Hydraulic stimulation method and corresponding hydraulic stimulation device

A method for hydraulic stimulation of rock of a borehole of a well having an internal casing. The method includes positioning a conduit in the casing, the conduit having along its external face expandable tubular sleeves, fixedly linked to the conduit, and at least one aperture before each sleeve placing the internal space of the conduit into communication with the space demarcated by the conduit and each sleeve. A fluid is injected into the conduit under a first pre-determined pressure, which is sufficient to expand the sleeves. For each of the areas of the wall of the well to be stimulated, the method includes: plugging the conduit downstream from a first area to be stimulated; perforating the wall of the conduit and, at least in certain cases, perforating the casing at the first area to be stimulated; and injecting a fluid into the conduit under a second stimulation pressure.

1. CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Section 371 National Stage Application of International Application No. PCT/EP2015/073961, filed Oct. 16, 2015, the content of which is incorporated herein by reference in its entirety, and published as WO 2016/078840 on May 26, 2016, not in English.

2. FIELD OF THE INVENTION

The invention relates to the field of drilling and especially but not exclusively to the field of oil drilling.

The invention can be applied especially for fracturing or fracking geological formations surrounding a well drilled into the ground by the injection of a hydraulic fluid under pressure. It can also be applied for re-fracturing or re-fracking these geological formations in order to retrieve quantities of fluid, produced by this well, that might have not been drained during the initial fracking.

3. PRIOR ART SOLUTIONS

Hydraulic techniques for fracking horizontal wells are widely known and consist of the injection of a fracking fluid into a borehole and conveying this fluid into contact with the formation to be fracked.

Sufficiently high pressure is applied to the fracking fluid to initiate and propagate a fracture in the formation and extract the gas or petroleum from it. Proppants such as, for example, sand or ceramic are generally carried along in this fracking fluid and deposited in the fracture in order to keep it open during production thus enabling the released hydrocarbons to flow up to the surface of the well.

The stimulation of horizontal wells by hydraulic fracking is generally done in several steps in order to create fractures distributed perpendicularly to the horizontal well throughout its length.

It is common practice to implement one of the following two hydraulic fracking methods depending respectively on whether the well is tubed and cemented or tubed but not cemented:fracking according to the technique known as “plug and perf” in which the fracking procedure is executed sequentially on several intervals of a horizontal well that is tubed and cemented. Starting from the bottom of the well, each interval is perforated, fracked (or stimulated) and isolated by means of a plug before the next interval is processed. Once this procedure has been performed, the plugs can be drilled and the production phase initiated. One drawback of this technique is that it requires a lengthy and difficult cementing operation because it is done in a horizontal well.fracking known as “open hole multi-stage or OHMS fracking” in which a steel casing is placed within the open well, the casing having sliding sleeves or frac sleeves which are disposed between two isolating packers. These sliding sleeves enable the interior and the exterior of the casing to be put into communication by means of holes or apertures once they occupy a position that does not cover them. These sliding sleeves can be activated by different systems by means of balls that are launched from the surface of the well and strike the sleeves, thus causing them to be moved. One drawback of this technique is that the number of stimulated areas is limited by the number of balls that can be used. In addition, the totality of the area isolated by two packers is exposed to pressure. It is therefore difficult to control the area of initiation and propagation of the fracture.

Other techniques combining sliding sleeves and cemented casings or liners can also be used.

The level of production of gas or petroleum from a well significantly decreases after some years.

It can be decided to re-frack a same well to extend its production period.

Re-fracking is especially sought as a way to augment the penetration of the initial fracture in the formation or develop a new network of fractures and thus extract quantities of petroleum or gas that are still trapped within the rock.

The productivity obtained attains and sometimes surpasses the productivity previously observed during the first fracking. Besides, re-fracking is advantageous in that it does not require the performance of all the operations of well drilling and completion in order to be put it into service, and this limits costs. In other words, it is sometimes more economical to re-frack a well than to drill a new well.

There are different re-fracking techniques. For wells, there have been fracked by what is called the “plug and perf” technique, it is possible to position and cement a new conduit of smaller diameter within the first cemented and fracked conduit and to reiterate the “plug and perf” procedure detailed here above. It is then necessary to ensure cementing quality that sets up tight sealing between the two conduits. The new operations of perforation must also pass through two walls of conduits.

