Patent Description:
It is increasingly common when drilling wells into hydrocarbon reservoirs for the well to extend laterally within the reservoir and for one or more additional wells to be drilled, branching off from the main well and extending laterally within the reservoir.

A current method, suitable for subsea wells, is to drill a relatively large diameter borehole through the overburden rock above the reservoir, and then case this bore. Drill string with a drilling BHA (bottom hole assembly) is then passed down through the casing and a narrower, but still quite large (e.g. <NUM> or 9½") bore is then drilled into the reservoir. A reasonably large diameter liner (e.g. <NUM> or 8⅝") is then run into the reservoir bore and cemented in place.

A drilling BHA is then passed down again and a smaller diameter bore drilled at the end of the lined bore in the reservoir ("lateral A"). A relatively narrow (e.g. <NUM> or <NUM>") production liner fitted with completion equipment and a liner hanger is then run into the narrower bore. The hanger is set in the <NUM> (<NUM>%") liner. The production liner is then completed; in a chalk/limestone reservoir this would be by passing acid through perforations in the production liner.

A valve is then closed to seal off the production liner before a hollow whipstock is run into the <NUM> (<NUM>%") liner to provide support for milling a side window in the <NUM> (<NUM>%") liner. A milling tool is run into the well and a side window milled away, then a drilling BHA is run in and a second narrow bore ("lateral B") drilled into the reservoir through the window. A narrow (e.g. <NUM> or <NUM>") production liner is then run into this second narrow reservoir bore, and the production liner completed as before. The relatively large diameter of the <NUM> (<NUM>%") liner makes it relatively simple to create lateral wells with <NUM> (<NUM>") liner.

The valve in lateral A is opened, normally being dissolved acid during the completion procedure in lateral B, and the well is then produced from both laterals.

The above procedure involved a considerable number of trips into and out of the well, each of which takes up expensive drilling rig time. Any way of reducing the number of trips is desirable.

In certain wells (e.g. the applicant's wells in the North Sea), the existing casing in the overburden is of relatively narrow diameter, e.g. <NUM> (8⅝"). If working with an existing well to drill multilaterals, this can present difficulties. Even if a new well is drilled with the intention to create multilaterals, the use of narrower casing in the overburden may have operational and cost advantages and may therefore be preferred. Also, generally, there is a desire to minimize the number of trips in a drilling operation, to save costs.

These considerations give rise to a need for an improved process for drilling multilateral wells.

<CIT> describes a technique for drilling into unstable formations comprising running in a ljner with a pre-installed whipstock and, adjacent the whipstick, a drillable region of liner wall; a drill string is then run into the liner and a window drilled in the wall of the liner through the drillable region.

Examples and various features and advantageous details thereof are explained more fully with reference to the exemplary, and therefore non-limiting, examples illustrated in the accompanying drawings and detailed in the following description. Descriptions of known starting materials and processes can be omitted so as not to unnecessarily obscure the disclosure in detail. It should be understood, however, that the detailed description and the specific examples, while indicating the preferred examples, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, additions and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure.

For example, a process, product, article, or apparatus that comprises a list of elements is not necessarily limited only those elements but can include other elements not expressly listed or inherent to such process, process, article, or apparatus.

The term substantially, as used herein, is defined to be essentially conforming to the particular dimension, shape or other word that substantially modifies, such that the component need not be exact. For example, substantially cylindrical means that the object resembles a cylinder, but can have one or more deviations from a true cylinder.

Additionally, any examples or illustrations given herein are not to be regarded in any way as restrictions on, limits to, or express definitions of, any term or terms with which they are utilized. Instead these examples or illustrations are to be regarded as being described with respect to one particular example and as illustrative only. Those of ordinary skill in the art will appreciate that any term or terms with which these examples or illustrations are utilized encompass other examples as well as implementations and adaptations thereof which can or cannot be given therewith or elsewhere in the specification and all such examples are intended to be included within the scope of that term or terms. Language designating such non-limiting examples and illustrations includes, but is not limited to: "for example," "for instance," "e.g.," "In some examples," and the like.

