GB2305953A

GB2305953A - Selective core sampling after logging

Info

Publication number: GB2305953A
Application number: GB9620876A
Authority: GB
Inventors: Terry Moore
Original assignee: Corpro Systems Ltd
Current assignee: Corpro Systems Ltd
Priority date: 1995-10-07
Filing date: 1996-10-07
Publication date: 1997-04-23
Anticipated expiration: 2016-10-07
Also published as: GB9620876D0; GB9520539D0; GB2305953B

Abstract

An apparatus and method for obtaining core samples at selected levels from a wellbore after the wellbore has been drilled and logged is disclosed. The apparatus comprises: an outer assembly 51 which may be incorporated into a drill string 52 and which includes a side exit mandrel 2 having a bore which starts centrally at the top of the mandrel and exits from one side at the lower end of the mandrel; and an inner assembly 50 which may be connected to a wireline 18, so that the inner assembly may be raised and lowered within the drillstring 52 and the outer assembly 51. The inner assembly includes a core barrel and a motor, driven by drilling fluid, which rotates the core barrel about its longitudinal axis. The core barrel is driven through the side exit mandrel 2, so that the core barrel exits at the side of the mandrel and engages the wellbore formation to one side of the wellbore.

Description

METHOD AND APPARATUS FOR SELECTIVE CORING AFTER LOGGING This invention relates to an apparatus and method for obtaining a core sample at a selected level in an existing wellbore.

BACKGROUND OF THE INVENTION During and after drilling of boreholes, as, for example, in the oil and gas industry, core samples are utilised to obtain accurate information relative to formations containing resources of interest. Such samples are commonly taken at the bottom of a bore hole during the boring process.

The core samples are taken by attaching a corebarrel, consisting of a strong rigid outer tube with a bit screwed on the lower end, and a thin flexible inner assembly, to the bottom of a string of drill pipe. The pipes are lowered to the bottom of the well where the combination of rotation, downward pressure and fluid allow the bit to drill into new information. The hollow centre of the drill bit forms a core which feed upwards to the inner assembly. On completion of the core run, the entire drilling assembly is withdrawn from the hole to enable the core to be recovered.

It is also very advantageous to be able to obtain substantial core samples at various levels in a hole, after the hole has been drilled and logged. After drilling, examination of open hole logs and bore hole cuttings can identify levels from which such additional core samples would be useful.

There are currently a number of methods available for obtaining samples from formations at various levels in an open hole. All such currently available methods have serious shortcomings. For example, in many cases only very small diameter cores can be obtained. In other methods multiple drill string trips into and out of the hole are required for each sample. With many methods the cores are short and/or do not represent the lithology changes along the bore hole.

The present invention is directed toward a method and apparatus for obtaining a core sample of sufficient length and diameter from the formation generally along the axis of bore hole, at a selected level in an existing bore hole and requiring only a single trip into the hole.

According to a first aspect of the invention there is provided an apparatus for obtaining core samples from a wellbore or the like, comprising: an outer assembly adapted to be incorporated into a drill string and including a side exit mandrel; and an inner assembly adapted for connection to a wireline whereby the inner assembly is adapted to be raised and lowered within said drillstring and said outer assembly, said inner assembly including a core barrel and drive means for rotating said core barrel about its longitudinal axis, said core barrel being adapted to engage the wellbore formation to one side of said outer assembly via said side exit mandrel.

The inner assembly is preferably adapted to cooperate with said outer assembly so as to prevent rotation of said drive means relative to said outer assembly.

Preferably, the rotation preventing means comprises corresponding keys and key slots formed on said inner and outer assemblies, which may be located on a drive housing enclosing said drive means and positioned above said core barrel, with the corresponding key slots or keys of the outer assembly located on a key subassembly portion of the outer assembly positioned above said side exit mandrel.

Preferably, the drive means comprises a hydraulic motor, most preferably a positive displacement drilling motor.

Preferably, the inner assembly further includes flow diverter means located between the drive means and the core barrel.

Preferably, means are provided for controlling the downforce applied to the core barrel by hydraulic fluid circulating through the apparatus.

According to a preferred embodiment of the invention, the downforce controlling means comprises a piston subassembly located between the drive means and the upper end of the inner assembly and adapted to restrict the flow of fluid between the inner and outer assemblies.

