GB2170842A - Drilling at least one deviated well - Google Patents

Abstract

For drilling deviated wells from a floating vessel, a curved well conductor (21) is run through a drilling guide (12) in a subsea template (10). Ribs on the well conductor (21) cooperate with slots in the drilling guide (12) to properly orient the well conductor (21). A casing string (27) is run through the well conductor and a well is drilled through the casing string. <IMAGE>

Description

SPECIFICATION Directional drilling from floating vessels The present invention relates to offshore directional drilling. More particularly, the present invention relates to the drilling of deviated wells from floating vessels.

In the petroleum industry, deviated wells are commonly drilled into petroleum bearing subterranean formations. Deviated wells differ from ordinary vertical wells in that the bottom of the well is offset from the top of the well by a substantial horizontal distance. The horizontal distance between the top and bottom of a deviated well can be as little as several hundred meters or as great as several kilometers. The required angle of deviation from the vertical depends on the desired horizontal distance between the top and bottom of the well and on the depth of the petroleum bearing formation. The angle of deviation increases with increasing horizontal distance and decreases with increasing formation depth. By directionally drilling deviated wells, a greater area of the petroleum bearing formation can be drained from a single drilling location.Directional drilling is extensively utilized in the development of offshore petroleum fields due to the great expense associated with each offshore drilling location.

One common method for directionally drilling wells is to first drill a long vertical section and then to kick-off from the vertical by using a bent sub or other directional drilling device.

Unfortunately however, the well does not always kick-off in just the right direction or at just the right angle. In addition, shallow rock layers can be fractured when the well is kicked-off, and this can result in lost circulation of drilling mud or in an underground blowout which causes petroleum to flow out of the well and into the fractures. In the case of offshore wells, such underground blowouts can result in petroleum flowing through the fractures and up through the sea floor, thereby polluting the sea. Deep rock layers are less susceptible to fracturing, so wells are typically not kicked-off until they have reached a depth of several hundred meters or more.

The need to drill a long vertical section at the top of the well decreases the area which can be drained from a single drilling location, resulting in the need for additional drilling locations.

A different approach which avoids some of these drawbacks is to use a slant drilling rig.

Slant drilling rigs are designed to drill wells that are inclined from the vertical. However, slant drilling rigs have problems of their own.

including the need for special equipment and the need to reorient the rig for each well. The need for reorientation of the rig is especially disadvantageous for offshore drilling due to space limitations on offshore platforms and vessels. In addition, the angle of slant is limited, and this limits the horizontal reach of the well. As mentioned above, limitations on horizontal reach result in the need to drill at additional locations at great expense in order to drain the desired area of the petroleum bearing formation.

One solution to these problems which is applicable for drilling deviated wells from offshore platforms is to use curved well conductors. Curved well conductors are used to impart the desired direction and angle of deviation for the well, and to eliminate the need to drill a long vertical top section or to use a special drilling rig. There are two basic approaches for using curved well conductors to drill deviated wells from offshore platforms. In one approach, curved well conductors which extend from the top to the bottom of the platform are used. A drill string and drill bit are run through the curved well conductors, and as a result, each well is already deviated and headed in the right direction from its start at the sea floor.However, due to the need for installation of the curved well conductors or guides therefor on the platform prior to installation of the platform at the offshore drilling location, this approach does not enable one to alter directional drilling plans on the basis of information learned from early wells. In a second approach for using curved well conductors to drill deviated wells from offshore platforms, this problem is overcome by installing the well conductors after the platform is installed. In this approach. a straight well conductor is driven from the platform into the sea floor and a special driving shoe positioned on the bottom end of the well conductor causes the conductor to bend into a curve as it is driven. Thus, with this approach, the direction of each well does not have to be established prior to platform installation. However. there are drawbacks.First, there is uncertainty as to the direction in which the well conductor will curve due to interactions between the driving shoe and the rock layers being penetrated.

Second, the horizontal reach of the well will be less than with first approach since the curvature of the well conductor does not start until some distance below the sea floor.

Although both of these approaches for using curved well conductors have their drawbacks, they can be used in combination to produce satisfactory results for most directional drilling from offshore platforms. However, neither approach is useful for wells drilled from floating vessels. Since drilling vessels have no structure running to the sea floor, there is no support available for running well conductors. Similarly, well conductors cannot be driven into the sea floor from a floating vessel due to the lack of support between the vessel and the sea floor. Consequently, deviated wells drilled from floating vessels are typically drilled through subsea templates using the technique described above where a long vertical section is drilled before kicking-off the well. As described above, there are several drawbacks to this technique.

