GB2577058A

GB2577058A - Coiled tubing or snubbing string drilling

Info

Publication number: GB2577058A
Application number: GB1814760.3A
Authority: GB
Inventors: Nasvik Håvard
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2018-09-11
Filing date: 2018-09-11
Publication date: 2020-03-18
Also published as: GB201814760D0; WO2020055262A1

Abstract

A method of drilling a borehole at an oil and gas installation uses a modified form of coiled tubing or snubbing string drilling and includes: circulating mud through a mud circuit during a drilling operation, wherein the mud circuit includes a topside mud lift pump 2 that elevates return mud up some of all of the height of a tower structure 6 of the installation before allowing it to flow 8 under the influence of gravity around the mud circuit back to the borehole. The extra mud column provides control of bottom hole pressure akin to managed pressure drilling within a narrow formation drilling window but without the need for extra components such as a recirculating rotating device and when circulation stops may maintain hydrostatic overbalance. The tower may be an existing rig derrick and the pimp should provide at least 50m elevation above the lowest topside point. The pump and any tubing may be dedicated to the method or may be primarily used for other purposes.

Description

(54) Title of the Invention: Coiled tubing or snubbing string drilling

Abstract Title: Raised mud height for pressure control in coiled tubing or snubbing string drilling (57) A method of drilling a borehole at an oil and gas installation uses a modified form of coiled tubing or snubbing string drilling and includes: circulating mud through a mud circuit during a drilling operation, wherein the mud circuit includes a topside mud lift pump 2 that elevates return mud up some of all of the height of a tower structure 6 of the installation before allowing it to flow 8 under the influence of gravity around the mud circuit back to the borehole. The extra mud column provides control of bottom hole pressure akin to managed pressure drilling within a narrow formation drilling window but without the need for extra components such as a recirculating rotating device and when circulation stops may maintain hydrostatic overbalance. The tower may be an existing rig derrick and the pimp should provide at least 50m elevation above the lowest topside point. The pump and any tubing may be dedicated to the method or may be primarily used for other purposes.

Fig. 1

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

11 18

1/1

Fig. 1

COILED TUBING OR SNUBBING

STRING DRILLING

The present invention relates to a method of drilling a borehole at an oil and gas installation using coiled tubing or snubbing string drilling, as well as to a method for providing related drilling equipment and to a related apparatus.

It is important to the oil and gas industry to have effective techniques to enable drilling of boreholes into geological formations for later extraction of hydrocarbons such as oil and gas. Various techniques exist in the art, including coiled tubing drilling and snubbing string drilling, for which the basic principles are well-known.

During a drilling operation the pressure of the drilling fluid (typically known as “mud”) provides a barrier against formation pressure and acts to prevent influx of fluid into the borehole. As a result the pressure of the mud is important. The mud pressure during drilling must remain greater than the formation pressure (pore pressure) at all points of the well depth. However, the mud pressure should also be lower than the fracture pressure at all points of the well depth in order to avoid a fracture of the well. It will hence be understood that there is a “drilling window”, defined as a range of pressure lower than the fracture pressure and higher than the formation (pore) pressure. A safe and efficient drilling operation must ensure that the pressure of the mud remains within the drilling window. In general both of the formation pressure and the fracture pressure will increase with depth and the increase includes a linear component based on hydrostatic pressure. The mud pressure will also increase with a similar hydrostatic pressure. It will be understood that a suitable selection for circulation pressure and density of the mud can allow for the mud pressure to remain in the drilling window throughout the entire well depth.

However, the control of the bottomhole mud pressure is complicated by the need to make connections during the drilling process in order to add further drill pipes as the depth of the borehole increases. When connections are made to add further drill pipes then the circulation of mud will stop and this change in flow conditions affects the mud pressure. In a traditional drilling operation the bottomhole pressure is the sum of the hydrostatic pressure and the circulation pressure. As a result, when circulation stops then there is a drop in mud pressure and this is typically proportional to the well depth. In order to allow for safe and effective drilling pressures both with circulation and without then it is commonplace to need to utilise a large extent of the drilling window, such that the bottomhole pressure with circulation is close to the fracture pressure and the bottomhole pressure without circulation is close to the formation pressure.