Preferably, the outer assembly includes a piston mandrel having a honed bore.

Preferably, the piston sub-assembly is adapted to be located, in use, within said piston mandrel, such as to force fluid through the inner assembly and to provide a download on said inner assembly.

According to a second preferred embodiment of the invention, the downforce controlling means comprises a piston and slide assembly located at the uppermost end of said inner assembly and including an outer barrel having a piston assembly at its lowermost end and being adapted for connection to a wireline at its uppermost end, and an inner barrel adapted for sliding movement within said outer barrel and having a lowermost end from which the remainder of the apparatus is suspended, in use.

Preferably, downward movement of the inner barrel within said outer barrel is restricted by resilient means, such as a stack of spring washers, located therebetween.

Preferably, the piston and slide assembly includes means for monitoring the operation of the apparatus, which may include fluid ports adapted to induce pressure variations in the hydraulic fluid in response to relative movement of the inner and outer barrels.

Preferably, the piston and slide assembly is adapted to cooperate, in use, with a load control housing forming part of the outer assembly and located at the uppermost end thereof.

The inner assembly may be adapted to receive spacer members between the drive means and the core barrel for varying the depth to which the core barrel may penetrate the wellbore formation.

The side exit mandrel preferably has an internal bore extending at an angle to the longitudinal axis of said mandrel and exiting at a point on the circumferential surface of said mandrel.

According to a second aspect of the present invention there is provided a method of operation of an apparatus according to the first aspect of the invention.

According to a third aspect of the present invention there is provided a method of obtaining core samples from a bore, comprising the steps of: connecting an outer assembly to a length of drill pipe or conductor, said outer assembly including a side exit mandrel; holding said outer assembly in a static position within said bore such that the side exit mandrel is positioned at the level at which a core sample is required; connecting an inner assembly to a wireline, said inner assembly including a core barrel and drive means; lowering said inner assembly within said outer assembly such that said core barrel passes through the side exit of said side exit mandrel and engages the side of said bore; driving said core barrel into the side of said bore to cut a core sample; and removing said inner assembly and said core sample from the bore.

Preferably, the side exit mandrel has an internal bore extending at an angle to the longitudinal axis of said mandrel and exiting at a point on the circumferential surface of said mandrel, and the path of the core barrel on exiting the side exit mandrel is defined by the angle of said internal bore.

Preferably, the core is cut using a rotary motor, which exerts a reactive force on said outer assembly by keying means provided on the inner and outer assemblies.

Examples of an apparatus and a method according to the invention will be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of an apparatus according to a first embodiment of the invention, showing the outer and inner assemblies; Fig. 2 is a sectional view to an enlarged scale through the inner assembly of the apparatus shown in Fig. 1; Fig. 3 is a sectional view to an enlarged scale through the piston and slide assembly of the inner assembly shown in Fig. 2; Fig. 4 is a schematic view of the outer assembly of an apparatus according to a second embodiment of the invention; Fig. 5 is a schematic view of the inner assembly of the apparatus according to a second embodiment of the invention; and Figs. 6a, 6b and 6c are schematic views of an apparatus according to the invention, showing different stages of the coring method of the invention.

The system in accordance with the invention comprises an outer assembly 51 which is incorporated in a drill string 52 and co-operates with an inner assembly 50 consisting of a wireline tool which includes a corebarrel 7.

Fig. 1 shows an apparatus according to a first embodiment of the invention being operated from a drilling platform 53. The outer assembly comprises a side exit mandrel 2, positioned above a rock bit 1.

The side exit mandrel 2 consists of a steel tube approximately 7.6 m (25 ft) long with a hole of approx.

63.5 mm (2 1/2") diameter starting centrally at the top, and exiting from one side at the lower end. The hole forms a long tapered side exit with an angle of approximately 10. The lower end of the mandrel 2 is fitted with three centralising blades, preferably straight, with the side exit hole exiting along the top of one blade. Both ends of the barrel are threaded with standard API connections.

The side exit mandrel 2 is connected at its upper end to a drive sleeve 3, consisting of a tube approximately 9.1 m (30 ft) long, machined with an internal bore of approximately 63.5 mm (2 1/2") diameter. The bore contains two opposing key slots of approximately 12.7 mm (1/2") width, travelling the length of the tube.

