WO2007051961A1

WO2007051961A1 - Nitrate test method

Info

Publication number: WO2007051961A1
Application number: PCT/GB2006/003577
Authority: WO
Inventors: David Ramsay Thrasher; Ian Vance
Original assignee: Bp Exploration Operating Company Limited
Priority date: 2005-10-31
Filing date: 2006-09-26
Publication date: 2007-05-10
Also published as: GB0522201D0

Abstract

A method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising: (a) taking a sample of the water that is to be treated with nitrate; (b) adding a water soluble nitrate compound into the water sample; (c) adding a water soluble iron compound to the water sample either before or after incubating the water sample for a predetermined period of time at a selected temperature and pressure under anaerobic conditions wherein the amount of water soluble iron compound that is added to the water sample is sufficient to react with any hydrogen sulfide that is already dissolved in the water sample or with any hydrogen sulfide produced during the incubation period; and (d) determining the level of iron sulfide precipitation in the incubated water sample.

Description

NITRATE TEST METHOD

The present invention relates to a method for determining an effective concentration of nitrate in an injection water or a produced water for reducing the production of hydrogen sulfide by sulfate-reducing bacteria (SRJB).

Many oil reservoirs have turned sour owing to the microbial production of hydrogen sulfide. The hydrogen sulfide is produced by SRB which convert sulfate (for example, from injected seawater) into hydrogen sulfide. These bacteria arise during the drilling for oil, when producing oil, and may also be indigenously present in the reservoir and in water injected into the reservoir. The production of hydrogen sulfide has health and safety implications. For example, the presence of dissolved hydrogen sulfide in produced fluids, in particular, produced water, may cause corrosion of metal components of downhole and/or topside equipment (in a production facility), in particular, corrosion of casing, tubing and valves. This corrosion, if left unchecked, may result in stress cracking of the metal components.

A method of inhibiting hydrogen sulfide production is to continuously add nitrate (for example, calcium nitrate or sodium nitrate) to the injection water. The nitrate will encourage the growth of nitrate reducing bacteria (NRB), nitrate reducing and sulfide oxidizing bacteria (NRSOB) and denitrifying bacteria (DNB) that are also naturally present in the injected water. It is believed that the presence of an effective amount of nitrate in the injection water results in NRB, NRSOB and DNB out-competing the SRB for assimilable carbon and electron donors thereby suppressing the production of hydrogen sulfide. In addition, at least some of the SRB may have the ability to switch to using nitrate as an electron acceptor. Furthermore, the increase in the population of NRSOB may result in any sulfide that is produced by the SRB being reoxidised to either sulfate or elemental sulfur. In addition, nitrate may be added to produced fluids in a flowline or into a vessel of a production facility, for example, an oil- water separator vessel.

US 2004/0126836 relates to methods for rapidly detecting and quantifying sulfide- producing bacteria, in a sample of a food product comprising meat, dairy products or fish. The food sample is combined with a quantity of growth medium. The growth medium preferably comprises an iron compound and organic and/or inorganic sulphur compounds and forms an iron precipitate when exposed to the sulfide producing bacteria (SRB). The growth medium and sample forms an incubation mixture. The number of SRB in the i sample is determined by using a visual detection method to identify a colour change, or by using fluorescence detection methods which detects trends in fluorescence production of the incubation mixtures which are correlated with SRB numbers.

Techniques currently employed in the oil industry for determining an effective dose of nitrate in injection water involve use of side-stream devices for examining biofilm growth and microcosm tests run in the laboratory, in addition to monitoring of hydrogen sulfide production in production wells.

Side-stream devices are operated at the site of water injection and comprise multiple removable test surfaces which may be analysed for the composition of sessile microbiological populations which grow upon the removable surfaces. Injection water that is dosed with nitrate is run through the side-stream device and changes in microbial populations are monitored and compared to the population in a replicate control device to which nitrate has not been added. Alternatively, the microbiological population may be compared with the population in the same device prior to dosing the injection water with nitrate. Thus, the side stream device may have previously been operated using injection water in the absence of added nitrate. An effective dose of nitrate is that which causes a shift in microbiological population from being dominated by SRB to one which is dominated by NRB, DNB or NRSOB.