This type of control can be adequate for a theoretical well profile with a linear increase in pressure as depth increases, but it places constraints on the drilling operation especially if the nature of the geological formations is such that there is a narrow drilling window. In addition, in most real-world situations the profile of the pressures for the borehole is not linear. Variations in the nature of the formation result in non-linear profiles for the formation pressure and fracture pressure. Weaknesses/faults can also impact on the pressure profiles. If the pressure profiles have a curve or other non-linear behaviour as depth increases then this can result in the well becoming impossible to drill with traditional methods since the drilling window varies too much with depth to enable the mud pressure with circulation to always remain below the fracture pressure, whilst the mud pressure without circulation always remains above the formation pressure. This gives rise to a known problem in relation to the control of mud pressure.

An existing solution to this problem is Managed Pressure Drilling (MPD), which creates a sealed system enabling bottomhole pressures to be maintained whilst connections are being made. This allows more effective drilling where there is a narrow drilling window and it can enable drilling of wells where traditional drilling techniques are impossible. MPD typically includes constant circulation of mud even during connections and the use of a pressurised fluid system with back-pressure applied at the surface to act through the entire mud circuit. However, there are disadvantages in terms of the cost and complexity of the drilling equipment. An operator will need to bring in a large number of additional components such as a Rotating Circulating Device (RCD), which is placed around the drill pipe near the surface in order to seal the borehole whilst allowing rotation of the drill pipe, and thus contain pressure whilst permitting on-going drilling as well as connections without a drop in pressure. MPD also requires added equipment to handle the flow of mud and generate the required back-pressure. The inventor has realised that it would be an advantage to provide a drilling method with at least some of the benefits of MPD but without all of the added complexity or expense.

Viewed from a first aspect, the invention provides a method of drilling a borehole at an oil and gas installation using coiled tubing drilling or snubbing string drilling, the method including: circulating mud through a mud circuit during a drilling operation, wherein the mud circuit includes a topside mud lift pump that elevates return mud up some of all of the height of a tower structure of the installation before allowing it to flow under the influence of gravity around the mud circuit back to the borehole.

Thus, the mud column is extended vertically above its usual height due to the use of the topside mud lift pump to provide added elevation of the mud above the installation up the tower structure. With this method it becomes possible to manage the pressure of the mud circuit during drilling in order to allow for narrower drilling windows without the need for the complexity of a conventional managed pressure drilling (MPD) system. In some ways the proposed method can be considered to allow for a mini-MPD arrangement. The pressure differential over the mud lift pump advantageously allows for lowered bottomhole pressure to compensate for the circulation friction pressure of the mud return while drilling, since there is a smaller bottomhole circulation pressure involved when using coiled tubing and snubbing drilling in accordance with the proposed method. As a result the method can drill with a narrower drilling window. When circulation is stopped and the pump is turned off, for example in order to allow for connections, then the system may be in hydrostatic overbalance with the bottomhole pressure held at a higher level than conventional drilling due to the increased hydrostatic pressure from the elevated mud column.

In contrast to known MPD systems there is no need for a fully contained mud circuit and with the proposed method there is no need to maintain a seal to keep the mud close to the surface at an above atmospheric pressure at all times. Additional advantages compared to traditional MPD systems include reduced equipment, such as avoiding the need for the RCD or for additional topside components of the mud circuit. This reduces the equipment costs and the man hours. Further, with the proposed method it can be possible to use a single mud system for the entire drilling operation before switching to completion fluid. This is a benefit compared to some MPD systems that require multiple mud systems with resulting added time and expense.

The mud circuit may include typical components of a mud circuit as used for coiled tubing drilling or snubbing string drilling. Thus, there may be a mud pit with a generally conventional structure and function, as well as mud lines of a known type. A significant difference to existing mud circuits is of course the addition of extra elevation via the use of the mud lift pump as set out above.

The oil and gas installation may be on land or offshore. It will be appreciated that the advantage of lowered bottomhole pressure can arise with any increased elevation via the topside mud lift pump. Ideally the mud lift pump would lift the mud as high as reasonably possible within the bounds of the installation in order to make best use of the advantages of the invention. It is beneficial to make use of a high structure already present at the installation such as a derrick or other tower type structure.

In one or more example embodiments the method makes use of an existing tower structure to gain advantages without the need for significant modification to the installation. The method may include using pre-existing tubing on an existing tower structure, such as pre-existing vent lines, in order to convey the mud up the height of the tower structure. In the alternative, the method may include installing suitable mud lines on an existing tower structure prior to using the mud lines within the mud circuit. In the case where an installation includes a tower some form of a vertical extension, such as a derrick with a flare tower, then this may be used to gain added height. The method may include installing a vertical extension on an existing tower in order to increase the elevation for the mud.

