WO2020076163A1

WO2020076163A1 - Plug and abandonment with overdisplaced cement

Info

Publication number: WO2020076163A1
Application number: PCT/NO2019/050215
Authority: WO
Inventors: Bård Marius JOHANSEN
Original assignee: Equinor Energy As
Priority date: 2018-10-12
Filing date: 2019-10-11
Publication date: 2020-04-16
Also published as: GB2577954A; GB201816652D0; GB201908761D0; GB2577954B; GB2577935B; GB2577935A

Abstract

A method of installing a plug in a well extending through a formation of the Earth and having a first seal between the casing and the tubing. The method comprises establishing a second seal within the tubing in proximity to said first seal, and establishing first passages through the tubing above both said first and second seals. A wet sealant is pumped into a space within the tubing above the second seal so that the sealant fills the tubing to a first height above the first passages whilst flowing through the first passages to fill the annulus to a second height greater than the first height, the sealant in the annulus between said first and second height representing a volume of over-displaced sealant. Pumping the wet sealant into a space within the tubing directly above the second seal comprises placing a landing structure within the tubing at or below said first height and above the first passages, and pumping a volume of sealant down the tubing and through the landing structure. Said volume of sealant is pumped down the tubing in a sealant train between a pair of wipers, wherein, upon landing on the landing structure, a lowermost wiper is arranged to open allowing sealant to flow through the landing structure.

Description

Plug and Abandonment with Overdisplaced Cement

Technical Field

The present invention relates to a method of establishing and verifying integrity of a plug at a downhole location within a well and more particularly to a method of establishing a permanent plug and abandonment plug, placing sealant in tubing, A- annulus and (if possible) B-annulus. Overdisplacement of sealant in the A-annulus compared to the tubing, are used for verification of A-annulus sealant integrity, B- annulus cement and or collapsed formation integrity and/or handling of control cable(s)/line(s) integrated in the A-annulus, to allow for the cable(s)/line(s) being integrated as part of the established permanent barrier plug.

Background

Oil and gas wells have in general three different purposes; as producers of hydrocarbons, injectors of water or gas for reservoir pressure support or for depositing purposes, or as exploration wells. At some point it is likely to be necessary to satisfactorily plug and seal these wells, e.g. after the wells have reached their end-of life and it is not economically feasible to keep the wells in service (so-called“plug and abandon”), or for some temporary purpose (e.g.“slot recovery”). Plugging of wells is performed in connection with permanent abandonment of wells due to decommissioning of fields or in connection with permanent abandonment of a section of well in order to permit construction of a new well bore (known as side tracking or slot recovery) with a new geological well target.

A well is constructed by drilling a hole into the reservoir using a drilling rig and then inserting sections of steel pipe, casing or liner into the hole to impart structural integrity to the wellbore. Cement is injected between the outside of the casing or liner and the formation (where the interval between the casing and the formation is referred to as the “B-annulus”) and then tubing is inserted into the casing to connect the wellbore to the surface (the interval between the tubing and the casing is referred to as the“A- annulus”). In some cases, a given casing may extend through one or more further casings to a deeper location, i.e. in a telescoping manner. Typically, the deeper the well, the more casings are required. For ease of reference, all of these entities inserted into the well are referred to here as“tubings”. When the well is to be abandoned, either temporarily or permanently, a plug must be established across the full cross-section of the well. This has traditionally been achieved by removal of the tubings from the well bore by pulling the tubings to the surface or by milling out a section of the tubings over an extent sufficient to form a plug, so-called “section milling”. Plugs are then established across the full cross-section of the well in order to isolate the reservoir(s) and prevent flow of formation fluids between reservoirs or to the surface. The plug location is typically above the hydrocarbon reservoir(s) to further prevent flow of formation fluids.

These traditional procedures have proved extremely costly due to the need to perform operations from drilling rigs, either floater or platform based. In order to reduce costs it is desirable to perform plugging of wells using intervention based methods, typically including pumping, wireline or coiled tubing. This allows for plugging to be executed from light well intervention vessels or ships. Performing plugging of wells as rigless activities, involving removal or milling of tubing, is also a costly and challenging operation, and in many cases not a possible option at all. Therefore, it is desirable to identify intervention based methods that allow for plugging of wells with the tubings remaining in situ.

Considering the case where a single casing is present between the production tubing and the formation, plugging procedures have been proposed that involve perforating, cutting, or otherwise partially destroying the production tubing in situ over a region of the well and thereafter pumping cement or another sealant into that location. The cement will pass through the cuts or perforations, leaving a volume of cement in the inner production tubing and the surrounding annulus, and a permanent A-annulus cement plug is established. At the depth of the A-annulus plug, integrity of additional surrounding tubings, with cement and/or collapsed formation on the outside, including integrity of the surrounding formation itself, needs to be known or verified in advance, or verified as part of the A-annulus cementing operation. This is necessary to fulfil the requirement of cross-sectional barriers after the A-annulus plug is established and verified. Due to the uncertainty of placement and contamination with other fluids, a rather long plug length is required per plug, e.g. 50 m, to ensure the required plug integrity. After the cement is placed and has cured, the cement plug is typically subjected to a large downwards force, for example 10 tonnes, and pressure tested to ensure that the cement is set properly. This constitutes integrity testing of the cement plug to ensure that it meets specified standards for permanent or temporary abandonment of a well. Of course, an improperly plugged well is a serious liability so it is important to ensure that the well is adequately plugged and sealed. WO 2015/044151 relates to a method of sealing a well in which a wireline is employed to position a tubing, typically referred to as a“stinger”, in a location within a wellbore where one or more openings have been created in a tubing installed in the wellbore to expose the formation. A sealant, e.g. cement, is injected through the stinger to form a plug at said location.

WO 2014/1 17846 relates to a method of plugging a well in which one or more explosive charges are detonated within a tubing or tubings extending through the well in order to remove, fragment and or cut one or more sections of the tubings around the entire circumference of the well to expose the surrounding formation or cement. The well is subsequently filled in the exposed region with a sealing material so as to form one or more plugs within the well.

US 2,918,124 A, US 2009/260817 A1 , US 2003/150614 A1 , US 5,667,010 A, US 3,053,182 A, WO 2012/096580 A1 and US 2005/028980 A1 describe methods relating to well plug and abandonment.

CA29941 13 discloses well abandonment using vibration to assist cement placement. Summary

According to a first aspect of the present invention there is provided a method of installing a plug in a well extending through a formation of the Earth, where the well comprises a casing, a tubing arranged concentrically within the casing, and a packer or other component providing a first seal between the casing and the tubing at a given longitudinal position such that an annulus is defined between the casing and the tubing above the first seal. The method comprises:

a) establishing a second seal within the tubing in proximity to said first seal;

b) establishing first passages through the tubing above both said first and second seals and pumping a wet sealant into a space within the tubing directly above the second seal so that the sealant fills the tubing to a first height above the first passages whilst flowing through the first passages to fill the annulus to a second height greater than the first height, the sealant in the annulus between said first and second height representing a volume of over-displaced sealant; and

c) allowing the sealant to set, wherein said step of pumping a wet sealant into a space within the tubing directly above the second seal comprises placing a landing structure within the tubing at or below said first height and above the first passages, and pumping a volume of sealant down the tubing and through the landing structure,

wherein said volume of sealant is pumped down the tubing in a sealant train between a pair of wipers, wherein, upon landing on the landing structure, a lowermost wiper is arranged to open allowing sealant to flow through the landing structure.

The method may further comprise: d) pressure testing the over-displaced sealant to determine the integrity of that sealant.

It will be appreciated that the first passages established through the tubing do not extend through the casing into the B-annulus. The sealant pumped into the A-annulus through the passages therefore remains substantially in the A-annulus.

In the event that the over-displaced sealant is determined to have sufficient integrity, the inside of the tubing may be further filled with sealant up to a third height. The first seal may be provided by a production packer, and step a) comprises establishing the second seal by setting a mechanical plug within the tubing. The method may alternatively comprise perforating or cutting the tubing and setting a bismuth plug or other plug of adhesive material within the tubing and extending into the surrounding space to also provide said first seal.

The step of pumping a wet sealant into a space within the tubing directly above the second seal may comprise placing a landing structure within the tubing at or below said first height and pumping a volume of sealant down the tubing and through the landing structure. Said volume of sealant may be pumped down the tubing in a sealant train between a pair of wipers, wherein, upon landing on the landing structure, a lowermost wiper is arranged to burst allowing sealant to flow through the landing structure. A further wiper may be located within the sealant volume such that, when the further wiper lands on said landing structure further pumping of the sealant is prevented.

The method may comprise, between steps d) and e), establishing passages through the tubing in the space up to said third height.

