The present invention relates to a method of sealing an annular space formed between an expandable tubular element arranged in a wellbore and a wall surrounding the expandable tubular element, whereby a pressure difference occurs between a first location in the annular space and a second location in the annular space axially spaced from the first location.
Wellbores for the production of hydrocarbon fluid are conventionally provided with one or more casings to provide stability to the wellbore wall, and to provide zonal isolation between different earth formation layers. Generally several casings are set at different depth, in a nested arrangement whereby the diameter of each (subsequent) casing is smaller than the diameter of the previous casing in order to allow lowering of the casing through the previous casing. The annular space between each casing and the wellbore wall is filled with cement to provide annular sealing and to support the casing in the wellbore. In most applications such cement layer provides adequate sealing functionality as long as the annular space is not too narrow.
Recently it has become practice to radially expand casings in the wellbore. In an attractive method of installing expandable casings, each subsequent casing is lowered through the previous casing and then radially expanded to substantially the same diameter as the previous casing. In this manner a wellbore of substantially uniform diameter is obtained. Such procedure is particularly advantageous for relatively deep wellbores or for extended reach wellbores. Furthermore, it has been proposed to expand casings against the wellbore wall such that a seal is created between the casing and the wellbore wall without a cement layer inbetween. Although such expansion against the earth formation is considered feasible, there may still be concerns regarding the effectiveness of the seal after the casing has been expanded against the formation. Experience has indicated that cement may not be a good solution for sealing a very narrow annulus in view of the possibility that the cement does not adequately flow into the annular space, and in view of possible shrinkage of the (narrow) annular cement layer upon hardening.
It is therefore an object of the invention to provide an improved method of sealing an annular space formed between an expandable tubular element arranged in a wellbore and a wall surrounding the expandable tubular element, which overcomes the drawbacks of the prior art.
In accordance with the invention there is provided a method of sealing an annular space formed between an expandable tubular element arranged in a wellbore and a wall surrounding the expandable tubular element, whereby a pressure difference occurs between a first location in the annular space and a second location in the annular space axially spaced from the first location, the method comprising:
It is thereby achieved that the fluid can be inserted in the annular space at a relatively low pumping pressure prior to expansion of the tubular element, since the annular space is relatively wide before the expansion process. Once the fluid is in the annular space and the tubular element has been expanded, the pressure required to induce longitudinal movement of the body of fluid through the annular space, and thus the sealing capacity of the annular body of fluid, increases. Such increase is almost exponential if the annular space becomes very narrow such as in case the tubular element is expanded (almost) against the wellbore wall. It will therefore be understood that the method of the invention is particularly advantageous for applications whereby the tubular element is radially expanded to near the wellbore wall, or even locally against the wellbore wall.
Preferably said fluid is a non-hardening fluid, so that any risk of shrinkage of the annular body due to hardening is avoided.
A suitable fluid for use in the method of the invention is a thixotropic fluid. Preferably the fluid is selected from a gel, a Bingham Plastic and a Herschel Bulkley fluid.
Examples of suitable gels for use in the method of the invention are:
In order to enhance the sealing and/or plugging properties of the body of gel in the annular space, suitably the body of gel comprises a plurality of solid particles of large particle size distribution.
Suitable solid particles to be included in the body of fluid, are:
The invention will be described hereinafter by way of example in more detail, with reference to the accompanying drawings in which:
In the drawings, like reference numerals relate to like components.
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During normal operation the casing 6 is lowered into the wellbore and suspended in the wellbore 1 from surface at the required depth. The annular space 7 is filled with brine (not shown). Subsequently the stream of gel 10 is pumped via the casing 6 into the wellbore 1 by means of the pump plug 12 which trails the stream of gel 10 in the casing (
Upon arrival of the pump plug 12 at the lower end of the casing 6, pumping is stopped and the pump plug 12 is removed from the casing 6 using a suitable retrieve string (not shown). At this stage the gel 10 fills the open-hole portion 13 of the wellbore 1 and extends into the annular space 7 thereby forming the annular body of gel 11.
