GB2248832A - Method for inhibiting scale formation - Google Patents

Abstract

A method for inhibiting the formation of scale in water having a high barium content and low pH comprises the addition of an aqueous solution of a phosphonate of formula: <IMAGE> wherein R = H, CH3, C2H5, M R<1> = H, CH3, CR3, C6H5, SO3H2 M = alkali meta or ammonium ion. The scale inhibitors may be present in the form of fully or partially neutralised salt and may be injected into production fluids obtained from an underground reservoir or injected into the reservoir via a wellbore as part of a squeeze technique.

Description

METHOD FOR INHIBITING SCALE FORMATION This invention relates to a method for inhibiting the formation of scale, in particular to a method for inhibiting the formation of scale in waters having a high barium content and low pH.

A scale inhibitor is a compound or mixture of compounds which inhibits crystal growth and thereby reduces the tendency of scale to form.

Scale inhibitors are used in the oil industry. Water flooding is one of the most successful and extensively used secondary recovery methods. Water is injected under pressure into the reservoir via injection wells and drives the oil through the rock into nearby producing wells. In this type of operation it is most important to ensure that the injection water is free from suspended particles or any chemicals or biological substance which might cause blockage of the pore channels in the reservoir rock. The oil in the reservoir is frequently associated with connate water which contains various cations, sometimes including barium, calcium and strontium.

Many oil fields are situated in offshore locations and for them the only source of injection water is the sea. Sea water contains sulphate and carbonate ions which, in the absence of treatment, would react with alkaline earth metal ions in the connate water to precipitate scale.

In order to prevent this, scale inhibitors are used.

Typical scale inhibitors include sulphonated ethoxylated hydrocarbons, polyacrylates, amine phosphonates and organic polyphosphonic acids.

The main types of scale occurring in oil production operations are calcium carbonate and barium, calcium and strontium sulphates.

Conventional scale inhibitors have proved effective in combating these types of scale. However scale formation encountered in reservoirs which have a high barium content in the formation waters coupled with a low pH is difficult to control with these conventional scale inhibitors.

We have now found that certain polyaminomethylene phosphonates may be used as effective scale inhibitors for waters having a high barium content and low pH.

Thus according to the present invention there is provided a method for inhibiting scale formation in waters having a high barium content and low pH which method comprises adding an aqueous solution of a compound of formula:

wherein R = H, CH3, C2H5, M R1 = H, CH3,CR3, C6H5, SO3H2 M = alkali metal or ammonium ion to an aqueous medium containing potential scale forming constituents.

Preferred compounds are those wherein R, R1 = H.

By high barium content we mean a content in the range 700 3000 ppm of the original formation water.

By low pH we mean a pH in the range 4 - 6.

The scale inhibitors according to the present invention may be suitably employed in either partially or fully neutralised form.

The neutralised scale inhibitors may be in the form of the alkali metal salt or ammonium salt, preferably in the form of the sodium salt.

The scale inhibitors of the present invention may be used either downhole as part of a squeeze technique or they may be injected on an offshore platform.

The scale inhibitors are suitably employed in concentrations in the range 5 - 200 ppm or in aqueous solutions with concentrations in the range 10 - 50X by weight.

The scale inhibitors of the present invention remain active and are thermally stable.

Scale inhibitors according to the present invention may be prepared by conventional methods. For example, by treatment of the appropriate polyamino compound with formaldehyde and phosphorous acid.

Suitable scale inhibitors include the compound of the following formula:

The potential scale inhibiting efficiency of this compound was evaluated using the static precipitation test against differing formation water/sea water ratios, pH conditions and concentrations.

The technique used was as follows.

For the purposes of the tests two brine mixing ratios were used for evaluating the scale inhibitor performance, these being 40:60 and 80:20 (formation water (FW): sea water (SW)). The formation water was prepared synthetically and chosen to represent formation water from the Miller Field in the North Sea which is known to have a high barium content and a low pH.

A synthetic formation water was prepared having an ion composition as follows achieved using the salt concentrations shown: Salt Composition Ion Composition (gm/l) (mg/l) CaCl2.6H2O 5.794 Ca 1060 MgCl2.6H2O 0.961 Mg 115 KC1 3.489 K 1830 SrCl2.6H20 0.335 Sr 110 BaCl2.2H2O 1.878 Ba 1050 NaCl 71.182 Na 28780 Cl 47680 Before the test, both the sea and the formation waters were filtered to at least 0.45 pm and the barium, calcium, strontium and sulphate levels confirmed prior to use.

An aliquot, either 40 or 80 mls depending on the ratio required, of the formation water was dispensed into a powder bottle.

The potential scale inhibitor compounds were used as standard solutions in distilled water. To each formation water aliquot an aliquot of scale inhibitor solution was added sufficient to give the required final inhibitor concentration. This was 1 ml or 2 ml respectively for a standard 1% w/v inhibitor solution in a final solution volume of 100 ml to give 100 or 200 mg/l respectively.

In order to evaluate the inhibitor performance at varying pH values a series of buffer solutions were prepared using mixtures of sodium acetate and acetic acid. Three solutions were prepared as follows: pH 4.5 13.6g sodium acetate trihydrate and 6g acetic acid pH 5.2 13.6g sodium acetate trihydrate and l.8g acetic acid pH 6.0 13.6g sodium acetate trihydrate and 0.535g acetic acid.

A 1 ml aliquot of the appropriate buffer solution was added to each powder bottle containing formation water.

The required aliquot of sea water was dispersed into a separate powder bottle. Distilled water may be added to each bottle to ensure a constant final mixture volume.

Each bottle was then capped tightly and both the formation and sea water bottles placed in an oven at 950C for approximately 45 minutes to enable the fluids to reach temperatue.

The bottles were removed from the oven and the sea water poured quickly into each of the bottles containing formation water which were recapped, shaken and returned to the oven.

The resultant brine mixtures were sampled after a two hour interval by removing each bottle from the oven and immediately withdrawing a sample by means of a disposable syringe. This was filtered (0.3 pm) and the filtrate analysed for scaling barium cation concentrations using standard techniques.

The results are presented in the Table. Inhibitor efficiency is expressed in terms of the concentration of scaling ions remaining in solution after the test compared to a blank with no inhibitor added.

TABLE

INHIBITOR EFFICIENCY CONC FW:SW FW:SW mg/l 80:20 40:60 pH 4.5 5.2 6.0 4.5 5.2 6.0 100 4 95 80 0 2 15 200 46 97 93 0 5 20

Claims (6)

1. Claims: 1. A method for inhibiting scale formation in waters having a high barium content and low pH which method comprises adding an aqueous solution of a compound of formula: wherein

   R = H, CH3, C2H5, M R1 = H, CH3,CR3, C6H5. SO3H2 M = alkali metal or ammonium ion to an aqueous medium containing potential scale forming constituents.
2. 2. A method according to claim 1 wherein R and R1 are hydrogen.
3. 3. A method according to claim 1 wherein the scale inhibitor compound is present in the form of the partial or fully neutralised salt.
4. 4. A method according to claim 3 wherein the salt is the sodium salt.
5. 5. A method according to any of the preceding claims wherein the scale inhibitor is present in a concentration in the range 5-200 mg/l in the aqueous solution.
6. 6. A method according to any of claims 1 to 4 wherein the scale inhibitor is present in a concentration in the range 10-50% by weight in the aqueous solution.