NL2018116B1

NL2018116B1 - Method for preparing a well for abandonment

Info

Publication number: NL2018116B1
Application number: NL2018116A
Authority: NL
Inventors: Adriaan Van Der Wal Jan
Original assignee: Adriaan Van Der Wal Jan
Priority date: 2017-01-03
Filing date: 2017-01-03
Publication date: 2018-07-25

Abstract

The present invention relates to a method for preparing a well for abandonment, for instance after drilling, in which a natural cap rock sealing a reservoir is drilled to gain access to the reservoir and the path from the drilling site to the reservoir is lined with casing, comprising the steps of: blocking a first end of the casing, the first location lying above the level of the reservoir, in particular at the location of the drilled cap rock; providing openings in the casing at a location between the first end of the casing and a second end, opposite to the first end and closer to the drilling site; and blocking the casing at the second end.

Description

Octrooicentrum

Nederland

E21B 33/13 (2017.01)

0 Aanvraag ingeschreven:	© Octrooihouder(s):
25/07/2018	Jan Adriaan van der Wal te Amsterdam.
(43) Aanvraag gepubliceerd:
-	© Uitvinder(s):
	Jan Adriaan van der Wal te Amsterdam.
© Octrooi verleend:
25/07/2018
	© Gemachtigde:
© Octrooischrift uitgegeven:	ir. H.Th. van den Heuvel c.s.
30/07/2018	te 's-Hertogenbosch.

Ξ) The present invention relates to a method for preparing a well for abandonment, for instance after drilling, in which a natural cap rock sealing a reservoir is drilled to gain access to the reservoir and the path from the drilling site to the reservoir is lined with casing, comprising the steps of: blocking a first end of the casing, the first location lying above the level of the reservoir, in particular at the location of the drilled cap rock; providing openings in the casing at a location between the first end of the casing and a second end, opposite to the first end and closer to the drilling site; and blocking the casing at the second end.

NL Bl 2018116

Dit octrooi is verleend ongeacht het bijgevoegde resultaat van het onderzoek naar de stand van de techniek en schrifteiijke opinie. Het octrooischrift komt overeen met de oorspronkelijk ingediende stukken.

Method for preparing a well for abandonment

The present invention relates to a method for preparing a well for abandonment.

In order to gain access to natural resources, such as hydrocarbons like oil and gas, located in reservoirs beneath the surface of the earth, typically the surface above these reservoirs is drilled until the reservoir is reached. The thereby obtained passage, from the drill site to the reservoir, is typically provided with a protective casing. This casing prevents that the resource can spread throughout all layers beneath the surface, and allows for transportation to the surface for production. Alternatively, the reservoir may be used to store greenhouse gasses such as carbon dioxide, as well as for fuel storage, such as gas storage or hydrogen storage.

Typically, the natural resources such as oil and gas are stored in permeable reservoir rock formations such as sandstone or carbonates. The hydrocarbons are stored in the void spaces between the grains of this permeable and porous formation. Usually the reservoir formation is overlain with an impermeable formation (also known as cap rock) such as salt (Zechstein in some areas of the Southern North Sea) or shale (clay) formations in order to contain the hydrocarbons. In order to access the resources this containment is broken, typically during the final phase of drilling. During this stage the lower section of the cap rock which naturally seals the reservoir and the reservoir is penetrated. Then the reservoir formation is drilled.

When the well is depleted from hydrocarbons and/or filled with greenhouse gasses, or when the formation has not enough permeability to be economically produced, the reservoir should be closed off and abandoned in order to prevent contamination of the layers beneath the surface of the earth with the resource and to prevent unwanted release of hydrocarbons into the environment and atmosphere. The current accepted process of closing of reservoirs is time consuming and is relatively environment unfriendly. Current abandonment of wells relies on installing mechanical and/or cement plugs in the casing above the reservoir, at multiple other zones along the casing, along flow zones and over casing weak points, such as liner tops, casing cuts, differential valves and collars. In the current abandonment process all elements of the abandoned casing that are prone to interact with the outside are filled with cement plugs, to minimize interaction. However, the placement of these cement plugs in the casing is a highly specialized process that may fail, especially when setting cement plugs deep in wells with high inclinations. Also the mixing process of cement is a manual exercise and prone to human error therefore not mixing the right cement recipe or slurry. This could lead to reduced reliability of cement plugs.

