WO1996034076A1

WO1996034076A1 - Water-based hydraulic fluid composition

Info

Publication number: WO1996034076A1
Application number: PCT/GB1996/000978
Authority: WO
Inventors: William John Sealy Neale
Original assignee: Houghton Vaughan Plc
Priority date: 1995-04-25
Filing date: 1996-04-24
Publication date: 1996-10-31
Also published as: AU5404296A; GB9508323D0

Abstract

A water-based hydraulic fluid suitable for use in controlling sub surface safety valves, comprises an aqueous solution of a water-soluble salt of a C4-C10 carboxylic acid as a lubricant and/or thickening agent. The carboxylic acid salt is preferably potassium heptanoate. The solution may also contain glycol and/or a polymeric thickener.

Description

WATER-BASED HYDRAULIC FLUID COMPOSITION

This invention relates to a composition and is more particularly concerned with a hydraulic fluid composition. In particular, the present invention is concerned with a hydraulic fluid composition suitable for use in controlling sub surface safety valves (SSSVs) which are used to shut off the flow of well fluids (oil or gas) in the event of an emergency in undersea wellheads. SSSVs are exposed to high temperatures and pressures in a hostile undersea environment and it is important for the hydraulic fluids used to control such valves to be capable of operating reliably in such an environment. For example, hydraulic control fluids for SSSVs may be required to exhibit satisfactory properties under test at pressures of up to 1350 bar and at temperatures of up to 200 °C. Also, the hydraulic control fluids are required to have a stable viscosity, to be resistant to decomposition and to remain deposit-free over an extended period of time even at elevated temperatures. The formation of deposits or viscous sludge within the fluid can prevent or slow operation oi the SSSV. Additionally, such a fluid is required to operate effectively over a large pressure drop in a relatively wide temperature range as the fluid travels to and from the safety valve on the sea bed.

Existing hydraulic fluids for SSSVs may be based on mineral oils, synthetic hydrocarbons or water-based fluids, with an increasing requirement for the last-mentioned type for environmental reasons. Whilst commercially available hydraulic fluids for SSSVs based on synthetic hydrocarbons may be stable up to 200 °C, they are not preferred for environmental reasons, whilst the more environmentally acceptable water-based fluids, eg those based on glycols, are only stable up to about 135 to 140 °C .

It is an object of the present invention to obviate or mitigate the above disadvantage.

In a first aspect, the present invention resides in a water-based hydraulic fluid comprising an aqueous solution of a water-soluble salt of a C₄-C₁₀ carboxylic acid as a lubricant and/or thickening agent.

In a second aspect, the present invention resides in the use of a water- soluble salt of a C₄-C₀ carboxylic acid as a lubricant and/or thickener in a water-based hydraulic fluid.

In a third aspect, the present invention resides in the use of a water- based hydraulic fluid according to said first aspect of the present invention for controlling operation of a hydraulically-operated device, more preferably a sub-surface safety valve.

The C₄-C₁₀ carboxylic acid may have a straight or branched chain, it is preferably a C_b-C carboxyiic acid, and is more preferably a heptanoic acid, most preferably n-heptanoic acid. Other examples include water- soluble salts of sekanoic-C₉, n-C₉ and neodecanoic acids.

The water-soluble salt is preferably an alkali metal salt, more preferably the sodium or potassium salt.

Preferably, the carboxylic acid salt used in the hydraulic fluid of the present invention is present in an amount, based on the total weight of the hydraulic fluid, of from about 5 wt%, more preferably from about 25 wt%, up to the limit of solubility of the salt. For the n-heptanoic acid salt, the upper limit may be about 60 wt%, whilst the range is more preferably about 10 to 50 wt% and most preferably about 40 to 45 wt%, based on the total weight of the hydraulic fluid.

The water used in the hydraulic fluid is preferably de-ionised water.

