Method and composition for the treatment of pipelines

Abstract

 Method and composition to remove solids, water and other contaminants from a pipeline.
The non aqueous gel composition is obtained from a cross-linkable polymer dissolved or dispersed preferably in a polyhydric alcohol such as monoethylene glycol (MEG).
Improved properties in terms of rheology and safety.

Description

[0001] The present invention relates to a method and composition for the treatment of pipelines to remove solids, water or other contaminants, and for related applications.

[0002] It is common practice to employ for these purposes devices known as 'pigs' which are propelled through the pipeline, for example by fluid pressure. Pigs are also used to control the interface between two successive, different fluids flowing through the same pipeline. It is well known, for example from U.S. Patent 3 209 771, to employ for this purpose gel pigs in the form of masses of gelled liquid. The liquid may be water, for which acrylamide copolymer or carboxymethylcellulose catalysed with aluminium sulphate is suggested as a gelling agent, or hydrocarbons, which may be. gelled by aluminium salts of fatty acids such as aluminium octoate, stearate or caprylate. Similar gelled pigs can be used for other purposes, such as the application of coatings or surface treatments to the pipeline wall, for example as described in Canadian Patent 957 910.

[0003] According to U.S. Patent 4 216 026, Bingham plastic fluid plugs, having a closed toroidal circulation when in motion through the pipe, are used to remove liquid or particulate debris from pipelines. The Bingham fluid may be based on mineral oils, which may be gelled with an organo-modified smectite, or on water, which may be gelled by means of xanthan gum (a high molecular weight, linear, natural polysaccharide produced by the micro-organism Xanthomonas Campestris) cross linked with a multivalent metal provided by aluminium sulphate, ferric sulphate or chromium chloride, or by means of other water-soluble polymers such as guar gum, carboxymethylcellulose or polyacrylamide with the addition of bentonite.

[0004] It is also known to remove unwanted moisture from pipelines, following cleaning operations and before their being put into service, by driving a liquid alcohol such as methanol, contained between two pigs, through the pipeline by means of a dry gas. Gelled methanol has been proposed for use in the fracture processing disadvantages for such purposes on account of its high vapour pressure, its toxicity and its flammability.

[0005] The present invention now provides a method and composition which overcome some or all of these disadvantages.

[0006] In the method of this invention the gelled mass is prepared by dissolving or dispersing a cross-linkable polymer in a hygroscopic organic liquid of low volatility, and causing the composition to form a viscoelastic gel by cross-linking the polymer with a salt of a multivalent metal.

[0007] In accordance with another aspect of this invention, a non-aqueous gel composition is provided which comprises a solution or dispersion of a cross-linkable polymer in a non-volatile hygroscopic organic liquid, the polymer being cross-linked by a multivalent metal.

[0008] It is an important feature of this invention that gels can be used which are essentially non-aqueous and can be prepared with the exclusion of water. The cross-linkable polymer is dissolved or dispersed directly in the organic liquid, and the metal salt is then added to affect the cross-linking and form the gel. The metal salt is preferably one that is soluble in the same organic liquid and may conveniently be dissolved in a further portion of the same liquid and then mixed with the solution or dispersion of the polymer in that liquid.

[0009] The organic liquid is preferably one of the class of polyhydric alcohols, and more especially monoethylene glycol (MEG). This substance has desirable properties of water-miscibility and low vapour pressure, and does not present a serious health hazard because it is not readily absorbed through the skin.

[0010] Considerable effort has been required to identify a gelling system that will form a strong viscoelastic gel in a polyhydric alcohol. However, it has been found that the cross-linkable polymer is preferably an hydroxylated polymer such as a polysaccharide, and more especially xanthan gum, being a class of polysaccharide of microbial origin as described above. The preferred metal species is ferric ions, and it has been found especially effective to employ this metal in the form of ammonium ferric sulphate.

[0011] To provide a gel according to the present invention which has sufficient strength and rigidity to meet the highest requirements for pipeline treatment, particularly preferred compositions contains from 2 to 12 grams gelling agent such as xanthan gum and from 0.2 to 0.6 grams metal salt such as ammonium ferric sulphate per litre of MEG. The amount of gelling agent employed may be varied to give a desired viscosity but, for the applications described herein, will most suitably be in the range 4.8 to 6.0 grams per litre.

[0012] An alternative polymer is a partially hydrolysed polyacrylamide. With xanthan gum, the rate of gel formation can be varied by employing modified xanthan gums having different rates of solvation or "hydration".

[0013] The cross-linking agent is preferably dissolved in a compatible and miscible liquid, preferably the same polyhydric alcohol, before addition to the gelling agent solution. The preferred concentration of metal salt in polyhydric alcohol for the purpose of addition is from 30 to 60 grams per litre.

