Hundreds of platforms for crude oil and natural gas production are gradually coming to the end of their lifetime. In the present, because of the insufficient profitability of exploitation in these reservoirs, their temporary or permanent shutdown is considered. The most expensive operation of these shutdowns is so-called plug and abandonment of the borehole, which is supposed to form the barrier impeding hydrocarbons to escape to the surface. As this procedure is required by legislation, plug and abandonment of the borehole provides space for innovation and increased efficacy.
Around the world, these activities achieve record level, especially in the Gulf of Mexico and in the North Sea. Plug and abandonment of the borehole represents the significant part of total costs for putting the borehole out of service. Therefore most of the boreholes are being plugged for the lowest cost possible based on the minimal requirements stated by regional regulation offices. Proper abandonment of the borehole minimizes the risk of unpredictable increase of costs related to the harming effects of the escape of hydrocarbons and also ecological disasters.
The reason for the realisation of the plug and abandonment of the borehole is that hydrocarbons could be escaping to the surface along the original casing or concrete. The procedure of the plug and abandonment of the borehole involves, especially milling of the specific section of the steel casing pipe, milling of the concrete dividing the casing pipe and the rock pillar, inserting the plug into this section and in the end an injection of concrete closing the borehole. The operation is repeated several times and the number of such operations depends upon the complexity of the borehole. In addition, many of the boreholes have casing pipes made of high-strength (alloyed) steel, able to resist pressure demonstrations of the reservoir. It is difficult to mill such casings and thus the research is focused on other non-conventional, methods of their elimination.
The most difficult part consists of the first two mentioned operations, i.e. milling of the casing pipe and the concrete which are the most challenging. Conventional rotary milling produces parings which must be removed prior to the process of concrete injection. However, removing of these parings may damage the mouth of the borehole. To avoid problems with integrity of the borehole and non-functional mouth of the borehole, it is necessary to dismantle this component, check it, clean it and repair it for significant costs. For example, companies operating in the North Sea, have to mill off and then thoroughly concrete at least two fifty-meter long sections of the borehole above each production horizon. The second disadvantage is the demand of heavy drilling rig, daily rent of which is very financially demanding. The third disadvantage is that during the milling, the milling cutter might get damaged and stuck in the borehole, or some part of the milling cutter might get stuck in the borehole.
So the demand of industry is technology that eliminates mentioned deficiencies, i.e. it does not generate problematic parings, it is possible to use simple, light, and thus cheap, drilling rig for its operation and it is also very reliable. Currently used technologies are based on hydraulic operated device—the milling cutter, the main part of which consists of the milling knives made of hard metal. The regular operation starts with lowering of the milling cutter into the required depth of the borehole, after which circulation of drilling fluid is activated. The circulation activates the milling knives, which are slid out and by their rotating the milling of the casing is performed. After completion of the milling, the circulation of the fluid is stopped and knives are slid into the tool which is lifted to the surface. The disadvantage of this method is the need for heavy, and thus expensive, drilling rig and for frequent replacement of the tool because of deterioration of the tool; these features are more striking for the boreholes located in the sea.
Furthermore, the transmission of the rotary moment onto the lamellas of the milling cutter requires significantly higher structural firmness and it has higher requirements than standard drilling tools.
Apart from the milling of casing, in the mining industry exists another operation, in which it is necessary to disrupt steel casing pipe or concrete. If we want to create new side branches in the existent vertical borehole, firstly we have to create an aperture in a casing wall. For that reason, the milling of the window or of the whole section of the casing pipe is used. The reasons are various, including: sticking of the tool, which it is not possible to mill off in another way, creating the entrance into another horizon or reservoir, collapse of the borehole in the lower part or geological reasons. So this procedure save costs for drilling new boreholes by increasing the production of the existing borehole. The disadvantages of the current method of milling the casing pipes for diverted and horizontal boreholes are the same as for the plug and abandonment of the boreholes.
It is necessary to solve mentioned disadvantages by developing a technology, which is able to effectively remove also alloyed steel and uses the tool highly resistant to deterioration.
