THERMITE REACTION CHARGE, METHOD FOR FORMING A THREEPHASED ROCK-TO-ROCK WELL BARRIER, AND A WELL BARRIER FORMED THEREOF

Abstract

This invention relates to a thermite reaction charge comprising bismuth oxide and aluminium adapted to react with a reaction rate giving a reaction time of 8 to 15 seconds for a thermite reaction charge of 30 to 100 kg from initialisation of the thermite reaction charge to at least 90% of the thermite reaction charge is reacted, a method for forming a three-phased rock-to-rock barrier by applying the thermite reaction charge and a well barrier formed thereof.

Claims

1. A thermite reaction charge, comprising bismuth oxide, Bi.sub.2O.sub.3 and a fuel metal comprising aluminium, characterised in that the thermite reaction charge is adapted to react at a reaction rate giving a reaction time of from 8 to 15 seconds for a thermite reaction charge of 30 to 100 kg from initialisation of the thermite reaction charge to until at least 90% of the thermite reaction charge is reacted, preferably from 9 to 14 seconds, and more preferably from 10 to 13 seconds.

2. The thermite reaction charge according to claim 1, wherein the thermite reaction charge is adapted by applying particulate bismuth oxide of a particle size in the range of from 1 mm to 1 cm and a fuel metal comprising particulate aluminium of a particle size in the range of from 1 mm to 1 cm, preferably particulate bismuth oxide of a particle size in the range of from 1 to 7 mm and particulate aluminium of a particle size in the range of from 1 to 7 mm, more preferably particulate bismuth oxide of a particle size in the range of from 1 to 5 mm and particulate aluminium of a particle size in the range of from 1 to 5 mm, most preferably particulate bismuth oxide of a particle size in the range of from 1 to 3 mm and particulate aluminium of particle size in the range of from 1 to 3 mm, wherein the particle sizes are determined by standard ISO 9276-1:1998 given for the median particle size (d50) as determined by ISO 9276-2:2001.

3. The thermite reaction charge according to claim 1, wherein the thermite reaction charge is adapted by applying particulate bismuth oxide having a particle size in the range of from 1 to 3 mm and a fuel metal comprising particulate aluminium having a particle size in the range of from 1 to 2 mm, where the particle sizes are determined by standard ISO 9276-1:1998 given for the median particle size (d50) as determined by ISO 9276-2:2001.

4. The thermite reaction charge according to claim 1, wherein the thermite reaction charge comprises: a monolithic planar solid disc of bismuth oxide, where: the disc of bismuth oxide is pressed to a density in the range of from 50 to 99%, preferably of from 55 to 95%, more preferably of from 60 to 90%, more of from 65 to 85%, and most preferably of from 70 to 80% of the theoretical maximum density of 8.9 g/cm.sup.3, and the disc of bismuth oxide has a thickness of from 0.5 to 20 cm, preferably of from 1 to 17.5 cm, more preferably of from 1.5 to 15 cm, more preferably of from 2 to 12.5 cm, and most probably of from 3 to 10 cm, and an outer diameter adapted to fit into an inner chamber of a thermite charge carrying tool, and a fuel metal comprising at least one monolithic solid object of aluminium.

5. The thermite reaction charge according to claim 4, wherein the thermite reaction charge comprises: a set of at least two of the monolithic planar solid discs of bismuth oxide, and a fuel metal comprising a set of at least two monolithic solid objects of aluminium, each shaped into a planar solid disc having an outer diameter similar to the monolithic solid discs of bismuth oxide, wherein the monolithic planar solid discs of bismuth oxide and the monolithic solid objects of aluminium are stacked in an interdigitated stack of alternating bismuth oxide and aluminium discs.

6. The thermite reaction charge according to claim 5, wherein the thickness of the monolithic solid objects of aluminium is adapted to give a stoichiometric ratio of Bi:Al based on either: the total content of bismuth oxide and aluminium of the thermite reaction charge, or: the content of bismuth oxide of the thermite reaction charge and the content of aluminium of the thermite reaction charge and content of aluminium of a thermite charge carrying tool applied to insert the thermite reaction charge into a well.

7. The thermite reaction charge according to claim 5, wherein the thermite reaction charge comprises: a set of at least two of the monolithic planar solid discs of bismuth oxide, where each has a through-going centre channel located at and in parallel with a rotational symmetry axis of the disc, and a fuel metal comprising one monolithic solid object of aluminium shaped into a rod adapted to fit into and fill the through-going centre channel of the monolithic planar solid discs of bismuth oxide, wherein the set of at least two of the monolithic planar solid discs of bismuth oxide are thread onto the aluminium rod, and an inner diameter of the centre channel and an outer diameter of the aluminium rod are both adapted such that when the aluminium rod fills the centre channel, the total amount of aluminium and bismuth present in the termite reaction charge corresponds to a stoichiometric ratio of Bi:Al.

