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Title:
TEST CELL FOR ASSESSING A WELL FLUID
Document Type and Number:
WIPO Patent Application WO/2012/001090
Kind Code:
A1
Abstract:
The present invention provides a test cell for assessing a well fluid comprising: a body, an inlet means and an outlet means; a formation sample shaped to fit inside said body; and a screen sample shaped to fit inside said body.

Inventors:
ALTERÅS, Eva (Strandveien 10, Hvitsten, N-1545, NO)
IBRAGIMOVA, Zalpato (Foldnesvegen 177, Straume, N-5353, NO)
MATHISEN, Anne Mette (Austlia 11, Nesttun, N-5223, NO)
SVANES, Grete Skalmstad (Midtunhaugen 147, Nesttun, N-5224, NO)
Application Number:
EP2011/060988
Publication Date:
January 05, 2012
Filing Date:
June 30, 2011
Export Citation:
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Assignee:
STATOIL ASA (N-4035 Stavanger, NO)
ALTERÅS, Eva (Strandveien 10, Hvitsten, N-1545, NO)
IBRAGIMOVA, Zalpato (Foldnesvegen 177, Straume, N-5353, NO)
MATHISEN, Anne Mette (Austlia 11, Nesttun, N-5223, NO)
SVANES, Grete Skalmstad (Midtunhaugen 147, Nesttun, N-5224, NO)
International Classes:
G01N33/28; E21B49/00; E21B49/08
Attorney, Agent or Firm:
GORDON, Kirsteen (4220 Nash Court Oxford Business Park South, Oxford Oxfordshire OX4 2RU, GB)
Download PDF:
Claims:
CLAIMS:

1. A test cell for assessing a well fluid comprising:

a body, an inlet means and an outlet means;

a formation sample shaped to fit inside said body; and

a screen sample shaped to fit inside said body.

2. A cell as claimed in claim 1 , wherein said formation sample is solid. 3. A cell as claimed in claim 1 or claim 2, wherein said body comprises a means to support a sample of formation.

4. A cell as claimed in any one of claims 1 to 3, wherein said body comprises a means to support a sample of screen.

5. A cell as claimed in any one of claims 1 to 4, wherein said body comprises an end cover.

6. A ceil as claimed in any one of claims 1 to 5, wherein said body comprises an end cap.

7. A cell as claimed in any one of claims 1 to 6, wherein said body comprises an end cover and an end cap and said end cover comprises said inlet means and said end cap comprises said outlet means.

8. A cell as claimed in any one of claims 1 to 7, wherein said sample of screen is positioned on the side of the formation sample that is opposite the inlet means.

9. A cell as claimed in any one of claims 1 to 8, further comprising a sample of filter cake.

10. A cell as claimed in claim 9, wherein said sample of filter cake is positioned in between said sample of formation and said sample of screen.

11. A cell as claimed in any one of claims 1 to 10, further comprising a sample of gravel pack.

12. A cell as claimed in claim 11 , wherein said sample of grave! pack is positioned in between said sample of formation and said sample of screen.

13. A cell as claimed in any one of claims 1 to 12, comprising heating means (e.g. a heating coil, heating jacket or heating tape). 14. A cell as claimed in any one of claims 1 to 13, comprising:

a holder comprising a means to hold a sample of formation and a means to hold a sample of screen.

15. A holder for a test cell as defined in any one of claims 1 to 14, comprising: a means to hold a sample of formation;

a means to hold a sample of screen; and

fixing means to fix the holder in the test cell.

16. An apparatus for assessing a well fluid comprising:

- a test cell as claimed in any one of claims 1 to 14; and

- a pump arranged to pump fluid through said cell via said inlet and outlet means.

17. An apparatus as claimed in claim 16, further comprising a pressure gauge to measure pressure.

18. An apparatus as claimed in claim 16 or claim 17, further comprising a computer arranged to receive pressure measurements. 19. An apparatus as claimed in any one of claims 16 to 18, further comprising a stand to support said cell.

20. An apparatus as claimed in any one of claims 16 to 19, wherein said stand rotatably supports said cell so that the cell may occupy a vertical or horizontal position.

21. A method of assessing a well fluid comprising:

- placing a sample of said well fluid in a test cell as defined in any one of claims

1 to 14;

- pumping a fluid through said cell via said inlet and said outlet means; and

- measuring the pressure during said pumping.

22. A method as claimed in claim 21 , wherein the method is conducted using an apparatus as defined in any one of claims 16 to 20.

23. A method as claimed in claim 21 or claim 22, further comprising the step of determining whether said well fluid is suitable for use in a formation.

24. A method as claimed in any one of claims 21 to 23, wherein said well fluid is a drilling fluid, a completion fluid or a breaker.

25. A kit for assessing a well fluid comprising:

- a test ceil as defined in any one of claims 1 to 14; and

- instructions for using said ceil e.g. in a method as defined in claims 21 to 24.

26. A kit for assessing a well fluid comprising:

- an apparatus as defined in any one of claims 16 to 20; and

- instructions for using said apparatus, e.g. in a method as defined in claims 21 to 24.

Description:
TEST CELL FOR ASSESSING A WELL FLUID

FIELD OF THE INVENTION The present invention relates to test cells for assessing well fluids, to apparatus comprising the test cells and to kits comprising the test cells. The invention also relates to methods of assessing well fluids using the test cells, apparatus and kits.