Additionally, cement plugs may degrade and shrink over time, after which their sealing effect fails. Such failing of the cement plug could ultimately result in the undesired release of hydrocarbons past the cement plugs, into the environment and surrounding atmosphere. Oil spills for instance could contaminate sea life around the well, whereas gas release into the atmosphere could seriously damage the protective ozone layer of the Earth.

It is an object of the present invention to provide an improved method for preparing a well for abandonment, for instance after drilling, which overcomes the abovementioned drawbacks.

The invention thereto provides a method for preparing a well for abandonment, in which a natural cap rock sealing a reservoir was drilled to gain access to the reservoir and the path from the drilling site to the reservoir is lined with casing, comprising the steps of: blocking a lower end of the casing; providing openings in the casing at an opening location between the lower end of the casing and a second location of the casing closer to the drilling site, wherein, preferably, the opening location is located at least at the location of the drilled cap rock and wherein preferably the second location is located above the location of the drilled cap rock; and blocking the casing at the second location. The lower end may for instance be located at, or slightly above the level of the reservoir. The second location may for instance be the second end of the casing, or the upper end of the casing, but could also be located between the upper end and the lower end of the casing. The second location for instance lies at the top or the upper end of the cap rock material located outside the casing. The lower end of the casing typically is the location of the casing that is provided with production openings or perforation, allowing the inflow of hydrocarbons from the reservoir into the production lines or casing, and is thus typically located below the level of the production packer used to isolate the completion/production tubing from the casing.

Casing according to the invention may be considered as the piping typically used for well construction to retrieve oil and gas from wells, or used to store gasses in reservoirs below the surface. Typically casing is cemented in place after the reservoir is drilled from the surface, to prevent flow outside the casing and/or contamination of the surroundings with the oil, gas or carbon dioxide for instance.

Tubing according to the invention may be considered as the piping typically used to complete the well after drilling providing a flow path from the reservoir to surface for production and to prevent contact of the hydrocarbons with the casing. Tubing can be replaced during the life of the well if necessary for flow rate and/or well integrity problems. After the life of the well, the tubing is typically cut above the production packer and removed from the well.

The production packer according to the invention maybe be considered as a sealing element typically used to isolate the tubing from the casing.

The casing thus protects the drilling hole, as well as the surrounding materials. Typically tubing is run through the casing to produce the hydrocarbons, in which these hydrocarbons flow through the tubing rather than the casing.

Cap rock is an impermeable formation such as salt or shales which are naturally keeping the hydrocarbons in the reservoir formations which can be sandstone or carbonates. Cap rock naturally prevents the migrations of hydrocarbons to surface.

By blocking the lower end of the casing, which could be located above the level of the reservoir, the reservoir is closed off. Closing off the reservoir prevents that the natural resources inside the reservoir can flow from the reservoir to the surface. Especially after drilling for hydrocarbons such as oil and gas, contamination of the surroundings with oil may thus be prevented. Also when the reservoirs are used for storing of greenhouse gasses such as carbon dioxide, the closing of the reservoir prevents the unwanted release of carbon dioxide to surface. Closing off the reservoir for instance occurs by placing a mechanical and/or cement plug in the casing.

Additional to mechanical and/or cement plug, use may also be made of chemical plugs. For instance, thermite plugs can be lowered into the casing and/or tubing and ignited to trigger a chemical reaction that produces sufficient heat to solidify the casing and cement behind and the thermite itself into one solid impermeable plug. These chemical plugs may also be applied when a mechanical and/or cement plug is already in place.

The level of blocking the lower end of the casing typically lies below the level of the drilled cap rock, and above the reservoir level, or the level of the natural resource.

By providing openings in the casing at a location between the lower end of the casing and a second location, closer to surface, the casing is opened above the blocked lower end of the casing. This opening allows for a natural flow of materials, in particular of cap rock formation, through the openings of the (previously impermeable) casing. This is particularly favourable when the casing is opened at a location wherein, on the outside of the casing, a large amount of mobile material is present to penetrate the opened casing. With this natural inflow of materials from outside the casing a new layer of naturally sealing materials, which extend from outside to inside the opened casing may be formed. This new layer of material, no longer interrupted by an impermeable casing, naturally closes off the reservoir on top of the applied blockage and forming an impermeable natural barrier, this method is called cap rock restoration. Therefore, the openings in the casing may be provided such that it allows cap rock material from outside the casing the flow into the casing. When a new cap rock is formed, and the block at the lower end of the casing would fail, the outflow of the natural resource is naturally prevented by the reinstated cap rock.