It is also possible to place an expandable sleeve within the first cemented and fractured conduit and reiterate the “plug and perf” procedure described in detail here above. The quality of the tight sealing between the expandable sleeve and the conduit is however vital. One drawback of this approach is that there is a risk that the sleeve will sag during the re-fracking due to generally limited resistance to external pressure for technologies of this type.

4. SUMMARY OF THE INVENTION

According to a first aspect, the invention relates to a method of hydraulic stimulation of the rock of a borehole having an internal casing, the said method comprising the steps of:positioning a conduit, in the casing of the well, the conduit being provided along its external face, with expandable tubular sleeves, fixedly linked to the conduit, the conduit having at least one aperture before each sleeve to place the internal space of the conduit into communication with the space demarcated by this conduit and each sleeve;injecting a fluid into said conduit under a first pre-determined pressure, this first pressure being sufficient to prompt the expansion of the said sleeves towards the wall of the casing so they get applied against this wall in a tightly sealed way;
the method furthermore comprising, for each of the areas of the wall of the well to be stimulated, the following steps:putting into position means for plugging the conduit downstream from a first area to be stimulated;perforating the wall of the conduit and, at least in certain cases, perforating the casing at the first area to be stimulated;injecting a fluid into said conduit under a second stimulation pressure different from the first pressure, this fluid being intended to surge into the respective apertures of the sleeves situated upstream to the plugging means positioned as well as in the perforations obtained during the perforation at the first area to be stimulated.

The invention proposes a method of hydraulic stimulation of a well by injection, into a conduit carrying several expandable sleeves, of a fluid under very high pressure intended to crack the rock. The conduit is placed in such a way that each area of the well to be stimulated (i.e. each area to be fracked or re-fracked) is situated between sleeves expanded against the wall of the well and mutually defining a confined space.

The conduit is configured in such a way that the same stimulation pressure prevails in the annular volume situated between the external surface of the conduit and the wall of the well at each area to be stimulated and inside the expanded sleeves situated on either side of the annular volume. Thus, the two sleeves demarcate the area to be stimulated.

Such a method can be implemented for applications of hydraulic fracking and re-fracking of the rock of a well (a gas or petroleum well for example).

Through this method, it is possible to carry out highly targeted and short-length fracking and re-fracking operations, in fracking one area at a time, with perfect impervious sealing or tight sealing between the fracked area and the surrounding areas.

It is possible especially to re-frack the areas that have not been sufficiently stimulated during a first fracking operation. In other words, this method increases the penetration of the initial fracture into the formation or develops a new network of fractures.

The method of the invention is carried out in successive phases, the sequencing of the different phases of re-fracking being done from the bottom to the top, i.e. from the downstream side of the well towards the upstream side.

The solution of the invention thus resolves the problems of the prior art and optimizes production simply, efficiently and at little cost.

According to one particular embodiment of the invention, the plugging means are placed downstream from the aperture of the sleeve situated downstream from the area to be stimulated.

According to one particular aspect of the invention, prior to the previous steps, the casing of the well has been perforated in at least one part of said areas and a hydraulic fracking operation has been implemented in the well.

According to one particular aspect of the invention, the sequencing of the steps for the positioning of means of plugging, perforation and injection of the stimulation fluid is done from the downstream side of the well towards the upstream side of the well.

According to one particular aspect of the invention, the step for the positioning of means for plugging the conduit, downstream from the first area of the wall of the well to be stimulated, comprises a step for injecting a ball into the conduit that will get firmly applied in a tightly sealed way against a seat made in proximity to the downstream extremity of the interior wall of the conduit.

According to one particular aspect of the invention, the step for the positioning of means for plugging the conduit, downstream from the first area of the wall of the well to be stimulated, comprises a step for injecting a plug into the conduit intended to get anchored in a tightly sealed way in proximity to the downstream extremity of the interior wall of the conduit.

