WELL ABANDONMENT

The present invention relates to methods and apparatus for well abandonment and in particular, though not exclusively, to a method and apparatus for perforating, washing and cementing an annulus across a longitudinal section of the well.

When a well has reached the end of its commercial life, the well is abandoned according to strict regulations in order to prevent fluids escaping from the well on a permanent basis. In meeting the regulations it has become good practise to create the cement plug over a predetermined length of the well. As a well is constructed by locating conduits such as casing, lining and tubing into the well, the cement plug must extend over all annuli present in the well. In many cases all conduits are removed leaving the outer casing, including the annulus bounded by the formation.

One method of creating the cement plug is to mill away the inner conduit to expose the annulus behind the conduit and then pump cement into the enlarged area to create the cement plug. This method of section milling is expensive as the length of the cement plug is typically one to two hundred metres, removal of a section of conduit weakens the structure of the well, and several trips must be made into the well to change the milling head for different sizes of conduit i.e. casing used through the wellbore. An alternative method perforates the conduit and pumps cement through the perforations to travel up the annulus and thereby create a plug within the annulus. This 'perf and plug' arrangement has disadvantages in that material present in the annulus can restrict the flow and the adhesion of the cement and thus the required sealing of the plug is not achieved. The material present in the annulus may be various particles, deposits, for example so-called filter cake, and fluids remaining from previous downhole operations, including remaining drill cuttings, cement residues, baryte deposits and/or drill fluid .

GB2499172 to Hydra Systems AS discloses a method and washing tool for combined cleaning of an annulus in a well across a longitudinal section of the well, and subsequent plugging of the longitudinal section, wherein the method comprises the following steps: (A) conducting a perforation tool into a casing to said longitudinal section; (B) forming holes in the casing along the longitudinal section; (C) by means of a washing tool conducted into the casing on a tubular work string, pumping a washing fluid through the tubular work string and out into the casing via the washing tool; (D) by means of a directional means associated with the washing tool, conducting the washing fluid out into the annulus via at least one hole at a first location within the longitudinal section, after which the washing fluid will flow via the annulus and onward into the casing via at least one hole formed in at least one second location within the longitudinal section; (E) pumping a fluidized plugging material out into the casing at the longitud inal section; and (F) placing the plugging material in the casing and in the annulus along the longitudinal section so as to plug the casing and the annulus.

WO2015115905 to Archer Oil Tools discloses a bottom hole assembly (BHA) which enables a one trip procedure to set a bridge plug and release from that bridge plug with the BHA. A perforating gun is fired and a straddle tool above the gun is repositioned to wash the perforations as the opposed packer cups are moved along the perforations. At the end of the washing step a dropped ball closes a circulation port above the upper packer cups and the ball seat is blown out. A disconnect releases the lower part of the straddle tool with the lower packer cups that are uphole oriented . Cement can now be delivered past the upper packer cups and pressurized to squeeze the cement into the perforations using the upper cups to hold the pressure. Another ported sub above the upper cups has a port opened to prevent swabbing as the BHA is removed.

GB2499172 and WO2015115905 provide the advantage of washing the annulus prior to pumping the cement in a perf and plug method. In both cases the tools include 'directional means' in the form of packer elements such as swab cups which aim to direct the wash fluid through the perforations. The wash fluid is also ejected through nozzles in the tool at a high velocity so that it provides sufficient force to sweep the material out of the annulus. A known disadvantage of these tools is that the swab cups can interfere with other functions as they prevent circulation between the tool and the perforated conduit. Additionally, the wash fluid experiences a high fluid velocity reduction on exiting the nozzles and thus may be ineffective at sweeping the material up and out of higher located perforations.

It is therefore an object of the present invention to provide a method of removing casing from a well which obviates or mitigates one or more disadvantages of the prior art.