By blocking the casing at the second location, for instance opposite to the lower end and closer to surface, the casing is further closed on top. This aids the formation of the impermeable natural barrier. The blocking at the second location may also act as an environmental barrier.

According to the present invention, a preselected part of the casing extending between the surface and the reservoir, is weakened and/or provided with openings on purpose, such as to allow natural material (cap rock), surrounding the casing, to flow through the casing, restoring the initial situation of the natural material, typically natural formation. This way, the breach of the reservoir, and in particular the breach of the cap rock material sealing of the reservoir from the surroundings, is restored using the natural materials available around the casing.

Preferably, the opening location is located at the level of the cap rock (salt and/or shale) material underneath the surface, for instance Zechstein levels. These levels are typically where the cap rock is naturally present, and where the mobile material around the casing is present. Preferably the opening location is located at the level where the cap rock is most mobile, such that salt and/or shale, which naturally sealed off the reservoir prior to drilling, can flow naturally back into place. These mobile sections can for instance be found by high gamma readings.

The present invention may be used for the preparation to abandonment of all kinds of wells such as vertical and directional drilled wells and multilateral wells and in all kinds of areas such as land wells, desert wells, arctic wells, offshore wells and deep water wells. A reservoir according to the invention may be considered as a region below the surface which either originally contained a natural resource, such as oil or gas, or is used to store a natural resource such as carbon dioxide. Such reservoir may be a pocket at least partially filled (or to be filled) with the natural resource, but may also be considered to be a region or layer(s) of material beneath the surface containing natural resources, or a region or layer(s) of material that may be used to store resources such as carbon dioxide. A natural resource according to the invention may be considered as a hydrocarbon, such as oil and gas, as well as gasses also referred to as greenhouse gasses, such as carbon dioxide. A well according to the invention may be an oil or gas well, as well as a gas storage well.

The openings in the casing may be provided by perforating the casing, preferably with a plurality holes of a diameter of at least 4cm, in particular at least 7.5cm, more particularly about 12.5cm. Such diameter of holes or perforations allow a sufficient inflow of material from outside the casing to inside the casing, in order to create the new layer of material sealing off the reservoir below. Perforation of the casing may be performed in a controlled application and a controlled and/or predetermined location of the openings in the casing.

The openings in the casing may be provided by perforating the casing with a gun assembly shooting holes in the casing. The gun assembly loaded with for instance explosive (directional) charges may be lowered through the casing from the drill site until the desired depth is reached and subsequently fired. The gun assembly may be similar to the device used to gain access to for instance oil rich reserves, which device is arranged to shoot holes in the casing at the level of the oil reserves. The casing openings may also be provided by another perforating medium or jetting medium such as a water jet, plasma jet or a sand jet.

The casing may also be filled with fluid containing a corrosion agent in order to provide the openings in the casing, wherein the openings in the casing are provided by corroding the casing by the fluid. This fluid may be present between the lower and second location of the casing, for instance between both blocked ends. The corrosion agent may be chosen from the group of hydrochloric acid, a mixture of hydrochloric acid and nitric acid, sulphuric acid or a carbon dioxide solution. The fluid containing the corrosion agent may for instance be sea water or a brine. Alternatively, the salt in seawater or the brine may be used as corrosion agent, and these fluids may be pressurized to act as abandonment fluid and act as corrosion agent, for instance when pressurized. Pressurizing the fluid in the casing may accelerate the corrosion process of the casing.

Pressurizing the fluid inside the casing further helps in preventing the unwanted release of hydrocarbons from the reservoir to the surface (on top of the placed plug at the lower end of the casing). When the pressure inside the casing exceeds the pressure of the reservoir, the hydrocarbons cannot flow naturally from the reservoir to the upper side of the casing. For instance, when the reservoir is located at 4000 meter below the surface, with a 1.20sg gradient, the pressure in the reservoir is about 470 bar. When the fluid inside the casing also has a 1.20sg (specific gravity) or slightly higher, the casing is in balance with the reservoir, and no outflow of hydrocarbons occurs. However, the casing could also be filled with seawater for instance, with 1.02sg, and pressurize this seawater with 70 bar. Together with the pressure of the seawater over 4000m, this also results in a pressure of about 470 bar.

