The present invention generally relates to a method for counteracting land subsidence in the vicinity of an underground reservoir accessible through a number of wellbores.

Land subsidence is a known negative side effect of production of natural resources, for example hydrocarbons, from an underground reservoir through a well. Extraction of hydrocarbons causes compaction of the reservoir rock leading to subsidence at ground level. Land subsidence causes environmental damage and effects water management. When earth layers are compacted near a natural fault, tension may build up and the earth layers will start shifting in a disruptive manner causing earthquakes. In recent years production of gas from the Groningen gas field in the north-eastern part of The Netherlands has led to numerous claims for damages to buildings resulting from earthquakes caused by the gas production. Many buildings in the area show visible damage, such as cracks in the walls.

The present invention has for its object to provide a method to prevent or minimize land subsidence due to activities, such as production of (natural) resources, involving transport of materials to and/or from an underground reservoir through a well having a well bore running in the direction of the reservoir.

According to the invention this object is achieved by the method steps of bringing reinforcing material into a casing present in the wellbore and making the reinforcing material flow out through perforations in the casing for forming a number of reinforcing objects of reinforcing material around the wellbore, whereby the method comprises the step of forming cracks in the reservoir surrounding the wellbore and filling the cracks with the reinforcing material for forming the reinforcing objects.

The reinforcing objects reinforce the reservoir soil surrounding the well bore and adequately prevent land subsidence. Furthermore, the reinforcing material replaces the fluids extracted from the reservoir to prevent the compaction of the reservoir rock. The present invention is specifically suitable for use in a reservoir of non-permeable soil, such as shale or rock.

According to a first preferred embodiment the reinforcing objects have a dome or arch shaped cross-section. Advantageously the dome or arch shape enables the reinforcing objects to withstand high loads from surrounding layers, for instance salt layers, and consequently attribute to prevent land subsidence.

According to a further preferred embodiment a suitable type of reinforcing material comprises grout.

In another preferred embodiment the reinforcing objects are formed around a substantially horizontal part of the wellbore to maximally support the well bore and to create an enhanced supporting surface in the surrounding soil.

In order to create reinforcing objects that completely surround the well bore the perforations are present in circumferential patterns in the casing.

Advantageously use is made of production perforations present in the casing for production of resources, such as hydrocarbons. Optionally additional

perforations are made to the production perforations present in the casing for production of resources, such as hydrocarbons.

Suitable equipment for use in the method includes a pump being used to bring the reinforcing material into the casing. In a practical preferred embodiment tubing provided with a nozzle head is used to make the reinforcing material flow out through the perforations in the casing.

In an optimized preferred embodiment the casing is dividable in sections by seals or packers.

According to a preferred elaboration of the second preferred embodiment the cracks are formed using the reinforcing material. The two separate steps of forming the cracks and forming the reinforcing objects are then effectively combined.

According to an alternative elaboration of the second preferred embodiment the cracks are formed using fracking liquids.

In an efficient elaboration of the second preferred embodiment the reservoir is hardened, for example by Nitrogen injection, prior to forming the cracks.

According to a general preferred embodiment the wellbore remains intact for production of the resources.

When performing the method according to the invention for reinforcing a number of adjacent wellbores, a pattern of reinforcing objects is defined around the adjacent wellbores creating a foundation platter. Such a foundation platter can support a plurality of buildings, such as a village, while leaving enough space for flow movements in the reservoir.

The invention will now further be described referring to the appending figures, in which: Figure 1 shows a schematic overview of a land area in which several well bores run in the direction of a reservoir, wherein the method according to the invention has been applied;

Figure 2 schematically shows a cross section through part of a first reservoir, including a well bore, to illustrate a first embodiment of the method according to the invention;

Figure 3A schematically shows a cross section through part of a second reservoir, including a well bore, to illustrate a second embodiment of the method according to the invention in a first time period;

Figure 3B schematically shows a cross section through the well bore of figure 3A to illustrate the second embodiment of the method according to the invention in a second time period; and

Figure 3C schematically shows a cross section through the well bore of figure 3A to illustrate the second embodiment of the method according to the invention in a third time period.

In all figures equal reference numerals denote equal features. Figure 1 shows a schematic overview of a production site for natural resources, such as hydrocarbons, in which several well bores B run from production wells P in the direction of a reservoir R. Parts D of the reservoir R are reinforced using the method according to the invention. A resulting foundation platter is created defining a pattern of reinforcing objects D around adjacent wellbores B. The dashed lines S indicate free space between the reinforcing objects D that leaves room flow movements of the hydrocarbons in the reservoir.

Figure 2 schematically shows a cross section through a well bore B1 of an underground reservoir R1 that has permeable soil to illustrate a first embodiment of the method according to the invention. Figures 3A through 3C schematically show a cross section through a well bore B2 of an underground R2 that has non-permeable soil to illustrate a second embodiment of the method according to the invention in different time periods. In each of the figures 3A through 3C A a section is cut out and shown in more detail.

In both reservoirs R1 , R2 the well bore B1 , B2 comprises a casing 10 that is inserted into a recently drilled section of the wellbore. The casing 10 is typically held into place with cementation 12. Casing 10 normally has a tubular shape receiving an internal elongate hollow tubing 11 that in the position of use runs in the direction of the reservoir. An annulus A surrounds the tubing 11 and is itself surrounded by the well wall formed by the casing 10 and the cementation 12.