Step d) may comprise: i. establishing second and third longitudinally spaced passages in the tubing between said first and third heights;

ii. setting a pressure test plug within the tubing at a height between said second and third passages; and

iii. establishing an increased pressure above said pressure test plug whilst detecting any pressure change directly beneath the pressure plug and using any detected pressure change to confirm an integrity of the over-displaced sealant.

Prior to establishing an increased pressure above the pressure test plug, the pressure test plug may be operated in order to create a reduced pressure beneath the pressure test plug.

The method may comprise repeating the procedure of steps i, ii, and iii for different regions of the space between the first and third heights.

Said second and third longitudinally spaced passages may extend through the sealant in the annulus and through the casing into the formation, the method comprising performing a leak-off test to determine a leak-off pressure at which fluid leaks from the inside of the tubing into the formation, wherein said increased pressure established above said pressure test plug is less than the determined leak-off pressure.

The method may be employed in a well in which there are no control lines in the annulus between said first seal and said first height, or control lines extend longitudinally over only a part of this region.

The method may comprise, following step e):

f) allowing the sealant placed up to said third height to set;

g) pressure testing the sealant in the annulus above said third height to confirm the integrity of that sealant; and

h) filling the inside of the tubing up to a further height below said second heights.

Between steps g) and h), passages may be established through the tubing in the space up to said further height.

The method may comprise repeating steps f) to h) one or more times in order to extend the plug length. The method may be employed in a well in which there are control lines present in the annulus between said first height and the or each further height.

Step g) may comprise:

i. establishing fourth and fifth longitudinally spaced passages in the tubing between said third and said further heights;

ii. setting a pressure test plug within the tubing at a height between said fourth and fifth passages; and

iii. establishing an increased pressure above said pressure test plug whilst detecting any pressure change directly beneath the pressure plug and using any detected pressure change to confirm an integrity of the sealant within the annulus.

The method may comprise, prior to establishing an increased pressure above the pressure test plug, operating the pressure test plug in order to create a reduced pressure beneath the pressure test plug. The method may further comprise repeating the procedure of steps i, ii, and iii for different regions of the space between the third and further heights.

Said fourth and fifth longitudinally spaced passages may extend through the sealant in the annulus and through the casing into the formation, the method comprising performing a leak-off test to determine a leak-off pressure at which fluid leaks from the inside of the tubing into the formation, wherein said increased pressure established above said pressure test plug is less than the determined leak-off pressure.

Said first height may be a height sufficient to provide at least a primary barrier of said plug and data obtained at step d) is sufficient to infer the integrity of the sealant within the annulus below the first height.

In circumstances wherein the casing is a first casing and is located concentrically within a second casing, the packer or component, or a further packer or component, may provide a third seal between the first and second casings at a given longitudinal position, such that a second annulus is defined between the first and second casings above the third seal. In such circumstances, the method may comprise

e) establishing second passages through the tubing, set cement in the first mentioned annulus and first casing, above said first height, and pumping a wet sealant into a space within the tubing so that the sealant fills the tubing to a third height above the second passages whilst flowing through the second passages to fill the second annulus to a fourth height greater than the third height; and

f) allowing the sealant to set.

Advantageously, this embodiment of the method yields a PP&A plug across multiple concentric casings, installed as a second plug atop the first plug of steps a) to d), moreover bonded therewith within the tubing.

When at least one control line is present in the first annulus between the second and third heights, step e) of the method may comprise establishing at least one blind passage across the first casing and the or each control line into the set cement of the first annulus, prior to pumping the wet sealant. Advantageously, this technique ensures that the wet sealant gets pumped into such blind passage(s) that are opened to the tubing, and so mitigates the possibility of leaks developing through control lines.

A fluid may be circulated through the second passages prior to pumping the wet sealant, for cleaning the second annulus.

The step of pumping the wet sealant into the space within the tubing may comprise placing a landing structure atop a landing packer within the tubing above the second passages, and pumping a volume of sealant down the tubing and through the landing structure.

The volume of sealant may be pumped down the tubing in a sealant train between a pair of wipers wherein, upon landing on the landing structure, a lowermost wiper is arranged to burst allowing sealant to flow through the landing structure into the space within the tubing, whereby the sealant fills the tubing above the first plug of steps a) to d) and flows through the second passages into the second annulus.

A further wiper may be located within the sealant volume intermediate the pair of wipers such that, when the further wiper lands on the lowermost wiper, further pumping of the sealant is prevented.

A viscous pill may be loaded in the cement train, for providing a liquid fundament to the sealant volume intended for the second annulus. This embodiment advantageously inhibits a heavy sealant from dropping down under gravity. The method may comprise, prior to steps a) to d), identifying a suitable location for said plug using casing and/or formation integrity data known a priori or collecting using a through tubing logging operation.

The method may comprise a step of verifying a maximum height of good sealant in the tubing by lowering a stroker tool with a spear attached into the tubular and operating the stroker tool to penetrate through poor sealant to good sealant.

The method may further comprise, after step (c) and prior to step (e), perforating the tubing directly above the first height, wherein perforating the tubing comprises cutting through one or more control lines located in the A-annulus adjacent to the tubing, wherein the inside of the tubing is further filled with sealant to a height above the highest perforation, using coiled tubing, coil hose, a dump bailer on wireline, or drill pipe deployed into the tubing, such that sealant flows into the one or more perforations.

The method may further comprise perforating the tubing at one or more additional locations at different heights above the first height.

The method may further comprise, after step (b) and prior to step (c): perforating the tubing at a first location directly above the first height, such that the wet sealant flows through the perforation from the A-annulus into the tubing to fill the tubing to an initial height above the perforation, wherein perforating the tubing includes cutting through one or more control lines located in the A-annulus adjacent to the tubing.

The method may further comprise applying a pressure differential between the tubing and the A-annulus to control the flow of wet sealant from the A-annulus to the tubing.

The method may further comprise setting a mechanical plug in the tubing to prevent further flowing of the wet sealant through the perforation into the tubing.

The method may further comprise: perforating the tubing at a second location directly above the initial height; and allowing or causing the wet sealant to flow through the perforation at the second location and to fill the tubing to a subsequent height higher than the initial height and above the perforation at the second location.

The method may further comprise: perforating the tubing at a third location directly above the subsequent height; and allowing or causing the wet sealant to flow through the perforation at the third location and to fill the tubing to a further subsequent height higher than the subsequent height and above the perforation at the third location.

The flow of cement from the A-annulus into the tubing may be controlled using a choke on the tubing and/or the A-annulus.

The method may be performed after the further wiper has landed on the landing structure.

The method may further comprise, after setting the mechanical plug, allowing the wet sealant to set.

The method may further comprise removing the mechanical plug.

The method may further comprise: removing at least a portion of the set sealant in the tubing, such that the top of the remaining set sealant is below the lowest perforation; and pressure-testing the sealant filling the one or more perforations to determine the integrity of that sealant.

Pressure-testing the sealant may comprise:

i. establishing second and third longitudinally spaced passages in the tubing between said first and third heights;

Prior to establishing an increased pressure above the pressure test plug, the pressure test plug may be operated in order to create a reduced pressure beneath the pressure test plug. The method may comprise repeating the procedure of steps i, ii, and iii for different regions of the space between the first and third heights. At least one perforation may be located longitudinally between the second and third longitudinally spaced passages. Establishing the second and third longitudinally spaced passages may comprise cutting through the control line that is located in the A-annulus adjacent to the tubing; and any detected pressure change may also be used to determine the integrity of the sealant filling the one or more perforations. The tubing may be further filled with sealant up to a third height using a dump bailer, coiled tubing, coil hose, or drill pipe deployed into the tubing.

According to a second aspect of the present invention there is provided a method of installing a plug in a well extending through a formation of the Earth, where the well comprises a casing, a tubing arranged concentrically within the casing, and a packer or other component providing a first seal between the casing and the tubing at a given longitudinal position such that an annulus is defined between the casing and the tubing above the first seal. The method comprises:

b) establishing first passages through the tubing above both said first and second seals and pumping a wet sealant into a space within the tubing directly above the second seal so that the sealant fills the tubing to a first height above the first passages whilst flowing through the first passages to fill the annulus to a second height greater than the first height, the sealant in the annulus between said first and second height representing a volume of over-displaced sealant; c) allowing the sealant to set;

d) pressure testing the over-displaced sealant to determine the integrity of that sealant.

Step (d) may comprise cutting a control line above and below a mechanical plug set within the tubing, wherein the control line is located in the annulus, and pressure testing the over-displaced sealant between the locations at which the control line is cut.