The mud lift pump may elevate the mud by at least 50 m above a lowest topside level of the installation, optionally by at least 75 m and further optionally by at least 100 m. For a land based installation the lowest topside level of the installation may be a ground level. For an offshore based installation, typically comprising a platform supported above sea level via a suitable foundation, the lowest topside level may be the lowest level of the platform. The mud lift pump may elevate the mud to a level that corresponds to a height allowed for by an existing tower structure of the installation. It is typical for oil and gas installations to include a derrick allowing for lines to be supported at 50-100 m or more above the lowest topside level and hence these elevations can readily be attained without the need for significant modification or rebuilding of the installation.

The mud lift pump used by the drilling method may be an existing pump with other purposes in relation to the oil and gas installation. In particular it may be a mud lift pump used for circulation of mud during drilling and/or during non-drilling operations. It will be understood that this allows for the drilling method to be used with minimal modification to an existing installation, and minimal added equipment. In the alternative the method may include the use of a dedicated and/or upgraded pump, optionally the method can include providing and installing a suitable pump before drilling commences.

The mud lift pump may be any suitable pump type. In example embodiments the mud lift pump is a centrifugal pump.

In example embodiments the mud lift pump is located at the lowest topside level of the installation. This is a typical location for mud lift pumps used for non-drilling purposes.

The method may optionally include Equivalent Circulating Density (ECD) control of the mud circuit with control of the pump speed to vary the bottomhole pressure. This may involve control of the pressure in a similar way to ECD-control in traditional MPD, although it will be appreciated that parameters may be changed via machine learning and artificial intelligence enhancements. Advantageously this enables existing control software to be easily adapted for use with the proposed method.

It will be understood that as well as providing a method for drilling the invention may also extend in another aspect to a method for adapting a coiled tubing drilling apparatus or a snubbing string drilling apparatus to use the method of the first aspect, the method for adapting the drilling apparatus comprising: providing a topside mud lift pump for elevating return mud above the lowest topside level of the installation or configuring an existing mud lift pump for that purpose; providing mud lines on a tower structure of the installation to allow for the mud circuit to extend vertically up the tower with mud flowing under the influence of gravity down the tower toward the borehole or configuring existing lines for that purpose; and arranging the mud lift pump and the mud lines for circulation of mud through the mud circuit during the drilling operation. This method may comprise providing features of a drilling apparatus as discussed above for the first aspect and/or as set out below in relation to another aspect.

Viewed from yet another aspect, the invention provides a drilling apparatus for drilling a borehole at an oil and gas installation using coiled tubing drilling or snubbing string drilling, the drilling apparatus comprising: a mud circuit for circulating mud during a drilling operation, wherein the mud circuit includes a topside mud lift pump that elevates return mud up some of all of the height of a tower structure of the installation before allowing it to flow under the influence of gravity around the mud circuit back to the borehole.

This apparatus may be configured to carry out the method of the first aspect and optionally any of the other features discussed above. The apparatus may be arranged so that when the pump is turned off in order to allow for connections then the mud circuit will be in hydrostatic overbalance with the bottomhole pressure held at a higher level.

It should be understood that although this apparatus allows for the pressure to be managed it nonetheless differs from a conventional MPD system since there is no fully contained mud circuit. The apparatus hence does not require a seal to retain an above atmospheric pressure close to the surface at all times since instead the hydrostatic pressure of the elevated mud allows for above hydrostatic pressure in the mud column. The apparatus advantageously omits a RCD.

The drilling apparatus may be on an installation that is on land or offshore and it may include features known for coiled tubing or snubbing string drilling in such situations. The drilling apparatus may include mud lines for carrying the elevated mud with the mud lines being supported on a tower structure. As with the method above the apparatus may make use of a high structure already present at the installation, such as a derrick or other tower type structure.

The mud lines for the elevated mud may include pre-existing tubing on an existing tower structure, such as pre-existing vent lines. In the alternative, the apparatus may include dedicated mud lines that are supported on an existing tower structure of the installation. In the case where an installation includes a tower some form of a vertical extension, such as a derrick with a flare tower, then this may be used to gain added height. The apparatus may hence comprise a vertical extension on an existing tower in order to increase the elevation for the mud.

