COMPOSITION USEFUL IN SULFATE SCALE REMOVAL

Abstract

The present invention discloses a novel aqueous composition for use in removing barium sulfate scale from a surface contaminated with such, said composition comprising: a chelating agent and a counterion component selected from the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA; and a scale removal enhancer. There is also disclosed methods to use such compositions.

Claims

1. A Method of removing sulfate scale off of a contaminated surface, said method comprising: providing a liquid composition comprising: a chelating agent selected from the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA; exposing said surface contaminated with barium sulfate scale to the liquid composition; allowing sufficient time of exposure to remove barium sulfate scale from the contaminated surface.

2. The method according to claim 1, wherein the liquid composition further comprises a scale removal enhancer that is selected from the group consisting of: potassium carbonate; potassium formate; cesium formate; cesium carbonate; and combinations thereof.

3. The method according to claim 1, wherein the sulfate scale is selected from the group consisting of: magnesium sulfate; barium sulfate; calcium sulfate; strontium sulfate; radium sulfate; and combinations thereof.

4. An aqueous composition for use in removing sulfate scale from a surface contaminated with such, said composition comprising: a chelating agent and a counterion component selected from the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA; and a scale removal enhancer.

5. The aqueous composition according to claim 4, wherein the scale removal enhancer is selected from the group consisting of: potassium carbonate; potassium formate; cesium formate and cesium carbonate and combinations thereof.

6. The aqueous composition according to claim 4, wherein the scale removal enhancer is present in the composition in an amount ranging from 5 to 20 wt % of the weight of the composition.

7. The aqueous composition according to claim 4, wherein the scale removal enhancer is present in the composition in an amount ranging from 5 to 15 wt % of the weight of the composition.

8. The aqueous composition according to claim 4, wherein the scale removal enhancer is present in the composition in an amount of approximately 5 to 10 wt % of the weight of the composition.

9. The aqueous composition according to claim 4, wherein the scale removal enhancer is present in the composition in an amount of approximately 5 wt % of the weight of the composition.

10. The aqueous composition according to claim 4, wherein the chelating agent and counterion are present in the composition in an amount ranging from 5 to 40% wt of the weight of the composition.

11. The aqueous composition according to claim 4, wherein the chelating agent and counterion are present in the composition in an amount ranging from 10 to 30% wt of the weight of the composition.

12. The aqueous composition according to claim 4, wherein the chelating agent and counterion are present in the composition in an amount ranging from 10 to 20% wt of the weight of the composition.

13. The aqueous composition according to claim 4, wherein the pH of the composition ranges from 10.5 to 11.5.

14. The aqueous composition according to claim 4, wherein the pH of the composition ranges from 10.8 to 11.2.

15. The aqueous composition according to claim 4, wherein the sulfate scale is selected from the group consisting of: magnesium sulfate; barium sulfate; calcium sulfate; strontium sulfate; radium sulfate; and combinations thereof.

16. The aqueous composition according to claim 4, wherein the sulfate scale is barium sulfate.

17. The aqueous composition according to claim 4, wherein the sulfate scale is radium sulfate.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] Features and advantages of embodiments of the present application will become apparent from the following detailed description and the appended figures, in which:

[0035] FIG. 1 is a picture showing the amount of scale produced in a tubing section when barium sulfate scale is left to accumulate;

[0036] FIG. 2 is a picture showing the barium sulfate scale crystals inside a tubing section;

[0037] FIG. 3 is a picture showing a close up of crystals removed from the tubing in FIG. 2;

[0038] FIG. 4 is a picture showing the experimental dissolution of crystals of barium sulfate scale over a period of time (at 0 hour; after 1 hour; and after 4 hours);

[0039] FIG. 5 is a graph depicting the temperature impact on the dissolution of barium sulfate scale.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0040] According to a preferred embodiment of the present invention, the sulfate scale removing composition provides a safety advantage over known compositions. By the addition of potassium carbonate to the K.sub.5DTPA, the same solubility numbers can be attained at a lower pH. Instead of 13.5 a pH of 11 was sufficient to get comparable solubility numbers. This represents a considerable difference and safety and environmental advantage.