So, the fluid may be pressurized before blocking the casing at the second location, in particular wherein the second location of the casing is blocked mechanically and/or hydraulically, to keep the fluid pressurized inside the casing after blocking.

The casing may be filled, on top of the blockage at the lower end, with fluid to a level corresponding to at least the location of the drilled cap rock material, or higher. The fluid in the casing should be filled to a level to allow opening or perforation of the casing at the level of the originally drilled cap rock and its mobile surroundings, to allow the inflow of these materials into the casing.

By providing the openings in the casing through corrosion of the casing, the complete casing, originally separating the drill hole and the surrounding cap rock material, may be removed. This allows the complete restoration of the initially present cap rock sealing the reservoir, irrespective of the location of the openings in the casing. Using a corrosion accelerator instead of perforation may take longer for the cap rock material to be restored, but may ultimately be more efficient and effective.

The method may further comprise the steps of detecting the location of mobile intervals of the cap rock material outside the casing and providing the openings in the casing on the location of the mobile intervals by local perforation of the casing. The mobile intervals of the cap rock material outside the casing are naturally the sections of cap rock material that can move relative easy. These intervals may move through the provided openings in the casing the easiest, accelerating the process of the formation of the new cap rock layer (barrier) on top of the reservoir.

In order to detect the location of the mobile intervals, electric logging may be used, for instance by detecting locations along the casing with high gamma reading. Such readings may be acquired during the drilling of the cap rock, of through a separate wireline run after the hole was drilled. Such readings may be used during drilling to determine the mobile salt sections in the layers of the earth, at which mobile salt sections typically heavy wall casing is required to prevent the casing from collapsing under the pressure of the mobile salt or shales.

Before providing openings in the casing, an original fluid inside the casing may be replaced with a replacement fluid, such as sea water or brine, wherein the replacement fluid preferably has a lower density compared to the original fluid, wherein the replacement fluid preferably is pressurized in the casing, to compensate for the loss of hydrostatic pressure. Such replacement is also referred to as displacement in the art. When abandoning wells after drilling, the casing is typically filled with a liquid with a high density to prevent the resources in the well from flowing to the surface. However, such practise could impede the inflow of natural materials through the openings in the casing. Therefore, the fluid inside the casing may be replaced by a lighter fluid, such as seawater or brine. When the casing is then provided with openings, the natural material flowing into the casing encounters a lighter fluid, which allows a faster inflow of material. To compensate for the loss of hydrostatic head keeping the hydrocarbons in place, the lighter fluid may be pressurized between the lower and upper blockings.

The blocking of the lower end of the casing may be achieved by squeezing a plug, preferably a cement plug, through the tubing into the perforated casing. The blocking may also be achieved by installing a mechanical plug in the tubing at a level below the production packer with a cement plug on top of the mechanical plug. The (cement) plug may for instance be located at the level of perforations of the casing used to introduce or expel natural resources into or from the reservoir, and is thus located below the level of production packers used in the production process. The top side of the plug preferably is located at a level below the level of the drilled cap rock. The plug may also partially be present at the level of the drilled cap rock, as long as cap rock material is still able to flow through the opened casing without being stopped by the placed plug.

After the lower end of the casing is blocked, pressure may be applied to the inside of the casing, in order to verify that the casing is blocked at the lower end. This pressure test may be used to check that the casing is actually blocked and that no unwanted release of material from inside the reservoir can occur. The pressure test is used to verify the integrity of the cement and/or mechanical plug(s).

The blocking of the lower end of the casing may be achieved by applying a first cement plug, a mechanical plug above the first cement plug and a second cement plug on top of the mechanical plug. Alternatively, the blocking of the lower end of the casing may be achieved by applying a first cement plug and a second cement plug on top of the first cement plug. By applying multiple plugs, either cement or mechanical, the blocking of the reservoir, and blocking of the casing, may be improved. Both the first and the second cement plug may be placed by the same cementing equipment. In particular, use may be made of the completion tubing, typically used to complete a well and readying it for production. Using the completion tubing for the cementing saves time to switch between different pieces of pipe, and ultimately is more efficient.

Another way of blocking the lower end of the casing may be achieved by deploying chemical plugs such as thermite plugs and solidification of the surrounding casing and cement layers due to the produced heat after ignition of the chemical reaction.

