WO2012001342A2

WO2012001342A2 - Sealing method and apparatus

Info

Publication number: WO2012001342A2
Application number: PCT/GB2011/000931
Authority: WO
Inventors: Robert D. Eden
Original assignee: Rawwater Engineering Company Limited
Priority date: 2010-06-30
Filing date: 2011-06-22
Publication date: 2012-01-05
Also published as: WO2012001342A3; GB201010998D0; CA2840538C; CA2840538A1

Abstract

According to the present invention there is provided an apparatus for forming a plug in a passsageway, the apparatus comprising a carrier which in use is lowered into the passageway, the carrier being dimensioned to define a clearance between the carrier and the passageway; a body of material supported on the carrier, said material having a melting point which is higher than the temperature within the passageway at the point at which the plug is to be formed and which expands as it solidifies; and means for melting the body of material such that melted material flows into the clearance defined between the carrier and the passageway.

Description

SEALING METHOD AND APPARATUS

The present invention relates to a method and apparatus for plugging a passageway. Such passageways include underground components which may be plugged to prevent leakage of hydrocarbon fluids from those components.

In the oil and gas extraction industries, abandoned wells have to be plugged to keep the contents of deep high pressure environments which communicate with those wells from invading levels at or adjacent the surface. Plugs can be inserted at any point in a well, for example adjacent the surface or at a substantial depth. Such plugs are commonly referred to as 'capping plugs', 'abandonment plugs', 'bridge plugs' or 'bridging plugs', these terms being used interchangeably by persons working in this technical field. Herein any reference to a 'plug' encompasses a 'capping plug', 'abandonment plug', 'bridge plug' or 'bridging plug'. Typically, plugs are formed by injecting cement or resin into the well so as to fill for example a fifty metre length of the well. Experience has proved however that such plugs are not particularly reliable and often leak.

The known plugs tend to leak for a variety of reasons. Firstly, as the well wall is typically not particularly clean and is also covered with a hydrocarbon film, it is difficult to produce a reliable contiguous seal. Often a contiguous seal of only a metre or so in length is formed with a plug fifty times that length. Furthermore, as cement and resin based plugs solidify they contract which tends to open up a gap between the plug and the well wall. Although when a plug is initially inserted there may be little dynamic pressure in the well, after the plug is in situ substantial pressures can build up and as a result a plug which appears initially to be working satisfactory may subsequently be found to leak. If hydrocarbons leak past the plug contamination of the surface environment or for example a sub-surface aquifer can result. It is well known in the industry that a significant proportion of abandoned wells leak. As a result leaking abandoned wells often have to be re-plugged which is an expensive and time consuming operation.

It is an object of the present invention to provide an improvement to existing methods and apparatus for sealing such structures. According to the present invention there is provided an apparatus for forming a plug in a passsageway, the apparatus comprising: a carrier which in use is lowered into the passageway, the carrier being dimensioned to define a clearance between the carrier and the passageway; a body of material supported on the carrier, said material having a melting point which is higher than the temperature within the passageway at the point at which the plug is to be formed and which expands as it solidifies; and means for melting the body of material such that melted material flows into the clearance defined between the carrier and the passageway.

The invention further provides a method for forming a plug in a passageway, wherein the method comprises: placing a carrier in the passageway, the carrier being dimensioned to define a clearance between the carrier and the passageway; melting in the passageway a body of material the melting point of which is higher than the temperature within the passageway at the point at which the plug is to be formed and which expands as it solidifies such that melted material flows into the clearance defined between the carrier and the passageway; and causing and/or allowing the melted material in the clearance to rapidly cool and solidify.

The present invention provides a simplified means by which a plug of sufficient strength to satisfy the operational requirements of most plug applications can be achieved. The plug may be used to repair leaks in a well casing or to shut off unwanted gas production or water production within an operational well. Moreover, the plug may be used as a vehicle to deliver molten metal or alloy to a well annulus to seal the well annulus.