According to one particular aspect of the invention, the step for the positioning of means for plugging the conduit, downstream from a second area of the wall of the well to be stimulated, comprises a step of injection of a plug within the conduit that is to get anchored in a tightly sealed way into the interior wall of the conduit.

Another aspect of the invention relates to a hydraulic stimulation device to implement the method of hydraulic stimulation of the rock of a borehole having an internal casing as described here above, the device comprising:a tubular conduit and several expandable tubular sleeves, the opposite extremities of which are connected fixedly and in a tightly sealed way to the external face of said conduit, the wall of the conduit comprising at least one aperture to make the interior of the conduit communicate with the interior of each sleeve;means of injection into said conduit of a fluid under a first pre-determined pressure, this first pressure being sufficient to prompt the expansion of said sleeves towards the wall of the casing so that they get applied in a tightly sealed way against this wall;means for plugging the conduit that are to get positioned, downstream from a first area of the wall of the well to be stimulated;means for perforating the wall of the conduit, and at least in certain cases the wall of the casing, at the first area to be stimulated;means of injection, into said conduit, of a fluid under a second stimulation pressure, different from the first pressure, this fluid being intended to surge into the respective apertures of the sleeves situated upstream to the plugging means, as well as into the perforations of the wall of the conduit and, and, at least in certain cases, into the perforations of the casing, obtained during the perforation at the first area to be stimulated.

This device can especially be used to stimulate the walls of a well (or rocky formations surrounding a well), a petroleum well for example, drilled in the ground in order to increase the permeability of the wall by formation of vacuums and thus facilitate the draining of the fluid produced by this well, i.e. in order to stimulate production.

This device is adapted to re-stimulating the old fracked portions but also to stimulate the new fracked portions.

Such a device is:simple to implement, compact, reliable, shows high performance and costs little; and hasexcellent resistance over time under a wide range of temperatures and pressures.

According to one particular aspect of the invention, the fluid under a second stimulation pressure is further intended to surge into the aperture of the sleeve situated downstream from the area to be stimulated.

According to one particular aspect of the invention, the plugging means comprise a plug bearing tight-sealing means capable of getting applied against the interior wall of the conduit and anchoring means for anchoring said plug into the interior wall of the conduit.

According to one particular aspect of the invention, said plug is fixedly attached to the perforation means.

According to one particular aspect of the invention, the perforation means get detached from said plug once the plug is positioned.

According to one particular aspect of the invention, the perforation means get detached from said plug once the perforations have been made.

According to one particular aspect of the invention, the perforation means carry mobile blocking means to cooperate with the circular grooves made in the interior wall of the conduit.

According to one particular aspect of the invention, a first circular groove is made downstream from the sleeve situated downstream from the area to be stimulated.

According to one particular aspect of the invention, a second circular groove is made upstream to the sleeve situated downstream from the area to be stimulated.

According to one particular aspect of the invention, the interior of the conduit comprises, at its downstream extremity, a seat capable of co-operating with a ball so as to plug said conduit.

According to one particular aspect of the invention, said apertures are equipped with a check valve, a device for plugging the aperture in the event of a flow rate of fluid greater than a pre-determined value, or an overflow valve.

In the appended figures and purely for the sake of simplification, only a fraction of the horizontal part of borewell A has been shown. It is of course possible for this horizontal portion to extend over a greater length. This portion is attached to a vertical portion leading into the open air, via an intermediate portion substantially shaped like an arc of a circle (not shown). For all the figures, it is considered that the “peak” of the well (which opens into the open air) is situated towards the left of the figures and that its bottom is situated towards to the right.

In all the figures of the present document, the identical elements are designated by a same numerical reference.

FIG. 1is a schematic view, along a longitudinal sectional plane, of a borehole A that has been fracked a first time and in which it is now sought to carry out a re-fracking of the rock. A casing3is disposed in the well A, the annular space4between the casing3and the formation of the well A being cemented.

The casing3can be continuous or formed by a succession of cylindrical steel tubes soldered or screwed to one another end-to-end.

ThisFIG. 1shows the hydraulic fracking areas Z1to Z5comprising several perforations extending radially in and around the casing3.

FIGS. 2 to 15illustrate an example of implementation of the method of hydraulic re-fracking of the tube well A according to the invention.