The casing may be blocked at a third location, between the lower and the second end, above the level of the drilled cap rock. The third block may for instance be used to contain and pressurize the volume or level of corrosion fluid, but may also be used to further seal the casing against undesired leakage. The third location lies above the level of the drilled cap rock, such that the block at the third location does not impede inflow of cap rock material through the openings in the casing.

The present invention will be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures, wherein:

- Figure 1 schematically shows different steps in drilling and abandoning wells, or different steps of the life cycle of wells;

- Figure 2 schematically shows a situation prior to abandonment of a well;

- Figure 3 schematically shows the preparation for abandonment after drilling according to a first aspect of the invention;

- Figure 4 schematically shows the situation of figure 3 after openings have been provided according to the invention;

- Figure 5 schematically shows the preparation for abandonment after drilling according to a second aspect of the invention;

- Figure 6 schematically shows a casing used in a typical Southern North Sea offshore well;

- Figure 7 schematically shows a casing of figure 6, prepared for abandonment according to the present invention

- Figure 8 schematically shows a casing schematic after the completion tubing is removed; and

- Figure 9 schematically shows a casing schematic after current abandonment methods without cap rock restoration.

Figure 1 schematically shows a reservoir (1) below the surface (2) under water (3). The reservoir (1) initially is filled with hydrocarbons (4), wherein the hydrocarbons (4) are enclosed in the reservoir (1) by a layer of cap rock (5). The surface (2) is drilled by a drill (6) in order to gain access to the reservoir (1), in which the cap rock layer (5) is penetrated. Typically, the drill (6) creates a path (7) from the surface (2) to the reservoir (1), which is path (7) is provided with a series of impermeable casing strings (8). The bottom end of this casing (8) is then perforated to obtain a collection of openings (9) allowing hydrocarbons (4) to flow through the collection of openings (9) and through the tubing (8) for collection above the surface (2). The drilling and creation of a path (7) and casing (8) as shown in figure 1 is well known in the art. Figure 1 shows the life cycle of a well very schematically. In practise, a number of additional components are used to get the hydrocarbons (4) from the reservoir (1) to the surface. Also, in practise the casing (8) is cut at least 6m below the surface (2), for instance in offshore Dutch wells, in the final stages of abandonment, for instance by abrasive cutting or mechanical cutting with knives.

Figure 2 schematically shows a situation prior to abandonment of a well, showing a reservoir (1) below the surface (2) under water (3). From the surface (2) to the reservoir (1) a cemented casing (8) is located, on the bottom provided with openings (9). The casing (8) extends through a layer of cap rock material (5). Also indicated is a production packer (46), typically left in the cemented casing (8) after use. This production packer (46) comprises a packer (52), a tail pipe (53) and a tubing cut-off (54), where the tubing used to get the produce to the surface is cutoff.

Figure 3 schematically shows the preparation for abandonment after drilling, starting from the situation as shown in figure 2. A first, or lower end (A) of the casing (8) is blocked, illustrated with a first mechanical and/or cement plug (10). The plug (10) blocks the collection openings (9) in the casing (8), thereby preventing possible remaining resources (4), or hydrocarbons (4), from flowing from the reservoir (1) into the casing (8). The second location, or end (B) of the casing (8) opposite to the first end (A) is also blocked, illustrated with a second cement plug (11), preventing the inflow of other materials in the casing (8) from above. Between the first (A) and second (B) end, the casing (8) is provided with openings (12), at the level of the cap rock material (5) on the outside of the casing (8). The openings (12) may be provided by a (non-shown) gun assembly, shooting holes (12) in the casing (8), but the openings (12) may well be provided in an alternative fashion.

Figure 4 schematically shows the situation of figure 3 after openings (12) have been provided in the casing (8). Indicated with arrows (C), these openings (12) allow the inflow (C) of cap rock material (5) from outside the casing (8) to inside the casing to form a new impermeable layer and natural barrier. The cap rock material (5) collects on top of the plug (10), creating a new layer, or new cap rock (13), closing of the reservoir (1) on top of the plug (10).