The apparatus incorporates a carrier for a body of meltable material which is used to form a seal against the wall of the passageway and provide a plug of the required strength. The carrier is sized so as to define a clearance between the carrier and the wall of the passageway (e.g. well). Once the meltable material has been heated and melts it flows into the clearance and then rapidly cools, solidifies and, due to the nature of the meltable material, expands to block further flow of gases or liquids through the clearance.

It was previously thought that to provide an effective seal apparatus of this kind used to plug a well or similar type of passageway would need to incorporate spaced radially extending fins and a downwardly depending "packer" dimensioned so as to be a tight fit within the well bore. Such an arrangement provides for very high integrity long life plugs that are resistant to the metallurgical phenomenon of 'creep' at elevated environmental temperatures and differential pressures. Surprisingly, however, the devisor(s) of the present invention have determined that such an arrangement is not in fact always required since creep control is not an operational priority for all well plugging operations. For example the plug may be used as a temporary platform from which a molten alloy 'squeeze' treatment can be delivered to repair leaks in a well casing or to shut off unwanted gas production or water production within an operational well. In a further application, the plug can be used as a temporary platform from which to deliver molten alloy to the well annulus to seal the well annulus. After the squeeze, the plug can then be reheated, for example using a pre-installed electrical heater, and removed from the well bore. Further, creep control is not required in the permanent abandonment of low pressure onshore wells. Moreover, the apparatus of the present invention could be used in applications in which a well casing is milled away at the intended sealing point prior to melting of the metal or alloy so that as the metal or alloy solidifies and expands it is squeezed into contact with the rock, ground etc which surrounds the well casing to form a seal, which may be permanent if desired. This may find use in, for example, C0₂ sequestration or the like where the apparatus of the present invention can be used to form a seal in the cap rock of a reservoir.

While not wishing to be bound by any particular theorem, it is currently thought that due to the speed at which suitable meltable materials cool and solidify it is not necessary to use a carrier including radially extending fins and/or a tight-fitting packer. Instead, in many applications a more simple and therefore cheaper carrier can be used that is a relatively loose fit within the passageway, optionally with a downwardly extending skirt of similar outer diameter to the carrier. As such, the apparatus of the present invention provides a "finless" and "packerless" means of deploying a sealing plug within a passageway. This affords a number of advantages over prior art systems incorporating fins and/or a packer, including easier manufacture, easier deployment and wider manufacturing tolerances since a close conformity between the size of the skirt and the passageway to be sealed is no longer required, and greater flexibility in the range of applications in which apparatus of a single size can be employed, all of which reduce the costs associated with plug deployment. Once the apparatus has been deployed within the passsageway it will typically be submerged in water already resident within passageway, often to a very signficant depth of, for example, around 300 to 400 m. Such depths of water provide a hydrostatic pressure of 3 to 4 MPa which is sufficient to prevent the water adjacent the hot molten material from being able to boil. Again without wishing to be bound by any particular theorem it is currently believed that the water, by virtue of having such a high specific heat capacity (around 4.2 J/cm³ K at 25 °C), contributes significantly to the rapid cooling of the melted material within the gap around the skirt, and that the melted material within the gap contacting the cooling water exhibits flow behaviour akin to the pahoehoe flow behaviour exhibited by certain types of lava flows. As a result, the solidified material around the carrier and optional skirt of the apparatus quickly cools, solidifies and expands to provide a strong and reliable seal.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 to 3 illustrate an assembly for forming a plug in a well in accordance with a first preferred embodiment of the present invention;

Figures 4 and 5 illustrate a second preferred embodiment of the apparatus of the present invention;

Figure 6 illustrates a cross-sectional view of part of the assembly of Figures 4 and 5; and

Figure 7 illustrates a cross-sectional view of a similar part of an assembly as shown in Figure 6 but in which the assembly is in accordance with a third preferred embodiment of the present invention.