Microcosm tests are carried out in a laboratory under conditions which simulate those in a water injection system. In one embodiment of such tests, bottles of sulfate-rich injection water are incubated under anaerobic conditions with various concentrations of nitrate. The activity of SRB is monitored by repeated chemical testing for the concentration of hydrogen sulfide (H₂S). As discussed above, the reduction of sulfate by SRB causes a rise in H₂S concentration. An effective dose of nitrate is one that attenuates the increase OfH₂S concentration, relative to that in control tests (where nitrate has not been added to the bottles of sulfate-rich injection water prior to incubation). Preferably, the dose of nitrate will completely inhibit the activity of SRB and hence the increase in H₂S concentration. In another embodiment of these tests a sand-packed column is flooded with a constant supply of sulfate-rich injection water and chemical testing of the effluent is carried out to monitor the concentration OfH₂S that is formed owing to the activity of SRB. When a steady state of H₂S production has been reached, nitrate is added at known concentrations to the injection water and the effect on H₂S production is observed. An effective dose of nitrate is one that inhibits the formation of H₂S in the sand-packed column.

However, there remains the need for more frequent tests to ensure that, as conditions change in the field due to changing fluid compositions or changes in production chemical types and doses, an effective nitrate dose can be maintained. There also remains a need for a test that can be carried out under field conditions.

It has now been found that the effective amount of nitrate for inhibiting the production of hydrogen sulfide by SRB in a hydrocarbon-bearing formation, a production well, a flowline or a production facility can be determined by adding a water-soluble nitrate compound to a sample of the water that is to be treated with nitrate and incubating the sample for a predetermined incubation period at a selected incubation temperature and pressure (where the sample is incubated either before or after the addition of a water- soluble iron compound) and thereafter detecting the level of iron sulfide precipitation.

Thus, the present invention relates to a method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising: '

(a) taking a sample of the water that is to be treated with nitrate;

(b) adding a water soluble nitrate compound to the water sample;

(c) adding a water soluble iron compound to the water sample either before or after incubating the water sample for a predetermined period of time at a selected temperature and pressure under anaerobic conditions wherein the amount of water soluble iron compound that is added to the water sample is sufficient to react with any hydrogen sulfide that is already dissolved in the water sample and with any hydrogen sulfide produced during the incubation period; and

(d) determining the level of iron sulfide precipitation in the incubated water sample.

With the present invention it is possible to detect very low concentrations of hydrogen sulfide produced by SRB upon incubation of the water sample. Thus, growth of SRB visibly alters the water sample owing to precipitation of iron sulfide (FeS) which causes the sample to become grey and eventually black (opaque). The iron sulfide arises from reaction of hydrogen sulfide (produced by any SRB that are present in the water sample) with the water soluble iron compound. The activity of SRB and hence the amount of produced hydrogen sulfide is related to the concentration of iron sulfide. It is envisaged that the presence of iron sulfide precipitate in the incubated water sample may be determined visually (using changes in visible light). For example, the colour of the sample may be matched against a colour detection chart to make a semiquantitative determination of the production of iron sulfide in the water sample. Alternatively, the presence of iron sulfide precipitate may be determined using a photometric device, with the level of iron sulfide precipitate being expressed by the absorption of light of a certain wavelength or by scattering of light of a certain wavelength. The level of iron sulfide precipitation may also be determined by particle counting techniques, for example, by counting iron sulfide particles under an optical microscope or by using an electronic particle sizing and counting device.

An advantage of the present invention is that the method for determining the effective dose of nitrate is simple and lends itself to frequent application thereby allowing the effective amount of nitrate that is dosed into the sampled water to be adjusted with a minimum of delay in response to changes in the operating environment. Also, the frequency of the sampling of the water can be adjusted in response to changes in the operating environment.

The water that is sampled may be water that is to be injected into a hydrocarbon bearing formation via an injection well for enhanced recovery of hydrocarbons from an associated production well. Typically, the injection water is seawater, a produced water, river water; lake water or aquifer water or a mixture thereof. Typically, one or more production chemicals are added to the injection water in order to treat the water and/or the hydrocarbons. For example, a corrosion inhibitor may be added to the injection water for corrosion control in the injection well and also in the production well. Where the injection water comprises produced water, the injection water may have a dissolved organics concentration of up to 10000 ppm (with the vast majority of the dissolved organics arising from the produced water). As discussed above, dosing of an effective amount of a water soluble nitrate compound into the injection water controls the production of hydrogen sulfide in the hydrocarbon bearing formation. Preferably, the amount of the water soluble nitrate compound that is dosed into the injection water is also sufficient to control the production of hydrogen sulfide in the production well. It is also envisaged that nitrate may be dosed into a flowline that carries produced fluids from a production well to a production facility or into a vessel of a production facility for controlling the production of hydrogen sulfide. Thus, the water that is sampled may be a water from a flowline or water from a vessel of a production facility, for example, from an oil- water separator vessel.