The first step of this method consists of the positioning, in the well A, of a hydraulic re-fracking device compliant with the invention. The device is shown inFIGS. 2 to 15but appears only partially in order to facilitate the understanding of the invention.

This device comprises a tubular conduit1made of metal that is traditionally positioned inside the horizontal part of the well A.

In practice, this conduit also comprises a vertical upstream extremity that leads into the surface of the well, as well as a curved intermediate portion to connect the vertical part to the horizontal part. This is a tubular conduit formed by several sections placed end-to-end so as to form a completion.

Against the external face of this conduit1, there extend several isolating devices, each comprising a single cylindrical (or approximately cylindrical) sleeve that is expandable and preferably made of metal. The opposite extremities of each sleeve21to26are connected fixedly and in a tightly sealed way, by means known per se, to the external face of the conduit1. About a hundred isolating devices can be provided on the external face of the conduit1, the spacing or span between two consecutive devices possibly being, for example, of the order of 15 m to 20 m.

In a manner known per se, the tubular conduit1comprises at least one aperture11that makes this internal space communicate with the interior of each sleeve21to26. Each of the apertures11can be provided with a valve system such as a check valve, a system for plugging the aperture11in the event of an excessive flow of fluid or an overflow valve.

The conduit1is disposed in the well A in such a way that the fracking areas Z1to Z5are situated appreciably at mid-distance between two consecutive sleeves. As shall be seen here below, the method of the invention described here below is aimed at refracking the wells Z1to Z5and at fracking an area Z6situated between the sleeves23and24(the area Z6has not been previously fracked).

The well is stimulated in successive steps. The sequencing of the different re-fracking phases is done from bottom to top (i.e. from the downstream side of the well towards the upstream side).

It can be noted that the conduit1is positioned in the well A with its downstream extremity open, i.e. it is not plugged, to enable the free circulation of fluid present in the well A during this first step. It must be furthermore noted that the sleeves21to26are in their original state, i.e. not expanded.

An original pressure P0prevails inside and outside the conduit1.

Then (FIGS. and3and4) an annular tight sealing is set up between the conduit1and the wall of the well A, on either side of each area Z1to Z5that is to be re-fracked.

To this end, the downstream extremity of the conduit1is closed, (FIG. 3). This closing is carried out by sending a fluid containing a ball13that gets applied against a seat14made at the open extremity of the conduit1, thus plugging this conduit. Any other means of plugging of the conduit1can be implemented, especially a plug.

Then, into the interior of the conduit1and the sleeves21to26, through the apertures11, a fluid is injected under a pressure P1, greater than P0. This pressure is such that it can give rise to the expansion of the sleeves and can apply the wall of the sleeves21to26against the interior wall of the casing3(FIG. 4) in a tightly sealed way.

FIG. 4is a view similar to that of the preceding figure after application inside the conduit of a pressure. The fluid surges into the apertures11communicating with the sleeves, the pressure of the fluid being chosen in such a way that it is sufficient to prompt the radial expansion of the sleeves towards the wall of the well A so that they get applied against this wall in a tightly sealed way. When this happens, the pressure P1prevails inside the sleeves21to26while, in the space between two consecutive sleeves demarcated by the conduit1and the wall of the casing3, only an original pressure P0prevails.

After the pressure P1gets relaxed, while the sleeves21to26remain expanded and supported against the wall of the well A, the conduit1is perforated at the height of the fracking area Z5(FIG. 5) by means of a classic perforation device (not shown) implemented in the “plug and perf” operations. Several perforations12distributed evenly on the periphery of the conduit1are then obtained in the wall of the conduit1. It is by this type of aperture that a fluid under high pressure is conveyed in order to frack the rock.

The perforations can for example be made by means of hollow-charge type explosives. These explosives can be let down into the well by “wireline”, “coiled tubing” or again “drill pipe” type tools.

In one alternative, it is possible to provide for a sliding sleeve (not shown) which is moved by the ball13in active position and enables the interior and the exterior of the conduit1to be put into communication by means of holes or apertures (which are covered by the sleeve in its inactive position).

