NO347165B1

NO347165B1 - A tool for pulse injection of a fluid for well stimulation purposes, and a method of performing a pulse injection to stimulate a well

Info

Publication number: NO347165B1
Application number: NO20210181A
Authority: NO
Inventors: Geir Ueland; Abbas Nabizadeh
Original assignee: Ags Solutions As
Priority date: 2021-02-11
Filing date: 2021-02-11
Publication date: 2023-06-19
Also published as: NO20210181A1

Description

A TOOL FOR PULSE INJECTION OF A FLUID FOR WELL STIMULATION PURPOSES, AND A METHOD OF PERFORMING A PULSE INJECTION TO STIMULATE A WELL

The present invention relates to a tool for pulse injection of a fluid for well stimulation purposes and a method of performing a pulse injection to stimulate a well.

Background

In petroleum technology, and in other fields, it is a well-known practice to use pressurised fluids for different purposes. One such purpose is well stimulation, to improve drainage from a formation comprising hydrocarbons.

Applying pressurised fluid in pulses have been shown to give improved results in well stimulation operations. From prior art there are many known tools and methods of applying pressurised fluid in pulses.

The patent publication WO2009132433 describes a system for injecting water into ground formation around a borehole, and for superimposing pulses onto the outflow of the injected water, by. The system includes a piston connected to a pulse-valve of the tool, a bias spring urging the piston towards its closed position. The piston is urged towards the open position by a differential between the supplied accumulator-pressure and the inground formation-pressure.

The patent publication WO2009089622 describes a system for automatically creating the pulses, in which a piston is acted upon by the pressure differential between a supplied accumulator pressure and the formation pressure. The changing levels of the pressure differential as the pulse-valve opens and as the pulse-valve closes are harnessed to actuate an inhibitor that restrain movement of the valve-piston, and delays opening and/or closing of the pulse-valve.

The patent publication WO2010034113 describes a tool including a suckback-chamber, which is timed to open to the ground formation just as a pulse-valve closes. A biasser (e.g a spring) drives the chamber open and sucks in some of the liquid from the ground. The chamber is then emptied, back to the ground, by the rising pressure as the pulsing tool is recharged.

The patent publication RU2583804 describes a device for pulse action on reservoir for use in development of oil deposits for pulse impact on bottom hole zone to increase reservoir recovery. The device includes housing with a radial channel, which is concentrically located in a casing pipe with a central channel, openings nozzle, nut and is rigidly connected with adapter, cylindrical valve and a spring.

The tools described, does however have drawbacks such as that they may be difficult to tune for different operational wellbore pressures, may have limitations with regards to operational wellbore temperature, etc.

Summary of the invention

In a first aspect, the present invention relates to a tool for applying pulses of pressurised fluid to a wellbore formation for well stimulation purposes, the tool comprising:

a housing comprising a wall for separating an inside of the housing from an outside of the housing, the wall having one or more side ports for providing flow paths for the pressurised fluid from the inside to the outside of the housing;

a piston, having a first side and a second side, the piston being movable in the housing between a first piston position and a second piston position via a first intermediate piston position, being biased by a biasing means towards the first piston position, and being arranged to move to the second piston position, against a biasing force produced by the biasing means, when exposed to a fluid pressure over a first threshold on its first side and to move back to or towards the first piston position via a second intermediate piston position when the fluid pressure on the first side drops below a second threshold;

a sleeve, the sleeve being movable between a first sleeve position and a second sleeve position and arranged in connection with and to move in response to movement of the piston and such that the one or more side ports are closed when the sleeve is in the first sleeve position and open when the sleeve is in its second sleeve position; and

a locking mechanism for releasably locking the piston to the sleeve, the locking mechanism being arranged to lock the piston to the sleeve when the piston is in the first intermediate piston position when the piston moves from the first piston position towards the second piston position and to release the piston from the sleeve in the second intermediate piston position when the piston moves from the second piston position towards the first piston position.

The locking mechanism may be part of what may be called a “delay mechanism”, a mechanism that delays the movement of the sleeve so not to move the sleeve instantly in response to movement of the piston. The locking mechanism may be a mechanism that releasably locks the piston in engagement with the sleeve, for the piston and the sleeve to move together when they are locked together. The delay mechanism may be said to comprise e.g. the roller balls, the housing, the sleeve and/or the piston. The delay mechanism may be said to have a locking means for releasably locking the sleeve to the piston.