The water sample may be incubated at any suitable temperature, for example, at a temperature within the range of 5°C±0.5°C to 120°C±0.5°C. High incubation temperatures may be employed as SRB are capable of surviving under the elevated temperature conditions found in the hydrocarbon-bearing formation, for example, at temperatures of over 75°C. Preferably, the sample is incubated at the temperature of the water at the point of sampling (for example, the temperature of the flowline or vessel) or, in the case of an injection water, at the temperature prevailing in the hydrocarbon bearing formation. The incubation temperature may be optimised in development tests prior to initiating the method of the present invention.

The water sample may be incubated at atmospheric pressure. However, it is also envisaged that the sample may be incubated at elevated pressure, for example, at the pressure of the flowline or vessel or, in the case of injection water, at the pressure of the hydrocarbon bearing formation. The incubation pressure may be optimised in development tests prior to initiating the method of the present invention.

The optimal incubation period may be determined in development tests prior to initiating the method of the present invention. Suitably, the predetermined incubation period is at least sufficient to detect a colour change in a control sample that contains no added nitrate. Preferably, the predetermined incubation period is at least 1 day, more preferably at least 5 days, most preferably, at least 10 days, for example, at least 20 days.

The samples are incubated under anaerobic conditions. This may be achieved by ensuring there is no air space above the water samples when they are placed in a sample bottle, vial or other suitable container.

Suitably, the water sample has a volume of at least 15ml, preferably, at least 25 ml so as to ensure that the water sample contains a sufficient population of bacteria.

Preferably, the water soluble nitrate compound is selected from the group consisting of sodium nitrate, potassium nitrate and calcium nitrate. Preferably, the water soluble nitrate compound is the same as the nitrate compound that is added to the sampled water for reducing the production of hydrogen sulfide by SRB. Sodium nitrate is preferred where there is an increased risk of calcium related scale in the formation, the production well or downstream thereof. Ammonium nitrate is to be avoided owing to explosive hazards.

Preferably, the water sample is contained in a sample bottle, vial or other suitable container that is formed from any material that is compatible with the water soluble nitrate compound. The person skilled in the art would be able to readily select a suitable material. For example, aqueous calcium nitrate solution is compatible with polymers such as polyethylene (PE), polypropylene (PP), polysulfone (PSO), polytetrafluoroethylene (PTFE), fluorocarbon rubber (Viton®), polyvinylchloride (PVC), neoprene, nitrile (NBR), ethylenepropylenediene polymer (EPDM), natural rubber, 316L stainless steel, and fibreglass but is incompatible with 304L stainless steel, mild steel, aluminium and brass.

The desired amount of the water soluble nitrate compound may be added to the water sample in the form of an aqueous nitrate solution or in solid form (with the nitrate compound subsequently dissolving in the water sample). However, it is also envisaged that the desired mass of a water soluble nitrate compound may be added to a sample bottle, vial or other suitable container prior to addition of the water sample. For example, the desired mass of the water soluble nitrate compound may be added in the form of an aqueous solution and is then fixed to the inner walls of the sample bottle, vial or container by evaporation of the water solvent, for example, by drying in air. Preferably, the bottle, vial or container that has the nitrate compound fixed to the inner walls thereof is filled to at or near capacity with the water sample. The bottle, vial or container is then sealed in order to maintain anaerobic conditions. Alternatively, the desired mass of water soluble nitrate compound may be added in solid form to the sample bottle, vial or other suitable container. The nitrate compound will then dissolve in the water sample when this is added to the bottle, vial or other suitable container under field conditions.