According to a first aspect of the present invention there is provided a method for the combined cleaning and plugging of an annulus in a well across a longitudinal section of the well, said annulus being located outside a conduit in the well, comprising the steps:

(a) perforating the conduit to form a plurality of holes in the conduit along the longitudinal section;

(b) passing a washing fluid through at least one of said holes at a first location on the longitudinal section and via the annulus to exit into the conduit through at least one of said holes at a second location on the longitudinal section; (d) pumping a fluidized plugging material through at least one of said holes to fill the annulus and conduit with fluidized plugging material and thereby plug said well along at least said longitudinal section of the well : and characterised in that, the method includes the step of creating pressure variations in the washing fluid.

In this way, by agitating the washing fluid this will assist in dislodging stuck material in the annulus so that it can be lifted and swept from the annulus and so improve cleaning and the subsequent placing of cement.

Preferably, the step of creating pressure variations is by use of a flow modifier device. Such a device may be the Agitator™ system provided by National Oilwell Varco. More preferably, the pressure variations are pressure pulses created in the washing fluid. In this way, the washing step can take a shorter time to complete as the cleaning action is more effective. Additionally, the velocity of the washing fluid is less critical. Preferably, the flow modifier device is located on a fluid carrying string run into the conduit. More preferably the flow modifier device is operated by pumping the washing fluid through the device to thereby create pressure pulses in the washing fluid. In this way, the pressure pulses are created downhole and no additional surface equipment is required.

Preferably, the washing tool is pulled out of the conduit along the longitudinal section while creating the pressure variations in the washing fluid. In this way, agitated washing is performed over the longitudinal section.

Preferably, the step of perforating is by operation of a perforating tool located on the fluid carrying string run into the conduit. The perforating tool may be a perforating gun as is known in the art. In this way, known tools may be used. Alternatively the perforating tool may be a punch tool being combined with an anchor to allow the punch tool to be operated by tension. In this way, the annulus is accessed quickly and inexpensively compared to section milling the conduit. Perforation of the longitudinal section may be performed in stages over the longitudinal section. In this way, a perforating tool having a length shorter than the longitudinal section can be used and thus the length of the longitudinal section is not restricted by the perforating tool length.

Preferably, the step of passing the washing fluid through the annulus is by operation of a washing tool located on a fluid carrying string run into the conduit. More preferably, the washing tool is located on the same string as the flow modifier device. In this way, the pulsed fluid output from the flow modifier tool can be directed immediately into the washing tool.

Preferably the fluidized plugging material is cement. More preferably, the fluidized plugging material is pumped from surface through a cementing tool.

The method may include the step of setting a bridge plug in the conduit. In this way, fluid entering the conduit is prevented from travelling downhole.

Preferably, the method includes the step of creating pressure variations in the fluidized plugging material. In this way, the cement is encouraged to distribute more evenly through the annulus and conduit. More preferably, the step of creating pressure variations in the fluidized plugging material is by use of the flow modifier device. More preferably, the pressure variations are pressure pulses created in the fluidized plugging material. More preferably the flow modifier device is operated by pumping the fluidized plugging material through the device to thereby create pressure pulses in the fluidized plugging material. In this way, the flow modifier device can be used to assist both in washing and cementing. The method may include providing the perforating tool, the flow modifier device and the washing tool on the same string and performing the method on a single trip in the wellbore. Alternatively, the method may include providing the perforating tool, the flow modifier device and the washing tool on the same string and performing steps (a) and (b) with the creation of pressure variations in the washing fluid, on a single trip in the wellbore.

The method may include providing the perforating tool, the flow modifier device, the washing tool and the cementing tool on the same string and performing the method on a single trip in the wellbore. The washing and cementing tool may be combined in a jetting tool. More preferably, the fluidised plugging material is pulsed on the same trip in the wellbore. Additionally, the fluidised plugging material is used to fill the conduit over the longitudinal section. In this way, a cement plug is created by a perf, agitated wash and agitated cement job in a single trip in the well.

Preferably, the string is a coiled tubing string . Alternatively, the string may be a drill string .

Preferably, the conduit is casing and the annulus is between the casing and the formation of the borehole. Optionally, the annulus is the B- annulus between inner casing and a surrounding casing. According to a second aspect of the present invention there is provided apparatus for combined cleaning and plugging of an annulus in a well across a longitudinal section of the well, said annulus being located outside a conduit in the well, comprising a string for running inside the conduit, the string including a perforating tool, a flow modifier device, and a washing tool. In this way, the annulus can be plugged and cleaned on a single trip in the well.