Brief Description of the Drawings

Figure 1 illustrates a region of interest of a well to be plugged using an A-annulus cementing method;

Figures 2 to 14 illustrate operations involved in providing a region of over-displaced cement in the A-annulus;

Figures 15 to 24 illustrate operations involved in verifying the integrity of the overdisplaced cement region;

Figure 25 illustrates the case where a sufficient length of A-annulus cement without control cable(s) integrated is present and the A-annulus PP&A plug functions as a single barrier (Primary or Secondary); Figure 26 illustrates the case where a sufficient length of A-annulus cement without control cable(s) integrated is present, and the integrity of the cement in the B-annulus is confirmed from the cement log, and A-annulus PP&A plug functions as a single barrier or combined (Primary & Secondary);

Figures 27 to 36 illustrate operations for the case where integrity of the B-annulus cement and/or collapsed shale is not known, and the overdisplaced A-annulus cement is used to verify integrity of a given interval of the B-annulus in the area with overdisplaced A-annulus;

Figures 37 to 42 illustrate verification operations for the case where a control line(s)/cable(s) is integrated through part the PP&A A-annulus cement plug;

Figure 43 illustrates the use of a verification of a region of overdisplaced A-annulus cement with control line present to verify a region of the PP&A plug with control line present;

Figure 44 illustrates the use of a verification of a region of overdisplaced A-annulus cement with control line present as a basis for forming a plug in that verified region; Figure 45 illustrates the use of a verification of a region of overdisplaced A-annulus cement with control line present, and verification of B-annulus cement in that same region, as a basis for forming a plug in that verified region; and

Figures 46 to 69 illustrate operations involved in providing regions of over-displaced cement in multiple concentric annuluses.

Figures 70 to 73 illustrate a method of cementing a region of the overdisplaced A- annulus containing control lines and set cement, where the method involves destroying/cutting a portion of the control lines, prior to placing cement in the volume previously occupied by the destroyed/cut portion of the control lines .

Figures 74 to 82 illustrate a method of cementing a region of the overdisplaced A- annulus containing control lines, performed while the cement is still wet.

Figures 83 to 88 illustrate an optional procedure to verify cementing of control lines. Figures 89 to 94 illustrate a process of cementing the A-annulus only, as an alternative to the process illustrated in Figures 9 to 14.

Detailed Description

The embodiments described here rely on a premise that a satisfactory plug can be achieved by combining a lower primary cement barrier and an upper secondary cement barrier. Both barriers are located above the production packer and are formed by placing cement within the central tubing and in the surrounding annulus, the “A- annulus”, between the tubing and the casing. During well establishment, the annulus between the casing and the formation, the“B-annulus”, will have been cemented in the area closest to the casing shoe. Shallower in the well, collapsed shale may be present outside the casing. Depending on the depth in the well where the primary and/or secondary barrier plugs are to be placed, the annulus between the casing and formation can either be filled with cement and/or collapsed formation. Both are acceptable for forming the outer cross section of the A-annulus PP&A plug, as long as the integrity of either the cement and/or collapsed formation is known/verified. In order to use this region of B-annulus cement, together with an A-annulus plug, for a primary barrier, an assessment of the integrity of the casing cement can either rely on“job” data obtained when the cementing was originally performed or available logging data. In order to use the same region of B-annulus cement also for a secondary barrier, an additional verification of the casing cement is required. This can either be obtained from already available cement logs or through other verification methods performed as part of the A-annulus cementing operation. In order to use a region of the collapsed formation in the B-annulus, either for the primary and/or the secondary barrier, integrity needs to be verified either through cement logging or through other verification methods performed as part of the A-annulus cementing operation.

It is known that control lines (electrical and/or hydraulic) within the A-annulus can affect the integrity of the cement in that annulus when forming the plug. The embodiments presented here further rely on the premise that control lines (electrical and/or hydraulic) may be present in the A-annulus over at least part of the barrier plug region, independently of whether the plug functions as a primary and/or secondary barrier only or as a combined barrier.

Figure 1 illustrates a region of interest of a well to be plugged using the A-annulus cementing method presented in the following description. The well comprises an outer cement casing or collapsed formation 1 which is located in the B-annulus between the casing 2 and the surrounding formation 3. A tubing 4 is located concentrically within the casing 2, so as to define an A-annulus 5 between the tubing and the casing. A production packer 6 is located close to the bottom of the tubing 4 and isolates the region of the A-annulus above the production packer 6 from the region below. An alternative to the production packer 6 as base for cement in the A-annulus can also be established if needed. Figure 1 also shows an upper portion of a liner 7 that is attached to the tubing 4 by a liner hanger 8. Also shown in the Figure is a single control line 9 which extends down through the A-annulus and is connected to a downhole pressure gauge 10. The control line in this example is an electrical cable. Of course, other types of control lines coupled to various gauges, components etc, may be present, including optical cables and hydraulic lines.

The region identified by the broken lines and reference numeral 1 1 is the region in which the plug, and in particular the primary and secondary barriers, are to be formed, and in which the tubing and the A-annulus therefore needs to be cemented.

Figures 2 to 45 illustrate a series of operations that are performed sequentially in order to form the plug in the well region 1 1 of Figure 1. The sequence presents establishment and verification of integrity of an A-annulus permanent plug and abandonment (PP&A) cement plug using the production packer in the A-annulus as the base for cement. The operations are as follows:

Figure 2. A mechanical plug 12 is run into the tubing 4 on a wireline and is located below the production packer but as close as possible to it (in some cases the mechanical plug 12 may be located above the production packer). A tool (not shown) is then run into the tubing 4 to punch holes 13 through the tubing 4 at a location just above the production packer 6 to allow for circulation between the tubing 4 and the A- annulus 5.

Figure 3. Fluid 14, for example seawater, is circulated down through the tubing, out through the holes 13, and up the A-annulus 5, in order to clean these areas in preparation for cementing. Cleaning chemicals to ensure optimal conditions for cementing can be introduced and, if well completion design allows, circulation can be carried out in both directions. Water 14 remains in the well at the end of this operation.

Figure 4. For scenarios where integrity of cement and/or collapsed formation needs to be verified as part of the A-annulus cementing operation, a logging BHA 15 is run internally in the inner tubing 4 for logging through the inner tubing and out into the B- annulus to identify areas in the B-annulus containing cement and/or collapsed shale 1. Using this data together with leak testing (performed after cement is placed in the A- annulus 5), the integrity of the B-annulus can be verified and confirmed sufficient to constitute the outer cross-section of the barrier plug.

Figure 5. A landing packer 16 for cement wipers, with integrated one-way valve (flapper valve, check valve, float or similar) 17 is then run in on a wireline to a position H1 just above but as close as possible to the punched holes 13, and is tested in situ. The one-way valve 17 is included to avoid U-tubing of cement from the A-annulus 5 and into the tubing 4, thereby allowing for overdisplacement of the A-annulus 5 when cement is placed in the tubing and the A-annulus.

Figure 6. A drift wiper 18 is then pumped down the tubing 4 and lands on the landing packer 16, sealing the landing packer 16 and creating an enclosed volume above.

Figure 7. The pressure in the tubing 4 above the drift wiper 18 is increased in order to shear an integrated burst disk 19 in the drift wiper. As the disk 19 shears, circulation is re-established from the tubing 4, down through the drift wiper 18 and landing packer 16, and into the A-annulus 5 through the punched holes 13.

Figure 8. Circulation 20 through the sheared burst disk 19 is performed to ensure sufficient circulation rate for performing the cementing operation, and to gather reference pressure/rate data prior to introducing cement into the well.

Figure 9. A“cement train” 21 is then pumped down the tubing 4 in a known manner. The cement train comprises a volume of cement 22 sandwiched between an upper wiper 23 and a lower wiper 24. The train further comprises an intermediate wiper 25 which divides the cement volume into two parts, an upper volume 26 and a lower volume 27. As will be discussed below, the upper volume 26 provides tubing cement whilst the lower volume 27 provides A-annulus cement. The total volume of cement 22 in the cement train may be around 2.5m³, dictated by the required length of the PP&A cement plug to be established. The cement train 21 is preferably pumped with as high a rate as possible down through the inner tubing 4, to minimize the risk of water bypassing the wipers 23,24,25 and contaminating the cement 22. Such a high rate is, for example, at least 500 litres per minute. Figure 89 illustrates an equivalent step performed in the same way, but using a cement train including only two wipers, and cement for the A-annulus only. In particular, the cement train 21 comprises the intermediate wiper 25 and the lower wiper 24, and the lower volume 27 of cement sandwiched between the intermediate wiper 25 and the lower wiper 24. The lower volume 27 provides A-annulus cement. In this alternative embodiment the cement train does not include the upper wiper 23 or the upper volume 26 of cement.