Suitably, the water soluble iron compound is a water soluble ferric, Fe³⁺, compound or a water soluble ferrous, Fe²⁺, compound that is capable of being reduced by reaction with hydrogen sulfide thereby generating iron sulfide (FeS) with the proviso that where the iron compound is added prior to the incubation period it does not provide assimilable carbon and is not a nutrient for the bacteria contained in the water sample. Preferably, the water soluble iron compound is ferrous sulfate, ferric chloride or ferrous chloride. Thus, ferric chloride (FeCl₃) and ferrous chloride (FeCl₂) react with hydrogen sulfide to generate iron sulfide as follows:

2FeCl₃ +3H₂S → S + 2FeS + 6HCl FeCl₂ + H₂S → FeS + 2HCl.

The water soluble iron compound may be added to the water sample(s) at a concentration sufficient to result in darkening of a sample having 1 to 2 mg/litre of free sulfide. Preferably, the water soluble iron compound is added to the water sample to give a concentration in the range 5 to 100 mg/1 iron preferably, at a concentration in the range 50 to 100 mg/1 iron.

The water soluble iron compound may be added to the water sample in the form of an aqueous solution of the water soluble iron compound or in solid form (with the iron compound subsequently dissolving in the water sample). Where the iron compound is added to the water sample prior to incubating the water sample, it is envisaged that the desired mass of the water soluble iron compound may be added to the sample bottle, vial or other suitable container prior to addition of the water sample. For example, the desired mass of iron compound may be added in the form of an aqueous solution and is then fixed to the inner walls of the sample bottle, vial or other suitable container by evaporation of the water solvent, for example, by drying in air. Preferably, the bottle, vial or other suitable container that has the iron compound fixed to the inner walls thereof is filled to at or near capacity with the water sample. The bottle, vial or container is then sealed in order to maintain anaerobic conditions. Alternatively, the desired mass of iron compound may be added in solid form to the sample bottle, vial or other suitable container. The iron compound will then dissolve in the water sample when this is added to the bottle, vial or other suitable container under field conditions. Preferably, the bottle, vial or other suitable container also contains the desired amount of nitrate compound (as described above).

Where the water soluble iron compound is added after the predetermined incubation period, it is preferred to leave the water sample for at least 2 hours, preferably, at least 6 hours before determining the level of iron sulfide precipitation thereby ensuring that the formation of any iron sulfide precipitate proceeds to completion (hereinafter "colour development period"). Preferably, the water sample is left at room temperature during the colour development period.

Where the water-soluble iron compound is added to the water sample prior to incubating the water sample, this allows the level of iron sulfide precipitation in the water sample to be determined before, during and at the end of the predetermined incubation period. Thus, the sample may be analysed to determine whether there is any darkening of the grey/black colour with time owing to an increase in the level of iron sulfide precipitate, or whether the amount of iron sulfide precipitate remains the same with time or decreases with time. It is also envisaged that the sample may be initially transparent in which case the sample is analysed to determine whether any grey/black colour develops with time. Thus, according to a first preferred embodiment of the present invention there is provided a method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising:

(a) taking a sample of the water that is to be treated with nitrate;

(b) adding a water soluble iron compound to the water sample in an amount sufficient to react with any hydrogen sulfide that is already dissolved in the water sample and with any subsequently produced hydrogen sulfide;

(c) adding a water soluble nitrate compound to the water sample;

(d) determining the level of iron sulfide precipitation in the water sample;

(e) incubating the water sample for a predetermined period of time under anaerobic conditions at a selected temperature and pressure; and

(f)

the level of iron sulfide precipitation has increased, decreased or remained unchanged upon incubation of the water sample.

As would be apparent to the person skilled in the art, the order of addition of the water soluble iron compound and the water soluble nitrate compound may be reversed or the water soluble iron compound and the water soluble nitrate compound may be added simultaneously to the water sample.

Where there is an increase in the level of iron sulfide precipitation upon incubation of the sample, the nitrate dose is insufficient for controlling the production of hydrogen sulfide in the sampled water. Where there is a decrease in the level of iron sulfide precipitation upon incubation of the sample, the nitrate dose is higher than required for controlling the production of hydrogen sulfide. Where there is substantially no change in the level of iron sulfide precipitation, the nitrate dose is sufficient for controlling the production of hydrogen sulfide. Thus, an effective amount of nitrate for dosing into the sampled water is a concentration where the sample is either still transparent after the incubation period or where there is no further darkening of colour after this incubation period.