The most common mechanical milling of casing sections is mentioned in various patents, as for example U.S. Pat. No. 8,555,955 One trip multiple string section milling of subterranean tubulars, U.S. Pat. No. 6,679,328 Reverse section milling method and apparatus, U.S. Pat. No. 8,225,884 Rotor underreamer, section mill, casing cutter, casing scraper and drill string centralizer, U.S. Ser. No. 13/179,997 Downhole cutting tool and method. Similarly, creating windows in a casing wall is performed mostly by milling, e.g. U.S. Pat. No. 7,537,055 Method and apparatus for forming a window in a casing using a biasing arm.
Patents U.S. Ser. No. 13/153,795 Method and system for abandoning a borehole, U.S. Ser. No. 13/694,208 Casing cutter, U.S. Pat. No. 7,823,632 Method and apparatus for programmable robotic rotary mill cutting of multiple nested tubulars disclose mechanical milling cutter-based devices explicitly aiming at plug and abandonment of the boreholes. U.S. patent Ser. No. 13/153,795 and U.S. Ser. No. 13/694,208 are mainly focused on milling several steel casing pipes nested into each other including their filling material (coaxial system).
Thermal removal of casing by heat arising from chemical reactions is mentioned, for example, in U.S. Pat. No. 4,889,187 Multi-run chemical cutter and method, U.S. Pat. No. 6,598,679 Radial cutting torch with mixing cavity and method. Thermal removal of parts of casing (for forming diverted, horizontal boreholes) is mentioned for example in U.S. Pat. No. 6,722,435 Window forming by flame cutting, U.S. Pat. No. 6,971,449 Borehole conduit cutting apparatus and process, U.S. Pat. No. 6,536,525 B1 Methods and apparatus for forming a lateral wellbore.
The temperature of the process may, but does not have to, exceed the temperature for evaporation of metals. Therefore mainly melting of casing by heat generated from exothermic chemical reactions of the mixture is used. It is necessary to highlight that this method does not directly use oxidation of removed (metal) material itself, which could bring additional heat into the process. The material being removed is heated by hot flow of liquid/reacted mixture.
WO 2013135583 A2 Method of well operation disclose a method of removing parts of a borehole (especially steel casing pipes, concrete, surrounding geological formation) by using an exothermic insert, which sufficiently melts the mentioned materials after ignition and these materials solidify into the form of the plug after they burn out. The disadvantages of this solution consist in the need of an exact determination of generating mixture heat amount for total or partial removing of casing. After ignition, it is not possible to suspend the burning of the mixture and this makes impossible to control the process while it is running.
Removing of objects in the borehole by directed especially by thermal action triggered by chemical reactions, is disclosed, for example, in U.S. Pat. No. 7,997,332 Method and apparatus to remove a downhole drill collar from a well bore.
Procedurally similar applications include also cutting of metal materials outside the borehole. Oxyfuel cutting is the most frequently used procedure for cutting steels. In the process, the metal is firstly preheated by the heat of fuel combustion, most frequently acetylene, which has the highest combustion temperature (around 3,500° C.) and the most concentrated primary flame among all of the technical gasses. The main process is the combustion of preheated machined metal by a stream of oxygen, which melts the metal with the heat of exothermic oxidative reactions and removes the products of combustion (slag) from the cutting place.
Plasma cutting is primarily the process of melting. Construction of plasmatron typically consists of central cathode and surrounding cooled jacket ended with a jet, which compress and directs plasma outflow with the temperature up to 30,000° C. The material is melted by the flow of plasma and it is being forced out of the cutting place by it. An electric circuit closes through the machined metal material, which functions simultaneously as an anode. Such system is disclosed, for example, in U.S. Pat. No. 6,963,0415 B2. Plasma-forming medium can be a gas such as O2, N2, Ar, H2 and others, including gas mixtures. The composition of the machined material determines the selection of plasma-forming gas. Chemical interaction of plasma with the machined material is usually an undesirable side effect. The exception to this is cutting of steels by oxygen plasma, in which the oxidation of iron increases the temperature of a melt, which is thus being removed faster.