In embodiments of the invention, the first and the second intermediate piston position may be the same position. In alternative embodiments, the first and the second intermediate piston position may be different pistons positions between the first piston position and the second piston position.

In embodiments of the invention, the locking mechanism may comprise a roller-delay mechanism. The delay mechanism may comprise one or more locking balls. The wall may comprise one or more recesses for the one or more locking balls. The piston and/or the sleeve may comprise one or more recesses for the one or more locking balls. The sleeve may comprise one or more openings for the one or more locking balls to pass through. Other types of locking mechanisms for releasably locking the piston and the sleeve together are possible. The delay mechanism may be called a roller-delay mechanism.

The biasing means acting on the piston may comprise a spring. The biasing means may further or alternatively comprise a pressurised fluid, such as a gas or a wellbore fluid. The biasing means may comprise e.g. a CO2, nitrogen, or propane gas or air.

In embodiments of the tool, wherein the biasing means comprises a spring, the spring may be arranged between the piston and an inner bottom surface of the housing. In another embodiment, the sleeve may have an inner bottom surface, and the spring may be arranged between the piston and said inner bottom surface of the sleeve. In yet another embodiment, the tool may comprise a first and a second spring, wherein both springs forms parts of the biasing means. The first spring may be arranged between an inner bottom surface of the sleeve and the piston, and the second spring may be arranged between an inner bottom surface of the housing and the piston or between an inner bottom surface of the housing and an outer bottom surface of the sleeve.

The tool according to the first aspect of the invention may be advantageous particularly due to the delay mechanism, as it may enable an improved oscillation of the pulses of pressurised fluid being released from the tool e.g. into a formation, which may improve the quality of penetration into the formation either in form of depth or direction or both. Other delay mechanisms than a roller-delay mechanism are conceivable and may be used to achieve the same effect of improved oscillation from a delay of movement of the sleeve.

Embodiments of the invention may also or alternatively be advantageous in that they are configured for ease of adaption to different wellbore pressures. It may be of importance for a tool for applying pulses of pressurised fluid to a wellbore formation for well stimulation purposes that it is correctly and accurately configured for a well pressure at a depth where the tool is intended to operate, for the tool to work as intended. Embodiments of the invention may be configured for adaption e.g. by facilitating a change of a biasing force of the biasing means, e.g. by facilitating a change of a fluid pressure inside a chamber in the housing that acts as part of the biasing means or a change of one or more springs.

The wall of the housing may in embodiments of the invention have an opening for a part of the piston and/or the sleeve to move downwards in the wellbore against a fluid pressure of a wellbore fluid. In such an embodiment, the wellbore fluid may form part of the biasing means. This may in some situations be an advantageous embodiment of the tool, e.g. in that it may be an embodiment that may be smaller and less complex to produce.

In embodiments of the invention, the tool may comprise an upper and a lower chamber, wherein the upper chamber is defined by inner walls of the sleeve and the piston, and the lower chamber is defined by inner walls of the housing and one or more outer walls of the sleeve. The one or more outer walls of the sleeve may comprise the outer lower surface of the sleeve. In such embodiment, the piston may be arranged to move at least partly towards a bottom of the first chamber before releasably locking itself to the sleeve to move with the sleeve, and then to move with the sleeve at least partly towards a bottom of the second sleeve following releasably locking itself to the sleeve. Each of the upper and the lower chamber may comprise one or more parts of the biasing means.

The tool may be configured to facilitate adjustment of a biasing means/biasing force. In some embodiments, the wall of the housing may have a part that may be detachable and re-attachable. The part of the wall may typically be a lower end of the wall. In some embodiments, a part of the sleeve may be detachable and re-attachable. The part of sleeve may typically be a lower end part of the sleeve. The part of the wall and/or the part of the sleeve may comprise a threaded portion, for forming a connection to a corresponding other threaded part of the wall and/or the sleeve respectively. Being able to detach a portion of the wall and/or the sleeve may make it easier to e.g. replace a spring or another biasing means or part of the biasing means.