Then, into the conduit1, a fracking fluid is injected under a fracking pressure P2(FIG. 6) different from the pressure P1, from the peak to the bottom, from upstream to downstream. A wellhead is provided to convey the fracking fluid into the well A. Since the conduit1is blocked at its downstream extremity by the ball13, the fracking fluid is directed, via the perforations of the conduit1, into the annular area situated between the external wall of the periphery1and the interior wall of the casing3, at the area Z5. The fracking pressure P2is such that it can break the wall of the well A and frack the rock without affecting the sleeve26.

As illustrated inFIG. 7, once the pressure P2is relaxed, a plug51is placed in the conduit1at the sleeve26downstream from the aperture21of the sleeve26and upstream to the perforations12situated at the re-fracturing area Z5.

The plug51is, for example, made of composite material and comprises a compressible membrane in the form of a cylindrical sleeve and anchoring teeth situated on either side of the membrane. The compression of the compressible membrane against the wall of the tubing ensures the tight sealing and the positioning of the teeth in a gripping position, so as to ensure that the plug51gets anchored in the conduit1. The body of the plug51can be solid and or it can include a longitudinal orifice at its center (thus enabling a circulation of fluid). This orifice is plugged by a ball injected into the conduit1, once the plug51is in position in this conduit. Such plugs can be drilled before the tools are put into production.

Other types of plugs well known to those skilled in the art can however be envisaged.

It can be noted that the sleeves21to26remain expanded.

As illustrated inFIG. 8, the conduit1is perforated again at the area Z4situated between the two sleeves25and26and downstream from the plug51.

A fracking fluid under a fracking pressure P2is then sent into the conduit1(FIG. 9).

The positioning of the plug51enables the passage of the fracking fluid from the interior of the conduit1into the internal space of each of the sleeves21to26via the corresponding aperture11. The fracking fluid furthermore penetrates the annular space via the perforations12situated between the sleeves25,26and the interior wall of the casing3. Thus the same pressure P2prevails on either side of the wall of the sleeves25,26, and this prevents the collapse of the wall of the sleeves25,26(i.e. within the expanded sleeves25and26as well as in the annular space situated between the sleeves25,26.

It can be noted that the plus51prevents the passage of the fracking fluid towards the area Z5which had been previously re-fracked.

Because no pressure differential exists between the interior of the sleeves25,26and the annular area facing the area Z4of the wall to be fracked, the fracking is truly localized at this annular wall without any risk of transmission of the fracking to an area that is not facing the one aimed at.

FIGS. 10 to 12illustrate the same steps of re-fracking of the area Z3situated between the sleeves24and25. A second plug52is placed upstream to the previously re-fracked area Z4slightly downstream from the aperture11of the sleeve25(FIG. 10). Once the wall of the conduit1is perforated at the area Z3(FIG. 11), a fracking fluid surges into the interior of the sleeves21to25and in the annular structure situated between the sleeves24and25, through the perforations12(FIG. 12). The same pressure P2prevails inside the expanded sleeves24and25as well as in the annular space situated between the sleeve24,25.

FIGS. 13 to 15illustrate the steps for fracking the area Z6situated between the sleeves23and24. A third plug53is placed upstream to the area Z3previously re-fracked and slightly downstream from the aperture11of the sleeve24(FIG. 13). Once the walls of the conduit1and the casing3are perforated at the level of the area Z6to be fracked (FIG. 14), a fracking fluid surges into the interior of the sleeves21to24and in the annular space situated between the sleeves23and24, through the perforations12(FIG. 15). The same pressure P2prevails within the expanded sleeves23and24as well as in the annular space situated between these sleeves23,24.

Although this is not illustrated in the figures, it can easily be understood that the re-fracking of the area Z2and then of the area Z1is implemented according to the same principle as the one described above for the areas Z3to Z5.

Once these steps of fracking (for the area Z6) and re-fracking (for the areas Z1to Z5) have been performed, the plugs can be drilled and the well A can be put into production.

The method of the invention can be implemented to frack a cased hole and thus be an alternative to the fracking technique known as “plug and perf” described in the prior art.

The drilled well can be a petroleum well, a gas well or a geothermal well for example.

