NO146642B - PROCEDURE AND METHOD OF INACTIVATING IONIZABLE SULFIDE IN A BROWN DRILL - Google Patents

Description

The present invention relates to the inactivation of hydrogen sulphide or soluble sulphide ion contaminants which frequently occur in wells that penetrate underground formations, such as oil wells, gas wells and the like. Removal or inactivation of sulphide ions is necessary to prevent poisoning of personnel, contamination of the area and extensive corrosion of steel pipes and tools used in the well.

A method for inactivating hydrogen sulphide in aqueous drilling fluids is described in US patent document no. 3 928 211. In this method certain metal compounds are added to the drilling fluid which form insoluble sulphide compounds. Zinc carbonate, basic zinc carbonate and zinc hydroxide are preferably used as such metal compounds. Alternatively, the zinc compounds can be added to the drilling fluid in the form of complexes with known organic dispersants, such as quebracho, lignosulfonates and lignites. These dispersants are used to keep the described zinc compounds colloidally dispersed in the drilling fluid.

From US patent no. 3 107 739 it is known to add certain zinc chelate compounds to drilling fluids to prevent hydratable clay materials and clay-like materials in a well from being hydrated and thereby swelling and cracking, so that the well walls can erupt. However, there is no question here of rendering sulphides harmless, and furthermore, carrying out the method according to the aforementioned US patent involves changes in the rheological properties of the drilling fluid as a result of the additions that are made.

Now it is important to be able to maintain the rheological properties of a drilling fluid, while additions are made to achieve one effect or another. According to the invention, it has now been shown to be possible to neutralize sulphides in a well drilling fluid by adding to the drilling fluid a specific, not previously used, chelate, while maintaining the desired rheological properties of the drilling fluid.

With the help of the invention, there is thus provided a method for inactivating ionizable sulphide in a well drilling fluid with a pH of 5 - 12 without adversely affecting the fluid's rheology, whereby an organo-Zn chelate is added to the fluid which adds zinc ions to the fluid. The method is characteristic in that the liquid is either added to a Zn-nitrilotriacetic acid chelate with a stability constant of 10-16, or a soluble zinc compound and nitrilotriacetic acid are added in excess over the zinc ion to form the same Zn-nitrilotriacetic acid in situ, and in that the added or in situ formed chelate is mixed with the liquid to form zinc sulphides, the chelate concentration being maintained at

0.71 - 1.43 g/l, which is sufficient to keep the sulphide ion concentration below the desired level.

The invention also provides an additive for use in the new method, which additive is distinguished by the fact that it contains Zn-nitrilotriacetic acid chelate with a stability constant of 10-16 or a combination of a soluble zinc compound and nitrilotriacetic acid, with a stability constant of 10- 16.

The drilling fluid is preferably an aqueous dispersed or non-dispersed fluid. However, the drilling fluid can also be an oil-based fluid or an emulsion. The oil can be any common liquid hydrocarbon, such as aliphatic hydrocarbon, aromatic hydrocarbon or a mixture thereof. The liquid is typically circulated in the well during drilling and other operations, so the concentration of the zinc chelate must be monitored. The liquid should also be monitored to detect the presence of hydrogen sulphide or sulphide ions which would indicate a need for the addition of zinc chelate or a need to increase the concentration level of zinc chelate in the liquid.

It is of the utmost importance that the zinc chelate not only removes the hydrogen sulphide, but does so without adversely affecting the rheological properties of the liquid, such as thickening or gelling of the sludge or a considerable increase in liquid loss. When drilling, completing or overhauling a well, this is important, because it is essential to maintain circulation and control of the well when undesirable and harmful conditions occur, such as in the presence of hydrogen sulphide. The Zn-nitrilotriacetic acid chelate used according to the invention has an ionization or stability constant of 10-16. as described by Stanley Chaber and Arthur E. Martell in Organic Sequestering Agents, John Wiley & Sons, Inc., New York, 1959. Such a stability constant prevents the formation of insoluble zinc hydrox<y>d, which would prevent reaction of the zinc ion with sulfide or make the zinc ion unavailable for the reaction with hydrogen sulphide. The stability constant is also such that the formation of highly soluble zinc compounds which would have a harmful effect on the rheology of the well fluid and thus the control of the well is prevented. Nitrilotriacetic acid, which is used as a chelant for the formation of the Zn-nitrilotriacetic acid chelate, has a stability constant as mentioned above of 11.0.