BACKGROUND TO THE INVENTION

During drilling of a wellbore, a drilling fluid is typically used to facilitate the operation. A primary reason for circulating drilling fluid is to remove cuttings (i.e. particles of crushed or cut formation or rock produced by drilling) from the wellbore as it is drilled. Drilling fluids therefore typically comprise suspended solids that are able to support the removal of cuttings. In a typical drilling operation, some of the suspended solids are deposited on the wellbore walls where they form a solid mass. This is generally referred to as filter cake. The filter cake itself may be useful, e.g. in applying a counterbalance hydrostatic pressure in the wellbore thereby preventing blow out and in providing fluid loss control, i.e. preventing loss of fluid into the formation.

Once the drilling operation is finished the wellbore must be completed, i.e. made ready to be put onto production. During the completion process, the drilling fluid remaining in the wellbore is sometimes removed therefrom. It is common to do this by displacing the drilling fluid in the borehole with a completion fluid prior to running a production screen into the well. The quality of the completion fluid and the drilling fluid should therefore be such that they do not plug the screens or subsequently reduce the productivity of the well. In some cases the completion process may also involve a step wherein some, or all, of the filter cake is removed. This is generally achieved using compositions referred to as breakers.

Before using a well fluid such as a drilling fluid, completion fluid or breaker in the field, it is conventional to test it in a laboratory to try to find correlations between properties of the fluid (e.g. solids content, mud weight, etc) and the likely impact of the fluid if it were to be used in the field. For instance, the ability of drilling and completion fluids to flow through production well screens is often tested before using the fluids downhole. Ideally the testing should serve as a reliable guideline to the subsequent performance of the well fluid in the field.

Various test cells and apparatus have therefore been developed for testing well fluids such as drilling and completion fluids. SPE94558, for instance, discloses a production screen test (PST) for investigating the flow-back properties of drilling and completion fluids through production screens. Figure 1 shows a schematic of the test set up described in SPE94558. It comprises a test cell (A) fitted with a sample of screen (B) at the bottom of the cell and a reservoir (C), e.g. of oil or brine. The test cell (A) and reservior (C) are fluidly connected by a flow line (D) connected to a pressure gauge (E).

A sample of, e.g. drilling fluid is tested in the PST by a process wherein a sample of the fluid is aged in the test cell for a given time period (e.g. 1 month). Depending on the fluid being tested, this may cause solids to settle out on the screen potentially causing blockages. This is tested by producing the fluid through the production screen using, e.g. mineral oil, at a constant rate. The direction of flow in the test is indicated by the arrows in Figure 1. During the production the pressure over the screen is monitored in order to determine the tendency of the fluid to plug the screen. A rise in pressure as production proceeds indicates plugging is occurring. After production of the fluid is completed, the test cell can also be opened and the appearance of the fluid cake on the screen evaluated. Together with the pressure build up results this gives a measure of the flow characteristics of the fluid through a production well screen after ageing. The test set up using the PST therefore provides a method of evaluating the potential usefulness of drilling and completion fluids and is included in their qualification. The limitation of this test configuaration, however, is that differences in the nature of wells are not taken into account. This is a drawback since it is known that the nature of the formation has a significant impact on the composition of a well fluid per se and on its performance in the wellbore. Significantly the potential affect of the well fluids on subsequent well productivity (e.g. on oil productivity) is not measured in this testing configuration.

Hence there is still a need for improved methods for qualifying well fluids such as drilling and completion fluids that provide a measure of the likely effectiveness of the fluid in the well but which additionally provide an assessment of the impact on the formation on the well fluid and vice versa. In particular methods that assess the extent to which well fluids cause productivity impairment, e.g. by screen blockage and/or incomplete clean up, are required.

Test cells, apparatus and kits have now been devised that enables well fluids to be tested in conditions that more closely replicate a cross section of a wellbore, particularly a horizontal wellbore. Methods of testing using the test cell, apparatus and kits herein described therefore give a measurement of the effectiveness of the fluid as a well fluid in addition to an assessment of the impact of the well fluid on well productivity and completion damage. Advantageously the test cells, apparatus, kits and methods of the invention also allow for different well conditions to be replicated.

SUMMARY OF INVENTION

Thus viewed from one aspect the present invention provides a test cell for assessing a well fluid comprising:

a body, an inlet means and an outlet means;

a formation sample shaped to fit inside said body; and

a screen sample shaped to fit inside said body.

In a preferred embodiment, the formation sample is solid.

In a particularly preferred embodiment, the body comprises a means to support a sample of formation. In a further preferred embodiment, the body comprises a means to support a sample of screen.

Viewed from a further aspect the present invention provides a holder for a test cell as hereinbefore described, comprising:

a means to hold a sample of formation;

a means to hold a sample of screen; and

fixing means to fix the holder in the test cell.

Viewed from a still further aspect the present invention provides an apparatus for assessing a well fluid comprising: - a test cell as hereinbefore described; and

- a pump arranged to pump fluid through said cell via said inlet and outlet means. Viewed from a yet further aspect the present invention provides a method of assessing a well fluid comprising:

- placing a sample of said well fluid in a test cell as hereinbefore defined

- pumping a fluid through said cell via said inlet and said outlet means; and

- measuring the pressure during said pumping.

In a particularly preferred embodiment the method is conducted using an apparatus as hereinbefore defined.

Viewed from a yet further aspect the present invention provides a kit for assessing a well fluid comprising:

- a test ceil or apparatus as hereinbefore defined; and

- instructions for using said ceil e.g. in a method as hereinbefore defined.

DETAILED DESCRIPTION

As used herein the term "test cell" refers to a device that is used within an apparatus to assess well fluids. The test cells described herein preferably replicate the conditions encountered downhoie by well fluids. The test cells of the invention therefore comprise a sample of formation and a sample of screen. Preferred test cells of the invention additionally comprise a sample of filter cake and/or a sample of gravel pack. Particularly preferred test cells of the invention comprise a sample of formation, a sample of filter cake, a sample of gravel pack and a sample of screen.

