Method And Apparatus For Treating Selected Reservoir Portions

Abstract

Selected portions of a subterranean reservoir are treated by seating a packer that forms a slideable seal above a perforated section of well casing and repetitively sealing and moving a depth-straddling sealing device to isolate and treat perforations that open into individual portions of the reservoir, while maintaining the slideable seal and a borehole fluid pressure that substantially equals the formation fluid pressure during the movement of the depth-straddling sealing devices.

Description

BACKGROUND OF THE INVENTION

This invention relates to treating a well containing a casing with a perforated section adjacent to a subterranean reservoir. The invention is particularly valuable in selectively treating an individual depth interval within an inhomogeneous reservoir formation to prevent or remedy a loss of permeability, competence, consolidation, or the like, within the reservoir.

Wells are commonly completed by installing and perforating a string of casing, which is surrounded by a sheath of cement or other grouting material. Such perforations are formed by means of bullets, explosive jets or abrasive streams, etc. The subterranean reservoirs are usually inhomogeneous with respect to their fluid permeability, fluid content, degree of mechanical competence, etc. In completing a well, it is not generally feasible to position the individual perforations so that their exact depths are known or so that they open into portions of the reservoir that are equally permeable or competent or oil-bearing, or the like.

The well-completing perforations are often treated or formed by applying a borehole fluid pressure that overbalances (or underbalances) the formation fluid pressure and causes an outflow or inflow of fluid through the perforations; by acidizing the perforations and adjacent portion of reservoir; by gravel-packing or sand-packing cavities that may have been washed out within the reservoir; by performing an in situ consolidation of sand or other grains in the reservoir formation; or the like. Such treatments tend to increase the chance that each of the perforations will provide an unimpeded fluid flow path between the interior of the casing and a competent and permeable portion within the reservoir. However, those treatments have been plagued with difficulties due to the placement or distribution of the fluids by which they are effected. For example, some perforations fail to accept fluid because they are plugged by perforation debris and/or open into an impermeable portion of the reservoir interval, such as a shale stringer, or the like. Any fluid that is injected or backflowed tends to bypass the less permeable perforations and flow through only those which are the most permeable.

A primary object of the present invention is the provision of a method and apparatus that makes it feasible to locate the open perforations and treat selected ones of the open perforations (and the adjacent portions of the reservoir) by causing an injection through them of only the particular treating fluid that is designed for that particular portion of the reservoir.

SUMMARY OF THE INVENTION

This invention relates to selectively treating the perforations and adjacent portions of a reservoir interval that are encountered at selected depths within a cased and perforated section of the borehole of a well. A tubing string containing a remotely actutable means for forming a slideable mechanical seal between the tubing and casing and a pair of remotely actuatable means for forming depth-straddling mechanical seals between the tubing and casing is inserted into the borehole. Open perforations are located by forming the depth-straddling seals at various depths and adjusting the pressure to induce fluid flow between the borehole and the reservoir at the straddled depths. The perforated section of the casing is isolated from the upper portion of the casing by forming and maintaining the slideable seal above the perforated section. Selected depths within the reservoir interval are treated selectively and sequentially, by moving the tubing string to the extent required, forming a depth-straddling seal that isolates at least one open perforation at each selected depth, and injecting fluid through the isolated perforations. During such movements of the tubing string, the slideable seal is maintained and the fluid pressure within the perforated section of the casing is kept substantially equal to that within the reservoir, in order to substantially prevent the flow of fluid between the borehole and the reservoir.

The maintaining of the slideable seal and the balancing of the fluid pressures, during the movement of the tubing string, is particularly important. It ensures that the fluid injected into a treated portion of the reservoir is kept substantially static within that portion of the reservoir while the depth-straddling seals are being released so that the tubing string can be moved. Such an injected fluid may comprise, for example, an acid followed by a sand consolidating solution that subsequently deposits a self-curing resin. In such a treatment, it is important that the injected fluid remain static during the deposition and curing of the resin. However, due to the acidization, the flow passageways have been opened so that they could permit a relatively rapid inflow or outflow of fluid, between the interior of the casing and the reservoir if the fluid pressure within the casing was not substantially equal to that within the reservoir and the perforated section of the casing was not isolated from the upper portion of the casing.

In a preferred embodiment, a substantially inert fluid, such as a brine, is positioned within the perforated section of the casing. Such a fluid can be positioned by inflowing and outflowing fluid through, respectively, the casing and the tubing, while the tubing extends into the perforated section of casing. The perforated section of the casing is preferably isolated from the fluid in the upper portion of the borehole while that section contains such an inert fluid. As known to those skilled in the art, where desirable, a circulation of a selected inert fluid into the perforated section of casing can be effected while that section is isolated by the slideable seal. For example, this can be done by utilizing a tubing string that contains a separate internal conduit that extends into the so-isolated section of casing.