The mud lift pump may be configured to elevate the mud by at least 50 m above a lowest topside level of the installation, optionally by at least 75 m and further optionally by at least 100 m. The mud lift pump may be configured to elevate the mud to a level that corresponds to a height allowed for by an existing tower structure of the installation.

The mud lift pump may be an existing pump with other purposes in relation to the oil and gas installation. In particular it may be a mud lift pump used for circulation of mud during non-drilling operations. Alternatively the apparatus may include a dedicated and/or upgraded mud lift pump. The mud lift pump may typically be a centrifugal pump. In example embodiments the mud lift pump is located at the lowest topside level of the installation.

The apparatus may include a control system for controlling the pump during the drilling operation. The control system may be arranged for Equivalent Circulating Density (ECD) control of the mud circuit with control of the pump speed to vary the bottomhole pressure.

The apparatus may comprise further features of the mud circuit and/or borehole components as required to complete the circuit such as a Blow-Out Protector (BOP) and so on. The apparatus may comprise features for coiled tubing drilling operations or snubbing string drilling operations, and advantageously such features can be generally conventional, i.e. the primary difference with the proposed apparatus is to the mud circuit to enable management of the mud pressure as discussed above.

The invention may be embodied as an oil and gas installation including a drilling apparatus as discussed above. This oil and gas installation may comprise a tower structure, such as a derrick, with the tower structure supporting mud lines for carrying the mud elevated by the mud lift pump. The oil and gas installation may be an onshore or an offshore installation with features as discussed above, for example in relation to the location of the lowest topside level and/or location of the mud lift pump.

Certain embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying Figure 1, which is a schematic diagram showing an example drilling method where the return mud is elevated via a derrick of an offshore platform.

As set out above, a drilling method is disclosed for drilling a borehole at an oil and gas installation. The drilling method involves coiled tubing drilling or snubbing string drilling, both of which are known techniques and hence which will not be described in more detail. In the example of Figure 1 the drilling is coiled tubing drilling. The drilling method makes use of a mud lift pump 2 to enhance the control of bottomhole mud pressure and thus allow for operation with a narrower drilling window. As shown in Figure 1, the mud lift pump 2 is mounted at a lower level of the platform and mud is lifted by the pump via an upward mud line 4. The upward mud line 5 extends up the height of a derrick 6 on the platform, in this case up the majority of the height of the derrick 6. This adds height to the mud column. The mud flows down the derrick 6 via a downward mud line 8 and continues in the mud circuit through the borehole. The downhole parts of the mud circuit are the same as for known coiled tubing drilling techniques (or snubbing string drilling). In the example of coiled tubing drilling the drilling apparatus further includes a coiled tubing injector (and BOP) 10.

This example advantageously makes use of the derrick 6 as a tower to provide suitable elevation for the mud return. This elevation and the pressure differential across the mud lift pump 2 act to reduce the bottomhole pressure. The mud lines 4, 8 on the derrick 6 could be existing vent lines, or they may be dedicated mud lines 4, 8 installed for the purposes of the drilling method.

The mud lift pump 2 is advantageously an existing pump on the platform, but it could be a dedicated pump installed for the purpose of this drilling method. Typically it is a centrifugal pump with a control system allowing for Equivalent Circulating Density (ECD) control of the pressure via variation of the pump speed.

In the example the height of the derrick 6 limits the height of elevation of the mud. It is typical for derricks to be at least 50 m in height. 75 m high towers/derricks may also be found in some existing platforms, or heights of this order could be implemented through an elongation of the derrick structure with a suitable pole or spire. 100 m towers could be feasible in some circumstances. The table below shows the potential reduction in bottomhole pressures for various mud densities at heights of 50 m, 75 m and 100m.

Mud (g/cc)	1.00	1.25	1.50	1.75	2.00
Height (m)	Reduction of bottomhole pressure (bar)
50	4.9	6.1	7.4	8.6	9.8
75	7.4	9.2	11.1	12.9	14.7
100	9.8	12.3	14.7	17.2	19.7

It will be appreciated that there are significant advantages to the proposed method via the ability to lower the bottomhole pressure as well as the ability to use the pump 2 to control the pressure of the mud during circulation. In addition, when circulation is stopped and the pump 2 is hence inactive, then the mud can be held in static overbalance with a full mud column to the top of the derrick 6. The drilling operation can be completed with a single mud system, hence avoiding the need to change between different mud systems as arises with known MPD techniques. There is no need for an RCD and the equipment is in general cheaper and simpler than MPD equipment. In fact if an existing mud lift pump can be utilised then there might be no need for an operator to rent any further equipment beyond that already present at the installation.