The test cells of the present invention comprise a body, an inlet means and an outlet means. By an inlet means is meant a means by which fluids can be passed from the outside of the test cell into the test cell. By an outlet means is meant a means by which fluids can be passed from the inside of the test cell to the outside of the cell. In preferred test cells of the present invention the inlet means and outlet means are positioned opposite to each other. Still more preferably the inlet means and the outlet means are positioned at opposite ends of the body of the test cell. The body of the test cells of the present invention may be any shape, e.g. rectangular or cylindrical. Preferably, however, the body of the test cell is cylindrical in shape. This shape most closely replicates the shape of the welibore in the field. Preferably the inlet and outlet means are positioned at the ends of the cylinder, i.e. on the circular cross sections thereof.

The body of the test cell may be any size that is convenient for testing. Typically, however, the body of the test cell will be about 10-100 cm in length, more preferably about 15-50 cm in length, still more preferably about 20-35 cm in length, e.g. about 22 cm in length. As used herein, the term length is used to refer to the longest dimension of the body. The cross section of the body at its ends (e.g. the diameter of a test cell that is cylindrical in shape) is preferably about 2-50 cm in width, more preferably about 5-30 cm in width, still more preferably about 10-25 cm in width, e.g. about 10 cm in width. In the case of non cylindrical test cells, the length of the cross section of the body at its end is preferably about 2-50 cm, more preferably about 5-30 cm, still more preferably about 10-25 cm, e.g. about 10 cm.

Preferred test cells of the present invention comprise a body comprising an end cover. The end cover may be integral to said body or may be formed separately therefrom. When the end cover is integral to said body, it may be hinged to the body. Alternatively the body and the end cover may be continuously formed as a single part. Preferably, however, the end cover is a separate part connectable to said body. Connection may be achieved by any conventional means, e.g. screw fitting, bayonet fitting or push fit fitting. When the end cover is a separate part, it facilitates access to the interior of the test cell and thus enables samples to be placed therein easily. It also allows for visual inspection of samples after testing to be carried out.

In preferred test cells of the present invention the body comprises an end cap. Like the end cover, the end cap may be integral to said body or may be formed separately therefrom. When the end cap is integral to said body, it may be hinged to the body.

Alternatively the body and the end cap may be continuously formed as a single part.

Preferably, however, the end cap is a separate part connectable to said body.

Connection may be achieved by any conventional means, e.g. screw fitting, bayonet fitting or push fit fitting. Particularly preferred test cells therefore comprise a body, an end cover, an end cap, inlet means and outlet means. In particularly preferred test cells of the invention the end cover comprises said inlet means. In further preferred test cells the end cap comprises said outlet means. Thus the end cover and end cap preferably provide the means for introducing fluids into and out of the test ceil respectively. Suitable inlet and outlet means can be integrally formed in said end cover and said end cap using conventional means, e.g. moulding or welding.

The test ceil may be made from any material(s) that can withstand temperatures of up to 150 °C and pressures of up 50 bar. Preferably the test cell comprises stainless steel. Preferably the different parts or components of the test cell are made of the same material. Preferably the body, inlet means and outlet means comprise stainless steel. When present, the end cover and/or end cap preferably also comprise stainless steel.

As mentioned above, the test cells of the present invention are designed to facilitate the testing of well fluids under conditions that mimic those encountered downhole. Thus the test ceils of the present invention comprise a sample of formation. Preferably a sample of formation from the well on which the well fluid is to be used is present. Alternatively a sample of formation from a similar well may be used. Alternatively a synthetic sample of formation may be used, e.g. a synthetic sample that replicates the particle size distribution of the formation in which the well fluid is to be used. The samples of formation may be taken from the well by conventional means. Typically a sample of formation will be taken during drilling. Generally the sample of formation will have to be cut or shaped to fit inside the body of the test cell. Thus when the body is cylindrical in shape, the sample of formation is preferably circular. In some embodiments, the body of the test cell is provided with inserts which protrude from the interior walls of the body (i.e. space fill) to enable close fitting to samples of formation that are smaller than the body. Optionally the inserts comprise support means for holding the sample of formation. The width of the samples of the formation (e.g. diameter for circular samples) is preferably the same as that of the body or marginally less to allow a close fit therein. Preferably there is no space between the walls of the body or its inserts and the sample of formation through which fluids can flow. The width (e.g. diameter) of the sample of the formation may be 1.5-49.5 cm, more preferably about 2.0-20.0 cm, still more preferably about 3.0-10.0 cm, e.g. about 5.0 cm in width. In the case of non- circular samples, the length of the sample of the formation may be 1.5-49.5 cm, more preferably about 2.0-20.0 cm, still more preferably about 3.0-10.0 cm, e.g. about 5.0 cm. Preferably the samples of formation have a thickness of 0.5-5 cm, more preferably 1-3 cm, e.g. about 2 cm. Conventional cutting equipment may be used to prepare samples of formation.

The sample of formation is preferably solid, e.g. a solid circular disc if the body of the test cell is cylindrical or a solid square or rectangular disc if the body of the test cell is oblong. The sample is preferably non-hollow. Thus fluids passing through the test cell must pass through the formation by virtue of its permeability.

Prior to using a sample of formation in a test, it is preferably saturated in a basefluid, e.g. brine, base oil or a mixture thereof. This is generally carried out once the sample of formation is put in position in the test cell.