In operating the present invention, the length of the tubing string between a surface located source of fluid and the perforated section of casing is preferably filled with a sequence of measured volume slugs of liquid that are arranged to position each fluid that is to be injected into each selected individual portion of the reservoir in the sequence in which the reservoir portions are to be treated. In such a process, in treating the first selected portion of the reservoir, just enough fluid is pumped into the upper end of the tubing string to displace the selected fluid (usually a series of slugs) into the reservoir. Then, after the tubing string has been moved and the depth-straddling seals have been reformed, and an additional measured volume of fluid (e.g., one or more additional series of slugs and/or a displacement fluid) is pumped into the tubing string.

DISCUSSION OF THE DRAWINGS

FIG. 1 is a schematic illustration of portions of a borehole, a reservoir, and an apparatus suitable for treating a reservoir in accordance with the present invention.

FIG. 2 is a schematic illustration of a sequence of series of slugs of fluid suitable for use in the invention.

FIG. 3 is a schematic illustration of an alternative arrangement of depth-straddling mechanical sealing means.

DESCRIPTION OF THE INVENTION

The present invention can be practiced by means of a unique combination that utilizes tools and techniques which are individually known to those skilled in the art. For example, the downhole equipment can include: (1) a pair of concentric tubing strings containing a Lynes Tool Company "Feeler Joint" and inflatable packer, slip-type "PIP" (without plug catcher sub) and control for the PIP mounted above inflatable-type straddle packers with means for injecting fluid between the packers; (2) a Baker Tool Company "Lock-Set" type releaseable packer with a polished bore above a control means for engaging and releasing the packer, spacer seal units, and a perforation washing type of arrangement for facilitating the movement of the swap cups within the casing; or (3) an analagous arrangement of Haliburton Tool Company equipment utilizing an internal seal and polished "Slick Joint" in the upper packer.

In the drawing, FIG. 1 shows a subterranean reservoir interval containing oil sands 1 and 2 and intervening shale stringer 3 penetrated by borehole 4. The borehole contains a casing string 5 which is surrounded by a cement sheath 7 and is penetrated by perforations, 8a through 8c, which form openings extending from within the casing to within the reservoir. Inserted into the perforated section of the casing is a pair of concentric inner and outer tubing strings 9 and 10 connected to a remotely actuatable means for forming a slideable mechanical seal consisting essentially of inflatable packer 12. The slideable seal is mounted above a pair of remotely actuatable depth-straddling mechanical sealing means consisting essentially of inflatable packers 13a and 13b. A tubing string port 14 is located between the packers 13 to provide a passageway for injecting or producing fluid into or out of a casing perforation such as 8b while the perforation is isolated by the depth-straddling seals.

In a preferred arrangement, such depth-straddling seal means includes at least one inflatable packer having a length sufficient to cover all of the perforations that are located within the perforated section of the casing. In FIG. 1 the perforations 8a and 8c are sealed-off by the packers 13a and 13b so that the perforation 8b is the only perforation through which fluid can flow. This ensures that, for example, when fluid is injected through perforation 8b and flowed into a void or cavity such as cavity 15 (which may comprise a portion of the reservoir from which sand was washed out) such a fluid injection will not cause a flow of fluid back into the borehole through a nearby perforation such as 8c. Such a backflow of fluid could displace or entrain sand and could cause sand to become lodged between components of the injection tubing string and the casing. Such an inflow of sand would be apt to stick the tubing string tool and/or cause other disadvantageous effects relative to a reservoir treatment operation.

FIG. 2 shows a train of two series of measured volume slugs of fluid arranged for injection into selected portions of the reservoir interval in which sand consolidations are to be effected. As shown, Series I is disposed to be the first injected and is designed for injection into at least one perforation which is open and is adjacent to a permeable portion of the reservoir that needs no acidization. Series II is disposed to be next injected, in a different portion of the reservoir, and contains an acidizing solution for opening perforations of somewhat impaired permeability and/or enhancing the permeability of the portions of the reservoir adjacent to those perforations.

FIG. 3 shows portions of a well treating apparatus differing from that of FIG. 1 in using a single injection tubing string and a remotely actuable means for forming a slideable seal that utilizes a mechanically deformable packer 17, and a depth-straddling means that utilizes opposing swab cups 18a and 18b. In such an apparatus, the swab cups are preferably used in conjunction with a remotely actuatable bypass means (not shown) for facilitating the movement of the cups within a liquid-filled casing.