The tool may comprise one or more elastomer seals to seal one chamber in the housing from another chamber in the housing, to avoid pressurised fluid from leaking into e.g. the upper chamber or the lower chamber.

One or more parts of the tool may comprise or be made of a ceramic material. Alternatively, or additionally, one or more parts of the tool may comprise or be made of e.g. a steel material, a hi-chrome alloy material, and/or a composite material. Other suitable materials may be used to construct one or more of the parts of the tool.

One or more parts of the delay mechanism/locking mechanism may advantageously comprise or be made from a ceramic material, such as e.g. the roller balls and/or the housing wall, and/or the piston and/or the sleeve. A ceramic material may be particularly advantageous due to material properties, such as chemical resistance properties, durability, and temperature resistance. By using a ceramic material, it may be possible to design the locking mechanism such that the roller balls can provide pressure tight seals, to stop pressurised fluid from leaking into the upper chamber or the second chamber. In an embodiment where the roller balls are made of steel, they may lack the hardness to be sufficiently durable to reliably provide such a seal.

The tool may also or alternatively be a tool for cleaning of old wells, cleaning of sand screens, and for other operations that involve application of pressurised fluid.

The tool may be used in conjunction with a fluid-delivering device such as e.g. a coiled tubing, a jointed pipe, a tubing or a hose wherein the fluid-delivering device may be used for providing pressurised fluid to the housing/the tool. The tool may comprise the fluid delivering device. The fluid delivering device may comprise e.g. a type of pumping device.

In a second aspect, the invention relates to a method of releasing a pressurised fluid from a tool, the method comprising the steps of:

a. providing a tool comprising a housing having a wall with one or more side ports, a piston, a sleeve, and a locking mechanism to releasably lock the piston to the sleeve for the two to move together, wherein the method comprises the steps of:

b. applying a pressurised fluid to the tool and the piston and by use of a force from the pressurised fluid moving the piston from a first piston position to a first intermediate piston position;

c. when the piston is in the first intermediate piston position releasably locking the piston to the sleeve by use of the locking mechanism, to have the sleeve move with the piston when the piston moves further; and

d. further applying the pressurised fluid to the tool and the piston and by use of the force from the pressurised fluid moving the piston further, to a second piston position, and moving the sleeve with the piston, thereby opening the one or more side ports and allowing the pressurised fluid to be released from the tool through the one or more side ports.

The pressurised fluid moving the piston may be a fluid with a fluid pressure over a first threshold.

The tool may be the tool according to the first aspect of the invention. The method may further comprise the steps of:

e. pushing the piston, with the sleeve, back from the second piston position, to the first piston position, by use of a biasing means arranged to push the piston towards the first piston position;

f. releasing the piston from the sleeve when the piston is in a second intermediate piston position between the first piston position and the second piston position; and

g. further moving the piston to the first piston position by use of the biasing means.

The movement of the sleeve may be a movement from a first sleeve position to a second sleeve position and back. The first intermediate position may be the same position as the second intermediate position. The first sleeve position may correspond with the first intermediate position, the second sleeve position may correspond with the second piston position.

The step of pushing the piston, with the sleeve, back from the second piston position, may happen as a result of a reduced force on the piston from the pressurised fluid resulting from a release of a pulse of the pressurised fluid from the tool, and happen as a fluid pressure on the piston from the pressurised fluid drops below a second threshold.

The steps involving moving the piston from the first piston position towards or to the second piston position may involve pushing the piston against a biasing means. The biasing means may comprise one or more or any combination of a wellbore fluid pressure, a fluid chamber of a gas or a liquid in a chamber in the tool, which may be a chamber inside the housing of the tool, one or more springs, or any other suitable object suitable for applying a biasing force onto the piston and/or the sleeve.

The method may further comprise the step of:

h. continuously or repeatedly performing step b, repeating steps c to g a number of times to create and release the series of pulses of pressurised fluid from the tool.

Thereby the method further relates to a method of releasing a series of pulses of pressurised fluid from a tool.

The step of providing the tool may be a step of providing the tool in a wellbore. The tool may be so provided as to neighbour a well formation to be stimulated by injection of fluid into the formation. The method according to the second aspect of the invention may be a method of performing a well stimulation operation by releasing a series of pulses of pressurised fluid into the wellbore formation.