The sample of formation may sit at the bottom of the body of the test cell. More preferably, however, the body comprises a means to support the sample of formation. Optionally the means to support and/or hold the sample of formation is provided in the body inserts.

Preferably the means supports the sample of formation above the bottom of the body of the test cell (i.e. so that the sample does not sit on the bottom of the cell). Preferably the support means holds the sample firmly in place. To facilitate holding or fixing the sample of the formation in the body, the sample may be placed in a formation fixing means prior to its placement in the body. In this case the formation fixing means are held by the support means. The formation fixing means may be, for example, a frame for holding the sample of formation around its circumference. Preferably the frame is provided with lugs or protrusions that interact with the support means to hold or fix the sample of formation in the body. The skilled man will readily identify other examples of suitable fixing means arrangements. Preferably the sample of formation is removeabie from the test cell.

Preferably said sample of formation is positioned in proximity to said inlet means. Preferably therefore the sample of formation is located in the half of the body comprising the inlet means. Still more preferably, the sample of formation is located about the middle of the body, e.g. in the middle third of the body according to the body length. Preferably said sample of formation is not in direct contact with the in flow of fluid through the inlet means. This means that when fluids are pumped into the test cell via the inlet means they first fill up a space below the sample of formation before passing through it. This helps to ensure that fluid passes through the sample of formation at the same rate across its entire cross section.

Preferably the means to support the sample of formation and/or the formation in formation fixing means comprises at least three ledges formed on the inside of said body. Still more preferably the means to support the sample of formation and/or the formation in formation fixing means is a continuous ledge formed on the inside of said body. The means to support the sample of formation and/or the formation in formation fixing means is preferably intergraily formed with said body. The sample of formation, optionally in its fixing means, is preferably held or fixed by the ledge. The sample may, for instance, sit on top of the ledge. More preferably the formation fixing means forms a compression fit with the ledge, thereby holding the sample of formation firmly in position. Alternatively the means to support the sample of formation and/or the formation in formation fixing means may be a slot or detents. Such supports are preferred with formation fixing means that comprise protrusions.

The test ceils of the invention further comprise a screen sample shaped to fit inside said body. As used herein, the term screen refers to a production screen through which oil and water is produced in the field. As described above, the body of the test cell may be provided with inserts that protrude from the interior walls of the body (i.e. space fill) to enable close fitting to samples of screen that are smaller than the body. Optionally the inserts comprise support means for holding the sample of screen. The width of the samples of the screen (e.g. diameter for circular samples) is preferably the same as that of the body or marginally less to allow a close fit therein. Preferably there is no space between the walls of the body or its inserts and the sample of screen through which fluids can flow. The width (e.g. diameter) of the sample of the screen may be 1.5-49.5 cm, more preferably about 2.0-20.0 cm, still more preferably about 3.0-10.0 cm, e.g. about 5.0 cm in width. In the case of non-circular samples, the length of the sample of the screen may be 1.5-49.5 cm, more preferably about 2.0-20.0 cm, still more preferably about 3.0-10.0 cm, e.g. about 5.0 cm. Preferably the samples of screen have a thickness that is equivalent to that used in wells. Conventional cutting equipment may be used to prepare samples of screen.

Preferably the body of the test cell is provided with means to support and/or hold the screen sample. Optionally the means to support and/or hold the screen sample are provided in the body inserts. To facilitate holding or fixing the sample of the screen in the body, the sample may be placed in a screen fixing means prior to its placement in the body. In this case the screen fixing means are held by the support means. The screen fixing means may be, for example, a frame for holding the sample of screen around its circumference. Preferably the frame is provided with lugs or protrusions that interact with the support means to hold or fix the sample of screen in the body. The skilled man will readily identify other examples of suitable fixing means arrangements.

Preferably the means to support the sample of screen and/or its fixing means comprises at least two, e.g. two or three, ledges formed on the inside of said body. Still more preferably the means to support the sample of screen and/or its fixing means is a continuous ledge formed on the inside of said body. Alternatively the means to support the sample of screen and/or its fixing means may comprise a slot. The means to support the sample of screen is preferably intergrally formed with said body. Optionally the means to support the sample of screen is formed in the inserts hereinbefore described.

Preferably said sample of screen is positioned in proximity to said outlet means. Preferably therefore the sample of screen is located in the half of the body comprising the outlet means. Thus any fluid produced through the ceil must pass through the screen. This replicates what happens in a wellbore. In a wellbore a screen is put in place around the wellbore and serves to prevent the passage of sand into the wellbore. A huge variety of different types of screens are available. Any commercially available screen may be purchased and a sample thereof shaped to fit inside the body of the test ceil. These may, for example, be made of different materials, have different sizes and comprise holes of different shape, size and density. Preferably the test cells of the invention comprise a sample of screen that will actually be used downhole. The samples can be cut or shaped to fit inside the body of the test cell as required using conventional means. Preferably the screen sample is removeable from the test cell.

In the test cells of the present invention, the sample of screen is preferably positioned on the side of the formation sample that is opposite the inlet means. This means that fluid introduced to the cell via the inlet means passes through the sample of formation then through the screen. Well fluid, e.g. completion fluid or drilling fluid, placed inside the cell prior to production passes through the screen sample. This mimics the flow of fluids through a production wellbore. Thus, when oil is being produced from a wellbore, it flows from the formation through the screen and into the wellbore from where it is produced. Similarly when a completion fluid is being flowed back through the test cell it passes through the production screens. Hence any plugging of the screen that causes a reduction in productivity will be detected. In a preferred embodiment of the present invention, the sample of formation and sample of screen, and optionally filter cake and gravel pack samples that are described below, are mounted in a holder for placement in a test cell. The holder preferably comprises means to hold the sample of formation and a means to hold the sample of screen. Thus a preferred test cell of the present invention comprises:

a body, an inlet means and an outlet means;

a formation sample shaped to fit inside said body;

a screen sample shaped to fit inside said body; and

a holder comprising a means to hold a sample of formation and a means to hold a sample of screen.