Preferably, the flow modifier device is a pressure pulse-generator actuated by fluid being circulated through the string. Alternatively the flow modifier device is a tubular member for mounting on the string, the member comprising a flow modifier for producing cyclic variations in the flow of fluid therethrough. Preferably, the flow modifier device is the Agitator™ system provided National Oilwell Varco.

Preferably, the perforation tool is a tubing punch. In this way, a controlled number of holes may be punched from the conduit without the use of explosives and without creating swarf and other cuttings. Circulation is also not required in the punch process.

Preferably the apparatus includes a cementing tool. Preferably, the cementing tool includes a plurality of radial ports. The cementing tool may have an outer diameter which is sized for the diameter of the conduit. In his way, the cement is encouraged to exit the cementing tool directly into the holes in the conduit. The cementing tool may include non-radial ports for the exit of cement. In this way, a stinger can form part of the cementing tool for the placement of cement in the conduit.

Preferably the washing tool is a fluid jetting tool. More preferably the washing tool and cementing tool are combined in the fluid jetting tool. In this way the fluidised plugging material can be distributed through the jetting tool.

Preferably, the fluid jetting tool includes a plurality of radial ports. The ports may include nozzles. In this way the pulsed washing/plugging fluid is accelerated on exiting the fluid jetting tool. The fluid jetting tool may have an outer diameter which is sized for the diameter of the conduit. In this way, the fluid velocity reduction on the washing/plugging fluid on exiting the fluid jetting tool and entering the annulus is minimised.

There may be a plug located on the string. In this way, a seal can be formed in the conduit at the end of the longitudinal section on the same run as the perf, agitated wash and agitated cementing is achieved. The plug may be a bridge plug as is known in the art.

In the description that follows, the drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results.

Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as "including," "comprising," "having," "containing," or "involving," and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes.

All numerical values in this disclosure are understood as being modified by "about". All singular forms of elements, or any other components described herein including (without limitations) components of the apparatus are understood to include plural forms thereof.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawing :

Figures 1(a) to 1(g) illustrate a method for the combined cleaning and plugging of an annulus in a well across a longitudinal section of the well, carried out on a single trip in a well, according to an embodiment of the present invention.

Reference is initially made to Figure 1 of the drawings which illustrates a method for the combined cleaning and plugging of an annulus in a well across a longitudinal section of the well, carried out on a single trip, according to an embodiment of the present invention. In Figure 1(a) there is shown a cased well bore, generally indicated by reference numeral 10, in which casing 12 lines the bore 14. A tool string 16 is run in the casing 12. Tool string 16 includes a perforating tool 20, a bridge plug 18, a jetting tool 22 and a flow modifier device 24. The jetting tool 22 and flow modifier device 24 may be formed integrally on a single tool body or may be constructed separately and joined together by box and pin sections as is known in the art. Other tools may be present. The tools may be arranged in any order. For example, the flow modifier device 24 may be located below the others if any of the others operate by use of a drop ball as the flow modifier device 24 may not provide an uninterrupted throughbore. Additionally the bridge plug 18 may be located at the end of the string 16.

Tool string 16 may be a drill string or coiled tubing having a central bore for the passage of fluid pumped from surface, as is known in the art. The perforating tool 20 is a perforating gun which is known in the art. The perforating gun produces multiple holes 32 through the casing 12. However, the perforating tool 20 may be any tool which can create individual holes 32 in casing 12. A punch tool may be used but would require an anchor in the string if it was to be set by tension.

The jetting tool 22 provides a plurality of radial ports 26 through which fluid can pass out of the tool 22. The ports 26 may include nozzles 28 to increase the velocity of the ejected fluid. The outer diameter of the tool 22 is sized so that ports 26 lie close to the wall 30 of the casing 12. This is done to encourage the fluid to pass directly into the holes 32 in the casing 12.