Zinc is a very favorable metal ion for reaction with the sulfide due to its special stability constant range and compatibility with drilling fluid rheology. The zinc ion is particularly effective in reacting with the soluble sulphide or sulphide ion to prevent stress cracking of steel pipes and tools used in the well and to prevent the escape of hydrogen sulphide which would act as a poison to personnel and the environment surrounding the well. The complexed zinc ion effectively traps the sulphide ion in the well fluid. Zinc effectively stops stress cracking of ferrous metals without being deposited on the metal and causing galvanic corrosion. Furthermore, zinc does not cause tensile cracking in secondary oxygen reactions. This zinc sulphide sweep does not have a harmful effect on the rheology of the well fluid and can be easily removed by conventional methods such as centrifugation, filtration or settling in the mud tanks. The zinc nitrilotriacetic acid chelate is active over a wide pH range and reacts virtually instantaneously upon its addition to the drilling fluid to effectively inactivate the hydrogen sulfide to prevent stress cracking of metals and release of hydrogen sulfide at the surface. It should contain at least 10% by weight zinc, preferably from 15 to 25% by weight zinc. The zinc nitrilotriacetic acid chelate is effective over a wide pH range of 5-12, but is preferably used in the pH range 6-11. The chelate can be used alone or together with conventional well fluid additives, and even other sulphide removing materials. The chelate can effectively remove zinc sulfide concentrations from only trace amounts such as 1 ppm up to large amounts such as thousands of ppm, by simply adding the amount of chelate necessary for stoichiometric reaction with the sulfide. The chelate must be present in the well fluid as a safety factor in a concentration of 0.71 - 1.43 g/l,

The zinc nitrilotriacetic acid chelate can be added to the well fluid as a pre-prepared zinc chelate, either in liquid form or in dry powder form. In addition, the chelate can be formed in the well fluid in situ by adding a water-soluble zinc compound and nitrilotriacetic acid in the desired weight ratio, which is from 4:1

to 99:1, preferably from 4:1 to 9:1. Preferred soluble zinc compounds for forming the chelate are zinc sulfate, zinc chloride and other readily available soluble zinc compounds. The zinc compound can advantageously contain an ion such as acetate, sulphate, ammonium, bromate, bromide, chlorate, chloride, formate, iodide, nitrate, silicofluoride, sulphoxylate or hydrosulphite.

Less soluble compounds such as zinc carbonate or basic zinc carbonate can also be used to prepare the chelate, but longer reaction times are then necessary. In order to be able to use less soluble zinc compounds, the chelates should be premixed and separated from the anion. The amount of the anion of the compound used to supply the zinc ion for the chelant must be kept low to prevent adverse effects. If e.g. zinc carbonate is used, the chelate should be premixed and purified by removing the carbonate, as the carbonate ion flocculates clays that occur in drilling fluids.

The chelate can also be formed in situ in the field, but the nitrilotriacetic acid and the soluble zinc compound should be added together and mixed so that the zinc ion does not precipitate the clays and the chelant is not adsorbed on solid particles. The nitrilotriacetic acid should be added in excess. For drilling fluids with high solids concentrations, the nitrilotriacetic acid and the zinc compound should be added slowly and mixed carefully with the drilling fluid before it is circulated in the well.

The production and use of the zinc nitrilotriacetic acid chelate used according to the invention should be obvious to those skilled in the art in light of the present description. However, the following literature references should be given as a supplement: US patents 2,801,994, 3,099,874, 3,107,739, 3,146,199, 3,431,202, 3,441,504, 3,462,239, 3,506,572, 3,578,508, 3,580 934,' 3,669,613, 3,697,498, 3,699,042, 3,810,882, 3,928,211 and 4,000,083, Stanley Chaber and Arthur E. Martell, Organic Sequestering Agents, John Wiley & Sons, Inc., New York, 1959,

Lars Gunnar Sillen and Arthur E. Martell, Stability Constants of Metal-Ion Complexes, Metcalfe & Cooper Limited, London, 1964, K. B. Yatsimirskii and V. P. Vasil'ev, Instability Constants of Complex Compounds, Consultants Bureau, New York, 1960.

This literature shows previously used cleaners and various practices regarding the production and use of drilling mud.

Examples

Samples were tested for reaction with hydrogen sulfide (H2S) in a standard water-based drilling fluid (pH = 9.0). The treatment level for the additives was calculated to give the same molar concentration of zinc metal in all examined samples. Hydrogen sulfide was developed from 0.1 M sodium sulfide (Na2S) with sulfuric acid and the hydrogen sulfide was bubbled into the drilling fluid sample in a Waring blender. For every 100 ml of 0.1 M Na2S used, 970 ppm H2S was developed for reaction in the drilling fluid. The hydrogen sulphide was bubbled through the sample until H2S release was detected with lead acetate paper indicating saturation of the sludge with H2S.

The effect of the addition on stress cracking was assessed using prestressed steel bearings in different drilling fluids at 66°C with rolling, i.e. aging or hot rolling of the sample for a specific time.