8. The thermite reaction charge according to claim 1, wherein the fuel metal of the thermite reaction charge comprises Al with Ca, Mg and/or Si in an amount to give a fuel mixture of Al, Ca, Mg and Si containing from 1 to 32 wt % Mg and from 1 to 68 wt % CaSi.sub.2, preferably of from 5 to 32 wt % Mg and from 10 to 68 wt % CaSi.sub.2, more preferably of from 10 to 32 wt % Mg and from 20 to 68 wt % CaSi.sub.2, and most preferably of from 15 to 32 wt % Mg and from 30 to 68 wt % CaSi.sub.2, the wt % is based on total weight of Al, Mg, Si and Ca present in the thermite charge.

9. The thermite reaction charge according to claim 1, wherein the thermite reaction charge further comprises CaO and/or SiO.sub.2 in an amount adapted to provide, after reacting the thermite charge, a slag phase having a melting point between 1800 and 1200° C., preferably between 1700 and 1200° C., more preferably between 1600 and 1200° C., more preferably between 1500 and 1200° C., and most preferably between 1400 and 1200° C.

10. A method of sealing a well with a rock-to-rock cross-sectional well barrier, where the well comprises a downhole completion comprising at least a casing, wherein the method comprises: installing a heat resistant bridge plug in an innermost casing at a location where the seal is to be formed, placing a thermite charge carrying tool on top of the heat resistant bridge plug, wherein the thermite charge carrying tool comprises an inner chamber filled with a thermite reaction charge and an igniter, and igniting the thermite reaction charge, characterised in that: the method further comprises applying a thermite reaction charge according to claim 1, wherein the thermite reaction charge is pressurised to an in-situ pressure of at least 5 MPa.

11. The method according to claim 10, wherein, the in-situ pressure is preferably at least 6 MPa, more preferably at least 8 MPa, more preferably at least 10 MPa, and most preferably at least 12 MPa.

12. The method according to claim 10, wherein the in-situ pressure is obtained by, prior to ignition of the thermite reaction charge, injection of gas to the inner chamber of the thermite charge carrying tool.

13. The method according to claim 10, wherein, the in-situ pressure is obtained by either: injecting a gas into the inner chamber of the thermite charge carrying tool prior to ignition of the thermite reaction charge, or: pressing the thermite reaction charge in the inner chamber of the thermite charge carrying tool by a piston prior to ignition of the thermite reaction charge, or: using gas from the initial thermite reaction phase to increase the pressure.

14. A thermite charge carrying tool, where the thermite charge carrying tool comprises a cylindrically shaped container having a bottom, a side-wall a top, a cylindrical inner chamber, and a cable interface arranged on the top, and an igniter adapted to ignite the thermite reaction charge, characterised in that the thermite charge carrying tool further comprises: a thermite reaction charge according to claim 1 being arranged within the inner chamber.

15. The thermite charge carrying tool according to claim 14, wherein the thermite charge carrying tool further comprises a piston arranged within the inner chamber adapted to press against the thermite reaction charge therein.

16. The thermite charge carrying tool according to claim 15, wherein the piston is actuated by the ambient hydrostatic pressure in the well.

17. The thermite charge carrying tool according to claim 15, wherein the thermite charge carrying tool further comprises one or more valves enabling injection of gas to the inner chamber for obtaining and maintaining a pressure, p.sub.i, within the inner chamber of at least 5 MPa, preferably of at least 6 MPa, more preferably at least 8 MPa, more preferably at least 10 MPa, and most preferably at least 12 MPa, and wherein the check and release valve is further adapted to open and release gas from the inner chamber if the pressure p inside the inner chamber becomes; p>p.sub.i+Δp, where Δp is 0.1 MPa, preferably 0.15 MPa, more preferably be 0.2 MPa, more preferably 0.3 MPa, more preferably 0.5 MPa, and most preferably 1 MPa.

18. A rock-to-rock cross-sectional well barrier in a well bore, where the well bore comprises a downhole completion comprising at least one casing, and characterised in that the rock-to-rock cross-sectional well barrier comprises: a first rock-to-rock well barrier element of bismuth, a second rock-to-rock well barrier element of steel on top of the first well barrier element, and a third rock-to-rock well barrier element of slag on top of the second well barrier element.