The flow modifier device 24 is a circulation sub which creates fluid pulses in the flow passing through the device. This can be achieved by a rotating member or a rotating valve. In a preferred embodiment the flow modifier device 24 is the Agitator™ System available from National Oilwell Varco. It is described in US6279670, US77077205 and US9045958, the disclosures of which are incorporated herein in their entirety by reference. It will be appreciated that a surface device could be used to create cyclic pressure variations in the fluid and preferably pulses. The flow modifier device 24 may create fluid pulses at any desired frequency. A typical frequency range may be 5.5 Hz to 21 Hz. Referring now to Figure 1(b), there is illustrated the perforating tool 20 positioned at a lower end 36 of a longitudinal section 38 selected over which a cement plug 54 is required . Guns have been actuated to create a spread of holes 32 through the wall 30 of the casing 12. The holes 32 are spaced circumferentially around the casing 12 and extend along the casing 12. The holes 32 provide multiple pathways between the annulus 42, bounded by the casing 12 and the formation 40, and the casing bore 44. The perforating tool 20 is pulled up to perforate the casing 12 at stages until it has provided holes 32 along the longitudinal section 38. Preferably, the placement of holes 32 extends above an upper end 46 of the longitudinal section 38. This is as illustrated in Figure 1(c). The annulus 42 is now accessible from the casing bore 44 along the full length of the longitudinal section 38. It will be appreciated that the perforation could be performed over the entire longitudinal section at one time. For this there may be a plurality of perforating guns which extend over the full length of the longitudinal section so that the perforation holes can all be made together on a single detonation event.

At this stage, the perforating tool 20 is released from the string 16 and allowed to fall into the casing bore 44. In this way, the perforating tool 20 can be considered as sacrificial. However, it will be appreciated that the perforating tool could be recovered to surface on the tool string 16. In this arrangement the bridge plug 18 would be arranged below the perforation tool 20 and the plug 18 would likely be set before detonation occurs.

In the preferred embodiment, plug 18 is now set in the casing 12 below the perforated holes 32. Plug 18 creates a seal across the casing 12 and provides a sealed section to the casing 12 preventing the passage of fluids across the plug 18 in either direction. Plug 18 is a bridge plug 18. Once the plug 18 is set in the casing 12, the string 16 is detached from the plug 18 using known methods and the string 16 is pulled to position the jetting tool 22 at a lower end 36 of the longitudinal section 38.

The jetting tool 22 is now aligned with the holes 32 at the lower end 36 of the longitudinal section 38. Washing can now begin. A washing fluid, typically viscosified brine, is pumped through the string 16 from surface. The fluid will pass through the flow modifier device 24 to produce a pulsed fluid. The pulsed fluid will exit the jetting tool 22 through ports 26 and nozzles 28, if present. The ports 26 and nozzles 28 are arranged to be perpendicular to the axis of the string 16. In this way, pulsed fluid is passes directly from the jetting tool 22 through the perforated holes 32 in the casing 12. It can be seen that the portion of the jetting tool 22 which includes the ports 26/nozzles 28 has a greater diameter than the drill pipe or coiled tubing forming the string 16 and the flow modifier device 24. This increased diameter is selected so that there is a minimal gap between the nozzle 28 and the perforated hole 32. In this way, there is a reduced fluid velocity reduction on the fluid exiting the jetting tool 22. Additionally, the reduced diameter of the flow modifier device 24 compared to the jetting tool 22 allows fluid to travel back into the casing bore 44 in the annulus 46 between the string 16 and the casing 12. The jetting tool 22 does not require to be rotated during washing, but may be rotated if desired. The pulsed washing fluid enters the annulus 42 between the casing 12 and the formation 40. Within the annulus 42 there is likely to be material 50 present in the form of various particles, deposits, for example so- called filter cake, and fluids remaining from previous downhole operations, including remaining drill cuttings, cement residues, baryte deposits and/or drill fluid . This material 50 can block the annulus 42 and adhere to the walls of the casing 12 and formation 40, thereby preventing cement from entirely filling the annulus 42 and adhering to the walls to create adequate cement bonding. The pulsing fluid hitting the materials 50 in the annulus 42 will cause them to break-up and dislodge from each other and the walls of the casing 12 and the formation 40. Pulsing the fluid is more effective than having fluid at a constant pressure as the pressure variations in the fluid give a pumping action to the fluid. This action assists in forcing the materials out of position and sweeping them upwards and preferably back though the perforation holes 32 into the casing bore 44. Thus the annulus 42 at the nozzles 42 is cleared of material and the walls of the casing 12 and formation 40 are washed clean. This is as illustrated in Figure 1 (d).