Preferably the holder holds the sample of formation at a first end thereof and the sample of screen at a second, preferably opposite, end thereof. Preferably the holder also comprises holder fixing means, e.g. lugs, protrusions, etc that interact with the support means of the body to fix or hold the holder in the body of the test cell. In order to more closely replicate a wellbore the test cells of the present invention preferably comprise a sample of filter cake. As used herein, the term filter cake refers to a solid mass comprising components derived from drilling fluid. Preferably the sample of filter cake present in the test cells is placed on the sample of formation. This replicates its position downhole. A filter cake is usually generated during the drilling of a wellbore by the deposition of solids from drilling fluid against the wellbore walls. Ideally the filter cake prevents the flow of cuttings, sand or fines into the formation where they may reduce formation permeability. At the same time, it is obviously essential that the filter cake is permeable to oil and water so that they can be produced from the well once the well is completed. Thus in order to recreate the downhole conditions in the test cell, in preferred test cells a sample of filter cake is positioned in between said sample of formation and said sample of screen. Preferably the filter cake is positioned on top of the sample of formation. In order to prepare a sample of filter cake, a high temperature, high pressure (HTHP) fluid loss cell is typically used. A HTHP cell comprises a production screen as well as means to control temperature and apply pressure therein. In a first step of filter cake production, a drilling fluid is hot rolled in an ageing cell, e.g. at reservoir temperature for 12-24 hours. During this time, the ageing cell is circulated in an oven, typically at a temperature of 60-140 °C. The drilling fluid is then produced through a screen in a HTHP cell at a temperature in the range 60-140 °C and a pressure of 500 psi (ca. 35 bar). Usually oil is used to drive the production. The production step normally takes 4 to 24 hours. The HTHP cell is then disassembled and the filter cake removed from the screen. Any conventional drilling fluid may be utilised to generate the filter cake by this method. Preferably the filter cake is made from the drilling fluid being tested.

In preferred test cells of the present invention there will exist a space or volume in between the sample of formation/if present sample of filter cake and the sample of screen. During testing this space or volume may be filled with well fluid, preferably a drilling fluid or completion fluid. The presence of drilling fluid simulates, for example, a poor displacement of drilling fluid, i.e. a situation wherein during completion the drilling fluid has not completely been displaced from the wellbore. The presence of completion fluid simulates a situation wherein completion fluid has been used to displace drilling fluid prior to running in of production screens. Still further preferred test cells of the present invention comprise a sample of gravel pack. As used herein the term gravel pack refers to a mass of solid particles, e.g. having an average particle size of e.g. 16/20 pm or 20/40μιη. Again this is to enable downhoie conditions to be more closely replicated. In a wellbore the grave! pack is typically packed around the screen to help prevent sand passing into the wel!bore. Thus in the test cells of the present invention the sample of grave! pack is preferably positioned in between said sample of formation and said sample of screen. When a sample of filter cake is present in the test cell, the sample of gravel pack is preferably positioned between the filter cake and the sample of screen. The gravel pack is placed on top of the sample of formation, or if present, filter cake. Any commercially available gravel pack may be employed. Preferably the gravel pack that is to be used downhoie will be utilised in the test cell. Preferably the gravel pack is saturated with carrier fluid prior to placement in the cell. Again this simulates what happens in wellbore treatment.

A particularly preferred test cell of the present invention comprises a sample of filter cake and a sample of gravel pack and said samples are positioned in between said sample of formation and said sample of screen so that the samples are present in the order: formation, filter cake, gravel pack and screen. Preferably there exists a space or volume (e.g. about 30-40 mis) in between the gravel pack and the screen into which well fluid, e.g. completion fluid or drilling fluid, may be filled prior to testing. This is intended to simulate incomplete removal of completion or drilling fluid as discussed above. Preferred test cells of the present invention are shown in the schematic diagrams of Figures 2a and 2b. Figure 2a shows the basic set up of a test cell 100 of the present invention. Thus it comprises a body 2, an inlet means 3 and an outlet means 4. The inlet means 3 are integral to end cover 5 whilst outlet means 4 are integral to end cap 6. Both end cover 5 and end cap 6 are separate but connectable to body 2. The body 2 also comprises a ledge 7 to support a sample of the formation 8. A sample of screen 9 is positioned on ledge 10 above the sample of the formation 8. The sample of formation 8 is thus in proximity to the inlet means 3 and the sample of screen 9 is in proximity to the outlet means 4. Space 1 1 may be filled with well fluid, e.g. drilling fluid, prior to testing. This simulates a wellbore wherein poor displacement of drilling fluid has occurred. Alternatively space 11 may be filled with completion fluid to simulate a wellbore wherein drilling fluid displacement has occurred.