19. (canceled)

Description

LIST OF FIGURES

[0073] FIG. 1 is a facsimile of FIG. 2.2 of [Ref. 2] showing a typical construction of a well including downhole completion.

[0074] FIG. 2 is a facsimile of FIG. 4.21 of [Ref. 2] showing a drawing of the structure of a permanent rock-to-rock well barrier according to prior art made by an iron oxide and aluminium thermite.

[0075] FIGS. 3a) to 3e) are drawings seen from the side schematically illustrating the method of forming a permanent rock-to-rock well barrier according to the present invention.

[0076] FIG. 4 is a drawing as seen from the side illustrating an example embodiment of a thermite charge carrying tool containing an example embodiment of a thermite charge according to the invention.

[0077] FIG. 5a) is drawing as seen from the side and above illustrating an exploded view of an example embodiment of discs made of bismuth oxide to be applied in another example embodiment of a thermite charge according to the invention.

[0078] FIG. 5b) is drawing as seen from the side of a thermite charge carrying tool loaded with a thermite reaction charge applying the discs shown in FIG. 5a).

[0079] FIG. 6 is a diagram showing measured pressure development in three full-scale tests of the thermite reaction charges according to the second aspect of the invention.

[0080] FIG. 7 is a drawing showing the construction of a test tool applied to test the barrier forming ability of the thermite reaction charges according to the second aspect of the invention.

[0081] FIGS. 8 to 11 show photographs showing resulting three-phased rock-to-rock well barriers being made in tests applying thermite reaction charges according to the second aspect of the invention.

[0082] FIGS. 12a) and 12b) are diagrams showing measured pressures versus time (12a)) and measured temperature gradients versus time (12b)) in comparison tests.

[0083] FIGS. 13a) and 13b) show a photograph of the resulting barrier (13a)) and the destroyed top of the test rig (13b)) in a failed test.

[0084] Verification of the Invention

[0085] The invention will be described in further detail by way of verification tests.

[0086] Experiment 1

[0087] A series of verification tests are made in a pilot-scale. Each test applied a cylindrical test tool constructed as illustrated in FIG. 7. The figure is a cut view seen from the side.

[0088] The test tool was prepared by cementing a cylindrically shaped rock (101) of outer diameter of approx. 20 cm and length of approx. 0.5 to 1.0 meter into a cylindrical concrete block (100) of outer diameter of approx. 40 cm and a height of 1 m. The rock should preferably have comparable physical and chemical properties with typical rock formations at actual locations for forming a well barrier. In these tests, the rock was commercially available slate from Oppdal, Norway.

[0089] A centre bore of inner diameter of 108 and coaxial with the rotational symmetry axis of the cylindrical body mm was made to go through the cylindrical rock cemented in concrete. Then a steel tube (102) of outer diameter of 88.9 mm was aligned coaxially into the centre bore and the gap between the bore wall and the outer surface of the steel tube was filled with Portland cement (103) to function as casing cement. The steel tube had an inner diameter of 76.3 mm (i.e. the steel tube had a thickness of 6.3 mm) and was approx. 2 m long such that it protrudes approx. 1 meter above the centre bore.

[0090] The steel tube (102) is provided with a bridge plug (104) at its lower part. The bridge plug may be made of cement or steel. A heat shield (105) of graphite was laid onto the bridge plug. Then a hollow cylindrically shaped thermite charge carrying tool (106) being closed in both ends was inserted into the steel tube and placed onto the bridge plug. The plugging tool was made of aluminium and had an outer diameter of 70.0 mm and a wall thickness of 3.0 mm, i.e. an inner diameter of 66.0 mm.

[0091] The inner space (107) was partly filled with 10 kg of a particulate bismuth oxide and particulate aluminium thermite reaction charge (108) where the bismuth oxide particles had a particle size of 1 to 3 mm and the aluminium particles has a particle size of 1 to 2 mm. The thermite reaction charge had a height of approx. 80 cm. An electric resistance igniter (109) was located inside thermite reaction charge. The inner space (107) was pressurised to a gas pressure of 235 bar by insertion of nitrogen gas before ignition. A pressure relief valve (110) set to release gas at pressures above 245 bar was applied in one of the tests.

[0092] FIG. 8 shows a photograph of the test tool cut in half and laid side by side after firing the thermite reaction charge and cooling. In the photograph we see that the heat from the thermite reaction charge has completely melted a section, marked with reference number (200), of the casing (102) together with the casing cement and the resulting plug has “eaten its way” a distance into the slate (101) and thus obtained a rock-to-rock well barrier. The barrier is seen to consist of three phases, a lower phase of bismuth (201), an intermediate phase of steel/molten casing (202), and an upper phase of slag (203) of mostly alumina. All three phases are observed to have rock-to-rock contact. The photograph shows also a remaining part of the plugging tool (106) and the bridge plug (104). The graphite heat shield (105) did loose and float upwards into the slag phase in this test.