The string 16 is pulled out of the casing 12 so that the jetting tool 22 moves upwards across the longitudinal section 38, while ejecting pulsed fluid through the perforation holes 32. The movement can be staged so that the jetting tool 22 is static at a longitudinal position for some time before being moved to the next position. Alternatively the movement can be continuous but performed slowly enough to provide sufficient cleaning. This is as illustrated in Figure 1 (e). The pulsed fluid will continue to flow up the annulus 42 to drain back into the casing bore 44 taking the materials 50 with it and thereby evacuating the annulus 42 of materials 50 leaving a clean annulus 42 over the longitudinal section 38. The jetting tool 22 will now be at the upper end 46 of the longitudinal section 38.

In the preferred embodiment a cement slurry is now passed down the string 16, through the flow modifier device 24 and out through the jetting tool 22. Thus, the cement slurry is also pulsed by being passed through the flow modifier device 24. This agitates the cement so that it releases any gas to prevent pockets of empty space forming in the cement which could result in an incomplete cement plug. Agitation also encourages the cement slurry to enter the annulus 42 through the perforation holes 32 and better adhere to the formation 40. The pulsed cement slurry is also released from the string 16 via the jetting tool 22. The jetting tool 22 can therefore be used as a cementing tool as well as a washing tool. In the embodiment shown, see Figure 1(f), release of the bridge plug 18 exposes ports 52 on a base 54 of the jetting tool 22 which can be selectively opened to allow for the deposit of the pulsed cement slurry into the casing bore 44, if required. Pulsed cement slurry may also pass through the ports 26 and nozzles 28. The pulsed cement slurry will be deposited in the entire casing bore 44 to the bridge plug 18 and through the perforation holes 32 to fill the cleaned annulus 42 along the entire longitudinal section 38. The string 16 is removed from well 10 and once set, a cement plug 54 is formed across the longitudinal section 38. This is as illustrated in Figure 1(g).

In the cementing step, the string 16 may be moved through the casing 12. This will result in the jetting tool 22 being located in the cement slurry as it is formed and then being drawn out of the plug of unset cement slurry. In this way, the jetting tool 22 and string 16 perform the task of a cement stinger as is known in the art. By passing the cement slurry through the flow modifier device 24, the benefits in placing cement found using a stinger will be enhanced by the agitation of the cement slurry. Additionally, jetting the pulsed cement slurry directly through the perforation holes 32 via the nozzles 28 will further ensure that the entire annulus 42 is filled with cement, so providing an ideal cement bond between the cement plug 54 and the formation 40.

It should be noted that while we refer to a cement plug as being required, the plug can be formed of any fluidised plugging material. A cement slurry is typically used but the Applicants are aware of gels and other materials which may be used alone or in combination with cement to provide a fluidised material which sets hard and bonds to both the casing 12 and the formation 40 of the well bore 14. A volume of the cement slurry is typically calculated for deposit to create the cement plug 54.

The principle advantage of the present invention is that it provides combined cleaning and plugging of an annulus in a well across a longitudinal section of the well in which pressure variations are created in the washing fluid to improve cleaning and the subsequent placement of the plugging material. A further advantage of the present invention is that it provides combined cleaning and plugging of an annulus in a well across a longitudinal section of the well in a single trip into a well bore. It will be apparent to those skilled in the art that modifications may be made to the invention herein described without departing from the scope thereof. For example, the string may include a separate cementing tool from the washing tool. Additionally, reference has been made to shallower and deeper, together with upper and lower positions in the well bore. It will be recognised that these are relative terms though a vertical well bore is illustrated the method and apparatus apply equally to deviated and horizontal well bores.