The biasing means may be referred to as a “biasing device”.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein:

Figs. 1 to 3 illustrates an example of a sequence of steps of the method according to the second aspect of the invention wherein the piston moves from the first piston position to the second piston position, and an embodiment of the tool according to the first aspect of the invention.

The tool is shown in a cross-sectional view in the figures. Note that the embodiment of the tool shown is merely an example, and that it may not be drawn to scale. Likewise, the method steps shown are merely meant to present a possible example of the method according to the invention.

See figure 1.

The tool 1 is shown having a housing 10, a piston 11, a sleeve 12, a locking mechanism 13, and a biasing device 15. The housing 10 comprises a wall 14. On a lower end side of the wall 14, the wall 14 has a lower end wall part 141 that is detachable from the wall 14, being connectable to and disconnectable from a side part of the wall by a threaded wall connection 143. The lower end wall part 141 has an inner surface 144 that forms a lower end inner surface 144 of the housing 10.

The sleeve 12 has a lower sleeve part 121, a mid sleeve part 122 and an upper sleeve part 123. The lower sleeve part 121 is connected to the mid sleeve part 122 through a threaded connection, and the mid sleeve part 122 is connected to the upper sleeve part through a threaded connection.

By having sections of the tool 1 made up by parts connected via threaded connections, parts that can be detached from and re-attached to each other, the tool 1 facilitates modifications, e.g. to adapt the tool 1 for use for different wellbore pressures, e.g. by replacing the biasing device 15 or a part of the biasing device 15, or by replacing the piston 11 with a piston 11 of a different size.

The piston 11 also has two parts, a lower piston part 111 and an upper piston part 112, that are connected together, to make up the piston 11, via a threaded connection.

The biasing device 15 (may also be referred to as a “biasing means”) comprises a first spring 151 and a second spring 152. Further, the biasing device 15 comprises a pressurised gas in a lower chamber 101 defined between the wall 14 of the housing, including the lower end wall part 141, a lowermost section of the lower piston part 111 and a lower outer surface of the lower sleeve part 121. The lower end wall part 141 comprises a valved port 142 that may be used e.g. to bleed off a fluid pressure in the lower chamber 101.

The second spring 152 is arranged in the lower chamber 101, between the lower end inner surface 144 of the housing 10 and an outer surface of the lower sleeve part 121. The spring adds a biasing force that acts on the sleeve 12 to push the sleeve 12 towards the first sleeve position, which is the position of the sleeve 12 in figure 1. When the piston 11 is releasably locked to the sleeve 12, the second spring 152 also, indirectly, biases the piston towards its first piston position, which is the position in which the piston 11 is in figure 1.

The first spring 151 is arranged in an upper chamber 102, defined between an inner wall 122 of the sleeve a side wall of the lower piston part 111 and a lower surface of an upper piston part 112. The first spring 151 is arranged for biasing the piston towards the first piston position.

The tool 1 has side ports 145 in the wall 14 of the housing 10 and is configured to release pressurised fluid in pulses through said side ports 145. The pressurised fluid is to be received in the housing 10 in the tool 1, and has to force the piston 11 and the sleeve 12 from the first piston position to the second piston position and from the first sleeve position to the second sleeve position respectively to open the side ports 145 for release of the fluid through the side ports 145.

When the sleeve 12 is in the first sleeve position, and uppermost position for the sleeve 12, its mobility upwards is blocked by a narrowing of the inner diameter of the housing 10. And upper sleeve edge 125, when the sleeve 12 is in the first sleeve position, abuts a sleeve-blocking edge 146 of the wall 14 of the housing 10. Furthermore, in this position, an upper part of the sleeve 12 blocks the side ports 145 so that pressurised fluid is not released from the tool.

The locking mechanism 13 comprises a set of locking balls 131. Further, the locking mechanism 13 comprises a wall recess 147, a sleeve side opening 124 and a piston recess 113 for each locking ball of the set of locking balls 131. The locking mechanism 13 is arranged to bring the piston 11 into a releasable locking engagement with the sleeve 12 when the piston 11 is moved a distance from the first piston position.