Most of the chelates were synthesized using the following steps: 1. Equimolar amounts of the chelating agent and the zinc salt were stirred in a minimal amount of water for 30 minutes. 2. The solution was made alkaline (pH 8-9) with

KOH.

3. Absolute ethanol was added to precipitate the chelates, which were filtered off and dried.

The stability of the organometallic chelates of lead, zinc and copper varies according to the metal, chelating agent, pH, etc. With zinc metal ion chelated with nitrilotriacetic acid (NTA), the compounds had little detrimental effect on the properties of the drilling fluid. Hydrogen sulphide reacts easily with Cu-, Pb-

and the Zn chelates of NTA and traps the sulfide ion as insoluble CuS, PbS, or ZnS. Although these compounds are very effective t^S scavengers, only ZnNTA (Zn-nitrilotriacetic acid chelate) also prevents sulphide stress cracking of prestressed steel bearings and has no harmful effects on the drilling mud. In reality, bearings placed in sludge treated with PbNTA and CuNTA will often crack before bearings in untreated sludge.

Zinc lignosulfonates containing zinc are effective

to prevent stress cracking in bearings. However, foaming is a serious problem/ and the zinc concentration is very low in this chelate.

Inorganic compounds of zinc (sodium zincate) and

lead (sodium plumbite) are effective H2S scavengers. However, these compounds do not prevent sulphide stress cracking of bearings in treated sludge.

The samples were examined as described above. Water-based sludge samples were prepared with the following additions: 2.5 7 g/l NaCl, 4.9 9 g/l CaC03, 4 5.65 g/l Wyoming bentonite,

79.88 g/l Southern bentonite, 11.41 g/l chromium lignosulfonate dispersant and a liquid loss additive. The pH was adjusted to 9.0 with NaOH. Various chelates and H2S scavengers were added in the amounts indicated in the table and with the results indicated there.

The rheology, liquid loss and pH of each sample were measured using a "Fann" direct reading viscometer, a liquid loss cell and a pH meter according to API Method 13B.

The zinc-NTA mixture was prepared by reacting sodium NTA (NTANa^) with each of the compounds ZnCl2 and ZnSO^. These mixtures were tested both as slurries and as oven-dried solids. The tests showed that these mixtures gave acceptable results in preventing stress cracking of steel bearings; but premixed and purified ZnNTA gave the best results.

Table 1

Identification of samples ( Tables 2 - 11)

Each sample was a 350 ml portion of water-based sludge with additions and treatment as indicated.

These tests show that the ZnNTA chelate has a high reactivity towards I^S with good rheology and wash loss.

These experiments show that drilling fluid with ZnNTA has good rheology and fluid loss after aging or hot rolling.

These samples show that drilling mud with ZnNTA has good rheology and fluid loss after reaction with H2S and ageing.

These tests show that drilling mud with ZnNTA has good rheology after prolonged aging in a hot state with H2S turnover.

Table 10, samples 2 4 - 26, shows that ZnNTA effectively removes K^S from drilling mud quickly and over a longer period of time with good rheology in the mud. ZnNTA reduces the sulfide ion concentration practically instantaneously compared to ZnCO 3 .

Samples 27 and 28 in Table 10 show that the ZnNTA chelate can be formed in situ and reacts virtually instantaneously to remove sulfide ion. ZnNTA reduces the sulfide ion concentration to only trace amounts.

Table 11, samples 29 - 33, shows that high ratios of zinc ion to NTA give higher gel strength or higher viscosity and flow limit at the high addition concentration of 14.3 g/l. The weight ratio of zinc ion to NTA should be in a molecular excess in relation to the zinc ion.

Claims (3)

1. Method for inactivating ionizable sulphide in a well drilling fluid with a pH of 5 - 12 without adversely affecting the fluid's rheology, whereby an organo-Zn chelate is added to the fluid which supplies the fluid with zinc ions, characterized in that the fluid is either added to a Zn -nitrilotriacetic acid chelate with a stability constant of 10-16 or a soluble zinc compound and nitrilotriacetic acid are added in excess over the zinc ion to form the same Zn-nitrilotriacetic acid chelate in situ, and by the added or in situ formed chelate mixing with the liquid to form zinc sulphides, the chelate concentration is maintained at 0.71 - 1.43 g/l, which is sufficient to keep the sulphide ion concentration below the desired level. 2. Additive for use in the method according to claim 1, characterized in that it contains Zn-nitrilotriacetic acid chelate with a stability constant of 10-16 or a combination of a soluble zinc compound and nitrilotriacetic acid, with a stability constant of 10-16. 3. Additive according to claim 2, characterized in that the soluble zinc compound is a compound which has an anion which is acetate, sulphate, bromate, bromide, chlorate, chloride, formate, iodide, nitrate, silicofluoride, sulphoxylate or hydrosulphite.