[0041] According to a preferred embodiment of the present invention, the sulfate scale removing composition provides improved rates of scale dissolution. This, in turn, reduces the down time or non-producing time for wells or equipment where the scale is being removed or treated. It also reduces the cost of such treatment, by limiting the treatment time and bring revenue generation back on-line faster.

[0042] According to a preferred embodiment of the present invention, a composition for removing sulfate scale permits the removal thereof at a much lower pH than what has been practiced to date. Indeed, such a composition can effectively remove barium scale under conditions where the pH is approximately 11 (ranging from 10.5-11.5 and more preferably from 10.8 to 11.2), rather than other scale removal compositions which require conditions where the pH is 13 or higher. According to another preferred embodiment of the present invention, there is provided a composition where the pH is 10 and removes 30 kg/m.sup.3 of BaSO.sub.4 scale.

[0043] According to a preferred embodiment of the present invention, a composition for removing barium sulfate scale permits the removal thereof with a higher dissolution capacity. This, in turn, allows reducing the volume of scale remover necessary. This also decreases transport costs and many other related items resulting from the usage of lower volumes of scale remover.

[0044] According to a preferred embodiment of the present invention, a composition for removing barium sulfate scale permits the removal thereof at substantially lower temperature than other barium sulfate scale removing treatments and with greater efficiency. This results in safer treatment conditions for individuals involved in this process.

Absolute Solubility of Barium Sulfate Scale

[0045] The inventors have noted that chelating agents such as EDTA (Ethylenediaminetetraacetic acid) or DTPA (diethylenetriaminepentaacetic acid) and the ability to dissolve barium sulfate depends substantially on the size and ion strength of the counterion. EDTA is a very poor choice to dissolve barium sulfate scale. It has respectable ability to dissolve calcium sulfate scale but in the presence of barium sulfate it is almost ineffective. Hence, having a composition capable of dissolving both barium sulfate and calcium sulfate scale efficiently would be very desirable.

[0046] In Tables 1 and 2 (absolute solubility testing) the absolute (or maximum) solubility increases with the size of the counterion from lithium to cesium. TEAH (Tetraethylammonium hydroxide) and TBAH (Tetrabutylammonium hydroxide) as organic bases (counterions) are showing the same trend. Information indicates that the size of the TBAH cation (including the hydrate layer) is comparable to potassium.

[0047] The solubility numbers for both were found to be very similar. In order to quantitatively compare the kg/solubility properly, the BaSO.sub.4:chelating agent ratio was calculated in g/mol and the Ba.sup.2+:chelating agent ratio was calculated in mol/mol. The mol:mol ratio indicates the number of molecules of the chelating agent needed to dissolve one ion of Ba.sup.2+ (complex). The highest ratio which was found was almost 0.5, which means that there needs to be, on average, 2 molecules of DTPA to dissolve 1 Ba.sup.2+ ion but mostly it can be much less.

[0048] Tests performed have indicated that, besides the nature of the counterion, an excess of the counterion also improves the solubility. K.sub.5DTPA was tested in conjunction with KCl, K.sub.2CO.sub.3 and KOOCH (potassium formate). Interestingly, here the counterion plays also a large role as K.sub.2CO.sub.3 (with the larger anion) was much more effective than KCl (with a small anion).

TABLE-US-00001 TABLE 1 Absolute solubility of Barium Sulfate Scale (when using a 40% solution of the scale removing composition) 40 wt % sol BaSO4 BaSO4 Ba2.sup.+ pH (kg/m3) (g/mol) (mol/mol) Li.sub.5DTPA 2 Na5DTPA 13.01 17 20.24 0.088 K.sub.5DTPA 13.25 46 62.16 0.266 K.sub.5DTPA + 10 wt % K.sub.2CO.sub.3 13.21 38 51.35 0.22 Cs5DTPA 13.4 52 72.2 0.309 Na.sub.4EDTA 13.11 9 7.89 0.034 K.sub.4EDTA 13.32 31 32.98 0.141 TEAH.sub.4DTPA 13.1 14 43.75 0.187 TBAH.sub.5DTPA 13.33 18 64.28 0.275