When the piston 11 and the sleeve 12 are in the first piston position and the first sleeve position respectively, as shown in figure 1, each of locking balls 131 of the locking mechanism 13 are in a resting, non-locking state, resting partly in a wall recess 147 and a corresponding sleeve side opening 124.

The locking mechanism 13 brings the piston 11 and the sleeve 12 into locking engagement when the piston 11 is pushed downwards in the tool to an intermediate piston position, a position where the piston recesses 113 corresponds with the sleeve side openings 124 to allow the locking balls 131 to move from the wall recesses 147 to the piston recesses 113. Each locking ball 131 then sits partly in a corresponding piston recess 113 and sleeve side opening 124.

The piston 11 is moved downwards when the fluid pressure of the pressurised fluid in the housing 10, acting on the piston 11, goes above a certain threshold so that a force or a combination of forces acting on the piston 11 against the biasing means 15 is greater than a force or a combination of forces acting against movement. The combination of forces acting on the piston 11 against the biasing means 15 may e.g. include gravity and a fluid force. The combination of forces acting against movement of the piston 11 may e.g. include a fluid force, a force from a spring and a friction force.

Figure 2 shows the same embodiment of the invention as figure 1, with the piston 11 in the intermediate piston position and the sleeve 12 in the first sleeve position. As described in the previous paragraph, the positioning of the sleeve 12 and the piston 11 of the tool 1 shown in figure 2 allows the locking balls 131 to move inwards into the piston recesses 113 and to lock the piston 11 and the sleeve 12 together for the two to move together if they are pushed towards their respective second positions.

In Figure 3, the tool 1 is shown after further movement of the piston 11. As the piston 11 and the sleeve 12 are interlocked due to the locking mechanism 13, the sleeve 12 has moved with the piston 11, into the second sleeve position.

With the sleeve 12 in the second sleeve position, the side ports 145 of the wall 14 are no longer blocked, and the pressurised fluid that has pushed the piston 11 and the sleeve 12 downwards can be released through the side ports 145.

As fluid is released through the side ports 145, the fluid pressure and thus the fluid force acting on the piston 11 and the sleeve 12 will be reduced. When the force has dropped low enough, under a certain threshold, where the force or a combination of forces acting on the piston 11 and the sleeve 12 to push piston 11 and the sleeve 12 towards their respective second positions is lower than a force or combination of forces pushing the piston 11 and the sleeve 12 towards their respective first position, the piston 11 and the sleeve 12 will start to move back to their respective first positions.

Typically, then, the sleeve 12 will move back so as to again block the side ports 145, and the piston 11 will move upwards via the intermediate piston position. Further upwards movement of the piston 11, from the intermediate piston position towards the first piston position, will then force each of the locking balls 131 back into its resting, non-locking position, from its piston recesses 113 to its wall recess 147.

The sleeve 12 will stop moving upwardly in the housing 10 when its movement is restricted by the sleeve-blocking edge 146 of the wall 14 of the housing 10. As long as the force acting on the piston 11 towards the first piston position is greater than the force acting on the piston towards the second piston position, the piston 11 will, after releasing the locking engagement from the sleeve 12, continue moving towards the first piston position.

When the sleeve 12 is so positioned as to again block the side ports 145, the fluid pressure in the housing 10 may again increase. Thus, the force acting on the piston 11 to move it towards the second piston position will again increase. When the piston 11 has reached the first piston position, a cycle is completed, and one pulse of pressurised fluid has been released from the tool 1.

The cycle described may be repeated for as long as pressurised fluid is provided into the housing 10. For each cycle, the tool 1 may release a pulse of pressurised fluid to be injected into e.g. a wellbore formation, in a scenario wherein the tool 1 is used in a wellbore, for wellbore stimulation purposes.