Figures 1 to 3 show an assembly according to the invention which can be used to form a bismuth or expanding bismuth alloy plug within a well casing 1. A solid plug is formed from an amount of bismuth or expanding bismuth alloy delivered in solid form on a carrier spool to the required depth within the casing 1. The carrier spool is made of a material which is resistant to elongation or compression as a result of creep, a suitable material is 1% manganese steel. In the exemplary embodiment shown in Figures 1 to 3 the carrier spool omits a downwardly extending skirt. In the embodiments shown in Figures 4 to 7 a cylindrical skirt 2 is connected to a tubular mandrel 3. In the embodiments shown in Figures 4 to 6, the skirt 2 is formed of concrete cast on to an inverted T-bar 4 (visible in Figure 6) secured to a lower end 5 of the mandrel 3. A single T-bar 4 is shown which is connected to the centre of the lower end 5 of the mandrel 3, but it will be appreciated that two or more such T-bars, or any other form of mounting point, could be used to support the cast concrete skirt 2. Moreover, the skirt 2 could be produced from any other suitable tough volume-filling material, such as cement (optionally with fibre reinforcement), or a plastic material which is then attached to the lower end 5 of the mandrel 3 using an adhesive 6 or some other form of fixing, such as a rivet, bolt, screw or the like, passing through a portion of the skirt 2 and the mandrel 3 as depicted in Figure 7. The skirt 2 can be formed of any appropriate material provided it can withstand the conditions to which it will be exposed during and after deployment. By way of further example, the skirt 2 could be made from steel or a suitable rubber-based material.

In the embodiments depicted in Figures 4 to 6, the lower end 5 of the mandrel 3 incorporates a frustoconical head 7, from which the cylindrical skirt 2 extends axially downwards so as to define a skirt region, the purpose of which will be described in more detail below. The head 7 does not have to be frustoconical, and may take any convenient form such as a flat radially extending flange, or may be completely omitted such that the skirt 2 extends directly from the lower end 5 of the mandrel 3 as in the embodiment shown in Figure 7 in which the outer diameter of the skirt 2 approximately matches that of the mandrel 3.

The mandrel 3 has an upper open end 8. The outer diameter of the mandrel 3 is suitably dimensioned to enable it to be slid down the casing 1 and to provide the required spacing in the form of an annular clearance between mandrel 3 and the well casing 1. In embodiments of the apparatus not including a head 7, such as the embodiment shown in Figure 7, the skirt 2 may have a similar outer diameter to the mandrel 3, or a larger or smaller diameter. With reference to Figure 7, since the skirt 2 has a similar outer diameter to the mandrel 3, the skirt 2 extends axially downwards from the lower end 5 of the mandrel 3 so as to define a substantially continuous curved peripheral surface made up of the lower portion of the mandrel 3 and the skirt 2.

In delivery form (shown in Figure 3), metal 9 to be melted to form a plug locates along the length of the mandrel 3 defining a cylinder. The metal is chosen so as to have a melting point that is higher than the temperature within the passageway at the point at which the plug is to be formed and to exhibit the property of expanding upon solidification. Suitable metals include pure bismuth and expanding bismuth alloys, for example an admixture of 95 % bismuth and 5 % tin, or an admixture of 52 % bismuth and 48 % tin, with of which may be doped with another metal, such as sodium. Expanding bismuth alloys typically contain around 50 % or more bismuth. In this form the carrier spool is inserted into the casing 1 (skirt end first if a skirt is present) and lowered to the required depth.

Thus positioned the bismuth or expanding bismuth alloy is melted in situ by a heater which normally locates within the mandrel 3 (but which is illustrated for clarity in Figure 4 outside the mandrel 3). The heater may remain within the mandrel 3 after operation, or may be removed either manually or automatically upon melting of a solder or the like having a suitable melting point relative to the operating temperature of the heater. The heater defines a cylinder, an upper portion of which comprises an ignition source 10 and a lower portion of which comprises a heater element 11. The heater element 11 may comprise an admixture of aluminium and iron oxide (a thermite mixture). The ignition source 10 may comprise a barium peroxide fuse, an electrical heater or an electrical match. It will be appreciated that other forms of both ignition source 10 and heater element 11 could be used. For example, the ignition source 10 and heater element 11 may be completely replaced with a suitable electric heater (not shown) located within the mandrel 3, or an intermetallic gasless pyrotechnic heat source, such as a nickel-aluminium powder admixture.