Particularly where the tubing string arrangement is one that contains a separate internal conduit through which a fluid can be circulated into a postion within the perforated section of the casing after that section has been isolated from fluid in the upper portion of the casing (by means of the slideable mechanical seal between the tubing and casing), the present invention can be used in or in conjunction with a wide varity of well treatment procedures. For example, procedures in which it can advantageously be used include: an in situ sand consolidation process, such as the epoxy resin sand consolidating processes of the Havenaar and Mays U.S. Pat. No. 3,294,166 or the Richardson U.S. Pat. No. 3,339,633, or the like, with or without an acidizing pretreatment of some or all of the portions of the reservoir which are consolidated; an emplacement of a sand or gravel pack to within the perforated section of casing and/or in the reservoir to fill voids such as the void 15 shown in FIG. 1 prior to or during or after a sand consolidation operation; an injectivity or productivity adjusting treatment for selectively injecting permeability increasing or decreasing fluid (e.g. an acidizing fluid, an oil solvent or plugging fluid or the like) into selected portions of the reservoir in which the permeabilities are too low or too high with respect to the injectivity or productivity of oil or water; etc.

The maintaining of a pressure on the fluid within the perforated section of casing that substantially equals the pressure on the fluid within the reservoir can be effected or facilitated in numerous ways. For example one or more inert fluids and/or treatment fluids that are adopted to provide a so-selected hydrostatic pressure can be spotted in at least a portion of that section; where the tubing string contains an interval conduit for flowing fluid past the slideable seal, the surface pressure and/or height of the column of fluid within the tubing string can be adjusted to adjust the pressure within that section; etc.

Claims

What is claimed is:

1. A process for selectively treating selected individual perforations and adjoining portions of a reservoir interval adjacent to a cased and perforated section of a well borehole comprising:

inserting into the well casing a tubing string containing a remotely actuatable sealing means for forming a slideable mechanical seal between the tubing and the casing and a pair of remotely actuatable sealing means for forming depth-straddling mechanical seals between the tubing and the casing;

locating open perforations by forming said depth-straddling seals at various depths and adjusting the pressure to induce fluid flow between the borehole and the reservoir when a perforation is open at a straddled depth;

isolating the perforated section of casing from fluid in the upper portion of the casing by forming the slideable mechanical seal above the perforated section;

treating a selected portion of the reservoir by positioning said tubing string to the extent required, forming said depth-straddling seals in a position that isolates at least one open perforation that opens into the selected portion of the reservoir, and injecting fluid through the straddled perforation; and

treating at least one other selected portion of the reservoir by moving the tubing string, reforming said depth-straddling seals and injecting fluid through the straddled perforation, while maintaining said slideable mechanical seal and maintaining a fluid pressure within the perforated section of casing that substantially equals the reservoir pressure.

2. The process of claim 1 including the steps of:

filling said tubing string with a series of measured volume slugs of fluid arranged to position the fluid to be injected into each of a series of selected individual portions of the reservoir in the sequence in which said portions are to be treated;

positioning said tubing string so that said depth-straddling seals isolate at least one open perforation opening into the first of said series of portions of the reservoir;

displacing sufficient fluid into the upper end of said tubing string to displace into the reservoir the fluid to be injected into that portion of the reservoir; and

subsequently moving said tubing string to position said depth-straddling seals for isolating at least one other open perforation opening into at least one other portion of the reservoir.

3. The process of claim 2 in which said sequence of slugs of fluid include different kinds of fluids for injection into different portions of the reservoir interval.

4. The process of claim 1 in which at least one of said depth-straddling mechanical seals are formed along a vertical interval having a length substantially equalling the length of the perforated section of the casing.

5. The process of claim 1 including the steps of:

providing a separate internal conduit within said tubing string; and

after said slideable mechanical seal has been formed, circulating a selected fluid into the perforated section by means of a fluid inflow and outflow through, respectively, said tubing string and internal conduit.

6. An apparatus for selectively treating selected individual portions of a reservoir adjoining a cased and perforated section of a well borehole, which comprises:

tubing string means inclusive of remotely actuatable sealing means for forming a slideable mechanical seal between the tubing and the casing and a pair of remotely actuatable sealing means for forming depth-straddling mechanical seals between the tubing and the casing;

means for moving said tubing string within the borehole of the well and actuating sealing by said depth-straddling sealing means at selected depths within the well;

means for adjusting the fluid pressure within the borehole at depths straddled by said depth-straddling seals;

means for detecting a flow of fluid resulting from said fluid pressure adjustments; and

means for displacing fluid through said tubing string and into the borehole at a depth straddled by said depth-straddling seals.

7. The apparatus of claim 6 in which said sealing means and means for moving the tubing string and actuating the seals are arranged for moving the string and repetitively actuating the depth-straddling seals while maintaining said slideable mechanical seal at a location near but above the perforated section of the casing.

8. The apparatus of claim 6 in which at least one of said depth-straddling sealing means is adapted to form a seal extending along a vertical interval substantially equalling the length of the perforated section of casing.