Thermally Stable Thiosulfate Hydroxyethylcellulose Suspension

The invention relates to aqueous salt suspensions of polymer. In particular the invention relates to a thermally, stable suspension of hydroxyethylcellulose in ammonium or sodium thiosulfate.

Fluidized polymer suspensions which employ an inorganic salt are known from U.S. Pat. Nos. 4,283,229, 4,883,536 and 4,883,537. These salts include alkali metal and ammonium ion carbonates, phosphates and sulfates. Assignee's copending applications, Ser. Nos. 600,745, 706,332 and 700,334 employ sodium formate as the suspending salt.

It is an object of the invention to provide thermal and storage stable aqueous suspensions of hydroxyethylcellulose containing at least 30% by weight ammonium or sodium thiosulfate.

The suspensions are prepared by dissolving the ammonium or sodium thiosulfates in water and dispersing the hydroxyethylcellulose into the solution.

The suspensions are prepared with fresh water and remain pourable after mild stirring for six months.

It has been discovered that hydroxyethylcellulose (well known as a nonionic thickening agent) can be protected against thermal degradation by thiosulfate salts. Hydroxyethylcellulose is available from the Aqualon Company as Natrosol.RTM. hydroxyethylcellulose (HEC). Grade HHXR is of high molecular weight. Grade EXR is low molecular weight. HEC is also available from Union Carbide and Daicel. HEC is widely used as a paint thickener and for other applications in food, cosmetics and pharmaceuticals.

Ammonium thiosulfate is a commodity chemical which is generally believed to be interchangeable with sodium thiosulfate. For example in photographic processing, "fixer" is usually sodium thiosulfate, while ammonium thiosulfate can also be substituted if desired.

The invention has industrial applicability for oil drilling fluids for deep wells where high temperatures create problems for existing drilling fluids. The following examples illustrate the practice of the invention.

An ammonium thiosulfate brine was prepared by dissolving 500 g ammonium thiosulfate in 500 g water. While 750 g of this solution was stirred with a Hamilton Beach mixer at 11,000 rpm, a 250 g portion of Natrosol.RTM. hydroxyethyl cellulose was added into the solution over 20 minutes. The procedure was repeated with a different HEC. Table 1 gives Brookfield LVF (25.degree. C.) viscosity results over time for the two grades of Natrosol.RTM. HEC tested in the suspensions of the invention.

Both suspensions containing 25 wt. % HEC and 37.5 wt. % ammonium thiosulfate were stable and remained pourable after mild stirring even after six months storage.

Table 2 gives the Brookfield LVF (25%) viscosities of similar suspensions prepared by the procedure described in Example 1, using either ammonium thiosulfate or sodium thiosulfate with different samples of HEC 250 HHXR (Examples 3 and 4).

Although brine viscosities were different in both cases, reflecting a difference in solubility of the HEC in both brines, the viscosities of the suspensions were stable for two months in each case.

This example illustrates the efficiency of ammonium thiosulfate as a thermal stabilizer for a sea water polymer solution.

HEC was added to sea water in the amount of either 1 or 2 pounds per barrel (ppb); (one ppb is equivalent in metric units to 2,847 kg/m.sup.3), with and without 30% by weight ammonium thiosulfate (A.T.). Yield values of lb/100 ft.sup.2 (metric equivalent 1 lb./100 ft.sup.2 =0.5 Pa) were determined initially for these combinations and then after 16 hours at 250.degree. F. (122.degree. C.). Table 3 gives comparative results in which the decrease in yield value is expressed as a percentage.

These results show the surprising degree of thermal protection provided by the simple addition of ammonium thiosulfate whereby it is also possible to prepare storage stable suspensions as illustrated in Example 1.

Test were run similar to Examples 1-4 except that both ammonium thiosulfate and sodium thiosulfate were used in addition to other components, as shown in Table 4 below. In these tests, Natrosol.RTM. 250 HHRP hydroxyethylcellulose available from Aqualon was used as the polymer. A.T.=Ammonium Thiosulfate, S.T.=Sodium Thiosulfate.

Table 5 summarizes the effects of addition of HEC suspensions in thiosulfates, as percent drop V after 16 hours at 250.degree. F. in properties.

Conclusions from Table 5

1. In FW and SW alone, neither ammonium thiosulfate (A.T.) nor sodium thiosulfate (S.T.) provided thermal stability to HEC ih suspension form by any of three indicators (Apparent Viscosity, Plastic Viscosity or Yield Point).

2. With 1 ppb NaOH present, A.T. is a more effective thermal stabilizer than S.T. in both FW and SW. Both provide some thermal stability. Plastic viscosity indicates this best.

3. The best thermal stability is provided with 1 ppb MgO present, in either FW or SW. Both S.T. and A.T. are about equally effective, with S.T. being slightly preferred. The best indicator is Plastic Viscosity or Apparent Viscosity.

Table 6 summarizes the effect of addition of 30% solid thiosulfate on thermal stability of HEC in different media, after 16 hours at 250.degree. F.

Conclusions from Table 6

1. With 1 ppb NaOH present, in FW neither A.T. nor S.T. provide much thermal stability. In SW, A.T. provides somewhat better stability than S.T. Yield Point indicates the best stability.

2. The best thermal stability is provided with 1 ppb MgO present, both in FW and SW. Both S.T. and A.T. are about equally effective. Plastic Viscosity and Apparent Viscosity are the best indicators.