Aqueous addition admixed to the plasma-forming medium utilizes, for example, a plasmatron, which is the subject-matter of U.S. Pat. No. 3,567,898. Water is introduced into a plasma outlet near to a root of an arc in a jet, providing instantaneous dissociation of water, which is endothermic, and so it cools down the plasma flow. Simultaneously, in the vicinity of the surface of the machined material, along with the decrease in the plasma temperature, the reverse recombination occurs and thereby effective heat is released exactly in the cutting place and acts thermally on the material being disintegrated. A stream of water further stabilizes the arc and contributes to removal of the melted metal material. In this case, the water is purely an assisting addition, the primary plasma-forming medium is gas. Alternatively, the streams of water are used for creating protective atmosphere around the cutting arc.
U.S. Pat. No. 5,006,687 discloses cutting with a plasma arc.
The disadvantage is that in these methods the streams of oxygen, heat generating dissociated gases or the stream of plasma act on the small surface and by point action, especially by melting, remove only a small amount of material. An electric arc as a source of energy generating plasma, hydrogen, and oxygen produced from the supplied water serving for exothermic recombination is transferred between an electrode inside the device and the material being cut itself. In this way, the material is disintegrated locally near the root of the arc which is suitable only for cutting. Plasma stream cannot act planary with sufficient potency suitable for bulk removal of material. Thus, there is no planar action of the heat and oxidizing reactions. The presence of the water vapour in the oxidizing gasses and gas mixtures accelerates the oxidation of most metals and their alloys at high temperatures (Saunders, S., Monteiro, M. & Rizzo, F., 2008. The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: A review. Progress in Materials Science). The effect of the water vapour on the metal oxidation kinetics is also heat-depending. Experiments confirm higher velocity constants of oxidation at high temperatures of pure vapour in comparison to oxygen (Yuan, J. et al., 2013. Comparison between the oxidation of iron in oxygen and in steam at 650-750° C. Corrosion Science, 75, pp. 309-317). The biggest effect of water vapour was observed at the Fe—Cr steels, which correspond to the material of casing pipes. Compared to the oxygen atmosphere, in case of pure water vapour, oxidative steel reactions proceed up to one order faster (Yuan, J. et al., 2014. Investigation on the Enhanced Oxidation of Ferritic/Martensitic Steel P92 in Pure Steam. Materials, 7(4), pp. 2772-2783).
Abovementioned studies confirm oxidizing abilities of water vapour, but these processes are related to heated water vapour, which oxidizing ability is given by level of thermal dissociation, which is not complete. However, the electric arc is able to dissociate all molecules of water, and thus further accelerating and amplification of oxidative reactions or, at high temperatures of plasma, the initiation of other reactions and processes can be assumed.
Other methods of disintegration comprise an electrospark perforation of pipes in U.S. Pat. No. 3,621,916 which functions on the principle of supplying energy by means of spark ignition, explosive detonation and subsequent formation of fluid flow that perforate a steel pipe wall and have a pulse-detonation character. On a short-term basis, the density of energy reaches a high level, sufficient to partially disintegrate the material, which is preferably used for perforation of casings in a borehole, but for planar removal of material it is insufficient.
Previous GA Drilling patents (PP 50058-2012 Multimodal rock disintegration by thermal effect and system for performing the method and PP 50006-2013 Generating electric arc, which directly areally thermally and mechanically acts on material, and device for generating electric arc) define processes of interaction of an electric arc with mostly rock material by means of thermal and mechanical acting. Just thermal and mechanical plasma effects melt the metal material, evaporate it, or force the melt and the vapour out of the place of plasma action. But they do not initiate chemical transformation of material which is preferable for effective removing of the material. The mentioned extent and way of interaction of electric arc with the disintegrated material in these patents are of point character, disintegrating a material linearly, and therefore insufficient for achieving the objective of planar removing of metal materials.
The mentioned disadvantage is eliminated by this invention, which extends the thermal effect of plasma by thermo-chemical, especially oxidizing, action of plasma.