[0093] FIG. 9 is a photograph showing a comparison of the test result of the test shown in FIG. 8 (here shown as the middle test result) with 4 similar test results, all of them performed as described above. As seen on the photographs, the intended three-phase rock-to-rock well barrier is obtained in all samples.

[0094] Experiment 2

[0095] A series of similar tests as described in example 1 was performed with a thermite reaction charge comprising an interdigitated stack of alternating discs of bismuth oxide and aluminium discs. The bismuth oxide discs were made of bismuth oxide powder pressed to a density of at least 60% of theoretical maximum density and had a thickness of 25 mm and a diameter of 64 mm. The aluminium discs had a thickness of 7 mm and a diameter of 64 mm. The thermite charge consisted of 9.4 kg bismuth oxide and 1.1 kg of aluminium. The initial pressure was set to 1.5 MPa and it was applied a pressure relief valve which opened at a gas pressure of 1.51 MPa. Otherwise, the test conditions and tools applied were the same as for example 1.

[0096] FIG. 10 is a photograph of the resulting three-phase rock-to-rock barrier. The photograph shows clearly the formation of a first well barrier (201) of bismuth metal, an intermediate well barrier (202) of steel, and third well barrier (203) of slag/aluminium oxide.

[0097] FIG. 11 is a photograph showing the resulting three-phase rock-to-rock barrier in a full-scale test with 90 kg of thermite comprising an interdigitated stack of alternating discs of bismuth oxide and aluminium discs. The bismuth oxide discs were made of bismuth oxide powder pressed to a density of at least 60% of theoretical maximum density and had a thickness of 25 mm and a diameter of 99 mm. The aluminium discs had a thickness of 7 mm and a diameter of 99 mm. The thermite charge consisted of 80 kg bismuth oxide and 10 kg of aluminium. The initial pressure was set to 15 MPa and it was applied a pressure relief valve which opened at a gas pressure of 15.1 MPa.

[0098] The test tool was similar to the test tool of the tests described in example 1, except for having larger dimensions. The length of the test tool was 2 m, the Oppdal slate block was approx. 1.8 meters long and had a diameter of 320 mm and the centre bore has an inner diameter of 220 mm. The casing had an outer diameter of 140 mm and an inner diameter of 122 mm. The thermite charge carrying tool was made of aluminium and had an outer diameter of 110 mm and a wall thickness of 5 mm, i.e. an inner diameter of 100 mm.

[0099] As is visible macroscopically in the photograph, the lowermost phase is Bismuth, that has a clean (tight) boundary with the steel, and then the dark, more voluminous oxide phase above that. From top to bottom the barrier is 1570 mm in height. As can also be seen, the casing pipe is melted away both within the barrier interval, but also significant parts of the casing pipe are melted away for a further 500 mm over the top of the barrier.

[0100] Comparison Tests

[0101] A series of small-scale tests with 600 to 800 g thermite charges were made in a test tool pressurised to 1.5 MPa. The test tool is cylindrical and around 420 mm tall and 220 in outer diameter. Inner chamber is approximately 210 mm in height and 160 mm diameter (4.2 litres). A crucible composed of Al.sub.2O.sub.3 is loaded into the chamber. The crucible has an inner volume for the thermite of around 140 mm height and 70 mm diameter. When the crucible is loaded with thermite there is approximately a litre or so of free volume in the test cell. There is one pressure sensor and several temperature sensors at various locations in the cell. The cell is pressurized with N.sub.2 gas, and the thermite is ignited by the use of a primer in the form of a small capsule of thermite that is initiated using electricity.

[0102] The first test applied a thermite charge of particulate bismuth oxide and aluminium of particle size of 50 micron, the second test applied a thermite charge of particulate tin oxide and aluminium of particle size of 50 micron, the third test applied a thermite charge of particulate bismuth oxide and magnesium with particle sizes of 1-2 mm, the fourth test applied a thermite charge of particulate bismuth oxide and aluminium of particle size of 2 mm, and the fifth test applied a thermite charge of 25 mm thick discs of powdered bismuth oxide pressed to at least 60% of theoretical maximum density and 7 mm thick aluminium discs.