TABLE-US-00002 TABLE 2 Absolute solubility of Barium Sulfate Scale (when using a 20% solution of the scale removing composition) at 60 C. 20 wt % sol BaSO4 BaSO4 Ba2.sup.+ pH (kg/m3) (g/mol) (mol/mol) K.sub.5DTPA 13.19 27 72.97 0.313 K.sub.5DTPA + 5 wt % K.sub.2CO.sub.3 13.32 41 110.81 0.475 K.sub.5DTPA + 5 wt %K.sub.2CO.sub.3 11.25 40 108.11 0.463 K.sub.5DTPA + 5 wt % K.sub.2CO.sub.3 10 33 89.19 0.3821 Cs5DTPA + 5 wt % CsCO3 11.1 35 Cs5DTPA + 10 wt % CsCO.sub.3 11.2 35 Cs5DTPA + 10 wt % 10.9 30 HCOOCs TEAH4DTPA + 10 wt % 11 21 K.sub.2CO.sub.3 TBAH5DTPA + 10 wt % 11.1 25 K.sub.2CO.sub.3

[0049] Moreover, the K.sub.5DTPA composition (at 40%) was determined to dissolve 30 kg/m.sup.3 of FeS for a g/mol total of 40.54.

[0050] Preferably, the dissolution of barium sulfate in an amount above 20 kg/m.sup.3. More preferably, dissolution of barium sulfate above 30 kg/m.sup.3 is desired.

Speed of Barium Scale Dissolution

[0051] A second set of tests were performed to study the speed of the barium sulfate scale dissolution. In order to determine the speed, a relatively small amount of BaSO.sub.4 (0.25 gthis equates to 2.5 kg/m.sup.3) was used and the time was measured until the solution became clear. Large differences were noted. The best results involved the combination of K.sub.5DTPA with K.sub.2CO.sub.3. This combination provided a dissolution time which was almost 4 times faster than K.sub.5DTPA alone.

[0052] The speed of dissolution of compositions according to preferred embodiment of the present invention were tested and studied. Table 3 summarizes the findings of the testing. The experiment involved the dissolution of 0.25 g of BaSO.sub.4 in a volume of 100 ml fluid at 60 C. under gentle stirring by magnetic stir bar.

TABLE-US-00003 TABLE 3 Speed of Dissolution of Barium Sulfate Scale Fluid Time pH K.sub.5DTPA (40%) 1 h 44 min 13.26 K.sub.5DTPA (40%) + 10% TBAH 1 h 38 min 13.4 K.sub.5DTPA (40%) + 20% TBAH 1 h 21 min 13.43 K.sub.5DTPA (40%) + 30% TBAH 1 h 20 min 13.49 K.sub.5DTPA (40%) + 10 wt % KCl 1 h 24 min 13.27 K.sub.5DTPA (40%) + 10% K.sub.2CO.sub.3 30 min 13.22 K.sub.5DTPA (20%) + 5% K.sub.2CO.sub.3 22-23 min 10.5-11

[0053] This testing indicates that both the extent of barium scale dissolution and the speed at which it is dissolved represent marked improvements over known compositions.

[0054] Preferred compositions of the present invention further comprises a scale removal enhancer selected from the group consisting of: K.sub.2CO.sub.3; KOOCH; CsCO.sub.3; CsCOOH and combinations thereof. Preferably, the scale removal enhancer is K.sub.2CO.sub.3. Preferably, the scale removal enhancer is present in an amount ranging from 5 to 30% by weight of the scale removal composition. More preferably from 5 to 20% by weight and even more preferably, the scale removal enhancer would be present in an amount of approximately 5-15 wt %, yet even more preferably from 5-10 wt % and most preferably in an amount of approximately 5 wt %.