Activation of the ignition source 10 triggers the heater element 11. Heat produced from the heater element 11 causes the bismuth or bismuth alloy 9 supported on the mandrel 3 to become molten. The molten material slumps into a volume defined between the mandrel 3 and the casing wall 1 (see Figure 1) and then quickly cools by rapid heat transfer from the molten material to the surroundings, primarily any water resident within the well bore. As the molten material solidifies it expands to fill the space between the mandrel 3 and the well casing 1 and thereby form a seal.

It has been established that the strength and integrity of the seal can be enhanced by using a carrier with a skirt 2, such as the embodiments shown in Figures 5, 6 and 7. The skirt 2 should have suitable dimensions that allow a small amount of the molten material to slump down passed the head 7 (if present) so as to reside, and then rapidly cool, within a gap defined between the peripheral surface of the skirt 2 and the casing wall 1 by rapid heat transfer from the molten material to the surroundings, again primarily any water resident within the well bore. For this to be achieved, the skirt 2 should have a diameter that is smaller than that of the well casing 1 so as to define a peripheral gap extending around the edge of the skirt 2, and the skirt 2 should also be of a sufficient axial length so that the molten material can slump sufficiently far from the heated mandrel 3 to very rapidly cool and solidify within the gap rather than slumping past the lower end of the skirt 2 and out of the volume resulting in an ineffective seal. An optimum size of skirt 2 should be selected for a particular well which will define a gap for molten alloy of sufficient volume to enable a reliable seal to be formed at reasonable cost.

The skirt 2 may have a diameter that is around approximately equal to that of the mandrel 3 (as shown in Figure 7), or may have a diameter which is larger, for example, around 50 to 100 % larger, than the diameter of the mandrel 3. The outer diameter of the mandrel 3 may be at least around 50 % of the inner diameter of the well casing 1 at the level the well is to be sealed, but may be at least around 60 % or around 75 to 90 % of the inner diameter of the well casing 1. This is to ensure that the radial dimension of the volume defined between the mandrel 3 and the well casing 1 is large enough to accommodate expansion of the molten material as it solidifies and to ensure that a sufficient volume of molten material is present to provide a seal with the required strength, but not so large as to waste costly material or to cause unequal cooling to occur across the radial dimension of the volume of molten material resulting in the volume possessing a heterogenous structure and thereby providing an unreliable seal. By way of example, a reliable seal can be formed in the manner described above using apparatus incorporating a tubular mandrel 3 having an outer diameter of around 7.5 cm (3 inches) in a cylindrical passageway similar to a conventional well bore having an inner diameter of around 11.5 cm (4.5 inches) and which therefore defines an annular clearance of around 2 cm (0.75 inches) between the mandrel 3 and the passageway for receipt of the molten material.

With regard to the axial length of the skirt 2, this also partly defines the volume and therefore affects the cost of the material that will reside within the gap between the skirt 2 and the well casing 1. A longer skirt 2 provides a greater volume to facilitate effective cooling of the molten material before it slumps passed the bottom of the assembly and thereby ensure an effective seal is formed around the skirt 2, but defines a larger volume for receipt of more molten material, which increases material costs. One way in which the skirt length can be defined is in relation to the overall length of the mandrel 3 since the length of the mandrel 3 typically defines the total volume of alloy material which is initially supported on the assembly before deployment (as shown in Figure 3) and which can therefore be used to form the seal. The skirt 2 may be at least around 10 to 20 % of the total length of the mandrel 3, or may be longer, such as at least around 30 to 40 % of the total length of the mandrel 3.