In a next step the expansion cone 14 and the packer 20 are brought to their respective collapsed states, and the packer 20 is removably attached to the lower end of the cone 14. The combined cone 14 and packer 20 are then lowered through the casing 6 by means of pipe string 16 until the cone 14 extends below the lower end of the casing 6, i.e. in the open-hole portion 13 of the wellbore 1. The cone 14 is then brought to its expanded state and pulled into the casing 6 using a force multiplier (not shown) thereby radially expanding a lower end portion of the casing 6. When the cone 14 and the packer 20 are fully located in the casing 6, the packer 20 is radially expanded so as to be anchored to the inner surface of the casing 6. After the packer 20 has been set, the cone 14 is detached from the packer 20 and brine is pumped via the pipe string 16 and the through-passage 18, into the interior of the casing 6 between the cone 14 and the packer 20. The cone 14 thereby moves upwardly through the casing 6 and gradually expands the casing 6 (
After the casing 6 has been fully expanded, or after expansion of a desired portion of the casing 6, the cone 14 and the packer 20 are removed from the casing. The open-hole portion 13 of the wellbore 1 is then cleaned, and the production tubing 22 and the production packer 24 are installed in conventional manner.
When the well is taken in production, hydrocarbon fluid flows from the reservoir zone 3 into the open-hole section 13 of the wellbore, and from there into the production tubing 22 to surface. The annular body of gel 11 seals the annular space 7 and thereby prevents that hydrocarbon fluid flows along the outside of the casing 6 in upward direction. In order that the body of gel 11 in the annular space 7 withstands the (high) fluid pressure of the hydrocarbon fluid entering the wellbore 1, the yield strength of the gel is selected such that the axial pressure difference across the body of gel 11 is lower than a minimum axial pressure difference across the body of gel 11 required to induce movement of the body of gel 11.
An example calculation of the minimum axial pressure difference across the annular body of gel required to induce movement of the body of gel for a given gel yield strength, is presented below.
A wellbore is drilled to a depth of 2000 m, with a diameter of 0.302 m (11.9 inch) in a lower section of the wellbore. The fluid pressure in the earth formation at the depth of 2000 m is 200 bar. An expandable casing is installed in the wellbore such that the lower end of the casing is positioned a short distance above the wellbore bottom. The outer diameter of the casing in unexpanded state is 0.244 m (9.625 inch). A stream of gel having a yield strength of 1000 Pa (0.01 bar), is pumped into the wellbore in the manner described above such that an annular body of gel of 2.28 m3 is contained in the annular space between the unexpanded casing and the wellbore wall. The length of the annular body of gel, before radial expansion of the casing, is 92.08 m. The maximum pressure at the lower end of the casing required to pump the gel in the annular space, is 63.74 bar which is well below the fracture pressure of the surrounding rock formation. The casing is then radially expanded to an outer diameter of 0.286 m (11.261 inch). The annular space thereby becomes narrower so that the length of the body of gel in the annular space increases to about 304.8 m (1000 ft). The effect of expansion of the casing on the minimum axial pressure required to induce longitudinal movement of the body of gel in the annular space, is twofold. Firstly the resistance of the body of gel to axial movement increases due to a longer contact surface with both the wellbore wall and the casing wall, and secondly the cross-sectional area of the annular body of gel decreases. In the present example it is found that the minimum axial pressure difference across the body of gel required to induce longitudinal movement of the body of gel through the annular space, increases from 211 bar before expansion of the casing, to 751 bar after expansion of the casing. In the present example, the axial formation fluid pressure difference across the body of gel is taken to be solely due to the hydrostatic column of formation fluid along the length of the body of gel, which is about 30 bar. Thus the actual axial fluid pressure difference across the body of gel is far below the minimum axial fluid pressure difference required to induce longitudinal movement of the body of gel. Therefore in the present example a gel with a lower yield strength could safely be applied if desired or, alternatively, the length of the body of gel in the annular space could be reduced.
Reference is further made to
Instead of pumping a gel into the wellbore, a fluid can be pumped which transforms into a gel some time after being pumped into the wellbore. Thus, such fluid obtains the desired yield strength and, optionally, the desired thixotropic properties after being inserted in the wellbore.
Number | Date | Country | Kind |
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04257820.3 | Dec 2004 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP05/56716 | 12/13/2005 | WO | 00 | 10/4/2007 |