If this hydraulic fluid is for non-SSSV applications, it may further contain an additive to depress the pour point or freezing point of the composition, for example a monomeric glycol, e.g. monoethylene glycol (MEG). Typically, such an additive is present in an amount of up to about 20% by weight of the hydraulic fluid. The fluid may also contain a polymeric thickener eg a polyalkylene glycol such as polyethylene glycol (PEC).

The hydraulic fluid typically has a viscosity of 1 to 15 cSt at 40 °C under the Institute of Petroleum Test IP71.

The hydraulic fluid of the present invention may be produced by dissolving the water-soluble carboxylic acid salt in water or by mixing the free carboxylic acid with the appropriately reactive anion (e.g. the hydroxide) of a water-soluble salt-forming cation with water.

The use of such water-soluble salt of a C₄-C₁₀ carboxylic acid not only provides the required thickening and/or lubricating properties but is also believed to be biodegradable so that the use of a hydraulic fluid according to the present invention under sea presents a minimum of environmental pollution problems. When the carboxylic acid salt is used in appropriately high proportions, it is also believed to impart microbial degradation-resistant properties to the hydraulic fluid.

Although the hydraulic fluid according to the present invention is particularly suitable for use as a control fluid for SSSVs, it may alternatively be used as a hydraulic control fluid in other subsea production control systems. It is fire resistant and is believed to be more environmentally acceptable than other previously used control fluids. It is therefore considered to be potentially useful as a fire-resistant hydraulic fluid.

The present invention will now be described in further detail in the following Examples.

Example 1

A water-based hydraulic control fluid for an SSSV was formulated by mixing 43 parts by weight of potassium n-heptanoate with 57 parts by weight of de-ionised water. Such a fluid had the following properties:-

(a) It was stable over a wide temperature range up to and including 240 °C,

(b) It had a decomposition temperature of 375 °C as determined by differential scanning calorimetry (DSC),

(c) It exhibited no deposit formation after 12 weeks at 220 °C or after 3 weeks at 240 °C.

(d) No change in viscosity was observed after 12 weeks at 220 °C,

(e) It exhibited a pH change of < 0.1 after 12 weeks at 220 °C, (0 It gave excellent corrosion protection - no effect on corrosion inhibition was observed after 12 weeks at 220 °C according to corrosion test IP135B and modified IP135B test using 50 % sea water,

(g) It gave good lubricity according to the Falex Armco Test and the

Shell Four Ball Test. The latter Test gave a wear scar of less than 0.42 mm after 60 sees under a 40 kg load,

(h) It exhibited good bio-resistance to bacteria, fungi and yeasts,

(i) It had good compatibility with elastomers of the type used in seals for hydraulic systems.

Example 2

Various concentrations (% by wt.) of potassium heptanoate in de-ionised water were made up and subjected to various tests and determinations, namely:-

(1) A standard test IP.135B to assess corrosion inhibition;

(2) A standard anti-bacterial shaker test (herein called "A-BST") to determine bacterial growth (after 14 days);

(3) Viscosity at 40 °C in centistokes (cSt);

(4) Fail Load (lbs) according to the Falex Armco test;

(5) Coefficient of friction (COF); and

(6) Mean Wear Scar Diameter (MWSD) in mm according to the Shell four-ball test (40 kg for 60 seconds). The results are given in Table 1 below. Table 1

Concn. IP.135B A-BST Viscosity Fail Load COF MWSD

5 Pass None 0.8 2400 0.07 0.38

10 Pass None 0.9 > 4500 0.03 0.39

20 Pass None 1.4 > 4500 0.04 0.36

30 Pass None 7.2 2600 0.03 0.44

40 Pass None 9.0 > 4500 0.03 0.44

50 Pass None 19.8 3200 0.02 0.40

60 - - Solid _ . .

In addition, no differences were observed when the thermal stability by DSC was checked at the various concentrations. The melting point of the salt is about 300 °C. Thermal decomposition occurs at about 375 °C.

Example 3

40 % salt solutions of various C₄ to C₁₀ carboxylic acids were made up and tested. The results are given in Tables 2 and 3 below.