Commonly, wells to be sealed contain a liquid, such as water. This is advantageous since this water can be used to cool the molten bismuth or expanding bismuth alloy as it slumps into the volume between the mandrel 3, skirt 2 if present and the well casing 1. If the water level is not at the optimum sealing level then further water can be introduced into the well so as to raise the water level to an appropriate level to assist in forming the seal at the optimum level. As the molten material slumps into the volume at the lower end of the assembly it contacts the water within the well and rapidly forms a solidified skin, in a similar way to that which occurs in undersea volcanic lava flows, exhibiting pahoehoe flow. The skin may initially re-melt or deform, but has sufficient structural integrity after a very short period of time to prevent rapid mass flow, and will rapidly solidify as cooling of the material continues until such time as a strong and reliable lower crust is formed. The underside of the solidified material contacting the water within the well is likely to be irregular but due to the pahoehoe nature of the material's flow the layer of material above the crust should have a more uniform structure and thereby provide a reliable seal against the wall of the well casing 1 , as the remainder of the molten material solidifies within the volume higher up the mandrel 3. It may be advantageous to use an assembly incorporating a relatively long skirt 2, for example, a skirt 2 that is around 50 to 100 % of the length of the mandrel 3 so that the skirt 2, which is generally formed of a relatively cheap material like concrete or plastic, can be submerged into the water within the well to a sufficient depth to ensure that the skirt 2 and the wall of the well casing 1 define an appropriate volume for receipt of the molten material taking into account the balance of cost against seal strength described above. Longer skirts 2 may be advantageous since they provide greater flexibility during deployment to ensure that the seal can be formed at the optimum position and at an acceptable cost. Longer skirts 2 also would not typically have a significant bearing on the total cost of the assembly because they are generally produced very cheaply using low cost raw materials, such as cast concrete (as in Figures 5 to 6) or moulded plastic (as in Figure 7). A further benefit is that a single, or a pre-specified range, of assemblies can be produced in large quantities but that will still suit a wide range of different applications.

The skirt 2 can be solid, for example a solid block of concrete, which may include fibre reinforcement, cast on to one or more supporting members attached to the lower end of the mandrel 3 as shown in Figures 1 to 6, or a solid block of plastic adhered to the lower end of the mandrel 3 as shown in Figure 7. Alternatively, the skirt 2 can be hollow or tubular so as to define an internal cavity for receipt of a coolant, such as water already resident within the well. In this way, the outer wall of the skirt 2 is cooler than if the skirt 2 is a solid block of material, and so in this way, the hollow skirt 2 can increase the rate of cooling of molten material flowing into the space defined between the skirt 2 and the wall of the well casing 1.

In addition to the above, in the embodiments depicted in Figures 5 to 6, the frustoconical head 7 is able to serve as a wedge that drives into the expanded bismuth or bismuth alloy plug and, in doing so, forces the plug against the casing wall 1 improving the integrity of the seal.

It was previously thought that radially extending fins 8 should be attached to the mandrel 3 to enable a satisfactory seal to be produced. In contrast to this general belief, it has surprisingly been determined that for many sealing requirements a safe and effective seal can be produced using a "finless" mandrel 3 as shown in Figures 1 to 7. While it was previously understood that the fins were required to force the expanding material against the casing 1 by minimising axial and promoting lateral expansion, to transfer heat from a heater element to the expanding material and to reduce up hole creep of the solidified material, it has unexpectedly been determined that this is not in fact the case.

A plug of sufficient strength to satisfy most industry standards for the sealing of wells (e.g. holding a pressure of 1000 psig for 10 minutes is a recognised standard in Canada for plugging disused oil wells) can be formed by using a relatively simply mandrel 3 without fins provided the radial clearance or spacing between the mandrel 3 and the well casing 1 is sufficiently wide to accommodate a large enough volume of molten material to form an effective seal but not so large as to facilitate undesirably high axial expansion of the material before lateral expansion establishes a seal, a low rate of heat transfer from the heater element to the bismuth or expanding bismuth alloy material, and/or allow unacceptable levels of up hole creep. In preliminary tests a "finless" mandrel 3 incorporating a cement skirt 2 exhibited the ability to hold a pressure of 2500 psig for 10 minutes, which more than exceeds the industry standard for sealing wells in Canada.