Figure 2b shows a more preferred set up of a test cell 200 of the present invention. The cell 200 comprises the same parts as described in relation to Figure 2a and the same reference numerals are used to indicate common features. Additionally the test cell 200 depicted in Figure 2b comprises a sample of filter cake 12 that is positioned on top of the sample of the formation 8. Furthermore the test cell also comprises a sample of gravel pack 13 that is positioned on top of the sample of filter cake 12. Space 11 may be filled with well fluid, e.g. drilling fluid, prior to testing. Again this simulates a wellbore wherein poor displacement of drilling fluid has occurred. Alternatively space 11 may be filled with completion fluid to simulate a wellbore wherein drilling fluid displacement has occurred. Figure 2c shows a further preferred set up of a test cell 300 of the present invention. The cell 300 comprises the same parts as described in relation to Figures 2a and 2b and the same reference numerals are used to indicate common features. The sample of formation 8 shown in Figure 2c is provided with formation fixing means 14. Together with ledges 7, the formation fixing means 14, hold the sample of formation 8 in place. A filter cake 12 is positioned on top of the sample of formation 8. A gravel pack 15 is then positioned on top of the sample of filter cake 12. A volume 16 exists above the sample of gravel pack 15 that may be filled with well fluid. The sample of screen 9 is provided in a screen fixing means 17. The ledges 10 interact with the screen fixing means 17 to hold the sample of screen 9 in place. The test cell 300 depicted in Figure 2c additionally comprises inserts 18, 19 inside the body 2 that allows smaller samples of formation and screen to be accomodated. The ledges 10 for supporting the screen fixing means 17 are provided integrally to these inserts 18, 19.

Figure 2d shows a preferred holder 400 of the present invention. The holder comprises features that have already been described in Figures 2a-c and the same reference numerals are used to indicate common features. The sample of formation 8 shown in Figure 2d is supported on ledge 31 of the holder 400. A filter cake 12 is positioned on top of the sample of formation and a gravel pack 13 is positioned on top of the sample of filter cake 12. A volume 11 exists above the sample of gravel pack that may be filled with well fluid. The sample of screen 9 is supported on ledge 32 of the holder 400. The ledges 31 , 32 are integrally formed in the walls of holder 400. The holder 400 additionally comprises lugs 33, 34, 35 and 36 for fixing the holder in the body of a test ceil. Preferred test cells of the present invention also comprise a heating means. This may take the form of a heating coil, heating jacket or heating tape. Preferably the heating means can maintain a temperature in the test cell in the range 20-150 °C, more preferably 50-100 °C. This means the test cell can replicate different downhole temperature conditions.

Preferred cells also comprise a pressure gauge to measure pressure. The pressure gauge may, for example, be integral with the outlet means.

In order to assess well fluids, the hereinbefore described test cells of the present invention are incorporated into an apparatus that additionally comprises a pump to pump fluid through the cell via the inlet and outlet means. Any conventional laboratory pump may be used for this purpose.

Preferably the apparatus also comprises a pressure gauge to measure pressure. This may be present at any point within the flow of fluid through the system. Typically the apparatus will be operated at a pressure within the range 0-50 bar, preferably 5-40 bar, e.g. up to 35 bar. Preferably the pressure gauge continuously measures pressure in the system during testing. Preferably the apparatus further comprises a computer arranged to receive pressure measurements. This means that the pressure within the system can be continuously measured and monitored.

Preferably the apparatus of the present invention further comprises a stand to hold or support the test cell. The stand may support the test cell in a vertical position and/or in a horizontal position. As used herein, the term vertical position means that the test cell is oriented so that an axis running through the centre of the test cell is perpendicular to the ground. This means that the cross sections of the samples present in the test cell are parallel to the ground. This configuration mimics the set up in a horizontal section of well. As used herein, the term horizontal position means that the test cell is oriented so that an axis running through the centre of the test cell is parallel to the ground. This means the cross sections of the samples present therein are perpendicular to the ground. This position mimics the set up in a vertical section of well. Still more preferably the stand rotatably supports the test cell (i.e. allows the test cell to be rotated) so that it may occupy a vertical or horizontal position or any position in between. This is particularly advantageous since it enables the test cell to be positioned in such a way that it mimics the orientation of various sections in a wellbore.

A preferred apparatus of the present invention further comprises a reservoir. This reservoir comprises fluid that is to be pumped through the test cell during testing. The reservoir may, for example, comprise oil, brine or a mixture thereof. Preferably the reservoir comprises oil since this simulates production and enables the cell to measure the affect on productivity. Preferably the reservoir is fluidly connected to the inlet means.

The apparatus preferably further comprises a means to collect the fluid from said outlet means. Preferably the collection means is fluidly connected to the outlet means.

The apparatus preferably also comprises one or more fluid tanks. This tank may comprise water, oil or a mixture thereof. Preferably the tank(s) is fluidly connected to the reservoir to enable it to be refilled during testing.

A preferred apparatus of the present invention is shown schematically in Figure 3. The apparatus comprises a reservoir 20, driven by pump 21 , which is fluidly connected to a test cell 104 as hereinbefore described. The test cell 104 comprises a sample of formation 22, a sample of filter cake 23, a sample of gravel pack 24 and a sample of screen 25. The test cell 104 also comprises an inlet 26 and an outlet 27. The outlet 27 is fluidly connected to collection means 28. The pressure in the system may be measured by pressure gauge 29 present on the flow line between reservoir 20 and test cell 400. Fluid tank 30 is fluidly connected to reservoir 20.

The present invention also relates to kits comprising the test cell or apparatus hereinbefore described. Preferred kits further comprise reference or calibration samples. The test cell, apparatus and kits of the present invention are designed for assessing well fluids. As used herein the term assessing means determining the suitability of a well fluid for use in a particular wellbore. The assessment may be qualitative, semiquantitative or quantitative. When a qualitative assessment is made, the result of the assessment may be that the fluid is or is not suitable for use in a wellbore. When the assessment is quantitative, an indication of the suitability of the well fluid in comparison to other fluids is provided. Preferably the assessment is quantitative.