In tubing 1 1 production equipment is received. An example of suitable production equipment is a Wireline Retrievable Oil Production (WROP) system of which a nipple 20 is shown. The WROP system has been described in the international patent application WO2014/01 1043 of the same applicant. Herein a so- called Wireline Retrievable Oil Production (WROP) system is described for the production of hydrocarbons.

Seals or packers 21 are present to close of sections of the casing 10.

Perforations 14 connect the interior of the casing 10 with the surroundings of the casing 10, i.e. the reservoir soil. Consequently, the perforations 14 also extend through the cementation 12. Typically, the perforations 14 are production

perforations for production of resources, such as hydrocarbons. Optionally additional perforations are made in the casing. Preferably the additional perforations are alternate to the production perforations. The preferred pattern for the

perforations is a circumferential pattern.

In general the method according to the invention for counteracting land subsidence due to activities involving material transport, such as production of resources, through wellbores comprises the step of bringing reinforcing material into the casing 10 present in the wellbore B1 or B2. A pump may be used to bring the reinforcing material into the casing 10.

The reinforcing material is made to flow out through the perforations 14 in the casing 10 to form a number of reinforcing objects D1 respectively D2 around the wellbore B1 respectively B2. Dedicated equipment provided with a nozzle head may be used.

Suitable reinforcing materials are injectable materials that will attach to the reservoir area and the outer well wall and will harden as a result of a reaction, for example a chemical reaction or a thermal reaction. The reinforcing material may comprise grout, resin, such as epoxy resin, polyurethane resin or polyester resin, glass fibre, silicon rubber, starch or any combination thereof. Several choices are available to the person skilled in the relevant art.

The reinforcing objects D are formed around a substantially horizontal part of the wellbore B. Optionally a sliding sleeve valve 30 is used to control fluid flow between the tubing 1 1 and the annulus A. Suitable sliding sleeve valves are known in the field of gas and oil exploration and production.

When applying the method according to the invention the wellbore remains intact for production of the resources. The resources flow out of the reservoir into the casing 10 in a direction of flow F that is substantially transverse to the casing 10. As seen in the direction F the reinforcing objects D1 and D2 have a dome or arch shaped cross-section. As seen in longitudinal direction of the casing 10 the reinforcing objects D1 and D2 have a general ring shape or donut shape.

The first embodiment of the method according to the invention comprises the steps described above and is suitable for a reservoir R1 of permeable soil, as illustrated in figure 2.

The second embodiment comprises additional steps prior to the steps described above and is suitable for a reservoir R2 of non-permeable soil, such as shale or rock, illustrated in figures 3A - 3C.

These additional steps relate to creating fracked reservoir areas FR2.

According to the second embodiment the method comprises the steps of forming cracks in the reservoir R2 surrounding the wellbore B2 and filling the cracks for forming the reinforcing objects. The cracks can either be formed using the reinforcing material itself or using fracking liquids. Suitable fracking liquids comprise water and proppants, such as sand or plastic. Prior to forming the cracks the reservoir R2 may be hardened, for example by Nitrogen injection.

The fracked reservoir areas FR2 are preferably alternately used for production of resources. In figure 3A as an example four fracked reservoir areas FR2 are shown in a first time period of which the second and the fourth FR2 (seen from left to right) are production areas and provided with arrows F. In the first and the third FR2 passageways from the perforations 14 to the tubing 1 1 are closed by dedicated equipment. An example of suitable equipment are sleeve valves or side doors 30 that are slideable over the outer surface of the tubing 1 1 , preferably under wireline control. By way of illustration next to some of the perforations 14 areas C are drawn to indicate closed passageways. The areas C are imaginary areas and have no physical embodiment.

Figure 3B shows the view of figure 3A in a second time period that is later than the first time period. The second and the fourth FR2 are no longer used as production areas, but have been transformed into reinforcing objects D2 using the method according to the invention. Optionally the passageways to perforations 14 are closed by dedicated equipment, for example the sleeve valves 30. Now the first and the third FR2 have become production areas.

Figure 3C shows the view of figure 3B in a third time period that is later than the second time period. The first and the third FR2 are also no longer used as production areas, but have also been transformed into reinforcing objects D2 using the method according to the invention. Optionally the passageways to perforations 14 are closed by dedicated equipment, for example the sliding sleeve valves 30. Water can be injected into the thus treated well bore to use the well bore as heat exchanger for geothermal application. Optionally a WROP - HEX is a Heat

Exchanger Plug-in device 40 as described in WO2014/011043 is used.

The invention is based on the general inventive thought to actively reinforce a well bore running towards an underground reservoir using reinforcing material and transporting it through the well bore to form reinforcing objects around the well bore. The reinforcing objects are formed by a combination of cavities and/or cracks filled with reinforcing material and/or reservoir formation, such as shale or rock.

Consequently the reinforcing objects will have an arbitrary shape generally similar to a ring or a donut shape and generally having a dome shaped or arch shaped cross- section.

Although the invention has been illustrated in the context of reinforcing well bores on a hydrocarbon production site, the invention is not limited to well bores for transport of hydrocarbons or other materials out of an underground reservoir. The method according to the invention can also be used to reinforce well bores used for transport of materials into an underground reservoir, for instance for the purpose of underground storage. Some examples of underground storage include disposal of nuclear waste or filling salt cavities with fluids. The method according to the invention can then also be used to reinforce well bores used for (continuous) transport of materials into and out of an underground reservoir, for instance for geothermal applications.

The invention is consequently not limited to the described and shown preferred embodiment. The invention relates generally to any embodiment falling within the scope of protection as defined in the claims and as seen in the light of the foregoing description and accompanying drawings.