[0014] The preferred gel compositions according to this invention have a highly viscous, semi-rigid structure rendering them useful under conditions similar to those employed for gel pigs based on water or hydrocarbon media. They are highly effective for drying operational pipelines, being completely non-aqueous while at the same time avoiding the various disadvantages of such substances as methanol. The preferred gels can accommodate up to 50X water without significant loss of gel structure. The preferred gels, being based on a water-miscible organic liquid, are completely dispersible in water, which simplifies the cleaning of equipment after use and removal of the gel in situations where water can be tolerated.

[0015] The present invention also provides a non-aqueous gel suitable for use in downhole stimulation, such as the breaking of formations in non-productive wells. It has been found, in accordance with a further aspect of this invention, that the gel can readily be broken by the addition of a gel breaking agent, whereby the gel can be pumped as a liquid out of a location when the treatment has been completed. With the preferred system of MEG viscosified with xanthan gum and cross-linked with ferric species, the preferred gel breaking agents are oxidising agents. Although the gel can be instantly broken by an agent such as calcium hypochlorite, it is preferred to use an agent providing a controlled delay in gel breaking, and especially ammonium persulphate. The latter agent may be advantageously used in combination with a mild reducing agent such as triethanolamine.

[0016] The following examples illustrate the practice of the present invention.

EXAMPLE 1

[0017] Preliminary experiments showed that non-aqueous gels in MEG could be produced by the use of a 15 to 25X hydrolised polyacrylamide (Dow type AP 273) or a commercially available xanthan gum (Kelzan XC).

[0018] Experiments were then conducted to form a stronger gel by cross-linking with readily available materials by a simple mixing procedure.

[0019] Various multivalent and borate cross-linking agents were tested by dissolving them in MEG, with or without an acid or base to control the "pH" of the system. By "pH" in this context is meant the apparent value obtained by subjecting an essentially non-aqueous material to a conventional pH monitoring system. Of the multivalent metal salts that were soluble in MEG, ferric sulphate produced a semi-solid, substantially rigid gel with little or no tendency to flow, but it was highly shear sensitive.

[0020] The gel produced with ammonium ferric sulphate was therefore preferred, since it was a highly elastic gel with strong cohesive forces.

[0021] The concentration of cross-linking agent in MEG forming the cross linker solution can be varied widely, but in the work described it was 50grams ammonium ferric sulphate per litre MEG. A solution of 100g NaOH per litre MEG was used as "pH" control solution.

[0022] Two methods were employed.

Method A - using aged base fluid

[0023] A base fluid was prepared with the following composition:

1,000 ml MEG

4.8g Kelzan XC

[0024] This pre-mixed base fluid was aged for 1 or 2 days and then placed in a blender and high shear applied. The pH control solution was added to achieve an apparent "pH" of 8-9 and the cross linker solution was added with high shear mixing for 1 minute. The amount of these additions was as follows:

0.2 ml "pH" control solution

8.0 ml crosslinker solution

[0025] The viscosity was then measured immediately after mixing and monitored for 6 hours. The results are shown in Table IV.

It was found that the 2-day old base fluid gave a better gel than the 1-day old, but the latter was still of good quality. Maximum viscosity was reached in each case after 2 hours. By way of comparison, it was noted that a typical hydrocarbon gel has an apparent viscosity of around 90 cp at 300 rpm.

[0026] A further trial was conducted with the pH control solution omitted. The cross linker solution was blended in the same proportion with a 1-day old base fluid, and a highly elastic cross linked gel was formed with an apparent viscosity of 228 cp at 300 rpm after 2 hours. This suggests that "pH" control is unnecessary.

Shear sensitivity.

[0027] The mature cross linked gel (over 2 hours old) was examined for shear sensitivity by returning it to the blender and subjecting it to very high shear for a further minute. It was found that the gel was reduced to a viscous liquid which poured easily. Table V sets out the viscosity immediately after 1 minutes high shear of the gel with various proportions of water added.

[0028] The above values are transient minima. Upon standing, the gels re-form in under 30 seconds in the case of the unwatered gel,and the physical structure is as good as before shearing.

Effect of water inclusion

[0029] To determine the maximum quantity of water that can be contained by the gel, an investigation was made into the time taken for the crosslinked structure to re-form after 1 min. mixing with various percentages of water. The results are shown in Table VI.

[0030] A higher concentration of gelling agent should lead to permissible uptakes of water above 50X for applications in which the return to the crosslinked structure is desirable.

Method B - with short pre-mix

[0031] For certain uses of the crosslinked gel, it may not be feasible to wait 1 or 2 days for maximum viscosity of the base fluid to be reached. Experiments were therefore conducted to see if xanthan gum could be more quickly solvated in MEG by reducing the apparent "pH" using acid additives.