The test cell, apparatus, kits and methods of the present invention may be used to test the suitability of any well fluid on the productivity of the well. Representative examples of well fluids that might be tested include drilling fluid, completion fluid, breaker fluid, stimulation fluid, corrosion inhibitors, scale inhibitors, bridging agents, sand control agents and mixtures thereof. The test cell, apparatus, kits and methods of the present invention are, however, particularly suitable for testing drilling fluid, completion fluid and breakers. The drilling fluid may be oil or water based. The completion fluid may be water or oil based. The breaker may be any conventional breaker used to disintegrate filter cakes or to break up emulsions.

During the methods of the present invention the pressure of the system is measured during pumping. Preferably the pressure in the system is measured continuously. Preferably pumping is carried out at a constant flow rate, e.g. at 10-500 ml/min, more preferably 20-200 ml/min, still more preferably 30-100 ml/min, yet more preferably about 50-80 ml/min. An increase in pressure in the system indicates there is a reduction in permeability within the test cell. This could be due to plugging or blockage of any of the formation, filter cake, gravel pack or screens. An increase in pressure corresponds to a situation in the wellbore wherein there is a reduction in productivity.

When testing well fluids using the test cell, apparatus and kits of the invention, the test cell is preferably set up with its samples (e.g. formation sample, filter cake, gravel pack and/or screen sample), the appropriate fluid is placed on top of the sample(s) and the test cell is allowed to stand. The test cell is preferably allowed to stand for 2 hours-7 days, more preferably 6 hours - 48 hours, still more preferably 8 hours - 24 hours, e.g. about 12 hours. This simulates the situation in a wellbore wherein well fluids may be present in the formation for a significant time before production is commenced. When testing a drilling fluid, the test cell comprises formation and screen samples. To test a drilling fluid it is filled into the space between the sample of formation and the sample of screen. A fluid, e.g. an oil, is then produced through the cell. This produces the drilling fluid through the screen and shows if any screen plugging that impairs productivity occurs.

Still more preferably, when testing a drilling fluid, the test cell comprises formation, filter cake and screen samples. Preferably the filter cake is made from the drilling fluid being tested. To test the drilling fluid, a sample of drilling fluid is placed in the space between the filter cake and the sample of screen. As in the above scenario a fluid, preferably an oil, is then produced through the cell. In this case, the affect of the drilling fluid on the filter cake and the screen is observed. Additionally the affect of the filter cake on productivity is also measured. When testing a drilling fluid, the test cell may also comprise a sample of gravel pack. This is preferably placed in between the sample of filter cake and sample of screen. As in the above scenarios a fluid, preferably an oil, is then produced through the cell. In this test cell the affect of the drilling fluid on the filter cake, gravel pack and the screen is determined. Generally, however, drilling fluid is displaced from a wellbore before a gravel pack is placed in the wellbore therefore this set up is less commonly used.

When testing a completion fluid, the test cell preferably comprises at least formation and screen samples. To test a completion fluid it is filled into the space between the sample of formation and the sample of screen. This mimics the location of a completion fluid after it has been used in a displacement operation. A fluid, e.g. an oil, is then produced through the cell. This produces the completion fluid through the screen and shows if any screen plugging that impairs productivity occurs. This test therefore provides a measure of the ability of a completion fluid to clean up the cell, i.e. to avoid any screen blockage. Moreover the test also shows the potential impact of the completion fluid on the formation.

Still more preferably, when testing a completion fluid, the test cell comprises formation, filter cake and screen samples. Preferably the filter cake is made from the drilling fluid that the completion fluid is being used to displace. To test the completion fluid, a sample of completion fluid is placed in the space between the filter cake and the sample of screen. As in the above scenario a fluid, preferably an oil, is then produced through the cell. In this case, the affect of the completion fluid on the filter cake and the screen is observed. Yet more preferably, when testing a completion fluid, the test cell comprises a sample of gravel pack. This is preferably placed in between the sample of filter cake and sample of screen. As in the above scenarios a fluid, preferably an oil, is then produced through the cell. In this test cell the affect of the completion fluid on the filter cake, gravel pack and the screen is determined.

The test cell, apparatus, kits and methods of the invention may also be utilised to test breakers. Breakers are sometimes employed to reduce the volume of filter cakes and in some cases to disintegrate them completely. Other breakers are used to break up emulsions. When testing a breaker, the test cell preferably comprises formation, filter cake and screen samples. Optionally a gravel pack may be present. The breaker is filled into the space or volume between the filter cake/if present gravel pack and screen. A fluid, preferably oil, is then pumped through the cell where it passes through the samples of formation, filter cake, gravel pack if present and screen and then out of the cell. This mimics the flow of a breaker when it is used in a wellbore. Any impact the breaker per se has on the formation will therefore be observed as well as any impact of the affect of the breaker on the filter cake and if present gravel pack. For instance when testing a filter cake breaker, if the breaker does not disintegrate the filter cake into sufficiently small pieces it may lead to gravel pack and/or screen blockage which will be detected as an increase in the pressure in the cell.

Once production in the above methods has been completed, the test cell is preferably disassembled. Preferably the end cover is removed and the sample of formation carrying the filter cake is removed for visual inspection. Preferably the end cap is also removed and the sample of screen removed for visual inspection. Preferably the gravel pack is also visually inspected.