The tool 1 and the method is highly beneficial over the prior art due e.g. to how it operates with the locking mechanism 13/delay mechanism that provides an advantageous oscillation of the pressure pulses that are released. The tool 1 may also be more reliable than at least some prior art.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

C l a i m s

1. A tool (1) for applying pulses of pressurised fluid to a wellbore formation for well stimulation purposes, the tool (1) comprising:

a housing (10) comprising a wall (14) for separating an inside of the housing (10) from an outside of the housing (10), the wall (14) having one or more side ports (145) for providing flow paths for the pressurised fluid from the inside to the outside of the housing (10);

a piston (11), having a first side and a second side, the piston (11) being movable in the housing (10) between a first piston position and a second piston position via a first intermediate piston position, being biased by a biasing means (15) towards the first piston position, and being arranged to move to the second piston position, against a biasing force produced by the biasing means, when exposed to a fluid pressure over a first threshold on its first side and to move back to or towards the first piston position via a second intermediate piston position when the fluid pressure on the first side drops below a second threshold; and a sleeve (12) , the sleeve (12) being movable between a first sleeve position and a second sleeve position and arranged in connection with and to move in response to movement of the piston (11) and such that the one or more side ports (145) are closed when the sleeve (12) is in the first sleeve position and open when the sleeve (12) is in its second sleeve position;

c h a r a c t e r i s e d i n t h a t the tool further comprises: a locking mechanism (13) for releasably locking the piston (11) to the sleeve (12), the locking mechanism (13) being arranged to lock the piston (11) to the sleeve (12) when the piston (11) is in the first intermediate piston position when the piston (11) moves from the first piston position towards the second piston position and to release the piston (11) from the sleeve (12) in the second intermediate piston position when the piston (11) moves from the second piston position towards the first piston position.

2. The tool (1) according to claim 1, where the locking mechanism (13) comprises a roller-delay mechanism, wherein the roller-delay mechanism comprises one or more locking balls (131).

3. The tool (1) according to any one of the preceding claims, wherein the biasing means (15) comprises a fluid and at least one spring (151, 152).

4. The tool (1) according to any one of the preceding claims, wherein the wall (14) of the housing (10) comprises a detachable and re-attachable lower end wall part (141).

5. The tool (1) according to any one of the preceding claims, wherein the sleeve (12) comprises a detachable and re-attachable lower sleeve part (121).

6. The tool (1) according to any one of the preceding claims, wherein the piston (11) comprises a detachable and re-attachable bottom piston section (111).

7. The tool (1) according to any one of the preceding claims, wherein the housing (10) comprises an upper chamber (102) and a lower chamber (101).

8. A method of releasing a pressurised fluid from a tool, the method comprising the steps of:

a. providing a tool (1) comprising a housing (10) having a wall (14) with one or more side ports (145), a piston (11), a sleeve (12), and a locking mechanism (13) to releasably lock the piston (11) to the sleeve (12) for the two to move together, wherein the method comprises the steps of:

b. applying a pressurised fluid to the tool (1) and the piston (11), and by use of a force from the pressurised fluid moving the piston (11) from a first piston position to a first intermediate piston position;

c. when the piston (11) is in the first intermediate piston position, releasably locking the piston (11) to the sleeve (12) by use of the locking mechanism (13), to have the sleeve (12) move with the piston (11) when the piston (11) moves further; and

d. further applying the pressurised fluid to the tool (1) and the piston (11) and by use of the force from the pressurised fluid moving the piston (11) further, to a second piston position, and moving the sleeve with the piston, thereby opening the one or more side ports (145) and allowing the pressurised fluid to be released from the tool (1) through the one or more side ports (145).

9. The method according to claim 8, wherein the method further comprises the steps of:

e. pushing the piston (11), with the sleeve (12), back from the second piston position, to the first piston position, by use of a biasing means (15) arranged to push the piston (11) towards the first piston position;

-f. releasing the piston (11) from the sleeve (12) when the piston (11) is in a second intermediate piston position between the first piston position and the second piston position; and

g. further moving the piston (11) to the first piston position by use of the biasing means (15).

10. The method according to claim 9, wherein the method further comprises the step of:

h. continuously or repeatedly performing step b and repeating steps c to g a number of times to create and release a series of pulses of pressurised fluid from the tool (1).

11. The method according to any one of claims 8 to 10, wherein the step of providing the tool (1) is a step of providing the tool according to any one of claims 1 to 7.

12. The method according to any one of claims 10 or 11, wherein the step of providing the tool (1) is a step of providing the tool (1) in a wellbore such as for the tool to neighbour a well formation to be stimulated by injection of fluid into the formation, and wherein the method is a method of performing a well stimulation operation by releasing a series of pulses of pressurised fluid from the tool (1) into the wellbore formation.