FIGS. 16 to 19illustrate a first technique for positioning the perforation device for perforating the conduit1(and in certain cases the casing3) in the conduit1.

For example, these figures illustrate the positioning of this device at the previously fracked area Z3which is now to be re-fracked.

As shown inFIG. 16, the perforation device or perforator6, which is cylindrical, is let down into the conduit1by means of a “wireline” type tool61.

At its upstream extremity (situated to the left), the perforator6comprises a perforation head62designed to draw explosive charges perpendicularly towards the exterior through holes63perforating the conduit1(and the casing3for a fracking application) and the rocky formation of the well A.

At its downstream extremity, the perforator6has two fingers64mobile in a direction perpendicular to the longitudinal axis of the conduit1and a plug52intended to ensure tight sealing between the area Z3to be re-fracked and the previously fracked area Z4.

The interior wall of the conduit1comprises a circular groove15, downstream from the sleeve25(itself situated downstream from the area Z3), intended to receive the fingers64when they are deployed. These fingers64are retracted when the perforator6is taken down into the well4then deployed when they are situated facing the groove15. This deployment is obtained by a system of springs65fixedly attached to each of the fingers64and the perforator6. The positioning of the perforator6is then provided (FIG. 16), the perforation tip62being situated facing the area Z3to be re-fracked (or to be fracked for the area Z6), appreciably at mid-distance between the expanded sleeves24,25situated on either side of the area Z3.

The plug52is then expanded and gets anchored in a tightly sealed way in the inner wall of the conduit1(FIG. 17). The conduit1can then be perforated at the area Z3by the perforation tip62, the perforations12obtained being visible inFIG. 18.

The fingers64are then retracted by means of the springs65and the perforator6is withdrawn from the conduit1. The perforator6gets detached from the plug52which remains in position in the conduit1(FIG. 19). The area Z3can then be re-fracked.

It can be understood that the same steps are implemented to isolate and perforate the other areas to be stimulated of the well A from downstream from upstream.

It can be noted that the groove15can be advantageously made at the coupling between two assembled portions of the conduit1.

FIGS. 20 to 22illustrate a second technique for positioning the perforation device of the conduit1(and in certain cases that of the casing3) in the conduit1at the previously fracked area Z3, which is to be re-fracked.

As can be seen inFIG. 20, the perforation device or perforator6has a smaller length than the one illustrated inFIGS. 16 to 19.

In the same way, the perforator6is brought down into the conduit1by a wireline type tool61and includes a perforation tip62, two fingers64mobile in a direction perpendicular to the longitudinal axis of the conduit1, and a plug52to set up tight sealing between the area Z3to be re-fracked and the previously re-fracked area Z4.

The inner wall of the conduit1comprises a first circular groove15, downstream, from the sleeve25(itself situated downstream from the area Z3) that is to receive the finger64when they are deployed. These fingers64are retracted when the perforator6is brought down into the well A and then deployed when they are situated facing the groove15. This deployment is carried out by a system of strings65. The positioning of the perforator6is then provided (FIG. 20), the perforation tip62being situated so as to be facing the sleeve25.

The plug52is then expanded and gets anchored in a tightly sealed way in the inner wall of the conduit1(FIG. 21).

The fingers64are then retracted and the perforator6is shifted in the conduit1, upstream. The perforator6gets detached from the plug52which remains in position in the conduit1. Once the fingers64are situated facing a second circular groove15, made in the wall of the conduit1upstream to the sleeve25and downstream from the area Z3, the fingers64are deployed and cooperate with the second groove15(FIG. 22).

The conduit1can then be perforated at the area Z3by the perforation head62, the perforations12obtained being visible inFIG. 22.

The fingers64are then retracted and the perforator6is withdrawn from the conduit1.

It will be understood that the same steps are implemented for (re-)fracking the other areas to be stimulated of the well A, from downstream from upstream, each sleeve downstream from the area to be (re-)fracked being situated between a first and a second groove.

Thus, these two techniques enable a relative positioning of the plugging means (plug52) and perforation means (perforator6) relative to the expandable sleeves.

An exemplary embodiment of the present invention is aimed at resolving the drawbacks of the prior art stimulation techniques.