisted by providing an abutment member 128 on the string 114 to engage the top of the open hole 5. The ECP 126 is preferably inflated with cement or resin material to provide a permanent installation.

The external and internal diameters of the ECP 126 are chosen so as to maximise the internal"drift" diameter of the assembly 120 while ensuring that the inflatable member will effectively grip and seal the annulus.

Turning to Fig. 5, the next step is to run in hole (Fig. 5A) with an assembly comprising a spring loaded key 130 attached to a whipstock 132. The

spring loaded key 130 (see also Fig. 14) incorporates a key 134 which finds the ramp 136 on the mule shoe and seats at the end of the ramp's travel, thus firmly locating the whipstock 132 at the required depth.

In some cases, the orientation of the whipstock is not important once it is set into the mule shoe; for example if the mother well is close to vertical, the required azimuth of the lateral to be drilled from the whipstock is easily achieved through directional drilling. In this situation the spring loaded key 130 and whipstock 132 can be permanently attached together.

In other cases it is desirable to orientate the azimuth of the whipstock 132 once it is seated with respect to the mule shoe 122. In this event, a downhole lockable swivel can be incorporated into the assembly, and the assembly orientated downhole using an MWD or gyro tool. Again, once the assembly and thus the whipstock are oriented in the desired azimuth, the key 130 and whipstock 132 must be locked together permanently.

In both cases the result is to establish a permanent orientation of the whipstock azimuth with respect to the mule shoe, which is utilised for re-entry operations as will be described below.

The running string 114 is now unlatched from the whipstock 132 and pulled out of hole (Fig. 5B).

It is now possible to drill the lateral or branch.

As seen in Fig. 6, a drilling assembly 114,116 is run into the well and the whipstock 132 is used to deflect the new hole 7 into the desired direction.

It should be noted that the diameter of the lateral can be the same as that of the mother well. Once the lateral has been drilled to the desired depth, the drilling assembly 114, 116 is removed from the hole, as seen in Fig. 7. The final stage is to run into hole with an assembly 140 designed to latch onto the whipstock 132 and remove the whipstock 132 and attached key 130 from the hole, as shown in Fig.

8.

The whipstock 132 and permanently attached key 130 are stored for future re-entry use.

At this stage, therefore, there is a mother well and lateral, with the mule shoe/ECP assembly 120 secured in the mother well immediately below the lateral.

The lateral can be the full diameter of the mother well, while the assembly 120 imposes a slight reduction in drift in the lower part of the mother well, but not such that the running in of tubing and other assemblies is prevented.

Fig. 9 illustrates re-entry to the mother well.

This is achieved by running in with a stiff assembly 142 having a smaller outside diameter than the drift of the mule shoe/ECP assembly 120. With no whipstock in the hole, the tubing assembly 142 will

follow the path of the mother well and pass through the assembly 120, and thus through to the end of the original borehole.

Alternatively, difficult circumstances where a positive re-entry is required can be dealt with as shown in Fig. 10. Here, a key assembly 144 similar to that employed with the whipstock is attached to a short length of casing 146 smaller than the diameter of the mother well and long enough to extend over the junction with the lateral. Once this has been landed into the assembly 120 the running string 114 can be unlatched and the tubing assembly 142 run into hole (Fig. 11), the tubing assembly 142 now being positively guided into the primary borehole.

To re-enter the upper lateral, the same key/whipstock assembly that was used during the drilling of the lateral is run into hole on a running string 150 (Fig. 12) and seated into the mule shoe/ECP assembly 120. The mule shoe ensures that the depth and azimuth of the whipstock are in alignment with the lateral. The running string 150 can then be unlatched and pulled out of hole, and the desired tubing 152 can be run into hole and is positively guided into the lateral by the whipstock (Fig. 13).

It will be noted that the present method gives the ability to drill a sidetrack in open hole without losing hole diameter in the new well. It is therefore possible to drill a further sidetrack from

the lateral and retain the same re-entry features.

This can be repeated as desired to achieve a multi- branch well.

Modifications may be made to the foregoing embodiment within the scope of the present invention.

For example, the orienting assembly can be constructed to allow the installation of a plug or choke so that the primary borehole can be isolated or choked back. It is also possible to incorporate an inflatable packer in the whipstock assembly to stabilise the whipstock during drilling, this packer being deflated and removed along with the whipstock.