Preferably, the colour of the incubated sample is assessed at intervals during the incubation period to determine whether there is any change in colour from transparent to grey/black or whether any darkening of the grey/black colour is seen. Typically, the colour of the sample may be assessed at time intervals of from 1 to 5 days (where the predetermined incubation period is in the range of 5 to 20 days). The results obtained are compared with the result at time zero (before incubation of the sample). Preferably, the level of iron sulfide precipitation in the water sample is also compared against a control sample that contains the water soluble iron compound but no added water soluble nitrate compound.

Preferably, the method of the present invention may be carried out by taking a plurality of water samples and dosing the samples with differing amounts of the water soluble nitrate compound. Suitably, at least 2, preferably, at least 3, for example, 2-10, preferably 4-10 water samples are dosed with differing amounts of the water soluble nitrate compound. Preferably, the water samples are dosed with the same amount of water soluble iron compound.

Thus, according to a second preferred embodiment of the present invention there is provided a method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising:

(a) taking a plurality of samples of the water that is to be treated with nitrate;

(b) adding different amounts of a water soluble nitrate compound to each of the plurality of water samples;

(c) adding a water soluble iron compound to each of the plurality of water samples either before or after incubating the water samples for a predetermined period of time at a selected temperature and pressure under anaerobic conditions wherein the amount of water soluble iron compound that is added to each of the plurality of water samples is sufficient to react with any hydrogen sulfide that is already dissolved in the water sample and with any hydrogen sulfide produced during the incubation period; and

(d) determining the level of any iron sulfide precipitation in each of the plurality of incubated water samples.

The assessment of the amount of iron sulfide precipitation in the plurality of water samples detects trends in hydrogen sulfide production which are correlated with inhibition of growth of SRB in the water samples by the addition of the water soluble nitrate compound. In particular, the assessment of the amount of iron sulfide precipitation in a plurality of water samples containing differing^' amounts of water soluble nitrate compound allows the minimum effective nitrate dose that is required to control hydrogen sulfide production in the sampled water to be readily determined.

Preferably, the level of any iron sulfide precipitation in each of the plurality of incubated water samples is compared with a control water sample which has been incubated under the same conditions as in step (c) but in the absence of added water soluble nitrate compound. Where the level of iron sulfide precipitation in a water sample is higher than in the control, the nitrate dose used in the sample is insufficient for controlling production of hydrogen sulfide in the sample water. Where there is a decrease in the level of iron sulfide precipitation compared with the control, the nitrate dose is higher than required for controlling the production of hydrogen sulfide. Where there is substantially no change in the level of iron sulfide precipitation compared with the control, the nitrate dose is sufficient for controlling the production of hydrogen sulfide. Thus, an effective amount of nitrate for dosing into the sampled water is a concentration where the sample is either still transparent after the incubation period or where the level of iron sulfide precipitation is the same as for the control.

Preferably, the water soluble iron compound is added after incubating the plurality of water samples so as to avoid the risk of addition of unwanted bacterial nutrients or assimilable carbon with the water soluble iron compound. ^■ ,

As would be evident to the person skilled in the art, a single sample of the water that is to be dosed with the water soluble nitrate compound may be taken and aliquots of this water sample may be removed to provide the plurality of water samples and the control water sample.

Where the water that is sampled is aproduced water (for example, from a flowline or a production facility), nitrate is generally added to the produced water at a concentration of at least 75 mg/litre. Thus, it is preferred to add the water soluble nitrate compound to the plurality of samples of the produced water at concentrations in the range 70 to 150 mg/litre using a spread of nitrate concentrations.

Where the water that is sampled is seawater, nitrate is generally added to seawater at a concentration of at least 15 mg/litre, for example 15 to 20 mg/litre. Thus, it is preferred to add the water soluble nitrate compound to the plurality of samples of the seawater at concentrations in the range 10 to 50 mg/litre using a spread of nitrate concentrations.