It would be highly advantageous to have an approach which enables one to drill deviated wells from floating vessels while overcoming the drawbacks discussed above, especially since many offshore petroleum fields must be drilled from floating vessels due to water depths beyond platform applicability. In addition, many small offshore petroleum fields must be drilled from floating vessels because it is uneconomical to use platforms. The present invention is aimed at providing such an approach.

According to the invention from one aspect there is provided an apparatus for drilling a plurality of deviated wells from a floating vessel located in a body of water, said apparatus comprising: (a) a subsea template positioned on the floor of said body of water, said subsea template having a plurality of drilling guides; and (b) a plurality of curved well conductors which extend through said drilling guides and which penetrate the floor of said body of water.

According to the invention from another aspect there is provided a method for drilling at least one deviated well from a floating vessel located in a body of water, said method comprising the steps of: (a) positioning a subsea template on the floor of said body of water; (b) lowering a curved well conductor from said floating vessel until the bottom end of said well conductor extends through one of a plurality of drilling guides in said subsea template; (c) excavating a hole beneath the bottom end of said well conductor and lowering said well conductor as the excavation progresses; (d) continuing step (c) until the top end of said well conductor is proximate said drilling guide; (e) lowering a casing strng from said floating vessel through said well conductor; (f) lowering a drill string and drill bit from said floating vessel through said casing string; and (g) drilling a deviated well from said floating vessel using said drill string and drill bit.

Briefly, according to a preferred way of putting the present invention into effect for drilling deviated wells from floating vessels, a subsea template before is positioned on the sea floor at a desired drilling location. The subsea template has a plurality of drilling guides which can be installed in the template before or after the template is lowered to the sea floor. A curved well conductor is assembled on the floating vessel from curved well conductor sections and is lowered into one of the drilling guides on the subsea template.

Ribs on the well conductor cooperate with slots in the drilling guide to properly orient the well conductor. Extending through the well conductor is a drill string which has either a drill bit or a jet nozzle at its end. Once the bottom of the well conductor is lowered through the drilling guide, the drill bit or jet nozzle is activated and the well conductor is lowered as a hole is excavated ahead of it.

When the top of the well conductor enters the drilling guide, it is secured to the drilling guide by a connection, The drill string is then released from the well conductor to drill a hole for a casing string, After reaching the desired depth for the casing string, the drill string is retrieved by the floating vessel. A casing string is then made up on the floating vessel from straight casing string sections and is lowered through the well conductor. As the casing string is lowered through the curved well conductor, it bends into a curve. A drill string and drill bit are then run through the casing string and a well is directionally drilled to a desired target point in a petroleum bearing formation.

The invention will be better understood from the following description given by way of example and with reference to the accompanying drawings, wherein: FIGURE 1 is a plan view showing a subsea template used for drilling deviated wells, FIGURE 2 is a perspective view, partly in section, showing a drilling guide of the subsea template, FIGURE 3 is a perspective view, partly in section, showing a curved well conductor used for drilling deviated wells, FIGURE 4 is a side view, partly in section, showing a subsea template, drilling guide, curved well conductor, and casing string after installation, and FIGURE 5 is a side view showing a floating vessel in the process of lowering a curved well conductor into a drilling guide on a subsea template.

Referring to FIGURE 1, a plan view of a subsea template used for drilling deviated wells can be seen. Template 10 is positioned on the sea floor and serves to guide wells drilled from a floating vessel. The template comprises structural members 11, permanent drilling guides 12, optional drilling guides 13 and well bays 14. Each well bay can hold one drilling guide. Permanent drilling guides 12 are affixed to the well bays by support members 15, whereas optional drilling guides 13 are affixed to the well bays by modules 16. Each module latches into an empty well bay via latch pins 1 7 which cooperate with pin receptacles 18. The template shown in FIGURE 1 has six permanent drilling guides, two optional drilling guides, and room for up to four additional optional drilling guides in the four empty well bays. This arrangement is illustrative and it should be recognized that the template could be designed to accommodate virtually any number of wells using permanent drilling guides, optional drilling guides, or any combination of the two. Arrows 9 indicate the direction of each well to be drilled through the drilling guides. The direction of each well drilled through a permanent drilling guide is determined prior to fabrication or positioning of the template on the sea floor. In contrast, the direction of each well drilled through an optional drilling guide need not be predetermined. The direction of each of the optional wells can instead be determined after the template has been positioned on the sea floor.

This is because the modules containing the optional drilling guides can be put into the template at any time. Thus, the direction of each well drilled through an optional drilling guide can be based on information learned from earlier wells drilled through the permanent drilling guides or through previously deployed optional drilling guides. In this manner, the optional drilling guides provide great flexibility for drilling plans.