Figure 10. The lower wiper 24 of the cement train 21 lands on top of the drift wiper 18 with reduced rate, sealing the drift wiper 18 and landing packer 16 and creating an enclosed volume above. The drift wiper 18 and/or the landing packer 16 form a landing structure. The landing of the lower wiper, and in particular the sealing of the drift wiper and landing packer, can be detected as an increase in the pressure in the tubing with further pumping from the surface (since any passage of fluid past the landing packer is prevented at this stage). Figure 90 illustrates an equivalent step performed in the same way for the alternative method in which the cement train includes only two wipers, and cement for the A-annulus only.

Figure 1 1. A burst disk 28 is integrated into the lower wiper 24. Pressure in the tubing 4 above the cement train 22 is increased, shearing the lower wiper burst disk. In particular, upon detection of the increase in pressure with further pumping from the surface as described with reference to Figure 10, the pressure in the tubing is increased still further, above a threshold pressure at which the lower wiper burst disk will open/burst. Figure 91 illustrates an equivalent step performed in the same way for the alternative method in which the cement train includes only two wipers, and cement for the A-annulus only.

Figure 12. Circulation 29 is re-established from the tubing 4, down through the lower wiper 24, drift wiper 18 and landing packer 16 and into the A-annulus 5 through the punched holes 13. Figure 92 illustrates an equivalent step performed in the same way for the alternative method in which the cement train includes only two wipers, and cement for the A-annulus only.

Figure 13. As pumping continues, cement from the lower volume 27 continues to be expelled from the cement train 21 through the lower and drift wipers 24,18. Cement then passes through the holes 13 into the A-annulus 5. Pumping at this stage is executed with a very low flow rate to ensure proper gravity placement of the“heavy” cement entering into the A-annulus 5. In particular, responsive to the opening of the lower wiper burst disk, which can be detected as a significant drop in pressure in the tubing, the pumping rate is reduced to the low flow rate. With a sufficiently low rate, possibly within the range of, 50-300, 150-200, or preferably less than 100 litres per minute, the heavy (high specific gravity compared to water) cement creates a cement- front in the A-annulus 5,“pushing” all the water present up the A-annulus, thereby creating a“perfect” cement displacement.

When the intermediate wiper 25 of the cement train 21 lands on top of the lower wiper 24, the lower volume 27 of the cement train will have been completely expelled into the A-annulus 5 and the upper volume 26 of the cement train is left in the tubing 4. The equivalent step is performed in the same way for the alternative method using two wipers and A-annulus cement only, with the exception that no upper volume of cement is left in the tubing, as illustrated in Figure 93. Whilst no upper volume of cement is left in the tubing, a portion of the A-annulus cement from the lower volume is in fact left in the tubing between the mechanical plug 12 and the landing packer 16. The region 30 of the A-annulus cement above the top of the upper wiper blade 23, or above the intermediate wiper blade 25 in the case that no upper wiper 23 is used, is referred to as the“overdisplaced A-annulus”, with the top of the overdisplaced annulus being at a height H2 above the mechanical plug 12. The height H2 may be around 70m above the upper wiper 23 of the cement train 21 . The distance between the upper and intermediate wiper blades is dictated by whether the established A-annulus PP&A plug is to function as primary or secondary barrier only, or as a combined barrier.

Figure 14, or Figure 94. The cement is then allowed to set before further operations are performed.

A set of verification tests are required to verify acceptable cement integrity of the A- annulus and tubing cement, or A-annulus cement only. With reference to the A-annulus cementing as illustrated in Figures 89 to 94, verification of cement integrity in the A- annulus and/or verification of integrity in the B-annulus, and/or handling of control line(s), can be executed as described with reference to Figures 16, 18 to 23, 27 to 35 and 70 to 88.

Figure 15. The proposed method uses the overdisplaced A-annulus cement 30 for verification of the A-annulus cement forming the barrier plug. As this overdisplaced cement is considered“worst case” from a cement contamination perspective, proving acceptable integrity of this“worst-case” cement makes it reasonable to conclude that the A-annulus cement further down, i.e. part of the PP&A plug, is of the same or better quality.

Figure 16. With the cement set, a conventional cement bond logging BHA 31 is run to identify/log the top of the cement 30 in the overdisplaced A-annulus. Other alternative technologies in the market, can also be considered for identifying the top-of-cement (TOC) in the A-annulus 5. A confirmation of the exact depth of the transition between cement and water in the A-annulus 5 provides proof of good volume control during the cement operation, i.e. the cement in the A-annulus 5 has been placed where planned and required. Figure 17. A spear 32 is then run into the tubing on a wireline stroker tool 33 to tag the top of the cement in the tubing 4. If required, the spear is designed to shear through the upper wiper 23 and tag the solid cement present below the wiper 23. A solid tag is obtained to verify integrity of the tubing cement.

Figure 18. A wireline punching/perforation tool 34 is lowered into the tubing 4, and a set of holes 35 is punched through the tubing 4 and into the A-annulus cement 30 at a given depth, as close to top of cement in the tubing as possible, but without penetrating the casing 2.

Figure 19. A leak/inflow test is performed by exerting a pressure down the tubing 4 and measuring the response pressure at the top of the A-annulus 5. The result provides an indication of the integrity of the overdisplaced A-annulus cement 30.

Figure 20. Further holes 36,37 are punched in the tubing 4 using the punching/perforation tool 34 at two different heights above the first punched holes 35. Again, these holes 36,37 extend part-way through the tubing 4 and into the A-annulus cement 30 but do not penetrate the casing 2.

Figure 21. The leak/inflow test is repeated after each punch sequence to verify that good integrity of the A-annulus cement 30 is obtained also for shorter sections of A- annulus cement, as the actual length of overdisplaced A-annulus cement inflow tested is reduced after each punch sequence.

Figure 22. A barrier verification system (BVS) 38 is installed in the tubing 4 between the lower set of holes 35 and the intermediate set of holes 36. This system comprises a mechanical bridge plug with a wireless pressure gauge attached beneath it. With the mechanical bridge plug set, continuously recorded pressure from the gauge is transmitted to the surface through a wireless transceiver 39 attached to a wireline cable 40. This can be used to monitor for leaks during leak testing.

Figure 23. The tubing 4 is pressured up above the mechanical plug of the BVS 38 to create a pressure differential across the two punched/perforated zones 35,36 closest to the cement plug in the tubing 4. With the pressure differential established, the pressure reading from the BVS 38 is used to confirm if leaks are present in the A- annulus cement 30. In the event that a very high pressure differential is required for testing, the mechanical bridge plug in the BVS 38 can be equipped with an atmospheric pressure chamber 41 on bottom. Once the system is installed, the atmospheric pressure chamber 41 is opened to the space 42 trapped between the mechanical plug and the tubing cement plug.

With pressure on top of the mechanical plug of the BVS 38 increased from the surface, the integrity of the A-annulus cement 30 between sets of holes 35, 36 is confirmed with a high differential pressure. If no leak is confirmed from the wireless gauge below the plug, the integrity is confirmed.

Figure 24. The barrier verification system 38 is removed. Apart from the stroker/spear tagging, all the performed tests will have provided a verification of the integrity of the overdisplaced cement 30 in the A-annulus. Therefore, it can be concluded that the integrity of the A-annulus cement part of the actual barrier plug, directly below the over displaced tested and verified A-annulus cement 30, will be of similar or even better quality. NB. As the control line 9 does not extend all of the way through the A-annulus cement forming the primary barrier, it is not necessary to test separately for a leak path along the control line 9 when assessing the integrity of the primary barrier. Embodiments of the method that are described hereinafter with reference to, respectively, Figures 37 to 45, Figures 46 to 69, and Figures 70 to 88, consider and address the leakage risk associated with control lines that extend over a substantial length of the annulus, encompassing the region of interest of the well to be plugged.

Figure 25. For wells where a sufficient length of A-annulus PP&A plug can be established without control cable(s) integrated, and the integrity of the cement in the B- annulus is confirmed from cement job data, an A-annulus PP&A plug 43 functioning as one single barrier (Primary or Secondary) is established and verified at this stage.

Figure 26. For wells where a sufficient length of A-annulus PP&A plug can be established without control cable(s) integrated, and the integrity of the cement in the B- annulus is confirmed from the cement log, an A-annulus PP&A plug 43 functioning as one single barrier or combined (Primary & Secondary) is established and verified at this stage. For wells where integrity of the B-annulus cement, and/or collapsed shale is not known, the overdisplaced A-annulus can be used to verify integrity of a given interval of the B- annulus in the area with overdisplaced A-annulus.