Impact of Temperature

[0055] The speed of dissolution of a composition according to preferred embodiment of the present invention was tested and studied under different temperature conditions. Table 4 summarizes the findings of the testing. The experiment involved the dissolution of 0.25 g of BaSO.sub.4 (2.5 kg/m.sup.3) in a volume of 100 ml of fluid at various temperatures under gentle stirring by magnetic stir bar. The composition tested comprised a 20 wt % solution of K.sub.5DTPA and 5 wt % K.sub.2CO3.

TABLE-US-00004 TABLE 4 Impact of Temperature on the Dissolution of Barium Sulfate Temperature in Time C. ( F.) (minutes) 25 (77) 225 40 (104) 50 60 (140) 22 80 (176) 3.5 90 (194) 1.5

[0056] Moreover, the compositions used are quite environmentally safe. This represents a major advantage over any known chemically-based methods of barium scale removal. Another advantage to the compositions according to preferred embodiments of the present invention includes the speed of dissolution which is considerably faster than any known commercial compositions. Another advantage of preferred compositions according to the present invention is that they can be deployed on wells according to a one-step process and thus are very desirable to operators which deal with barium sulfate scale issues often.

[0057] Compositions according to the preferred embodiment provide substantial improvement in sulfate scale removal starting 40 C. More preferably, the preferred compositions according to the present invention can be used at temperatures of at least 50 C., even more preferably at temperatures of at least 60 C. In some cases, the compositions according to preferred embodiments of the present invention can be exposed to temperatures of up to 80 C. and even up to 90 C. and higher and still provide excellent sulfate scale removal performance.

[0058] Compositions according to the present invention which exhibit a pH below 12 are considered non-regulated by Transport Canada, this provides a substantial advantage to any operator with respect to reduced transportation costs and related costs. According to a preferred embodiment of the present invention, water is the sole solvent used in the preparation and dilution of the composition. The preparation of a composition according to the present invention is carried out by exposing the various components to water and ensuring complete and proper dilution and obtaining an homogeneous solution.

[0059] Preferably, the aqueous composition according to the present invention have a pH ranging from 10.5 to 11.5. More preferably, the aqueous composition according to the present invention have a pH ranging from 10.8 to 11.2.

[0060] According to a preferred embodiment of the present invention, there is provided a one-step process for removing sulfate scale inside a wellbore, said process comprising: [0061] providing a liquid composition comprising: [0062] a chelating agent selected from the group consisting of: Li.sub.5DTPA; Na.sub.5DTPA; K.sub.5DTPA; K.sub.5DTPA; Cs.sub.5DTPA; Na.sub.4EDTA; K.sub.4EDTA; TEAH.sub.4DTPA; and TBAH.sub.5DTPA; [0063] optionally, a scale removal enhancer; [0064] exposing a surface contaminated with barium sulfate scale to the liquid composition; [0065] allowing sufficient time of exposure to remove barium sulfate scale from the contaminated surface. A person skilled in the art will understand that what is meant by one-step is that there is a single treatment step in the process (or method) to remove the sulfate scale buildup. Preferably, The sulfate scale is selected from the group consisting of: magnesium sulfate; barium sulfate; calcium sulfate; strontium sulfate; radium sulfate; and combinations thereof.

[0066] When the surface contaminated is deep underground or a hard to access tubing or piping, the exposure consists of circulating the liquid composition through the tubing or piping until it has been established that the scale has been removed beyond a desirable predetermined point. Hence, in some cases, it is quite possible that the entirety of the scale present is not removed but the amount of removal is sufficient to re-start operations and provide the desired productivity and/or circulation through the affected tubing/piping. The liquid composition can also be heated in order to improve the removal of the scale and the speed at which the removal is effected or heated naturally by geological heat.

[0067] According to another preferred embodiment of the present invention, the method of treatment of BaSO.sub.4 scale wherein the fluid is spotted, i.e placed in a tube/tank/pipe/equipment in a soaking operation. This may in some instances be somewhat less efficient than circulating the fluid due to the surface reaction nature of the fluid, but it is used in some cases to remove enough scale to run tools or reestablish circulation in an exchanger completely plugged off by scale, for example.