In particularly preferred methods of the invention a standard or reference sample is initially tested and the performance of well fluids compared to the standard. For drilling and completion fluids, the standard is generally a drilling fluid or completion fluid currently in use in a well. Since the performance of a currently used fluid is known, its use as a reference provides a useful measure of whether more or less plugging of the screens, gravel pack or filter cake are likely to occur. For breakers the standard could be brine which would not be expected to cause any disintegration or break up. The methods of the invention may be utilised to test different combinations of well fluids, e.g. drilling fluids and completion fluids, drilling fluids and breaker fluids, drilling fluids, completion fluids and breaker fluids. The methods of the invention may also be used to explore the effect of different well conditions, e.g. by pumping well fluid at different rates, by varying the temperature, pressure etc.

During the methods of the present invention, however, the temperature of the cell is preferably maintained at a constant temperature during pumping. Preferably the temperature of the cell is in the range 20-150 °C, more preferably 30-100 °C, still more preferably 40-75 °C. Preferably this is achieved using a heating jacket as described above.

The invention is described with reference to the following non-limiting examples and Figures wherein:

Figure 1 is a schematic of the test set up using a PST tester of the prior art;

Figure 2a is a schematic of a test cell of the present invention;

Figure 2b is a schematic of a preferred test cell of the present invention;

Figure 2c is a schematic of a further preferred test cell of the present invention;

Figure 2d is a schematic of a preferred holder of the present invention;

Figure 3 is a schematic of an apparatus of the present invention;

Figure 4 shows the results of qualification testing on a drilling fluid;

Figure 5 shows the results of qualification testing on a drilling fluid wherein a gravel pack is present in the test cell; and

Figure 6 shows the results of qualification testing on a breaker. EXAMPLES

General

The test set up used is as described above and as shown in Figure 3. Similarly the test ceil used is as described above and as shown in Figure 2c. The test ceil was used in a substantially vertical position.

Example 1: Qualification of Drilling Fluid Test Setup for back production consists of: piston cylinder for 1 liter of oil or formation water, Gilson pump, test flow cell and pressure gauge.

The test cell is provided with a sample of formation, a sample of filter cake A1 or A2 (made from drilling fluid A using the process hereinbefore described and having different thicknesses) and a sample of screen (mesh size 250 micron). A rubber seal is placed on top of the sample of filter cake. Drilling fluid is placed over the filter cake and then the screen is put in place.

Brine or base oil, depending on the test being run, is then pumped into the test cell. Brine or base oil is therefore produced through the cell for approximately approx. 20 min at the rate 50 ml/min until 1 liter of oil/formation water is produced. During the run the pressure is constantly monitored.

The results are shown in Figure 4. The results show that the filter cakes A1 and A2 produced by the drilling fluid tested are permeable and both of oil and water can be produced at a stable and steady pressure therethrough. This was achieved after 5-10 minutes. No significant difference in pressure was found between the production of brine and formation water. Example 2: Qualification of Drilling Fluid

Test Setup for back production consists of: piston cylinder for 1 liter of oil or formation water, Gilson pump, test flow cell and pressure gauge. The test cell is provided with a sample of formation, a sample of filter cake A1 , A2, B, or C (made from drilling fluid A, B or C using the process hereinbefore described and in the case of drilling fluid A having two thicknesses), a gravel pack (55-60 ml, approx 2.5 cm height) and a sample of screen. The gravel pack size is 16/30 μηι and it is saturated in 1.03 sg NaCI brine. Saturation time in brine is 1.5-2 hours. The saturated gravel pack is subsequently placed between the filter cake and screen in the flow cell

The cell is then filled with drilling fluid A, B or C in between the gravel pack and the screen. Alternatively brine and/or base oil could be used. Formation water or base oil is then produced through the cell for approximately 20 min at the rate 50 ml/min until 1 liter of fluid is produced. During the run the pressure is constantly monitored.

The results are shown in Figure 5. The results show that all of the filter cakes, the gravel pack and the screen are permeable to both base oil and formation water as both are able to pass through the gravel pack and the screen. Filter cakes B and C showed less pressure build up than filter cake A. The test results indicate that each of drilling fluids A, B and C would be suitable for use in the field with the gravel pack and screens tested. In all cases, the cell is preferably opened after testing to enable visual observation of the filter cake, gravel pack and screen samples.

Example 3: Qualification of emulsion breakers Test method:

- Gravel Packing, approx. 50 ml, saturated with 30 ml test emulsion * , is placed in a test cell comprising a sample of formation and a sample of filter cake.

- The test cell is heated to 90 °C for 2 hours

- Breaker solution is placed over the gravel packing in concentrations recommended by the supplier

- The screen is placed over the breaker solution

- Shut-in period for 18 hours at 90 °C

- Base oil (a total of 3 liters) is produced through the cell

- The first liter is produced at a rate of 10 ml/min - The second liter is produced at a rate 50 ml/min (to 250 ml), at a rate of 75 ml/min (to 600 ml) and at a rate of 100 ml/min (to 1000 ml)

- The third liter is produced at a rate of 100 ml/min

- Visual observation of the liquid phase produced and the gravel packing is carried out ( * Test emulsion: a blend of an oil-based mud and brine solution in proportion 50%/

50%)

Picture 1 in Figure 6 shows the gravel packing after the back production with breakers A and B at 90°C.

Picture 2 in Figure 6 shows the back produced base oil (total of 3 liters) with breaker B.

The results show the extent of degraded emulsion and the purity of the gravel packing after back production at 90°C. Less oil-like coating on the edge of the gravel pack indicates that emulsion breaker B has a greater affect (picture 1b: breaker B) than emulsion breaker A (picture 1a: breaker A).

The back produced base oil shows a difference between colors that can give us an indication of how much of the emulsion is degraded and removed from the gravel packing (picture 2).