Figure 5 schematically shows the preparation for abandonment after drilling, starting from the situation as shown in figure 2. A first, or lower end (A) of the casing (8) is blocked, illustrated with a first mechanical and/or cement plug (10). The plug (10) blocks the collection openings (9) in the casing (8), thereby preventing possible remaining resources (4), or oil (4), from flowing from the reservoir (1) into the casing (8). The second end (B) of the casing (8), opposite to the first end (A) is also blocked, illustrated with a second mechanical and/or cement plug (11), preventing the inflow of other materials in the casing (8) from above. The casing (8), between the first (A) and second (B) end is shown to be partially filled with a fluid (14) containing a corrosion agent (15). The corrosion agent (15) over time corrodes the casing (8), thereby producing (non-shown) openings in the casing (8) similar to the openings (12) shown in figures 3 and 4. The corrosion spots (16) leading to the openings are indicated in figure 5, wherein, when the corrosion of the casing (8) at these spots (16) continues, openings will be formed in the casing, allowing inflow of natural material surrounding the casing, such as cap rock material (5), from flowing into the casing (8) on top of the plug (10).

Figure 6 schematically shows a typical casing schematic (20) used in a standard North Sea offshore well. On the right side of figure 6, different sections of casing (20) are indicated. On the left side, different layers of material present beneath the surface are indicated. Starting at about 30 meter depth (or true vertical depth), a North Sea Group layer (21) is present. This layer (21) is encased using a conductor casing (22) and a surface casing (23). Casing is named according to their outside diameter in inch. The conductor casing (22) may be 30” and the surface casing (23) maybe 13-3/8”.

Below the North Sea Group layer (21), at about 1000m depth, a Cretaceous layer (24) lies, followed by Jurassic (25) and Triassic (26) layers starting at about 2000m depth. These layers (25, 26) are encased using production casing (27), which may be 9-5/8”.

Starting at about 2500m depth, the Zechstein Group (28) begins, which is encased using production liner (29), which may be 7” or 7-5/8” (heavy wall casing). Below the Zechstein Group (28), the Rotliegend (30) and Carboniferous (31) layers are present. These layers (30, 31) may be encased using 5” production liner (32). The hydrocarbons may typically be found in the Rotliegend (30) formations. Although these formations, dimensions and depths may be used in a typical Southern North Sea well, they may differ for other locations. The Zechstein Group (28) referred to is generally the layer in which cap rock material is found, whereas the Rotliegend generally is the layer with the drilled reservoir (1). The crosses, or x-marks in figure indicate the location of the liner hangers or dv collars. In figure 6 for instance a dv collar (47), and two liner hangers (45) are shown. The triangles on the casing indicate casing shoes (55)

Figure 7 schematically shows the casing schematic (20), prepared for abandonment. The lower end of the casing (20) is blocked with a cement plug (40), blocking the collection openings (39) allowing to flow through the collection of openings (39) and through the casing (20). The collection opening (39) are in figure located at the Rotliegend layer (30). Above the plug (40), the casing (20) is provided with openings (42), at the level of the Zechstein (28). At this layer (28) cap rock material is present. The casing (20) is further blocked at a second end by another plug (41). Indicated with arrows (C), the openings (42) allow the inflow (C) of cap rock material from the Zechstein (28) from outside the casing (20) to inside the casing (20) to form a new impermeable layer and natural barrier, which collects on top of the plug (40), creating a new layer, or new cap rock (43), closing of the casing (20) on top of the plug (40).

The upper plug (41) could be located at the layers (24, 25, 26) between this layer (21) and the Zechstein layer (28) as well.

Figure 8 schematically shows a casing schematic (20) after the completion tubing is removed, wherein the drilling hole is cased successively by a conductor casing (22) , an intermediate casing (44), a surface casing (23), a production casing (27), a liner casing (29) and a production liner (32). The production liner (32) is provided with collection openings (39), such that hydrocarbons may enter the string of casings. At the transitions between different sections liner tops (45) are present. Located in liner casing (29) is a production packer (46), which is to provide a seal between the outside of the production tubing and the inside of the casing string, such that only hydrocarbons entering through the openings (39) will flow through the casing parts. The production packer (46) comprises a packer (52), a tail pipe (53) and a tubing cut-off (54). At the transition between surface casing (23) and production casing (27) a DV collar (47) is arranged, which collar (47) is typically used in the cementing process.