Although the terms first, second, etc. can be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present inventive concept.

While preferred examples of the present inventive concept have been shown and described herein, it will be obvious to those skilled in the art that such examples are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the disclosure. It should be understood that various alternatives to the examples of the disclosure described herein can be employed in practicing the disclosure. It is intended that the following claims define the scope of the disclosure and that methods and structures within the scope of these claims and their equivalents be covered thereby.

In order to aid understanding of the invention a technique according to the prior art is first described, with reference to <FIG>. A solution to drilling multilateral wells exists, and is proven, by drilling a <NUM> (9½") x <NUM> (11¼") bore <NUM> such that it extends laterally into the hydrocarbon-bearing reservoir <NUM> (represented in <FIG> as the region beneath the broken line <NUM>) and setting an <NUM> (8⅝") expandable liner <NUM> in the bore. The <NUM> (8⅝") liner <NUM> is hung from a liner hanger <NUM> set in large diameter casing <NUM> in the overburden rock <NUM> (the region above the broken line <NUM>). The term "<NUM> (9½") x <NUM> (11¼")" is understood to mean a <NUM> (9½") drilled bore which is then under-reamed to <NUM> (11¼").

In the procedure, an <NUM> (8½") bore <NUM> is then drilled into rock at the end of the <NUM> (8⅝") liner, and a <NUM> (<NUM>") production liner <NUM> run into the bore <NUM>. The liner <NUM> is hung from a liner hanger <NUM> set at the end of the <NUM> (8⅝") liner. The production liner <NUM> is then completed by injecting acid through perforations <NUM> in the liner <NUM>. A ball valve <NUM> is then closed to shut off the liner <NUM>. This completed part of the well is known as "Lateral A".

A hollow whipstock <NUM> is then run into the <NUM> (9½") bore <NUM> and set adjacent the <NUM> (<NUM>") production liner hanger <NUM>. A milling tool is then run into the well and a window <NUM> milled in the <NUM> (8⅝") liner <NUM>. A drilling BHA is then run into the well and a second <NUM> (8½") bore <NUM> is then drilled through the window <NUM> into the rock. A second <NUM> (<NUM>") liner <NUM> is then run into the second <NUM> (8½") bore <NUM> and hung from a liner hanger <NUM> set in the large diameter casing <NUM> in the overburden. The second production liner <NUM> is then completed via perforations <NUM>. This second completed part of the well is known as "Lateral B".

Both laterals may now be put on production.

This procedure involves a large number of runs into the well and is also challenging to perform when an <NUM> (8⅝") casing has been set in the overburden so that it is only possible to set a liner of less than <NUM> (8⅝") diameter in the reservoir.

A procedure according to the invention is now described which involves fewer runs into the well and is more suitable for a scenario where it is not possible to set a <NUM> (8⅝") liner in the reservoir.

On the applicant's wells in the North Sea, <NUM> (8⅝") casing is utilized in the overburden, and hence is not always an available option in the reservoir.

A solution proposed by the inventors is to install a tapered liner, typically <NUM> (7¾") or <NUM> (7⅝"), tapering to <NUM> (<NUM>") or <NUM> (<NUM>"). The <NUM> (7¾") or <NUM> (<NUM>%") section may accommodate components to stimulate, and later temporarily isolate the <NUM> (<NUM>") section below. This section may also accommodate a nipple for a whipstock to enable sidetracking. The liner hanger for the tapered liner can be installed in either a <NUM> (<NUM>%") section, a <NUM> (9⅝") section or in a <NUM> (10¾") section of casing in the overburden. The <NUM> (<NUM>") liner below the <NUM> (7¾") or <NUM> (7⅝") section may be stimulated and isolated temporarily. A whipstock may then be installed in the <NUM> (7¾") or <NUM> (<NUM>%") section; a window may be be milled before drilling of an additional lateral well, typically with a <NUM> (6½") bit to accommodate a <NUM> (<NUM>") liner.