In possible variations of the above the oil and gas installation may be an onshore installation or a floating offshore installation. Existing installations may be adapted to use the proposed method by reconfiguration and repurposing of an existing mud pump and/or existing lines on a tower structure such as a derrick, and/or by installation of a new pump or new mud lines.

Claims

CLAIMS:

1. A method of drilling a borehole at an oil and gas installation using coiled tubing or snubbing string drilling, the method including: circulating mud through a mud circuit during a drilling operation, wherein the mud circuit includes a topside mud lift pump that elevates return mud up some of all of the height of a tower structure of the installation before allowing it to flow under the influence of gravity around the mud circuit back to the borehole.

2. A method as claimed in claim 1, wherein when circulation is stopped and the pump is turned off then the system is in hydrostatic overbalance.

3. A method as claimed in claim 1 or 2, comprising use of an existing tower structure of the installation to provide a support for lines carrying the elevated mud.

4. A method as claimed in claim 3, comprising using pre-existing tubing on the existing tower structure, such as pre-existing vent lines, in order to convey the mud up the height of the tower structure.

5. A method as claimed in claim 3, comprising installing suitable mud lines on the existing tower structure prior to using the mud lines within the mud circuit.

6. A method as claimed in any preceding claim, wherein the mud lift pump elevates the mud by at least 50 m above a lowest topside level of the installation.

7. A method as claimed in any of claims 3 to 6, wherein the mud lift pump elevates the mud to a level that corresponds to a height allowed for by mud lines supported by an existing tower structure of the installation.

8. A method as claimed in any preceding claim, wherein the mud lift pump is an existing pump with other purposes in relation to the oil and gas installation.

9. A method as claimed in any preceding claim, wherein the mud lift pump is located at the lowest topside level of the installation.

10. A method as claimed in any preceding claim, comprising Equivalent Circulating Density (ECD) control of the mud circuit with control of the pump speed to vary the bottomhole pressure.

11. A method for adapting a coiled tubing or snubbing string drilling apparatus to use the method of the first aspect, the method for adapting the drilling apparatus comprising: providing a topside mud lift pump for elevating return mud up some of all of the height of a tower structure of the installation or configuring an existing mud lift pump for that purpose; providing mud lines on the tower structure of the installation to allow for the mud circuit to extend vertically up the tower with mud flowing under the influence of gravity down the tower toward the borehole or configuring existing lines for that purpose; and arranging the mud lift pump and the mud lines for circulation of mud through the mud circuit during the drilling operation.

12. A drilling apparatus for drilling a borehole at an oil and gas installation using coiled tubing or snubbing string drilling, the drilling apparatus comprising: a mud circuit for circulating mud during a drilling operation, wherein the mud circuit includes a topside mud lift pump that elevates return mud up some of all of the height of a tower structure of the installation before allowing it to flow under the influence of gravity around the mud circuit back to the borehole.

13. A drilling apparatus as claimed in claim 12, wherein the apparatus is configured to carry out the method of any of claims 1 to 10.

14. A drilling apparatus as claimed in claim 12 or 13, including mud lines for carrying the elevated mud with the mud lines being supported on a tower structure of the installation.

15. A drilling apparatus as claimed in claim 12, 13 or 14, wherein the mud lift pump is configured to elevate the mud by at least 50 m above a lowest topside level of the installation.

16. A drilling apparatus as claimed in any of claims 12 to 15, wherein the mud lift pump is configured to elevate the mud to a level that corresponds to a height allowed for by an existing tower structure of the installation.

17. A drilling apparatus as claimed in any of claims 12 to 16, including a control system for controlling the pump during the drilling operation, wherein the control system is arranged for Equivalent Circulating Density (ECD) control of the mud circuit with control of the pump speed to vary the bottomhole pressure.

18. An oil and gas installation including a drilling apparatus as claimed in any of claims 12 to 17.

5

19. An oil and gas installation as claimed in claim 18, wherein the tower structure is a derrick supporting mud lines for carrying the mud elevated by the mud lift pump.

Intellectual

Property

Office

Application No: GB1814760.3