FIGURE 2 is a perspective view, partly in section, which shows one of the optional drilling guide modules in greater detail. Optional drilling guide 13 is held within module 16 by struts 19. The module is designed so that it can be lowered from a floating vessel into an empty well bay on the subsea template. The drilling guides themselves, whether permanent or optional, are essentially identical. Each drilling guide is curved in the desired direction for the well to be drilled through it, and has slots 20 which serve to properly orient the curved well conductor, as will be described below.

Although curved drilling guides are preferred, straight drilling guides with orientation slots can also be used to run the well conductors.

However, if straight drilling guides are used, they must be slightly larger in diameter to permit the passage of curved well conductors.

Guide funnel 8 at the top of drilling guide 13 facilitates entry of the well conductor.

FIGURE 3 is a perspective view, partly in section, showing curved well conductor section 21. Each well conductor is made up of a number of such sections. Various means of joining the well conductor sections together can be used, such as standard squnch joints 22, which are well known to those skilled in the art. To show how the well conductor sections fit together, FIGURE 3 includes bottom end 23 of a second well conductor section.

The standard squnch joints cooperate to align the curved well conductor sections so that the direction of curvature is constant. Orientation ribs 24 of the well conductor fit into the slots of a drilling guide so that the direction of curvature of the well conductor will correspond to the desired direction for the well. In the case where curved drilling guides are used, this direction will also correspond to the direction of curvature of the drilling guide. Although the direction of curvature of the well conductor section shown in FIGURE 3 is parallel to the longitudinal plane in which the orientation ribs lie, this does not have to be the case. Likewise, means other than ribs and slots can be used to properly orient the curved well conductors in their respective drilling guides.In fact, if the drilling guide is long enough and its curvature great enough, the drilling guide could orient the well conductor without the need for additional hardware. The key is simply to provide a means which assures that the direction of curvature of the well conductor when installed will correspond to the desired direction of the well being drilled.

FIGURE 4 shows a side view, partly in section, of a subsea template, curved drilling guide, curved well conductor and casing string which have been fully installed. Subsea template 10 rests on sea floor 25 at a location to be directionally drilled from a floating vessel.

Affixed to the template is curved permanent drilling guide 12. A curved optional drilling guide would look much the same. Contained within permanent drilling guide 12 and extending into the sea floor is curved well conductor 21. The well conductor is secured to the drilling guide by conductor connection 26 and is made up of a number of curved well conductor sections as described above. The individual sections and their orientation ribs are not shown in FIGURE 3. Extending through well conductor 21 is casing string 27. The casing string is attached to the well conductor by casing connection 28 and consists of a number of casing sections which are not individually depicted.Although casing string 27 was straight prior to being run through the curved well conductor, it underwent bending as it was lowered through the well conductor so that its resulting direction of curvature corresponds to the direction of curvature of the well conductor and drilling guide. Thus, the bottom end of the casing string points in the desired direction for the well. Once the well conductcr and casing string are installed, a well is drilled through the casing string from a floating vessel using conventional drilling techniques and equipment.

FIGURE 5 shows a floating vessel in the process of lowering a curved well conductor into a curved drilling guide. Floating vessel 29 is shown in two different positions for a reason that will be explained below. Subsea template 10 has previously been positioned on the sea floor and has been anchored thereto with piles (not shown). The template contains curved drilling guide 12 through which curved well conductor 21 is to be run. The sections which make up the well conductor were assembled and lowered from the floating vessel in a manner similar to the conventional assembly and lowering of a straight casing string. Although derrick 32 on the floating vessel is straight, it is able to handle the curved well conductor sections due to the small amount of curvature in each section.For example, if a 400-foot curved well conductor made up of ten 40-foot long sections is used, and a 30 degree angle of deviation is desired, each section will only curve 3 degrees.

Curved well conductor 21 is lowered from the floating vessel by means of drill string 30, which extends through the well conductor and terminates with a conventional underreaming drill bit or jet nozzle (not shown). A swivel or flexible joint (not shown) releasably attached to the top of the well conductor allows it to hang from the drill string with its center of mass directly beneath derrick 32. The well conductor is lowered by adding drill string sections. If an underreaming drill bit is used rather than a jet nozzle, the swivel in the flexible joint suspending the well conductor must be adapted to permit rotation of the drill string, unless the drill bit is attached to a turbo drill, which operates without rotation of the drill string. Such swivels and flexible joints are well known to those skilled in the art.

Although the drill string is made up of straight sections of drill pipe, its length gives it sufficient flexibility to extend through the curved well conductor and to rotate therein. However, this rotation is not commenced until after the bottom of well conductor 21 has been lowered through drilling guide 12.