In a further aspect of the present invention, it is envisaged that following development of the iron sulfate precipitate, hydrogen sulfide maybe produced from the iron sulfide precipitate by adding an acid to the water sample. A test paper coated with an agent that is sensitive to hydrogen sulfide may be exposed to the vapour phase above the water sample and an assessment of the activity of SRB is obtained by observing the extent of the colour change of the test paper upon exposure to the produced hydrogen sulfide. Suitably, the acid that is added to the sample may be selected from hydrochloric acid, sulphuric acid and nitric acid. Preferably, the addition of the acid lowers the pH of the water sample to a value of less than or equal to 2, more preferably, less than or equal to 1. Preferably, the test paper is coated with a metal acetate or rrietal silicate that changes colour upon reaction with hydrogen sulfide. Preferred metal acetates or metal silicates include lead acetate, iron acetate, copper acetate, nickel acetate, tin acetate, lead silicate, iron silicate, copper silicate, nickel silicate or tin silicate, particularly, lead acetate. It is also envisaged that a sub-samples may be taken from the water sample during the incubation period to detect hydrogen sulfide that is evolved upon addition of the acid. However, care must be taken to avoid introducing air to the water sample.

The present invention will now be illustrated by reference to the following Examples and to Figures 1 to 3. Examples

Typical Test Procedure The test procedure is as follows:

1. The water that is to be treated with nitrate is sampled. This water may be produced water taken from a flow line, storage container or process vessel or the water may be injection water. If applicable, a secondary water source may be mixed with the water sample at a desired ratio to simulate anticipated operational changes.

2. A predetermined amount of ferrous chloride (Fe(II) chloride) or ferrous sulfate (Fe(II) sulfate) is added to the water sample which is shaken well to ensure thorough mixing. The amount of ferrous chloride or ferrous sulfate is determined from development tests to give a concentration of free iron (II) ions in solution sufficient to give an observable degree of blackening with l-2mg/l of free sulfide. The desired mass of ferrous chloride or ferrous sulfate may be added in aqueous solution and fixed to the inside of a suitable clear glass bottle by air drying. When the water sample is subsequently added to the bottle, the ferrous ions will dissolve in the water to give the desired concentration of free ferrous ions. It is preferred that the bottle is sized such that it is filled to capacity with the water sample.

3. Sub-samples of the water from step 2 (containing free iron(II) ions) are added to 3 or more bottles, depending on the desired number of nitrate doses that are to be tested.

4. Different doses of nitrate are added to the sub-samples, with at least one bottle having zero nitrate as a control. The desired mass of nitrate may be added in aqueous solution and fixed to the inside of a suitable clear glass bottle by air drying. When the sub- sample is added to the bottle, the nitrate ions will dissolve in the water to give the desired concentration of free nitrate ions. It is preferred that the bottle is sized such that it is filled to capacity with the sub-sample to be tested.

5. The degree of colour (blackness) present in each sample at this time is determined (time zero). The degree of colour is determined using at least one of the following techniques: by eye (e.g. by comparing the colour against colour charts); by using a photometric device (e.g. adsorption of light of a certain wavelength or scattering of light of a particular wavelength); or by particle counting techniques (e.g. under an optical microscope).

6. The samples are then incubated at a temperature corresponding to the temperature conditions encountered in a flowline or vessel. Alternatively, the samples may be incubated at the temperature encountered in the subterranean formation. Incubation of the samples may be carried out at atmospheric pressure, but could also be done at elevated pressure in completely full serum vials fitted with a flexible rubber septum to transmit pressure to the contents, as described in : Vance, I. & Hunt, RJ. Technical Note; Modification of a French Press for the Incubation of Bacteria at Elevated Pressures and Temperatures. J. Appl. Bact. 58, pp 525-528, 1985, which is herein incorporated by reference.

7. An assessment is then made of whether the grey/black coloration due to the presence of iron sulfide solids increases, stays the same, or decreases after the incubation period (the incubation period and the degree of darkening is determined in development tests). As would be evident to the person skilled in the art, step 2 may be omitted and the ferrous chloride or ferrous sulfate may be added directly to the sub-samples, before or after addition of the nitrate (or simultaneously with the nitrate). For example, the desired mass of nitrate and the desired mass of ferrous chloride or ferrous sulfate may both be fixed to the walls of the sub-sample bottles. Alternatively, the ferrous chloride or ferrous sulfate and the nitrate may be added in solid form to the sub-samples. The ferrous sulfate may be added after an appropriate incubation period that promotes the development of SRB activity.