Table 2

NaOH salts

Acid Melting Point Viscosity Decomposition Temp (DSC) °C 40 °C (cSt) (DSC) °C n-C₄ 256 3.8 -330 n-C₅ 235 4.7 -366 n- 236 8.8 -368 n-C₇ 244 22.5 -362 n-C₈ 247 - - n-C₉ 250 61.4 -352

Neo- decanoic 375 8.5 > 375

Cekanoic

Co 283 12.7 -327 Table 3

KOH salts

Acid Melting Point Viscosity Decomposition Temp.

(DSC) °C 40 °C (cSt) (DSC) °C n-C₄ 355 2.0 > 400 n-C₅ 317 2.4 >400 n-Q 315 4.4 > 400 n-C₇ 301 9.0 > 400 n-C₈ 293 18.1 >400 n-C₉ 276 38.2 > 400

Neo- decanoic 433 6.0 >400

Cekanoic

295 8.4 >400

Example 4

12 % by weight salt solutions of various carboxylic acids in de-ionised water were made and tested for Fail Load (lbs) and COF (coefficient of friction) according to the Falex Armco test. The results are given in Table 4 below:-

Table 4

Acid Salt Fail Load COF n-C₄ K 2100 0.12 n-C₄ Na > 4500 0.07 n-C₅ K 4000 0.08 n-C₅ Na > 4500 0.06 n-C₇ K > 4500 0.04 n-C₅ Na >4500 0.05 Example 5

To show the effect of addition of the anti-freeze agent, glycoi (MEG - monoethyleneglycol), solutions of 21.25 wt% of potassium heptanoate and various quantities of glycol in de-ionised water were made up and the viscosity of such solutions at 40 °C was measured. The results are given in Table 5 below:-

Table 5

Glycol (%) Viscosity (cSt)

5 1.7

10 1.9

20 2.4

30 3.1

40 3.9

It is proposed to use solutions containing glycol mainly as hydraulic fluids for non-SSSV applications.

Claims

1. A water-based hydraulic fluid comprising an aqueous solution of a water-soluble salt of a C₄-C,₀ carboxvlic acid as a lubricant and/or thickening agent.

2. A water-based hydraulic fluid as claimed in claim 1 , wherein the device is a sub-surface safety valve.

3. A water-based hydraulic fluid as claimed in claim 1 or 2, wherein the carboxylic acid is a C₆-C,₀ carboxylic acid.

4. A water-based hydraulic fluid as claimed in claim 1 , 2 or 3, wherein the carboxylic acid is a heptanoic acid.

5. A water-based hydraulic fluid as claimed in any preceding claim, wherein the carboxylic acid is n-heptanoic acid.

6. A water-based hydraulic fluid as claimed in any preceding claim, wherein the water-soluble salt is an alkali metal salt.

7. A water-based hydraulic fluid as claimed in claim 6, wherein the water-soluble salt is the sodium or potassium salt.

8. A water-based hydraulic fluid as claimed in any preceding claim, wherein the carboxylic acid salt is potassium n-heptanoate.

9. A water-based hydraulic fluid as claimed in any preceding claim, wherein the carboxylic acid salt is present in an amount, based on the total weight of the hydraulic fluid, of from about 5 wt7o , up to the limit of solubility of the salt.

10. A water-based hydraulic fluid as claimed in claim 9, wherein the carboxylic acid salt is present in an amount, based on the total weight of the hydraulic fluid, of from about 25 wt7o, up to the limit of solubility of the salt.

1 1 . A water-based hydraulic fluid as claimed in any preceding claim, having a viscosity of 1 to 15 cSt at 40 °C.

12. A water-based hydraulic fluid as claimed in any preceding claim, further containing at least one of an additive to depress the pour point or freezing point of the composition and a polymeric thickener.

13. The use of a water-soluble salt of a C₄-C,₀ carboxylic acid as a lubricant and/or thickener in a water-based hydraulic fluid.

14. The use of a water-based hydraulic fluid as claimed in any one of claims 1 to 12 for controlling operation of a hydraulically-operated device.