Standard API connections are applied to both ends.

The upper end of the drive sleeve 3 is connected to a load control housing 4, consisting of a steel tube approximately 6.1 m (20 ft) long with a 76.2 mm (3") smooth bored hole through the centre and API connections top and bottom.

The inner assembly 50 is shown in greater detail in Fig. 2. It comprises at its lower end a corebarrel 6.

This may be a standard double tube, mining type corebarrel, made up to 7.62 m (25 ft) in length. It is fitted with steel inner tubes containing the liner in a triple tube configuration. The corehead is oversized to 63.5 mm (2 1/2") diameter and is not fitted with external gauge cutters. A reaming shell fitted with hard metal only runs immediately behind the bit.

A ball valve 7 and seat are provided at the top of the corebarrel 6, and are included in all barrels to allow fluid to flow in only an upward direction through the inner tube.

A drill pipe spacer 8, comprising an appropriate length of standard drill rod, is connected to the core barrel 6 above the ball valve 7. The length of the spacer 8 depends on the length of the side exit mandrel 2, the length of the core barrel 6 and the length of the core required.

The drill pipe spacer 8 includes at its upper end a crossover/flow diverter 9. This is a steel fitting connecting the drill rod 8 to a motor 10 and contains flow ports. The diverter 9 functions as a flow splitting device which allows a portion of the fluid flow to escape the corebarrel 6 ! This is achieved by restricting the flow downwards with a smaller nozzle than that allowing fluid to escape. The purpose of this tool is to allow higher flow rates to be applied (for increased motor output and downloads) which would not be able to flow through the corebarrel.

The motor 10, which is connected to the drill pipe 8 by the crossover/flow diverter 9, is a standard 60.3 mm (2 3/8") positive displacement drilling motor, as stocked by numerous suppliers. Preferably the motor is a high torque and low RPM type. The motor 10 is fitted with keys at a position normally reserved for the lower stabiliser (approx. 305 mm (1 foot) above the bearing).

The keys match with key slots provided in the drive sleeve 3.

Above the motor 10 is connected a standard 57 mm (2 1/4") slimhole drilling jar 11, either hydraulically or mechanically operated. The jar 11 is adapted for transmission of upward blows only and is set specifically for this operation.

A sinker bar 12 is connected to the upper end of the jar 11 and comprises a steel bar of 57 mm (2 1/4") diameter with a 15.9 mm (5/8") bore approximately 2.1 m (7 ft) long.

The main function of the sinker bar 12 is to apply a download to cock the jar 11, and to supply a dense mass which will create the force for the upward blow of the jar 11.

Above the sinker bar 12 is a piston and slide assembly 13, shown in detail in Fig. 3, comprising a steel tube 14 with a 15.9 mm (5/8") bore approximately 2.4 m (8 ft) long, fitted with a piston 16 of 76 mm (3") diameter which travels freely along the length of the load control housing 4. A load spring 14a is located so that it is depressed by the download of the motor 10. The piston 16 is connected to the wireline 18 via a sacrificial weak link 17 and retained on the steel tube 14 by a collar.

The steel tube 14 has a recess at the top end where a stack of spring washers 14a are located. These are held in position by a cap nut 14b. The steel tube has fluid ports 14c drilled at specific intervals to allow fluid to exit from the 15.9 mm (5/8") bore.

Sliding over the smaller tube 14 is a larger tube 15 with a piston assembly 16 fixed on the lower end. The larger tube 15 is able to slide freely up and down the smaller internal tube 14 but is arrested at the top by the spring washers 14a. The larger outer tube 15 is attached to the wireline 18 by a sacrificial weak link 17.

The objective of the piston assembly is three fold: 1 To apply a hydraulic downforce to the motor assembly when the piston is allowed to move to the lower piston position; 2 To monitor progress of the coring operation, which will be noted by a series of pressure fluctuations as the downward movement of the inner shaft 14 exposes the ports 14c set into the member; 3 To restrict the download of the motor 10 when starting the operation by prefixing the compressive load of the spring washers 14a.

Figs. 4 and 5 illustrate the outer and inner assemblies respectively of a second embodiment of the invention.