Field Test for Effective Nitrate Dose in Produced Water Test Procedure

Produced water from a North Sea reservoir and seawater were commingled at a volumetric ratio of 95:5. Aliquots (100 ml) were incubated in clear glass serum vials fitted with butyl rubber septa, at a temperature of 35°C for 29 days in the presence of different doses of nitrate (60, 80 and 120 mg/1 nitrate) in the form of calcium nitrate (45% w/w aqueous solution).

The septa were removed briefly to allow the addition of a tablet of ferrous sulfate and were then replaced to prevent the loss OfH₂S. The tablets were a commercially available health product comprising ferrous sulfate (14 mg of Fe), ascorbic acid (60 mg), dicalcium phosphate, microcrystalline cellulose, sodium carboxymethyl cellulose, stearic acid, magnesium stearate and hydroxypropyl methylcellulose. The abscorbic acid acts as an antioxidant although the presence of an antioxidant is not an essential feature of the present invention. The bottles were left at room temperature overnight to promote colour development. In addition, the optical density of duplicate bottles was determined in a spectrophotometer at a wavelength of 670 nm. Visual Appearance of Bottles

The appearance of the bottles before the addition of the ferrous sulfate tablets (at the end of the incubation period), and 30 minutes and 24 hours after the addition of the tablets is shown in Figure 1. The water in the two bottles at the front, left side of each frame of Figure 1 was not dosed with nitrate. The two bottles on the front, right side were dosed with 60mg/l nitrate, those on the back, left side were treated with 80 mg/1 nitrate and those on the back, right side were dosed with 120 mg/1 nitrate.

Within 30 minutes of adding the ferrous sulfate tablets, the water in the bottles dosed with 60 or 80 mg/1 nitrate started to turn dark grey/black. After 24 hours, all of the water in the bottles dosed with 60 or 80 mg/1 nitrate had turned intensely black and opaque. In contrast, the water in the bottles dosed with 120 mg/1 nitrate had turned an orange/brown colour.

Optical Density Measurements of Duplicate Bottles

The optical density, H₂S concentration and residual nitrate concentration of the duplicate samples of commingled produced water and seawater, 24 hours after addition of the ferrous sulfate tables are given in Table 1 below. The H₂S and residual nitrate concentrations were determined using standard chemical quantitative analytical techniques. Table 1 Duplicate bottle test results.

Figure 2 illustrates, in graphical format, the' arithmetic mean (closed circles) and range (open circles) of optical density measurements of the duplicate samples of commingled produced water and seawater, 24 hours after the addition of ferrous sulfate tablets.

Figure 3 shows a significant correlation (p<0.01) between the optical density measurements and H₂S concentrations in the duplicate samples of commingled produced water and seawater 24 hours after the addition of ferrous sulfate tablets.

The results of this field trial show that the addition of ferrous sulfate in tablet form is a simple and effective method for determining the extent of activity of sulfate-reducing bacteria (SRB) in waters dosed with nitrate.

Claims

1. A method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising:

(a) taking a sample of the water that is to be treated with nitrate;

(b) adding a water soluble nitrate compound to the water sample;

2. A method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising:

(a) taking a sample of the water that is to be treated with nitrate;

(c) adding a water soluble nitrate compound to the water sample;

(d) determining the level of iron sulfide precipitation in the water sample;

(e) incubating the water sample for a predetermined period of time at a selected temperature and pressure under anaerobic conditions; and

(f) determining whether the level of iron sulfide precipitation has increased, decreased or remained unchanged upon incubation of the water sample.

3. A method of determining an effective amount of nitrate for dosing into water for controlling hydrogen sulfide production, the method comprising:

(c) adding a water soluble iron compound to each of the plurality of water samples either before or after incubating the water samples for a predetermined period of time at a selected temperature and pressure under anaerobic conditions wherein the amount of water soluble iron compound that is added to each of the plurality of water samples is sufficient to react with any hydrogen sulfide that is already dissolved in the water sample and with any hydrogen sulfide produced during the incubation period; and (d) determining the level of any iron sulfide precipitation in each of the plurality of incubated water samples.

4. A method as claimed in Claim 3 wherein the amount of nitrate that is dosed into the water for controlling the production of hydrogen sulfide is adjusted in response to an increase or decrease¹ in the level of iron sulfide precipitation in the incubated water samples by comparing the level of any iron sulfide precipitation in each of the plurality of incubated water samples with a control water sample which has been incubated under the same conditions as in step (c) but in the absence of any added water soluble nitrate compound.