Figure 27. Using a perforating gun (not shown), lower perforations 45 are formed. The perforations may be placed as close to top of cement in the tubing 4 as possible or, if available, as dictated by through-tubing logging data. These perforations 40 extend through the tubing 4, over-displaced A-annulus cement 30, casing 2, and casing cement 1 , into the surrounding formation 3.

A leak-off test is performed by applying pressure from the top of the tubing. This test determines the pressure at which fluid is caused to leak through the lower perforations 45 into the formation 3. This also confirms of course that the lower perforations 45 actually extend through to the formation. NB. The perforations 45 can, if required, be circumferentially arranged such that they do not cut the control line 9.

Figure 28. A second, upper set of perforations 46 are then formed, again extending into the formation 3. The distance between the lower and upper perforations may be in the range of 30-50 m.

Figure 29. The barrier verification system (BVS) 38, as described in detail above, is installed in the tubing 4 between the upper and lower perforations. A leak-off test is performed on the upper perforations 46 to verify that these perforations also extend through to the formation 3.

Figure 30. By pressuring-up on top of the BVS 38, a pressure differential is created between the two perforation zones (region 47). If a high differential pressure is required for verification, the BVS atmospheric chamber 41 can be activated to establish an increased pressure differential. The executed leak test, confirmed through the wireless gauge integrated below the mechanical plug of the BVS 38, is intended to verify the integrity of the cement in the region between the upper and lower perforations 45,46, including both the A-annulus cement and the casing cement. The cement in this region within the A-annulus may include the control line 9. Flowever, as the upper and lower perforations 45,46 potentially do not cut the control line 9 during the perforation operation, the test primarily looks at the integrity of the cement per se rather than looking at leaks along and through the control line 9. Figure 31. For wells where through-tubing logging data is available, and similar logging responses are found in sections forming parts of the A-annulus PP&A plug and in the overdisplaced A-annulus, the performed B-annulus verification in the overdisplaced A- annulus can be used as confirmation of the integrity of the B-annulus cement and/or collapsed shale part of the A-annulus PP&A plug. The established and verified A- annulus PP&A plug may then function as both the primary and secondary barriers 48,49 of the PP&A plug.

Figure 32. For scenarios where through-tubing logging data is not available or cannot be obtained, the interval with confirmed integrity by leak-testing through perforations may be used for establishing the required barrier plug(s). The tubing section between the upper and lower perforation 46,47 can then be filled with cement to form permanent barrier plugs. This may be done either with drill pipe, coiled tubing, or a wireline conveyed method. A method for filling this inner tubing using a wireline intervention is described in the following sequence.

Figure 33. A dump bailer (not shown) run on wireline filled with cement is used to dump cement 50 in the area between the lower and the upper perforations 46,47. Further runs of the cement dump bailer are performed in the event that additional cement is required.

Figure 34. The cement 50 is allowed to set.

Figure 35. The top of the cement 50 is tagged with a stroker/spear 32,33 run into the tubing on a wireline 51 .

Figure 36. A PP&A cement plug 52 is now established in the area where the B- annulus integrity has been verified and confirmed with a leak test performed in a perforated interval.

For wells where a control line(s)/cable(s) 9 are to be integrated through part of or through the entire PP&A A-annulus cement plug, an additional set of verifications as described in the following section can be performed to confirm acceptable integrity of the established barrier with the integrated line(s)/cable(s). Specifically, for wells where hydraulic control cable(s) 9 are to be integrated as part of the A-annulus PP&A plug, hydraulic control cable(s) will be filled with a sealing material from the surface prior to performing the A-annulus cementing operation. The sealing chemical will be pumped, and verified as set, prior to establishing the A-annulus cement plug.

Figure 37. With the A-annulus PP&A plug established, and integrity of the cement verified, a junk catcher 53 is positioned on top of the upper wiper 23.

Figure 38. Using a mechanical or explosive wireline cutter/puncher (not shown), a number of cuts 54 (in this example three) are formed in the tubing 4. The cuts extend part way into the overdisplaced A-annulus cement 30 and cut through the control line 9. The cuts 54 are placed as close to top of the cement in the tubing as possible, or as dictated by logging through-tubing data if available. The junk catcher 53 potentially collects debris produced during cutting and may be used to confirm a successful cut of the control cable 9.

Figure 39. The tubing 4 is pressured up to perform an inflow test of the overdisplaced A-annulus 30, through the established cuts 54, by monitoring for leaks in the A-annulus on top of the overdisplaced cement 30. This inflow test may identify if leaks exist through the control line/cable 9 itself by monitoring the internal pressure in the lines/cables 9 during the test.

Figure 40. A further set of cuts 55 are formed using the mechanical wireline cutter at a location shallower in the tubing 4. The distance between the upper and the lower cut zones 54,55 may be in the range of 30 to 50 m. Again, the cuts 55 extend part way into the A-annulus cement and cut through the control line 9.

Figure 41. The junk catcher 53 is pulled out of the tubing 4. Pieces of control cable/line 9 might be collected from the catcher 53 at the surface and used as confirmation the control cable 9 has actually been cut.

Figure 42. The BVS 38, as described in detail previously, is installed in the tubing between the lower and upper cuts 54,55. By pressuring up above the mechanical plug of the BVS, a pressure differential between the two cut zones is created. The integrated wireless gauge below the mechanical plug is used to confirm that there are no leaks. If a high differential pressure is required, the atmospheric chamber 41 integrated as part of the BVS 38 can be employed. As it is known that that the control line(s)/cable(s) 9 have been cut at the top and bottom of an interval, any potential leak path through the line(s) 9 themselves is exposed. Assuming that no leak is detected during the leak test, this verifies the integrity of the overdisplaced cement 30 in the A- annulus including the integrated control line(s)/cable(s) between the two cut zones.

Figure 43. With an approved leak test, this can be used as verification of the A- annulus PP&A plug integrity established below the tested interval, with similar control line/cable as tested, integrated as part of the PP&A plug. Dependent on integrity status of the B-annulus cement and/or collapsed shale in the B-annulus at position of the PP&A plug, the established barrier (PP&A cement plug) can work either as a single, or a combined barrier.

Figure 44. For scenarios where integrity of the B-annulus cement and/or the presence of collapsed shales in the area of the A-annulus PP&A plug below the tested interval (between cut zone 1 and 2) is not known or not possible to verify, and the integrity of the B-annulus in the tested interval is known or confirmed as acceptable, a cement plug in tubing can be established and verified with cement bailing and tagging as described previously with wireline dumping. With cement in place, the PP&A plug is established and verified.

Figure 45. For scenarios where integrity of the B-annulus cement and/or collapsed shale in the area of the A-annulus PP&A plug below the tested interval (between cut zone 1 and 2) is not known or not possible to verify, and the integrity of the B-annulus in the tested interval is not known, B-annulus verification can be done with a perforation at top bottom of the interval, as described in detail previously. With acceptable integrity of the B-annulus, and integrity of cemented A-annulus with integrated control line(s)/cable(s) confirmed, cement plug in tubing is established and verified in the tested area, with cement bailing and tagging as described previously with wireline dumping. With cement in place, the PP&A plug is established and verified.

The embodiment described hereinafter with reference to Figures 46 to 69 maintains reliance on the premise that a satisfactory plug can be achieved by combining a lower primary cement barrier and an upper secondary cement barrier, in situations where the well to be plugged comprises a plurality of concentric annuluses about the central tubing as a result of the presence of multiple concentric casings. Both barriers are still located above the production packer and are again formed by placing cement within the central tubing and in each surrounding annulus, initially between the tubing and the first casing, then between the first casing and a next casing intermediate the formation or a third casing, and so on. Figure 46 illustrates a region of interest of a well to be plugged using a multistage cementing method, typically at shallow depth (50 to 500 meters measured depth). The well comprises an outer cement casing or collapsed formation 1 which is located between an outer casing 56 and the surrounding formation 3, in an annulus that may be referred to as the D-annulus. A tubing such as a production tubing 4 is located concentrically within an inner casing 2, and the inner casing is located concentrically within the outer casing 56, whereby an A-annulus 5 and a B-annulus 57 are defined. The A-annulus 5 between the tubing 4 and the inner casing 2, and the B-annulus 57 between the inner casing 2 and the outer casing 56 are expected to be free of solid content, i.e. allowing for circulation if communication to the two annuluses is established. Also shown in the Figure is a single control line 9, which extends down through the A-annulus 5 and which is connected, for example, to a downhole pressure gauge (no shown). The control line in this example is an electrical cable. Alternative or further types of control lines coupled to various gauges, components etc, may be present, including optical cables and hydraulic lines.

The region identified by the broken lines and reference numeral 1 1 is the region in which the plug is to be formed, and in which the tubing 4 and the A-annulus 5 and the B-annulus 57 therefore need to be cemented.