Except where indicated otherwise, details of the embodiment of Figs. 4 and 5 are the same as that of Figs. 1 to 3. The outer assembly comprises, from the lower end, a side exit mandrel 21 approximately 6.4 m (21 ft) long, having stabiliser blades whose dimensions are appropriate to the hole size. The side exit mandrel 21 is connected at its upper end to a wear sub 22, 610 mm (2 ft) long, having a 66.7 mm (2 5/8") bore, which in turn is connected to an extension collar 23 approximately 5.03 m (16.5 ft) long, also with a 66.7 mm (2 5/8") bore.

Above the extension collar 23 is connected a piston mandrel 24 of length 9.14 m (30 ft), having a honed bore of diameter 63.5 mm (2.5"). Another extension collar 25 of length 9.45 m (31 ft) and bore 73.7 mm (2.9") is connected at the upper end of the piston mandrel 24, and to this is connected a key sub-assembly 26 which is approximately 610 mm (2 ft) long, with a 66.0 mm (2.6") bore. Drive keys are fitted within the bore of the key sub-assembly, for engagement with a drive shaft 28, described later.

The complete outer assembly is attached to the lower end of a drill pipe 52, and has a bit 20 connected to the lower end.

The inner assembly comprises, from the upper end, an inward flow diverter 27, which allows the fluid flowing down the drill pipe 52 to enter into the inner assembly. The drive shaft 28, connected to the diverter 27, is a three piece assembly of 71 mm (2.8") outside diameter containing four longitudinal keyways to match the keys fitted in the outer key sub-assembly 26.

Connected to the drive shaft 28, which is approximately 9.14 m (30 ft) long, is a pair of sinker bars 29, or heavy walled extensions, each being 4.57 m (15 ft) long. These correctly position the drive shaft 28 and piston sub-assembly 30 (described later) in relation to the outer assembly, and apply download to the bit 20.

The piston sub-assembly 30, approximately 0.3 m (12") long, is positioned between the sinker bars 29 and a further lower sinker bar 31. It contains two sets of two seals and its purpose is to restrict the flow of fluid down the outside of the inner assembly. This restriction will force the fluid inwards via the inward flow diverter 7, positioned higher. In use it sits within the honed bore of the piston mandrel 24.

The lower sinker bar 31, approximately 4.8 m (16 ft) to 6.1 m (20 ft) in length, is positioned to enable accurate spacing between the motor 32 and the piston sub-assembly 30, and to apply download to the bit 20.

The motor 32, approximately 2.7 m (9 ft) to 4.0 m (13 ft) in length and connected to the lower sinker bar 31, is typically a positive displacement motor having first 32a and second 32b stage motors. At the lower end of the motor 32 is a flow splitter 33, approximately 225 mm (9") long, which is fitted with standard type nozzles that can be changed on site to alter the motor/core barrel hydraulic characteristics.

The core barrel, connected to the flow divertor 33, is typically 7.9 m (26 ft) long and may be a standard "Longyear BV3" double or triple tube core barrel.

In this embodiment, two items used in the first embodiment are not utilised, namely the piston and slide assembly 13 and the drilling jars 11. Wireline jars may be used instead of drilling jars to break the core or to jar the inner assembly loose. When operating the coring device, a wireline winch is used to control the forward progress and to limit the download applied to the drill bit 20. The piston sub 30, working within the honed bore of the piston mandrel 24, restricts the exterior flow of drilling fluid around the outside of the inner assembly. The fluid is instead forced through the inner assembly and thus is used to operate the motor 32. This flow will also produce a down load to the core barrel 34, which is useful when operating in higher angle holes (i.e.

greater than 10).

DETAILED DESCRIPTION OF THE CORING OPERATION Figs. 6a, 6b and 6c show the selective coring operation carried out from a drilling platform 53. Fig. 6a shows the core barrel assembly 50 being lowered in the outer assembly 51 by the wireline unit 55 before coring.

Fig. 6b shows the core run being made into the reservoir 56. Fig. 6c shows the full core barrel being retrieved by the wireline 18.

The outer assembly 51, comprising the tricore drill bit 1 and the outer mandrels 2, 3, 4, is secured to the drill pipe 52 by the appropriate API connections.

There is no requirement for either drill collars or heavy weight drill pipe. The assembly is positioned within the well bore at the point from which the core is required, and hung off on the pipe ram and within the BOP stack 54. The slip joint 5 is attached to the top drillpipe and set in the slips in a position to allow safe tide and wave movement.