5. A method as claimed in any one of the preceding claims wherein the level of iron sulfide precipitation in the incubated water sample(s) is determined visually by (a) matching the colour of the sample(s) against a colour detection chart to make a semiquantitative determination of the level of iron sulfide precipitation in the water sample; (b) using a photometric device wherein the level of iron sulfide precipitation is expressed as an absorption of light at a certain wavelength or by scattering of light of a certain wavelength; or (c) by counting iron sulfide particles under an optical microscope or using an electronic particle sizing and counting device.

6. A method as claimed in any one of the preceding claims wherein the water sample(s) is taken from water that is to be injected into a hydrocarbon bearing formation via an injection well for enhanced recovery of hydrocarbons from an associated production well.

7. A method as claimed in any one of Claims 1 to 5 wherein the water sample(s) is taken from a flowline that carries produced fluids from a production well to a production facility or is taken from a vessel of a production facility.

8. A method as claimed in any one of the preceding claims wherein the predetermined incubation period is determined in development tests.

9. A method as claimed in any one of the preceding claims wherein the incubation period is at least 1 day.

10. A method as claimed in any one of the preceding claims wherein at the end of the incubation period, hydrogen sulfide is produced from the iron sulfide precipitate by adding an acid to the incubated water sample(s) and a test paper coated with an agent that is sensitive to hydrogen sulfide is exposed to the vapour phase above the incubated water sample(s) and an assessment of the activity of the sulfate reducing bacteria (SRB) is obtained by observing the extent of the colour change of the test paper upon exposure to the produced hydrogen sulfide.

11. A method as claimed in Claim 10 wherein the test paper is coated with a metal acetate or metal silicate that changes colour upon reaction with hydrogen sulfide.

12. A method as claimed in any one of the preceding claims wherein the water sample(s) is incubated at a temperature within the range of 5°C±0.5°C to 120°C±0.5°C.

13. A method as claimed in Claim 12 wherein the water sample(s) is taken from a flowline or vessel and is incubated at the temperature of the flowline or vessel at the point of sampling.

14. A method as claimed in Claim 12 wherein the water sample(s) is taken from an injection water and is incubated at the temperature prevailing in the hydrocarbon bearing formation into which the water is to be injected.

15. A method as claimed in any one of Claims 3 to 14 wherein the plurality of water samples are dosed with the same amount of water soluble iron compound.

16. A method as claimed in any one of the preceding claims wherein the iron compound is a water soluble ferric, Fe³⁺, compound or a water soluble ferrous, Fe²⁺, compound that is capable of being reduced by reaction with hydrogen sulfide thereby generating iron sulfide (FeS) with the proviso that where the iron compound is added to the water sample(s) prior to incubation of the sample(s), the iron compound does not provide assimilable carbon and does not act as a nutrient for the bacteria contained in the water sample.

17. A method as claimed in Claim 16 wherein the water soluble iron compound is selected from the group consisting of ferric chloride, ferric sulfate, ferrous chloride and ferrous sulfate.

18. A method as claimed in any one of the preceding claims wherein the water soluble iron compound is added to the water sample(s) at a concentration in the range 5 to 100 mg/1 iron.

19. A method as claimed in any one of the preceding claims wherein the water soluble iron compound is added to the water sample(s) in the form of an aqueous solution of the water soluble iron compound or in the form of a solid.

20. A method as claimed in any one of the preceding claims wherein the water soluble nitrate compound is selected from the group consisting of sodium nitrate, potassium nitrate and calcium nitrate.

21. A method as claimed in any one of the preceding claims wherein the desired amount of the water soluble nitrate compound is added in the form of an aqueous nitrate solution or in the form of a solid.

22. A method as claimed in any one of claims 3 to 21 wherein the water that is sampled is a produced water and the water soluble nitrate compound is added to a plurality of samples of the produced water at concentrations in the range 70 to 150 mg/litre using a spread of nitrate concentrations.

23. A method as claimed in any one of claims 3 to 22 wherein the water that is sampled is seawater and the water soluble nitrate compound is added to the plurality of samples of the seawater at concentrations in the range 10 to 50 mg/litre using a spread of nitrate concentrations.