Figures 47 to 69 illustrate a series of operations that are performed sequentially in order to form a permanent plug and abandonment (PP&A) cement plug, by circulating in the required sealant into the tubing 4 and the A-annulus 5 and B-annulus 57. The sequence establishes an A-annulus and B-annulus PP&A cement plug using one or more production packer(s) and/or other fundaments (e.g. bismuth or epoxy plugs) in the A-annulus 5 and B-annulus 57 as the base(s) for cement and, in alternative embodiments with extended further steps, verifies the integrity of that cementing.

Figure 47. A mechanical plug 12 is initially run into the tubing 4 on a wireline and positioned at bottom of target depth for the planned PP&A plug in the tubing 4, and then tested.

Figure 48. A fundament 6 is established above the mechanical plug 12, as close as possible to the plug 12, noting that in some embodiments the mechanical plug 12 may be located above the fundament 6. That fundament 6 may be implemented by one or more production packers, or by setting a bismuth plug or other plug of adhesive material such as epoxy illustrated in the Figure, apt to isolate the region of each A- annulus 5, 57 above the fundament 6, from the region respectively below it.

Figure 49. A fluid 14, such as seawater, is circulated down through the A-annulus 5, in order to verify the establishment of the fundament 6.

Figure 50. A tool (not shown) is then run into the tubing 4 to punch through passages 13 through the tubing 4 at a location just above the fundament 6, to allow for circulation between the tubing 4 and the A-annulus 5. The fluid 14 may be circulated again, down through the tubing 4, out through the passages 13, and up from the fundament 6 into the A-annulus 5, in order to clean these areas in preparation for cementing. Cleaning chemicals may be introduced to ensure optimal conditions for cementing and, if well completion design allows, circulation can be carried out in both directions.

Figure 51. A landing packer 16 for cement wipers, with integrated one-way valve (flapper valve, check valve, float or similar) 17 is then run in on a wireline to a position just above but as close as possible to the punched passages 13. The one-way valve 17 is included to avoid U-tubing of cement from the A-annulus 5 and into the tubing 4, thereby allowing for overdisplacement in the A-annulus 5 when cement is placed in the tubing 4 and the A-annulus 5.

Figure 52. A“cement train” 21 is then pumped down the tubing 4 in a known manner. The cement train comprises a volume of cement 22 sandwiched between an upper wiper 23 and a lower wiper 24. The total volume of cement 22 in the cement train may be around 2.5 m³, as dictated by the required length of the PP&A cement plug to be established and the respective volumes of tubing 4 and A-annulus 57 to cement.

Figure 53. The lower wiper 24 of the cement train 21 lands on top of the landing packer 16 and seals it, thereby creating an enclosed volume.

Figure 54. A burst disk 28 is integrated into the lower wiper 24. Pressure in the tubing 4 above the cement train 21 is increased, shearing the lower wiper burst disk. Circulation 29 is re-established from the tubing 4, down through the lower wiper 24, the landing packer 16 and into the A-annulus 5 through the punched passages 13.

Figure 55. Pumping continues with a very low flow rate within the tubing 4, whereby cement continues to be expelled from the cement train 21 through the lower wiper 24. Cement then passes through the passages 13 into the A-annulus 5. The low flow rate of the pumping again ensures proper gravity placement of the“heavy” cement entering into the A-annulus 5. When the upper wiper 23 of the cement train 21 lands on top of the lower wiper 24, the volume of cement 22 in the train 21 has been completely expelled into the tubing 4, with a portion of that volume displaced into the A-annulus 5 through the passages 13, and a second portion of that volume left in the tubing 4. The region 30 of the A-annulus cement above the top of the upper wiper 23 is referred to as the“first overdisplaced A-annulus”, with the top of the first overdisplaced annulus being at a height H2 above the mechanical plug 12.

Figure 56. The cement is then allowed to set, before further operations are performed.

Figure 57. When the cement is set, a conventional cement bond logging BFIA 31 is run to identify/log the top of the cement 30 in the first overdisplaced A-annulus 5. Further steps may be performed to verify the integrity of the cemented A-annulus 5, as described hereinbefore, such as sequential punching and inflow testing.

Figure 58. A tool 58 is then run into the tubing 4 to punch one or more blind passages 59 through the tubing 4, through the control line 9, and partway into the cemented A- annulus 5, at a location above the upper wiper 23 of the cement train 21. Three blind passages are shown in the example. The purpose of this step is to cut the control line(s) integrated in the cement.

Figure 59. The tool 58 further punches second passages 60 (two pairs of axially- opposed passages are shown in the example), through each of the tubing 4, the control line 9 and the cemented A-annulus 5, at a location intermediate the blind passages 59 and the upper wiper 23 of the cement train 21. The second passages 60 allow circulation between the tubing 4 and the B-annulus 57, through the first overdisplaced A-annulus 5.

Figure 60. A fluid 14 may be circulated again, down through the tubing 4, out through the second passages 60, and up from the fundament 6 into the B-annulus 57, in order to clean these areas in preparation for cementing.

Figure 61. A second landing packer 16 for cement wipers, again with integrated one way valve 17, is then run into the tubing 4 on a wireline to a position just above, but as close as possible to, the second passages 60. The one-way valve 17 is included to avoid U-tubing of cement from the B-annulus 57 and into the tubing 4, thereby allowing for overdisplacement of the B-annulus 57 when cement is placed in the tubing 4 and the B-annulus 57.

Figure 62. A second drift wiper 18 is then pumped down the tubing 4 and lands on the second landing packer 16, sealing the second landing packer 16 and creating an enclosed volume.

Figure 63. The pressure in the tubing 4 above the second drift wiper 18 is increased in order to shear a burst disk 19 integrated in the second drift wiper 18. As the disk 19 shears, circulation is re-established from the tubing 4, down through the second drift wiper 18 and the second landing packer 16, into the B-annulus 57 through the second passages 60. Circulation 20 through the sheared burst disk 19 of the second drift wiper 18 is performed to ensure sufficient circulation rate for performing the cementing operation, and to gather reference pressure/rate data prior to introducing cement into the well.

Figure 64. A second“cement train” 61 is then pumped down the tubing 4 in a known manner. The cement train again comprises a volume of cement 22 sandwiched between an upper wiper 23 and a lower wiper 24. The second train 61 further comprises an intermediate wiper 25, which divides the cement volume into two parts, an upper volume 26 and a lower volume 27. The upper volume 26 provides tubing cement whilst the lower volume 27 provides B-annulus cement. The total volume of cement 22 in the second cement train 61 may be comparable to the volume in the first cement train 21 , and is in any case dictated by the required length of the PP&A cement plug to be established. The second cement train 61 is preferably pumped with as high a rate as possible down through the inner tubing 4, to minimize the risk of water bypassing the wipers 23, 24, 25 and contaminating the cement 22.

Figure 65. The lower wiper 24 of the second cement train 61 lands on top of the second drift wiper 18, sealing both the second drift wiper 18 and the second landing packer 16, and creating an enclosed volume. A burst disk 28 is integrated into the lower wiper 24 of the second cement train 61.

Figure 66. Pressure in the tubing 4 above the second cement train 61 is increased, shearing the lower wiper burst disk 28. Circulation 29 is re-established from the tubing 4, down through the lower wiper 24, second drift wiper 18 and second landing packer 16 and into the B-annulus 57 through the second passages 60.

Figure 67. As pumping continues, cement from the lower volume 27 continues to be expelled from the second cement train 61 through the lower and drift wipers 24, 18. Cement then passes through the second passages 60 into the B-annulus 57. When the intermediate wiper 25 of the second cement train 61 lands on top of the lower wiper

24, the lower volume 27 of the cement train has been completely expelled into the B- annulus 57 and the region of tubing 4 located intermediate the upper wiper 23 of the first cement train 21 and the second landing packer 16. The upper volume 26 of the second cement train 61 , intermediate the upper wiper 23 and the intermediate wiper

25, is left in the region of tubing 4 extending between the upper and lower wipers 23, 24 and so fills the blind passages 59 across the punched control line 9.

The region 61 of the B-annulus cement above the top of the upper wiper blade 23 of the second cement train 61 is referred to as the“overdisplaced B-annulus”, with the top of the overdisplaced annulus being at a height H3 above the mechanical plug 12.

Figure 68. The cement is then allowed to set before further operations are performed. A set of verification tests may be performed to verify acceptable cement integrity of the established barrier plug, i.e. the A-annulus, B-annulus and tubing cement forming the barrier plug, as previously described.