In preferred embodiments, a coolant is introduced into the carrier body defined by the mandrel 3 after the plug material has been melted. The coolant can be delivered to the mandrel 3 in any convenient manner. For example, simply by ensuring that the casing above the plug is filled with water is generally sufficient providing that the water can penetrate into the mandrel 3 after plug material has been heated. Alternatively, a body of coolant can be provided which is released a predetermined period after heating. Introduction of the coolant causes material adjacent the mandrel 3 to solidify before material further from the mandrel 3. By ensuring the volume of the space between the mandrel 3 and the casing 1 is not too large the rate of heat transfer from the hot molten material to the coolant is sufficiently rapid and uniform to ensure that a suitably high proportion of the total volume of the molten material solidifies rapidly enough to enable a satisfactory seal to be formed. While coolant can be introduced into the mandrel 3 to fill its core along substantially its full length, this is not always necessary and appropriate cooling can be generally achieved by cooling just the end regions of the mandrel 3. By not cooling a section of the mandrel 3 intermediate the ends the molten material adjacent the non-cooled section of the mandrel 3 cools less quickly than molten material nearer to the cooled end regions of the mandrel 3. This creates a bulge of molten material around the non-cooled section of the mandrel 3 which can enhance the strength and integrity of the final seal. While not wishing to be bound by any particular theory, it seems that the solidified and expanded portions of the bismuth or expanding bismuth alloy adjacent the cooled end regions of the mandrel 3 limit axial expansion of the still molten intermediate portion of molten metal and force it to expand radially as it solidifies contributing to a stronger final seal.

It will be appreciated that the formation of a plug as described above has a wide range of applications, such as sealing passageways in nuclear waste containers or securing objects, such as cables, components of bridges or the like, to carriers anchored to a solid base such as a rock.

Claims

1. An apparatus for forming a plug in a passsageway, the apparatus comprising: a. a carrier which in use is lowered into the passageway, the carrier being dimensioned to define a clearance between the carrier and the passageway;

b. a body of material supported on the carrier, said material having a melting point which is higher than the temperature within the passageway at the point at which the plug is to be formed and which expands as it solidifies; and

c. means for melting the body of material such that melted material flows into the clearance defined between the carrier and the passageway.

2. An apparatus according to claim 1 , wherein the outer diameter of the carrier is at least around 50 % of the inner diameter of the passageway, at least around 60 % of the inner diameter of the passageway, or around 75 to 90 % of the inner diameter of the passageway.

3. An apparatus according to claim 1 or 2, wherein the elongate body is tubular.

4. An apparatus according to claim 3, wherein the tubular body receives a heater element.

5. An apparatus according to any one of claims 1 to 4, wherein the carrier is dimensioned such that said clearance between the carrier and the passageway is an annular clearance.

6. An apparatus according to any preceding claim, wherein the carrier comprises an elongate body of a material resistant to creep.

7. An apparatus according to any preceding claim, wherein the carrier supports a skirt that extends axially from a lower end of the carrier, the skirt being dimensioned to define a further clearance between the skirt and the passageway.

8. An apparatus according to claim 7, wherein the skirt is solid or hollow.

9. An apparatus according to claim 7, wherein the skirt is tubular and defines an opening at its lower end.

10. An apparatus according to claim 7, 8 or 9, wherein the skirt is dimensioned such that said further clearance between the skirt and the passageway is an annular clearance.

1 1. An apparatus according to any preceding claim, wherein the passageway is a well.

12. A method for forming a plug in a passageway, wherein the method comprises: a. placing a carrier in the passageway, the carrier being dimensioned to define a clearance between the carrier and the passageway;

b. melting in the passageway a body of material the melting point of which is higher than the temperature within the passageway at the point at which the plug is to be formed and which expands as it solidifies such that melted material flows into the clearance defined between the carrier and the passageway; and

c. causing and/or allowing the melted material in the clearance to rapidly cool and solidify.

13. A method according to claim 12, wherein the method further comprises cooling the carrier such that molten material adjacent the carrier cools and solidifies.

14. A method according to claim 13, wherein said cooling is applied to ends of the carrier but not to a region of the carrier in between said ends.

15. A method according to claim 12, wherein the carrier comprises an elongate tubular body which is cooled by introducing coolant into the tubular body.

16. A method according to claim 15, wherein the coolant is introduced into ends of the tubular body but not to a region of the tubular body in between said ends.

17. A method according to any one of claims 12 to 16, wherein the method comprises submerging the carrier and associated body of material within a liquid in the passageway.

18. A method according to claim 17, wherein said liquid is water or oil.

19. A method according to any one of claims 12 to 18, wherein the passageway is a well.