EP0435756B1 - Method and device for stimulation of an underground formation by injection of a fluid coming from an adjacent zone along fractures proceeding from a drain hole in a low-permeable intermediate layer - Google Patents

Description

The present invention relates to a method for stimulating an underground zone by delayed injection of pressurized fluid from a neighboring zone by means of a deflected drain passing through an intermediate layer which is very slightly permeable. Throughout the rest of this text, the term “diverted drain” will designate any borehole of which at least part is horizontal or relatively slightly inclined relative to the horizontal.

The method according to the invention more particularly makes it possible to stimulate the production of an oil zone separated from an underlying zone containing a pressurized fluid, such as an aquifer zone or possibly oil zone.

Various techniques well known to specialists are used to stimulate the production of oil zones. One of them consists essentially of injecting a pressurized fluid into the production formation, capable of draining the stagnant oil in the rocks due to its viscosity.

US-A-2,736,381 relates to the stimulation of a formation by placing in communication with an underlying zone by a vertical well and production by another vertical well laterally offset from the first, drilled through layers of separation.

The fluid used is for example pressurized water injected by drains drilled through the formation. It may also be water existing deep in the production basin itself in the form of an underlying aquifer.

In certain types of basins, the aquifer is located under the oil zone and separated from it by a very low permeable layer due in particular to the presence of hydrocarbon products heavy and very viscous ("tarmat").

We can consider using this underlying water to stimulate the production of an oil zone. The water table being located at a depth greater than that of the oil zone, its internal pressure is higher. The at least partial exhaustion of the zone in production has the effect of increasing the overpressure of the water in the underlying tablecloth relative to the fluids of the upper formation. The possible injection into the oil formation of this overpressure water must make it possible to drain the oil and to promote production.

Attempts to stimulate production areas above aquifers have so far not produced the expected results. Vertical wells or drains were drilled through the oil zone, so as to put the oil deposit in communication with the water table. However, it has been found that this type of well mainly produces water. This negative result can be explained by the fact that the water in the aquifer tends to escape directly to the surface through the well thus created instead of entering the oil formation. This phenomenon persists if a shutter member is lowered into the well because the water in the aquifer tends to bypass it through the surrounding formations. A possible plugging near the surface leads to a certain diffusion of water in the oil zone. However, the results are not very significant since the volume of the deposit penetrated by the water remains relatively small.

It is also known, in particular from US-A-3,313,348 for example, to improve the drainage of a reservoir formation by carrying out hydraulic fracturing in a deviated drain, drilled through it.

The method according to the invention makes it possible to stimulate a reservoir zone of an underground formation, separated from a neighboring zone containing a pressurized fluid by an intermediate layer less permeable than the two zones on either side, by performing a hydraulic fracturing in a deflected drain portion after closing this deflected drain portion to delimit a fracturing zone.

It is characterized in that the deflected drain is pierced so that over at least part of its length, it crosses the intermediate layer, the method comprising the delayed opening by hydraulic fracturing carried out entirely in this part of the drain, 'at least one communication channel connecting the neighboring area to the reservoir area, so as to promote the drainage of the reservoir area by the pressurized fluid.

The method comprises, for example, carrying out from the drain, fractures making an oil zone communicate with an underlying aquifer zone.

The method also includes, for example, carrying out from the drain, fractures making an oil zone communicate with a second underlying oil zone.

The drain can be pierced in an intermediate layer which is not very permeable. It can also be drilled in a non-oil-bearing intermediate layer.

The injection of pressurized fluid into an underground zone such as an oil deposit whose internal pressure is weakened due to production, by fracturing an intermediate layer from a drain drilled along it and suitably confined for preventing the fluid from escaping to the surface, has the effect of pushing the oil towards the production wells. The use of a deviated drain (horizontal or slightly inclined on the horizontal) makes it possible to increase the volume of the area invaded by the injected fluid. When the neighboring zone is an active aquifer zone, that is to say permanently replenished by water inlets, the injection pressure does not undergo any significant drop throughout the drainage phase. The effect obtained is more lasting.

• la Fig.1 montre de façon schématique une coupe d'une zone de production pétrolière surmontant une zone contenant un fluide sous pression telle qu'une zone aquifère et un premier mode de mise en oeuvre de la méthode selon l'invention; et.
• la Fig.2 montre de façon schématique la même coupe avec un second mode de réalisation de la méthode.

Other characteristics and advantages of the method and the device according to the invention will appear better on reading the following description of an embodiment described by way of nonlimiting example, with reference to the appended drawings where:

• Fig. 1 schematically shows a section through an area of petroleum production overcoming an area containing a pressurized fluid such as an aquifer area and a first mode of implementation of the method according to the invention; and.
• Fig.2 shows schematically the same section with a second embodiment of the method.

The oil zone Z1 produces oil by at least one production well P1. This well can be vertical as shown in FIG. 1 or alternatively be horizontal. In certain types of basins, below the oil zone Z1 and separated from it by a very low permeability layer L, there is a neighboring zone containing a fluid under pressure. The pressure of this fluid is higher than the pressure prevailing in the production zone Z1 because the neighboring zone is at a greater depth. This fluid can be water or even a petroleum fluid. When the neighboring area is aquiferous, it is generally of the active type i.e. it is supplied by external water supplies and the pressure which reigns there remains substantially constant. The intermediate layer L can for example be an area made almost impermeable due to the presence of very heavy and viscous hydrocarbon products. This layer L can also be of a non-oil type.