Field Testing Results

[0068] An International E&P company operating in the WCSB utilizing downhole chokes on their wells has had ongoing issues with sulfate blockage. As production pressures declined the chokes need to be removed and it was found that barium sulfate (BaSO.sub.4) scale deposition in the tubing was making the process very difficult, if at all possible to continue production. Various commercially available dissolvers were deployed with no effect. Mechanical solutions were inhibited by large scale tubing deposition resulting in stuck pipe.

[0069] A barium sulfate scale dissolver according to a preferred embodiment of the present invention (was deployed in an attempt to remove the scale deposits and retrieve by completely freeing the choke of scale. While the composition (K.sub.5DTPA 20 wt % and 5 wt % K.sub.2CO3) according to a preferred embodiment of the present invention would have been able to perform without agitation at low temperatures, in order to optimize its performance, agitation along with the elevated temperatures were employed to expedite dissolution. The wells in the field where the testing was carried out typically have BHT (bottom hole temperature) of 110 C.

[0070] A volume of approximately 500 gallons of a composition according to the present invention were delivered and loaded into a pressure truck. A wireline unit deployed a scraper brush into the wellbore and was used to create agitation around the scale as the composition was periodically spotted and left to soak. Over the next few hours the bottom hole agitator reached its target depth and once contact was established the choke, it was successfully retrieved.

[0071] Utilizing the composition according to the present invention along with agitation from the bottom hole agitator the operator was able to remove enough scale to retrieve the choke and recommence the production of the well. Utilizing the composition according to the present invention, the operator was able to solubilize over 80 kg of scale thus allowing the choke to be removed and sized accordingly to current flow rates and pressures. This highly effective product is capable of solubilizing more than twice as much barium sulfate scale than the leading competitions claimed rates, many of which failed prior to the deployment of the present invention

[0072] Moreover, the composition according to the preferred embodiment of the present invention showed no damage to wellbore metals and seals for the period of time for which they were employed which allows long soaks to be performed (+24 hr). With a high temperature stability of 130 C./270 F. and a lower pH profile than most dissolvers of pH 10.5 to 11, the composition according to a preferred embodiment of the present invention provides a substantially increased level of performance and safety to operations. Advantageously, the speed and efficiency of the scale dissolving agent were noted to be beyond anything that had ever been proposed to or deployed the operator.

Laboratory Testing of Scale Dissolution

[0073] The sample selected for the solubility testing origins from an oilfield tubular containing sulfate scale crystals originally used for demonstration purposes. FIGS. 1 and 2 show the inside of an oilfield tubular containing sulphate scale similar to most deposits encountered.

[0074] Crystals of barium sulfate scale were removed from the tubular to be used for the solubility testing. 200 cc of composition (K.sub.5DTPA 20 wt % and 5 wt % K.sub.2CO3) was used. A weighted portion of oilfield sulphate scale sample was submerged in 200 cc of each de-scaling composition. A small magnetic stirrer is added to create a very minimal vortex, creating a small movement of fluid without rigorously stirring the fluid. The fluid was heated to 70 Celsius.

Results

[0075] 25.165 grams of oilfield sulphate scale was weighted and added to the fluid. The stirrer and heater were started. After 1 hour, a slight colouring of the fluid was observed. After 4 hours at temperature when no continued visual reduction of scale was observed, the fluid was filtered and the filter rinsed with water, dried and weighed. The maximum scale solubility was reached and subsequently calculated.

[0076] The composition according to a preferred embodiment of the present invention was able to dissolve 52.97 grams per litre of scale at 70 Celsius.

[0077] The testing was also carried out with a commercially available product (Barsol NS), which is alkali/EDTA based and with EDTA. The Barsol NS product was capable of dissolving 24.19 grams per litre. EDTA attained a poor dissolution of only around 6 grams per litre. Under identical conditions, the composition according to a preferred embodiment of the present invention has shown to have more than double the performance of Barsol NS.

[0078] While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be appreciated by those skilled in the relevant arts, once they have been made familiar with this disclosure that various changes in form and detail can be made without departing from the true scope of the invention in the appended claims.