In particular, with reference to Figure 69, a spear 32 is run into the tubing 4 on a wireline stroker tool 33 to tag the top of the cement in the tubing 4. If required, the spear 32 is designed to shear through the upper wiper 23 of the second cement train 61 and tag the solid cement present below the wiper 23. An environmental PP&A plug is thus obtained, corresponding to the cross section of the well and cemented regions between the upper and intermediate wipers 23, 25 of the second cement train 61 .

Figures 70 to 73 illustrate a method for mitigating or eliminating the risk of leaks along a control line or control line path in the A-annulus. The method is to be performed after the operations shown in Figures 1 to 14 have been completed.

Figure 70. The operations shown in Figures 1 to 14 have been completed. The control line 9 extends partially into the A-annulus cement forming the primary barrier. In Figure 70 the control line 9 is shown as being adjacent to the tubing 4. In some circumstances the control line 9 may be more radially distant from the tubing 4, as shown e.g. in Figures 1 to 14.

Figure 71. The tubing 4 is perforated at one or more locations to create one or more perforations 61 , which creates passages through the tubing 4 into the set cement in the A-annulus 5, cutting the control line(s) present in the cemented annulus at the depth of perforation. The lowermost perforation 61 is preferably close to the upper wiper blade 23, i.e. directly above the first height. Alternatively, cutting of the control cable(s) present in the cemented annulus, can be done using a wireline conveyed punching tool, e.g. the tool 34 described in relation to Figure 18. Wireline conveyed milling and/or reamer tools, capable of moving a section of tubing, cement and control cable(s) inside the milled/reamed cement, could also be utilized. Independent of method utilized for cutting control cable(s) inside the cemented annulus, the casing 2 will not be penetrated. Tools for such milling or cutting are known to the skilled person.

Figure 72. Wet sealant 63, e.g. cement in a fluid state, is added to the tubing to a height above the highest location at which the control cable(s) have been cut. The cement is added using coiled tubing, coil hose, a dump bailer on wireline, or drill pipe deployed into the tubing. The wet cement flows into the perforations and fills the one or more gaps in the control line created by cutting through the control line.

Figure 73. The cement is then allowed to set.

The integrity of the established PP&A cement plug, with integrated cut control cable(s) can be verified in form of tagging with use of the tagging spear as shown in Figure 17. Alternatively, a wireline, coiled tubing or drillpipe conveyed milling assembly can be used for dressing and tagging of the cement plug.

Figures 74 to 82 illustrate a method for mitigating or eliminating the risk of leaks along a control line or control line path in the A-annulus, which may be used in addition to, or as an alternative to, the method of Figures 70 to 73. The method of Figures 74 to 82 is to be performed after the operations shown in Figures 1 to 13 have been completed.

Figure 74. The operations shown in Figures 1 to 13 have been completed. The cement in the A-annulus 5 is in a fluid state. Figure 75. A punching/perforation assembly 71 is prepared on wireline, and is ready to run in hole as soon as it is confirmed that the intermediate wiper 25 has landed. The wireline punching/perforation assembly 71 is lowered into the tubing 4. The punching/perforation assembly includes a punching/perforation tool 72, and a plug 73. The punching/perforation assembly must be capable of creating holes in the tubing and at the same time cut the control line(s) in the annulus outside the tubing, without penetrating through the outer casing 2. A tool capable of logging temperature can be included in the assembly cutting the control cable(s) to allow for identifying top of cement in the annulus.

Figure 76. The punching/perforation assembly 71 is lowered to a position just above the upper wiper blade 23. The punching/perforation tool 72 is then used to perforate/punch through the tubing 4 and cut through the control cable(s) 9 as close to the top of the cement in the tubing as possible, i.e. directly above the upper wiper blade 23. The method as described herein is suitable for all type of control cable(s) (electrical and/or hydraulic and/or bumper wires) as long as the perforator/puncher is suitable for cutting the line(s) present. It is desirable to punch/perforate as close to tubing collars (connections between tubing joints) as possible. The likelihood of the line(s) being in direct contact with the tubing (limited/no gap) is higher close to connections, typically because of tubing clamps present at the connection. I.e. the probability of achieving a successful cut of the control line(s), independent of cutting method, is higher when executed close to tubing connections. The one or more perforations 74 extend through the tubing and the control line, but do not penetrate the casing 2.

Figure 77. The punching/perforation assembly 71 is raised to a higher position and the wet sealant, i.e. cement in a fluid state, flows from the A-annulus into the tubing through the one or more perforations. It is envisaged that the difference in density between the cement and the water, and the pressure head provided by the overdisplaced cement in the A-annulus, will itself cause the cement to“fall” and replace the water in the tubing. Flowever, if necessary, a choke on the tubing and/or A-annulus, and/or an applied pressure differential, is used to control the flow of the cement from the A-annulus into the tubing. If required, the perforation/punching tool 72 is then used to create a second set of perforations 75 at the higher position, again punching through the tubing 4 and cutting the control line(s) 9, ideally directly above the cement level in the tubing. Figure 78. The operations described for Figure 77 are repeated, i.e. the cement flows from the A-annulus into the tubing through the second set of perforations, and if required the punching/perforation procedure can be repeated (e.g. created a third set of perforations) until sufficient length of cement, with cut control cable(s) integrated, is achieved.

Figure 79. The cement flows from the A-annulus into the tubing through the third set of perforations, and the punching/perforation assembly 71 is raised to facilitate placement of the plug 73. As set out previously, a choke on the tubing and/or A-annulus may be used, if necessary, to control the movement of the cement from the A-annulus into the tubing. It is likely that a controlled placement of cement can be achieved with choke only, i.e. the plug as included in figure 55 and 55, could be considered optional

Figure 80. The plug 73 is set within the tubing, to prevent any further flow of cement from the A-annulus into the tubing. The cement covers all perforations, i.e. the TOC in the tubing is higher than the highest perforation. The plug is set at a position below the top of the cement in the A-annulus. Top of the cement in the A-annulus can be identified using a temperature log while waiting for cement to set.

Figure 81. The cement is allowed to set.

Figure 82. The plug 73 is removed.

Figures 83 to 88 illustrate a verification procedure that may be performed in a pilot phase. In particular, the procedure may be performed in a pilot well to verify the integrity of the cemented A-annulus with integrated cut control line(s) after the method of Figures 70 to 73 and/or the method of Figures 74 to 82 has been performed.

Figure 83. The cement in the tubing is drilled out, to a height below the lowest perforation position. The drilling apparatus required to drill out the cement is conveyed into the tubing using, for example, wireline, coiled tubing or drill pipe.

Figure 84. The TOC in the A-annulus including cement bonding, can verified using cement bond logging or other alternative logging tools, e.g. as set out previously for the BFIA 31 in Figure 16. Figure 85. Alternatively to logging, the integrity of the cement in the A-annulus with integrated cut control cable(s) can be verified using the leak-testing procedure described previously in relation to Figures 18 to 26 and/or Figure 27-30 as illustrated in Figure 85. Verifying the integrity of the cement in the A-annulus includes verifying the integrity of the cement filling the perforations, and in particular the cement filling the gap(s) in the control line(s). In the leak-testing procedure at least one perforation is located longitudinally between the second and third longitudinally spaced passages, i.e. between the sets of holes 35, 36.

Figure 86. The barrier verification system 38 is removed. Cement is then added to the tubing to replace the cement that was previously drilled out. The cement is added using a dump bailer (81 ) on wireline. Alternatively, the cement may be added using coiled tubing, coil hose or drill pipe, as illustrated in Figure 87.

Figure 88. The cement is allowed to set, and then the TOC in the tubing is verified.

The various operations described above may be used each time it is necessary to install a PP&A plug. Alternatively, the operations, or various subsets thereof, may be used to verify a specific PP&A plug or set of plugs, i.e.“pilot” plugs, with the results being used to verify that a reliable plug can be provided in a well of a given type. Thereafter, for wells of the same or similar type, further verification is not required (or the verification operations are reduced). A same or similar well type may have, for example, an identical or similar type of control line at a similar location. In some cases, where verification of the pilot plug(s) has been performed, plugging of further wells may be carried out without the need to provide overdisplaced cement.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.

Claims

CLAIMS:

1 . A method of installing a plug in a well extending through a formation of the Earth, where the well comprises a casing, a tubing arranged concentrically within the casing, and a packer or other component providing a first seal between the casing and the tubing at a given longitudinal position such that an annulus is defined between the casing and the tubing above the first seal, the method comprising:

c) allowing the sealant to set,

wherein said step of pumping a wet sealant into a space within the tubing directly above the second seal comprises placing a landing structure within the tubing at or below said first height and above the first passages, and pumping a volume of sealant down the tubing and through the landing structure,

2. A method according to claim 1 and comprising:

3. A method according to claim 1 or claim 2 and comprising:

e) in the event that the over-displaced sealant is determined to have sufficient integrity, further filling the inside of the tubing with sealant up to a third height.