The proposed design can be run, for example, in any scenario where the restricting inner diameter is less than <NUM> (<NUM>/<NUM>") above the reservoir, but is not limited to this and can be beneficial for any size of overburden casing.

In this way the multilateral system is more flexible and can be utilized with more restrictive inner diameters above the reservoir. It is also more efficient as the liner to serve as junction is run in combination with the first branch's reservoir liner, rather than a "dedicated parent liner".

<FIG> illustrates a system according to the invention. Parts corresponding to those shown in <FIG> are designated by the same number but in the <NUM> series. The overburden casing <NUM> immediately above the reservoir <NUM> is <NUM> (8⅝") or <NUM> (9⅝") casing. In this procedure a bore <NUM> is drilled into the reservoir <NUM> with perhaps an <NUM> (8½") drill bit, all the way to the desired end point (TD) of a first lateral well (Lateral A). Optionally, the bore may be under-reamed up to <NUM> (9½").

A tapered liner is then run into the bore <NUM>, the tapered liner having a <NUM> (7¾") section <NUM> and a <NUM> (<NUM>") production section <NUM>. The liner is pre-fitted with completion equipment and one or more valves <NUM> as well as a nipple (not shown) for receiving a whipstock.

The liner is cemented in place and then completed and stimulated, and then the valve <NUM> closed.

In the method described above, Lateral A is completed with a saving of at least two runs into the hole, with associated time and expense. Compared to the prior method at least one two runs into the well are avoided: one drilling run and one run to set a liner. The need to set a liner hanger to support the production liner for Lateral A is avoided. In the narrower well in which the system operates, there is less space for setting a liner, so that this step can be problematic. In the method according to the invention, the need to set a liner hanger for Lateral A is avoided. This also saves time and equipment cost.

After Lateral A is completed, a hollow whipstock <NUM> may then be run into the well and installed at the far end of the <NUM> (7¾") liner section <NUM>. The hollow whipstock includes a fluid loss valve (not shown), and is used to close off Lateral A during drilling and completion of Lateral B. After completion of Lateral B the valve will be opened to allow Lateral A to come onto production.

The procedure from this point is similar to the known procedure described above with reference to <FIG>. A milling tool is run into the well and a side window <NUM> milled in the <NUM> (7¾") section <NUM>. A <NUM> (6½") bore <NUM> may be drilled into reservoir rock by passing a suitable drill bit through the milled window <NUM>. A <NUM> (<NUM>") liner is then run into the well, through the window <NUM> and into the <NUM> (6½") bore <NUM>. Completion is then performed and Lateral B is thus established. Production may then commence from both laterals.

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

Claim 1:
A process for drilling a multi-lateral well, where the process comprises:
a) drilling a first bore of a first bore diameter through overburden rock (<NUM>) above a reservoir (<NUM>) and casing the bore;
b) drilling a second bore (<NUM>) of a second, smaller, bore diameter through the first bore and into the reservoir (<NUM>); characterized by:
c) running into the bores an assembly comprising:
i) a liner hanger;
ii) a tapered liner having a first section (<NUM>) with a first liner outer diameter and a second section (<NUM>) with a second, smaller liner outer diameter;
iii) a whipstock nipple and valve in the first section (<NUM>) of the liner;
iv) completion equipment (<NUM>) installed in the second section (<NUM>) of the liner;
whereby the liner hanger is anchored in the first bore and the first and second sections of liner extend into the second bore (<NUM>);
d) performing a completion operation in the second section (<NUM>) of liner;
e) running a whipstock (<NUM>) into the first section (<NUM>) of liner and milling a window (<NUM>) in the first section (<NUM>) of liner, adjacent the whipstock (<NUM>);
f) drilling a branched, third bore (<NUM>) through the window (<NUM>), running into the third bore (<NUM>) a second liner and performing a completion operation in the second liner.