Then the inner assembly, comprising the mining corebarrel 6 (complete with corehead, reamer shell, inner barrels and bearing assembly), is fixed to a section of standard BQ drill pipe 8 using standard mining connections. A flow diverter 9 is installed into the top of the drill pipe. A suitable slimhole positive displacement drilling motor 10 with preaffixed keys located externally on the motor body is connected to the flow diverter 9. A fishing jar 11 is fixed to the top of the motor. A sinker bar 12 is attached to the top of the jar with the piston assembly 13 installed last. The piston is attached to the wireline 18 by a sacrificial weak link 17 and a circulating head 19 is fitted to the top of the drill pipe when the assembly is lowered clear of the tooljoint.The coring barrel is run in at the same time as fluid is pumped into the drill string at approximately 50-75 litres per minute (10-15 GPM) (see Fig. 6a). A pressure increase will be noted when the piston assembly 13 enters into the bore of the mandrel of the load control housing 4. At this time lowering will cease and various pressure measurements will be recorded using different pump rates. Using a fixed GPM (fluid supply rate), lowering will continue. An increase in pressure will indicate the core bit has contacted the side wall and is starting to drill (see Fig. 6b). Gentle lowering of the wireline is continued at the same rate as coring. Alterations in both speed of rotation (RPM) and wight on bit (WOB) can be achieved by increasing the flow rate and lowering the piston to the bottom of its travel on the slide bar. A steady pressure and no progress will indicate a full barrel or difficulties which will require removal of the internal coring assembly.

It should be noted that the purpose of the keyways manufactured within the mandrel of the drive sleeve 3 and the corresponding keys affixed to the top of the motor body, are for the specific purpose of transferring the reverse torque produced by the mud motor to the static drill pipe. Without this feature, the motor body would revolve, and not the corebarrel and drill bit attached to the drive shaft.

To remove the inner assembly it is necessary to stop pumping and pull upwards on the wireline 18 (Fig. 6c).

If overpull is recorded, stop at the given weight and wait for the jars to act. A reduction of overpull will signal the core has been sheared and the assembly can be winched to surface. Pumping can recommence when the piston and slide assembly 13 is clear of the load control housing 4. At surface, the complete corebarrel (only) is removed for emptying purposes.

Although the operation has been described with reference to the embodiment illustrated in Figs. 1 to 3, the operation of the embodiment illustrated in Figs.

4 and 5 is similar.

In order to obtain a further core from the same zone, additional sections of relevant drill pipe must be added at the top of the barrel, equivalent to the amount of core cut on the previous run. A different empty barrel is attached and lowered downhole as previously.

In order to obtain a further core from a different zone, it is necessary to stand back the inner assembly, then remove the compensating wireline pulley from the elevators. Remove the circulating head, release the pipe rams and remove the slip joint. The drill string is then positioned at the next interval where it is again retained in the pipe rams.

On jack-ups or fixed installations, the slip joint and pipe rams are not required, since landing in the slips is sufficient.

It is clear that the assembly of the above mentioned tools is capable of cutting cores from the lower side of a non-rotating static drillstring. It is also clear that the majority of tools assembled are standard items in regular use for different purposes. Only the outer mandrels and the piston assembly are custom made for the purpose of this operation.

The apparatus and method according to the invention make it possible to cut and recover cores from the sides of pre-drilled holes. This may be undertaken through the bore of drill pipe/tubing or a similar conductor which is held in a static position, when using jack-ups the pipe being held in the slips, and when using semi-submersibles the pipes being hung off on the pipe rams. It is not necessary to create a ledge or secure the coring assembly within the predrilled well bore.

The path of the corebarrel 6 into the well bore is controlled by the angled hole through the lower mandrel 2.

The core is cut using a downhole motor operated by drilling fluid, with the reactive torque controlled by the keyways cut within the drive sleeve mandrel 3.

The option exists to break off the core by: a) straight wireline pull; or b) utilisation of a fishing jar The use of a piston and slide bar device fitted with a by pass port and spring allows additional hydraulic down force to be applied by means of the piston. The bypass ports and spring supply positional information and enable finite control.