4. A method according to claim 3, wherein said first seal is provided by a production packer, and step a) comprises establishing the second seal by setting a mechanical plug within the tubing.

5. A method according to claim 3, wherein step a) comprises perforating or cutting the tubing and setting a bismuth plug or other plug of adhesive material within the tubing and extending into the surrounding space to also provide said first seal.

6. A method according to any one of claims 1 to 5, wherein the sealant train is pumped down the tubing at a high rate, the landing on the landing structure is detected as an increase in the pressure in the tubing, and the opening of the lowermost wiper is detected as a decrease in the pressure in the tubing, and responsive to the detection of the opening of the lowermost wiper, a pumping rate is decreased such that the sealant flows through the landing structure at a low rate, wherein the high rate is optionally 500 litres per minute or more, and the low rate is optionally in the range of 50-300, 150-200, or preferably equal to or less than 100 litres per minute.

7. A method according to any one of claims 1 to 6, a further wiper being located within the sealant volume such that, when the further wiper lands on said landing structure further pumping of the sealant is prevented,

the method optionally further comprising, between steps d) and e), establishing passages through the tubing in the space up to said third height.

8. A method according to any one of claims 2 to 7, wherein step d) comprises: i. establishing second and third longitudinally spaced passages in the tubing between said first and third heights;

9. A method according to claim 8 and comprising, prior to establishing an increased pressure above the pressure test plug, operating the pressure test plug in order to create a reduced pressure beneath the pressure test plug; and

optionally comprising repeating the procedure of steps i, ii, and iii for different regions of the space between the first and third heights.

10. A method according to claim 8 or 9, wherein said second and third longitudinally spaced passages extend through the sealant in the annulus and through the casing into the formation, the method comprising performing a leak-off test to determine a leak-off pressure at which fluid leaks from the inside of the tubing into the formation, wherein said increased pressure established above said pressure test plug is less than the determined leak-off pressure.

1 1. A method according to any one of claims 2 to 10, wherein there are no control lines in the annulus between said first seal and said first height, or control lines extend longitudinally over only a part of this region.

12. A method according to any one of claims 3 to 1 1 and further comprising, following step e):

f) allowing the sealant placed up to said third height to set;

h) filling the inside of the tubing up to a further height below said second heights; and optionally, between steps g) and h), establishing passages through the tubing in the space up to said further height;

and optionally repeating steps f) to h) one or more times in order to extend the plug length;

wherein control lines are optionally present in the annulus between said first height and the or each further height.

13. A method according to claim 12, wherein step g) comprises:

iv. establishing fourth and fifth longitudinally spaced passages in the tubing between said third and said further heights;

v. setting a pressure test plug within the tubing at a height between said fourth and fifth passages; and

vi. establishing an increased pressure above said pressure test plug whilst detecting any pressure change directly beneath the pressure plug and using any detected pressure change to confirm an integrity of the sealant within the annulus,

the method optionally comprising, prior to establishing an increased pressure above the pressure test plug, operating the pressure test plug in order to create a reduced pressure beneath the pressure test plug; the method optionally further comprising repeating the procedure of steps i, ii, and iii for different regions of the space between the third and further heights.

14. A method according to claim 13, wherein said fourth and fifth longitudinally spaced passages extend through the sealant in the annulus and through the casing into the formation, the method comprising performing a leak-off test to determine a leak-off pressure at which fluid leaks from the inside of the tubing into the formation, wherein said increased pressure established above said pressure test plug is less than the determined leak-off pressure.

15. A method according to claim 1 , wherein said first height is a height sufficient to provide at least a primary barrier of said plug and data obtained at step d) is sufficient to infer the integrity of the sealant within the annulus below the first height.

16. A method according to any one of the preceding claims and comprising, prior to steps a) to c), identifying a suitable location for said plug using casing and/or formation integrity data known a priori or collecting using a through tubing logging operation; and optionally comprising a step of verifying a maximum height of good sealant in the tubing by lowering a stroker tool with a spear attached into the tubular and operating the stroker tool to penetrate through poor sealant to good sealant.

17. A method according to claim 2, wherein said casing is a first casing and is located concentrically within a second casing, and said packer or component, or a further packer or component, provides a third seal between said first and second casings at a given longitudinal position, such that a second annulus is defined between the first and second casings above said third seal, the method comprising, following step (d):

e) establishing second passages above said first height through the tubing, the cement in the first mentioned annulus, and the first casing, and pumping a wet sealant into a space within the tubing so that the sealant fills the tubing to a third height above the second passages whilst flowing through the second passages to fill the second annulus to a fourth height greater than the third height; and

f) allowing the sealant to set.

18. A method according to claim 17, wherein at least one control line extends longitudinally within the first annulus above the first height, and wherein step e) further comprises establishing, above said second passages, at least one blind passage through the tubing, the or each control line, and into the cement in the first annulus, prior to pumping the wet sealant.

19. A method according to claim 17 or 18, comprising a step of circulating a fluid through the second passages for cleaning the second annulus, prior to pumping the wet sealant.

20. A method according to any of claims 17 to 19, wherein said step of pumping a wet sealant into the space within the tubing comprises placing a landing structure atop a landing packer within the tubing above said second passages, and pumping a volume of sealant down the tubing and through the landing structure,

wherein said volume of sealant is optionally pumped down the tubing in a sealant train between a pair of wipers wherein, upon landing on the landing structure, a lowermost wiper is arranged to burst allowing sealant to flow through the landing structure, the method optionally comprising the step of locating a further wiper within the sealant volume intermediate the pair of wipers such that, when the further wiper lands on said lowermost wiper, further pumping of the sealant is prevented, the method optionally further comprising loading the sealant train with a viscous pill to provide a liquid fundament for the sealant within the second annulus.

21. A method according to any one of claims 3 to 20, further comprising, after step (c) and prior to step (e), perforating the tubing directly above the first height, wherein perforating the tubing comprises cutting through one or more control lines located in the A-annulus adjacent to the tubing,

wherein the inside of the tubing is further filled with sealant to a height above the highest perforation, using coiled tubing, coil hose, a dump bailer on wireline, or drill pipe deployed into the tubing, such that sealant flows into the one or more perforations; and optionally further comprising perforating the tubing at one or more additional locations at different heights above the first height.

22. A method according to any one of claims 1 to 20, further comprising, after step (b) and prior to step (c): perforating the tubing at a first location directly above the first height, such that the wet sealant flows through the perforation from the A-annulus into the tubing to fill the tubing to an initial height above the perforation,

wherein perforating the tubing includes cutting through one or more control lines located in the A-annulus adjacent to the tubing.

23. A method according to claim 22, further comprising applying a pressure differential between the tubing and the A-annulus to control the flow of wet sealant from the A-annulus to the tubing,

and optionally further comprising setting a mechanical plug in the tubing to prevent further flowing of the wet sealant through the perforation into the tubing.

24. A method according to claim 22 or 23, further comprising:

perforating the tubing at a second location directly above the initial height; and allowing or causing the wet sealant to flow through the perforation at the second location and to fill the tubing to a subsequent height higher than the initial height and above the perforation at the second location.

25. A method according to claim 24, further comprising:

perforating the tubing at a third location directly above the subsequent height; and

allowing or causing the wet sealant to flow through the perforation at the third location and to fill the tubing to a further subsequent height higher than the subsequent height and above the perforation at the third location,

wherein the flow of cement from the A-annulus into the tubing is optionally controlled using a choke on the tubing and/or the A-annulus.

26. A method according to any one of claims 20 to 25 when dependent on claim 7, wherein the method is performed after the further wiper has landed on the landing structure.

27. A method according to any one of claims 23 to 26 comprising setting the mechanical plug and further comprising, after setting the mechanical plug, allowing the wet sealant to set,

and optionally further comprising removing the mechanical plug.

28. A method according to claim 21 or claim 27, further comprising: removing at least a portion of the set sealant in the tubing, such that the top of the remaining set sealant is below the lowest perforation; and

pressure-testing the sealant filling the one or more perforations to determine the integrity of that sealant.

29. A method according to claim 28, wherein pressure-testing the sealant comprises performing the method of claim 8 or 9, wherein

at least one perforation is located longitudinally between the second and third longitudinally spaced passages;

establishing the second and third longitudinally spaced passages comprises cutting through the control line that is located in the A-annulus adjacent to the tubing; and

any detected pressure change is also used to determine the integrity of the sealant filling the one or more perforations,

wherein the tubing is optionally further filled with sealant up to a third height using a dump bailer, coiled tubing, coil hose, or drill pipe deployed into the tubing.