To lower the curved well conductor through the curved drilling guide, the floating vessel initially takes position A as shown in FIGURE 5. Position A is located to the side of centerline 31 of the top of the drilling guide. This permits the bottom end of the curved well conductor to enter the top of the drilling guide. As the bottom end of the well conductor enters the drilling guide, the well conductor is rotated using the drill string until the orientation ribs of the well conductor align with the slots of the drilling guide. The well conductor is then lowered from position A as far as it will go. Next, the floating vessel is moved over to postion B, which is located on the opposite side of centerline 31 from position A. This causes the well conductor to be brought into a position which aligns its curvature with the curvature of the drilling guide.

Once aligned, the well conductor can be lowered from the floating vessel until its bottom end passes all the way through the drilling guide.

At this point, the drill bit or jet nozzle is activated to excavate a hole for the well conductor. As the hole is excavated, the well conductor is lowered from the floating vessel.

Since the drill bit or jet nozzle is carried by the well conductor, the direction of the hole will automatically be correct. Referring again to FIGURE 4, the excavation continues until the top end of the well conductor enters the top of the drilling guide and conductor connection 26 engages, thereby securing the well conductor to the drilling guide. The drill string is then released from the well conductor and excavation is continued to the desired depth for casing string 27. The floating vessel then retrieves the drill string along with the drill bit or jet nozzle. Casing string 27 is then made up on the floating vessel from standard straight casing sections and is lowered through the curved well conductor using conventional techniques.When fully lowered, casing connection 28 secures casing string 27 to well conductor 21, At this point, the well is ready to continue drilling using conventional drilling techniques.

The preferred way of performing the present invention as thus described avoids the problems associated with the use of direction drilling techniques which require that a long vertical well section be drilled before the well is kicked-off in the desired direction. Two of the problems avoided are the risk of fracturing a shallow formation and the risk of deviating the well at an angle or direction other than that desired. In addition, greater horizontal reach can be achieved using the preferred way of performing the present invention described above since a long vertical section at the top of the well does not have to be drilled. This is especially adantageous for the directional drilling of large, shallow petroleum fields because it cuts down on the number of drilling locations required, thus resulting in great savings.If desired, a deviated well drilled with the approach described above can be further deviated simply by using a bent sub or other conventional directional drilling device. This combined approach may be desired, for example, where the well needs to be drilled horizontally through the petroleum bearing formation, Even in the cases where conventional directional drilling techniques are used in conjunction with those described hereinabove, directional drilling accuracy will be improved.

This is because the kick-off point will be deeper and the angle smaller, thus lessening the effect of inaccuracies in the direction and angle of deviation, As described above, the advantages achievable will offer substantial improvements in the drilling of directional wells from floating drilling vessels. With these advantages, more petroleum fields can be economically developed. As also described above, the approach described above utilizes conventional hardware and techniques to the extent possible to further minimize expenses and complications.

Claims (10)

1. An apparatus for drilling a plurality of deviated wells from a floating vessel located in a body of water, said apparatus comprising: (a) a subsea template positioned on the floor of said body of water, said subsea template having a plurality of drilling guides; and (b) a plurality of curved well conductors which extend through said drilling guides and which penetrate the floor of said body of water.

2. An apparatus as claimed in claim 1, wherein said drilling guides are curved.

3. An apparatus as claimed in claim 1 or 2, wherein said well conductors comprise a plurality of curved conductor sections.

4. An apparatus as claimed in claim 1, 2 or 3, wherein said drilling guides and said well conductors have means to orient said well conductors in desired directions.

5. An apparatus as claimed in any preceding claim, wherein at least one of said drilling guides is adapted for installation on said subsea template after said subsea template has been positioned on the floor of said body of water.

6. An apparatus for drilling a plurality of deviated wells from a floating vessel located in a body of water, substantially as hereinbefore described with reference to the accompanying drawings.

7. A method for drilling at least one deviated well from a floating vessel located in a body of water, said method comprising the steps of: (a) positioning a subsea template on the floor of said body of water; (b) lowering a curved well conductor from said floating vessel until the bottom end of said well conductor extends through one of a plurality of drilling guides in said subsea template; (c) excavating a hole beneath the bottom end of said well conductor and lowering said well conductor as the excavation progresses; (d) continuing step (c) until the top end of said well conductor is proximate said drilling guide; (e) lowering a casing strng from said floating vessel through said well conductor; (f) lowering a drill string and drill bit from said floating vessel through said casing string; and (g) drilling a deviated well from said floating vessel using said drill string and drill bit.

8. A method as claimed in claim 1, wherein said drilling guidd is installed in said subsea template after said subsea template has been positioned on the floor of said body of water.

9. A method as claimed in claim 7 or 8, wherein the position of said floating vessel is changed between steps (b) and (c) to facilitate the lowering of said well conductor through said drilling guide.

10. A method for drilling a plurality of deviated wells from a floating vessel located in a body of water, substantially as hereinbefore described with reference to the accompanying drawings.