[0103] FIG. 12a) show a diagram illustrating measured pressures in the test tool as a function of time. The first test (curve marked “BiOx+Al (50 micron)”) show a very rapid pressure increase comparable to an explosion from 1.5 to about 3.2 MPa in less than a second. The second test (curve marked “SnOx+Al (50 micron)”) also show a very rapid pressure increase from 1.5 to about 2.6 MPa in less than a second. The third test (curve marked “BiOx+Mg based fuel Al (1-2 mm)”) does also rise rapidly after a few tenths of a second delay to about 2.5 MPa. The fourth and fifth tests, (curve marked “2 mm Granular” and “7 mm disc”, respectively) applied a thermite charge comparable to the thermite charges applied in experiment 1 and 2. As seen in FIG. 12a), these thermite charges created a significantly slower and more controlled pressure build-up.

[0104] FIG. 12b) is a diagram displaying measured pressure build-up given as the pressure gradient in bar/s for a series of five small-scale tests with 600 to 800 g thermite charges applying a particulate bismuth oxide of particle size 1-3 mm and particulate aluminium of various particle sizes. The curve marked “A” shows the measured pressure gradient with aluminium particles of 0.05 mm, the curve marked “B” shows the measured pressure gradient with aluminium particles of 0.125 to 1 mm, the curve marked “C” shows the measured pressure gradient with aluminium particles of 0.5 to 1.5 mm, the curve marked “D” shows the measured pressure gradient with aluminium particles of 1 to 2 mm, and the curve marked “E” shows the measured pressure gradient with aluminium particles of 2 mm. As expected, the reaction kinetics increased significantly with lesser particle sizes of the aluminium fuel metal.

[0105] Another result in the small-scale tests was that if the pressure was reduced before ignition, that the gaseousness of the thermite became more and more, until eventually the safety burst disc on the pressure cell actually punctured due to getting hot condensed Bi gas on it. It was necessary to contain the pressure in the test tool, then eventually the majority of the thermite products condense and accumulate into density separated solids.

[0106] FIG. 13a) is a photograph showing the resulting barrier formed in a half-scale test with approx. 10 kg of the same thermite charge of particulate bismuth oxide and magnesium with particle sizes of 1-2 mm applied in the small-scale test shown in FIG. 12a). The test was performed in a similar test tool as applied in experiment 1 with an initial pressurisation to 1.5 MPa. As seen from photograph 13a), the test failed by not being able to melt the casing (102) such that the well barrier did not become a rock-to-rock barrier and consisted of only two phases, a lower bismuth phase (201) positioned onto the bridge plug (104) and a slag phase (203) mainly consisting of magnesium oxide. The photograph also indicate that slag has been violently hurled upwards in the casing. This is confirmed by the photograph in FIG. 13b) which shows the top of the test setup after the test having an accumulation of granular material which turned out to be thermite reaction products. I.e., a part of the plug forming material was blown away such that the thermite products accumulated only a few cm in the base of the test setup, and no casing was melted, proving that loss of control of the thermite reaction (too high reaction kinetics) is not likely to yield a successful barrier.

[0107] The above results of the “BiOx+Mg based fuel (1-2 mm)” thermite charge does not form the intended three-phase rock-to-rock barrier while the “2 mm granular” thermite mixture does, indicate that the limit for how fast the thermite reaction can proceed (and build up pressure) is somewhere between the reaction velocity/-pressure gradient of the 1-2 mm particulate bismuth oxide and magnesium thermite and the 2 mm bismuth oxide and aluminium thermite.

[0108] Thus, in summary, these test results (and other not displayed here) indicate that a reaction velocity corresponding to a pressure gradient of less than 5 MPa/s provide a controllable thermite able to form the intended three-phased rock-to-rock well barrier. This corresponds to a reaction rate giving a reaction time for a thermite reaction charge of 30 to 100 kg of from 8 to 15 seconds from initialisation of the thermite reaction charge to at least 90%, see FIG. 6.

REFERENCES

[0109] 1 World Oil Magazine February 2020, retrievable on the internet: http://www.worldoil.com/magazine/2020/february-2020/special-focus/special-focus-2020-forecast-international-drilling-and-production [0110] 2 Khalifeh, M. and Saasen, A., “Introduction to Permanent Plug and Abandonment of Wells”, Springer Open, 2020, https://doi.org/10.1007/978-3-030-39970-2, ISBN 978-3-030-39969-6. http://creativecommons.org/licenses/by/4.0/3 [0111] 3 Wang, L. et al., “The behaviour of nanothermite reaction based on Bi.sub.2O.sub.3/Al”, Journal of Applied Physics 110, 074311 (2011); https://doi.org/10.1063/1.3650262 https://aip.scitation.org/doi/abs/10.1063/1.3650262?ver=pdfcov&journalCode=jap