[0032] A base fluid of the following composition was mixed at very high shear for 1 hour:

1000 ml MEG

4.0g boric acid

4.8g Kelzan XC

[0033] 8.0 ml crosslinker solution was then mixed in and the viscosity of the resulting gel is shown in Table VII.

[0034] The measured viscosities were found to be higher with this mixing method than with Method A, but the gel structure was not of as high a quality as that produced with Method A. After the 2 hour old gel had been mixed at high shear, it reformed within 5 minutes but with a structure of still lower quality. It is not clear whether the effect of the boric acid is simply due to a lowering of the apparent pH or whether it modifies the xanthan gum to permit more rapid solvation.

Example 2

[0035] Experiments were conducted to determine how the gel described in Example 1 could be controllably reduced to a viscous liquid after a reproducible time delay.

[0036] Samples of the crosslinked gel prepared by Method B were mixed with various oxidising agents as gel breakers.

[0037] The crosslinked gel system consisted of:

1000 ml MEG

4.0g boric acid

4.8g Kelzan XC

8 ml crosslinker solution

[0038] Addition of calcium hypochlorite to this gel broke the structure, but with no time delay. The viscosity was drastically reduced as soon as the calcium hypochlorite was blended into the gel.

[0039] More controllable gel breakage was obtained with ammonium persulphate, which was added as a solution of 30 grams ammonium persulphate per litre MEG. It was also found advantageous to add a small quantity of triethanolamine (a mild reducing agent) to assist in the gel breaking effect.

[0040] Various quantities of the gel breaker solution together with 4 ml triethanolamine per litre were blended into the crosslinked gel at high shear rates and the samples were placed in a water bath at 65°C. Signs of gel breakage were monitored visually and by stirring, and the breakdown was taken to be when the gel had reverted to a viscous liquid. A colour change from green to brown was also evident upon breaking. The crosslinked gel had a semi-solid structure which was barely pourable, whereas the broken gel was totally liquefied and easily pourable.

[0041] The break times obtained at 65°C are shown in Table VIII.

[0042] It has thus been shown that the use of gel breaking agents with the MEG gel system can give controlled break times of the gel, with the structure reverting to a viscous liquid after break. This capability gives the system a possible use in fracturing operations where a non-aqueous fluid is required.

Claims

1. A method of treating the interior of a pipeline or other surface by applying a gelled mass to the surface, characterised in that the gelled mass is prepared by dissolving or dispersing a cross-linkable polymer in a hygroscopic organic liquid of low volatility, and causing the composition to form a viscoelastic gel by cross-linking the polymer with a salt of a multivalent metal.

2. A method according to claim l'in that the gel is prepared in the absence of water.

3. A method according to claim 1 or 2, characterised in that an hydroxylated polymer is dissolved or dispersed in a polyhydric alcohol, and the metal salt is dissolved in the same alcohol and mixed with the polymer solution.

4. A method according to claim 3, characterised in that the polymer is a cross-linkable polysaccharide and the metal salt is a ferric salt.

5. A method according to claim 4, characterised in that the polymer is xanthan gum and the cross-linking agent is ammonium ferric sulphate.

6. A method according to any of claims 3 to 5, characterised in that from 2 to 12 grams polymer is dissolved or dispersed per litre of polyhydric alcohol, and from 0.2 to 0.6 grams multivalent metal salt is added per litre of polyhydric alcohol.

7. A method according to claim 6, characterised in that the metal salt is added in solution in the alcohol having a concentration in the range of 30 to 60 grams per litre.

8. A method according to claim 6 or 7, characterised in that from 4 to 6 grams xanthan gum is dissolved per litre of mono-ethylene glycol, and about 0.4 grams ammonium ferric sulphate is added per litre of glycol.

9. A method according to any of claims 5 to 8 characterised in that the polymer solution or dispersion is allowed to age for at least one day before the addition of the multivalent metal.

10. A method according to any preceding claim, characterised in that the gel is subsequently broken by the addition of a gel breaking agent prior to its removal from the surface.

11. A method according to claim 10, characterised in that the gel breaking agent is ammonium persulphate.

12. A method according to claim 11, characterised in that the gel breaking is accomplished with the additional introduction of triethanolamine.

13. A method according to claim 11 or 12, characterised in that the amount of ammonium persulphate added is from 0.12 to 1.2 grams per litre of organic liquid in the gel.

14. A non-aqueous gel composition characterised in that it comprises a solution or dispersion of a cross-linkable polymer in a non-volatile hygroscopic organic liquid, the polymer being cross-linked by a multivalent metal.

15. A composition according to claim 14, characterised in that the organic liquid is a polyhydric alcohol, the polymer is xanthan gum and the cross-linking metal is ferric iron.

16. A composition according to claim 15, characterised in that it contains 4-6 grams xanthan gum and about 0.4 grams ammonium ferric sulphate per litre of mono-ethylene glycol.