Figure 9 shows the casing schematic (20) of figure 8, after abandonment. At the bottom, the casing is plugged with a first cement plug (40), below the packer (46). This plug (40) also covers the bottom liner top (45). Above the production packer (46) a second cement plug (48) may be present. Further above a third cement plug (49) may be located, which also covers the upper liner top (45). Located at the level of the DV collar (47) a fourth cement plug (50) may be placed. Finally, on top, an upper plug (41) may be applied acting as an environmental or surface barrier. The liner casing (29) between the shown third (49) and second (48) plug may be provided by openings, either through filling that space with a corrosive agent to corrode the casing (29), or by perforating the casing (29). In case corrosive agent is used, the casing between the second cement plug (48) and the third cement plug (49) may be pressurized. To do so, a mechanical plug (51) may be provided.

Although the figures show the preparation of a well for its abandonment after extracting resources from the well, the same principles apply when underground reservoirs are used to store greenhouse gasses such as carbon dioxide. The resources are then brought into the reservoir by the tubing. Eventually these wells need to be abandoned as well.

It will be apparent that the invention is not limited to the exemplary embodiments shown and described here, but that within the scope of the appended claims numerous variants are possible which will be self-evident to the skilled person in this field.

Claims

Conclusions

A method of preparing a source to leave behind, wherein a natural cap rock sealing a reservoir is pierced to gain access to the reservoir and wherein the path from the hearing site to the reservoir is lined with the steps comprising the steps from the:

a. blocking a lower end of the coating;

b. provided with openings in the liner at an opening location between the lower end of the liner and a second location of the liner, closer to the hearing site, wherein, preferably, the opening location is provided at least at the location of the pierced cap rock, and wherein the second location is preferably provided above the location of the pierced cap rock;

c. blocking the covering at the second location.

Method according to one of the preceding claims, wherein the openings in the coating are provided such that they allow inflow of cap rock material into the coating from outside the coating.

Method according to claim 1 or 2, wherein the openings are provided in the coating by perforating the coating, preferably with a plurality of openings with a diameter of at least 4 cm, in particular at least 7.5 cm, more in particular approximately 12.5 cm.

A method according to any one of the preceding claims, wherein the openings in the coating are provided by perforating the coating with a gun assembly, which makes holes in the coating and / or through a jet of perforating medium, such as a water jet or a sand jet.

A method according to any one of claims 1-3, wherein during step b) the coating is filled with a fluid, which fluid comprises a corrosion agent, at least to the opening location, and wherein the openings are provided in the coating by corroding the coating with the fluid.

The method of claim 5, wherein the corrosion agent is selected from the group consisting of hydrochloric acid, a mixture of hydrochloric acid and nitric acid, sulfuric acid or a carbon dioxide solution.

The method of claim 5, wherein the fluid comprising a corrosion agent is seawater or a saline solution.

The method of any one of claims 5-7, wherein the coating is filled with a fluid to a level corresponding to the location of the pierced cap rock material.

A method according to any of claims 5-8, wherein the fluid is pressurized before blocking the coating at the second location, wherein the second location of the coating is blocked, in particular mechanically and / or hydraulically, to lower the fluid keep pressure in the coating after blocking.

The method of any one of the preceding claims, further comprising the steps of:

a. detecting the location of mobile intervals of the cap rock material outside the coating; and

b. provided with the openings in the coating at the location of the mobile intervals by locally perforating the coating.

The method of claim 10, wherein electrical logging is used to detect the location of the mobile intervals, for example, by detecting locations with a high gamma value.

A method according to any one of the preceding claims, wherein before openings are provided in the coating, an original fluid present in the coating is replaced with a replacement fluid, such as seawater, the replacement fluid preferably having a lower density compared to the original fluid present in the coating, the replacement fluid preferably being pressurized in the coating to compensate for the loss of hydrostatic pressure.

A method according to any one of the preceding claims, wherein blocking the lower end of the coating is achieved by pressing a plug, preferably a cement plug, through the coating.

The method of any one of the preceding claims, wherein after the lower end of the coating is blocked, pressure is applied to the inside of the coating to verify that the coating is blocked at the lower end.

A method according to any one of the preceding claims, wherein blocking of the lower end of the coating is achieved by applying a first cement plug at the lower end, applying a mechanical plug above the first cement plug and a second cement plug on top of the mechanical plug.

The method of any one of the preceding claims, wherein the coating is blocked at a third location, between the lower end and the second end, above the level of the pierced cap rock.

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