The method according to the invention comprises drilling from the surface S of a deviated drain D (horizontal or slightly inclined relative to the horizontal) so as to remain in the intermediate layer. When the drain D is drilled, it is provided with a casing or casing C over its entire length. Known sealing means close the annular space between the casing and the drain. Over at least a portion d1 of its length, where it crosses the production zone, the casing C is provided with lateral openings O putting the crossed formations in communication with the interior of the drain D. At this stage no communication exists between the two zones Z1 and Z2. The drain can be drilled at any time, before the production of the oil zone Z1 or possibly during the production period.

During this period, when it turns out that zone Z1 requires stimulation, fracturing operations are carried out on the intermediate layer from the previously drilled drain D. For this purpose and by a method well known to specialists, one descends in the casing C expandable obturation members B1, B2 as far as the part provided with lateral openings O and they are placed so as to confine a limited portion of the casing. By a tube (not shown) communicating the confined part with a hydraulic system, a fluid is applied under sufficient pressure to fracture the walls until the establishment of communication channels F between the two zones Z1 and Z2. The previous sequence of operations may be repeated at several different points in the drain, so as to widen the fractured area.

The fracturing operations being completed, a blocking block B3 is installed in the casing so as to isolate the drain from the outside environment. The fluid, in this case the water of zone Z2, being at a higher pressure and the drain being confined by the obturation block B3, the water enters zone Z1 by the fractures F of casing C and flushes out the oil accumulated in the formation.

According to the embodiment of Fig.2, the drain D is arranged so that it can be used as a production well. For this purpose, the casing C is provided in its part which crosses the production zone Z1, with another portion of length d2 provided with lateral orifices. This other casing portion d2 is, depending on the case, more or less distant from the portion d1 through which the water coming from the zone Z2 can enter and laterally offset with respect thereto. Between the two portions d1 and d2 is disposed a shutter member B4 whose closure can be triggered from the surface installation.

During the production period, when stimulation of production is necessary, the length portion d1 of the casing is fractured, in the same way as in the previous embodiment and the organ is closed B4 so as to isolate from one another the two parts d1 and d2 of the casing. Water from the underlying zone, penetrates by fracturing in the production zone Z1 and drives the oil towards the production wells and in particular towards the part d2 of the casing C open to the external environment. The drain thus participates in production.

The method according to the invention thus makes it possible to best use the high pressure fluid available in situ for the purpose of stimulating oil production.

The position of the vertical and / or horizontal production wells relative to the deflected drains D or vice versa of the drains with respect to the wells according to the order in which they will have been drilled is chosen according to the rules of the art so as to optimize oil production .

Claims (10)

1. A method for stimulating a zone-reservoir of a subterranean formation separated from an adjacent zone containing a pressurised fluid by an intermediate layer that is less permeable than the two zones on either side, by hydraulically fracturing a section of inclined drain after blocking off this section of inclined drain so as delineate a fracturing zone, characterised in that at least a portion of the inclined drain is drilled in such a way that it passes through the intermediate layer, the method consisting in subsequent opening, by hydraulic fracturing effected entirely within this section of the drain, of at least one communication channel linking the adjacent zone to the zone-reservoir to encourage drainage of the zone-reservoir by the pressurised fluid.
2. A method in accordance with claim 1, characterised in that fractures are made from this drain to link a petroleum zone (Z1) and a second, subjacent water-containing zone (Z2).
3. A method in accordance with claim 1, characterised in that fractures are made from this drain to link a petroleum zone (Z1) with a second subjacent petroleum zone (Z2).
4. A method in accordance with one of the previous claims, characterised in that the drain (D) is drilled through an intermediate layer that is not very permeable.
5. A method in accordance with claim 4, characterised in that the drain (D) is drilled through an non-petroliferous intermediate layer.
6. A method in accordance with one of the previous claims, characterised in that the subterranean zone (Z1) has a communication link with the surface via at least one vertical production well (P) whose position relative to the inclined drain is selected so as to optimise production.
7. A method in accordance with claim 4 or 5, characterised in that the subterranean zone (Z1) communicates with the surface via at least one inclined production well whose position relative to the drain is selected so as to optimise production.
8. A method in accordance with one of the previous claims, characterised in that the section of the drain drilled along the length of the intermediate layer is sealed so that a section of the drain may be used as a production well.
9. A device for stimulating, by hydraulic fracturing effected in a section of an inclined drain bored through a subterranean formation, after confining this portion thus obtained by intermittently sealing the drain, production of a zone in the formation separated from an adjacent zone containing fluid at a pressure level greater than that of the subterranean formation zone by an intermediate layer that is substantially impermeable, having a conduit (C) arranged in the inclined drain, a block for sealing the conduit (C) to isolate this section from outside and means for applying hydraulic pressure for fracturing, characterised in that the inclined drain is drilled through an intermediate zone, a section (d1) of the conduit being provided with openings (O), the device having sealing blocks (B1, B2) to cut off at their opposite ends the sections of the conduit passing through the section (d1).
10. Device in accordance with claim 9, characterised in that the conduit (C) has a second section of length provided with holes in the section passing through the subterranean zone (Z1), this length section (d2) being laterally offset from the first length section (d1) in the intermediate layer (L) and a sealing block (B4) being arranged in the conduit (C) between the two length sections (d1, d2).

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