Claims

1. An apparatus for obtaining core samples from a wellbore or the like, comprising: an outer assembly adapted to be incorporated into a drill string and including a side exit mandrel; and an inner assembly adapted for connection to a wireline whereby the inner assembly is adapted to be raised and lowered within said drillstring and said outer assembly, said inner assembly including a core barrel and drive means for rotating said core barrel about its longitudinal axis, said core barrel being adapted to engage the wellbore formation to one side of said outer assembly via said side exit mandrel.

2. An apparatus according to claim 1, wherein said inner assembly is adapted to cooperate with said outer assembly so as to prevent rotation of said drive means relative to said outer assembly.

3. An apparatus according to claim 2, wherein said rotation preventing means comprises corresponding keys and key slots formed on said inner and outer assemblies.

4. An apparatus according to claim 3, wherein the keys or key slots of the inner assembly are located on a drive housing enclosing said drive means and positioned above said core barrel, and the corresponding key slots or keys of the outer assembly are located on a key sub-assembly portion of the outer assembly positioned above said side exit mandrel.

5. An apparatus according to any preceding claim, wherein the drive means comprises a hydraulic motor, preferably a positive displacement drilling motor.

6. An apparatus according to any preceding claim, wherein the inner assembly further includes flow diverter means located between the drive means and the core barrel.

7. An apparatus according to any preceding claim, further comprising means for controlling the downforce applied to the core barrel by hydraulic fluid circulating through the apparatus.

8. An apparatus according to claim 7, wherein said downforce controlling means comprises a piston subassembly located between the drive means and the upper end of the inner assembly and adapted to restrict the flow of fluid between the inner and outer assemblies.

9. An apparatus according to claim 8, wherein the outer assembly includes a piston mandrel having a honed bore.

10. An apparatus according to claim 9, wherein said piston sub-assembly is adapted to be located, in use, within said piston mandrel, such as to force fluid through the inner assembly and to provide a download on said inner assembly.

11. An apparatus according to claim 7, wherein said downforce controlling means comprises a piston and slide assembly located at the uppermost end of said inner assembly and including an outer barrel having a piston assembly at its lowermost end and being adapted for connection to a wireline at its uppermost end, and an inner barrel adapted for sliding movement within said outer barrel and having a lowermost end from which the remainder of the apparatus is suspended, in use.

12. An apparatus according to claim 11, wherein downward movement of the inner barrel within said outer barrel is restricted by resilient means, such as a stack of spring washers, located therebetween.

13. An apparatus according to claim 11 or 12, wherein the piston and slide assembly includes means for monitoring the operation of the apparatus, which may include fluid ports adapted to induce pressure variations in the hydraulic fluid in response to relative movement of the inner and outer barrels.

14. An apparatus according to any of claims 11 to 13, wherein the piston and slide assembly is adapted to cooperate, in use, with a load control housing forming part of the outer assembly and located at the uppermost end thereof.

15. An apparatus according to any preceding claim, wherein the inner assembly is adapted to receive spacer members between the drive means and the core barrel for varying the depth to which the core barrel may penetrate the wellbore formation.

16. An apparatus according to any preceding claim, wherein the side exit mandrel has an internal bore extending at an angle to the longitudinal axis of said mandrel and exiting at a point on the circumferential surface of said mandrel.

17. A method of operation of an apparatus according to one of claims 1 to 16.

18. A method of obtaining core samples from a bore, comprising the steps of: connecting an outer assembly to a length of drill pipe or conductor, said outer assembly including a side exit mandrel; holding said outer assembly in a static position within said bore such that the side exit mandrel is positioned at the level at which a core sample is required; connecting an inner assembly to a wireline, said inner assembly including a core barrel and drive means; lowering said inner assembly within said outer assembly such that said core barrel passes through the side exit of said side exit mandrel and engages the side of said bore; driving said core barrel into the side of said bore to cut a core sample; and removing said inner assembly and said core sample from the bore.

19. A method according to claim 18, wherein the side exit mandrel has an internal bore extending at an angle to the longitudinal axis of said mandrel and exiting at a point on the circumferential surface of said mandrel, and the path of the core barrel on exiting the side exit mandrel is defined by the angle of said internal bore.

20. A method according to claim 18 or 19, wherein said core is cut using a rotary motor, which exerts a reactive force on said outer assembly by keying means provided on the inner and outer assemblies.