10136 Development of Low Corrosive Environmental Friendly Calcium Carbonate Scale Dissolver for HTHP Wells (51300-10136-SG)

7/25/2019 10136 Development of Low Corrosive Environmental Friendly Calcium Carbonate Scale Dissolver for HTHP Wells (…

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INTRODUCTION

Calcium carbonate, CaCO3, is one of the most common scale depositions found in oilfield productionwells and surface facilities. It can be deposited all along the water paths from injectors through thereservoir to the surface equipment, especially in high temperature and high pressure (HTHP) wells,where temperature is up to 250oC and pressure is up to 20000psi.

Calcium carbonate scale forms when the solution is supersaturated with respect to Ca2+ ions and HCO3-

ions. The two major factors causing CaCO3 deposition in the oil and gas industry are reduction inpressure and high temperature during production1,2. Pressure drop leads to the loss of carbon dioxide(CO2) from aqueous solution. This causes an increase of pH and an associated increase ofsupersaturation. High temperature is another driving force causing CaCO3 self-deposition. The solubilityof calcium carbonate decreases with temperature increase, hence CaCO3 crystallization frequently occursat high temperature3. In addition, the kinetics of calcium carbonate scale formation is a function oftemperature, i.e. slow kinetics at low temperature. As the temperature increases, the formation ofcalcium carbonate will accelerates and precipitation may occur at an earlier stage

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Carbonate scale formation can impair production by blockage of tubing and flowlines, fouling of

equipment and concealment of corrosion. The effects of carbonate scale can be dramatic and costs canbe enormous6.

Effective techniques are needed to solve the scale deposition and keep producing wells healthy. In mostcases, scale prevention through chemical inhibition is the preferred method of maintaining wellproductivity. In order to minimize the formation of scale deposits, scale inhibitors treatment withpolycarboxylates or phosphorous containing compounds (such as phosphonates or phosphate esters) arecommon practice in oil industry5. However, while the most polymer scale inhibitors showed a goodthermal stability, some phosphonate inhibitors had a limited usage for being applied at temperatures over170oC.

Hydrochloric acid (HCl) has been widely used as a low cost carbonate scale dissolver. Its reaction rate

with carbonate minerals is very rapid, and results in high corrosion rates and tubular damage, especiallyat high temperature condition. Corrosion inhibitor packages are often incorporated into the HCl acidtreatment, however success has been limited, especially at high temperature

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In order to overcome the concerns of HCl acid, organic acids are usually used as an alternative to HCl inhigh temperature condition. The main organic acids that are frequently used in scale dissolvingtreatments are acetic, formic, and citric acid. These acids are less reactive with carbonate scale thanhydrochloric acid. The retarded nature of these acids makes them a good alternative to HCl acid.However, organic acids are more expensive than HCl. These organic acids offer an alternative to mineralacids with lower corrosion implications, but have typically been deployed in fields where operatingtemperatures do not exceed 100°C1. At elevated temperatures where the corrosion rate is much higher,control can be problematic. To ensure that the remedial treatment of calcium carbonate scale can be

effectively performed in high temperature fields without adversely affecting the integrity of productionassemblies an effective product with low corrosivity is required2. These products must be capable of

withstanding HTHP conditions whilst exhibiting good performance and in particular be compatible withthe mild steels.

In addition, there is a requirement for scale removing chemicals with good environmental characteristicsin order to cope with stringent environmental regulations.

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In this paper, a low corrosive environmental friendly calcium carbonate scale dissolver has beendeveloped to provide an effective treatment to remove calcium carbonate scale formed in HTHP wells.This study details the results of the laboratory work performed to develop the environmental friendlycalcium carbonate scale dissolvers for at HTHP applications.

EXPERIMENTAL PROCEDURE

Scale Dissolver Tests – Static Jars

Test condition of scale dissolving

Temperature: 90oC / 170°C heated by ovenMass solid: 1g calcite scale (0.5-1.0mm size particle)Scale dissolver: 10mlTime: 24 hours

1g calcite scale samples were weighed accurately to four decimal places before addition of dissolversolutions in test container autoclave (170°C) or plastic jar (90oC).

100µl samples were taken after 0.5, 2, 4 and 24 hours. These samples were diluted in deionised waterand analysed by Inductively Coupled Plasma (ICP).

After 24 hours, the test solutions were transferred to an empty jar while solutions remain hot, and thencooled to room temperature to check the risk of jelly precipitates. For the tests run at 170°C, the sampleshould be cooled down to less than 90°C before transferring solution. The remaining solid was filteredin to pre-weighed filter papers. Wash with deionised water, oven dried and reweigh to determine %weight loss.

Thermal Stability Tests (170°C)

The scale dissolver SD-C was thermal aged at 170°C for 5 days. The samples were sparged withnitrogen prior to thermal ageing. The sample was assessed visually before and after ageing, and digitalimages were recorded prior to and after ageing. The test conditions are summarised as follows:

Test condition of thermal stability at 170°C

Scale dissolver: SD-CConcentration: neatTest Temperature: 170oCTest Duration: 5 days

The dissolving performance tests were carried out with the above thermal aged sample based on theprocedure detailed in static jar scale dissolver procedure.

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After 24 hours, SD-A and SD-B achieved 100% dissolution at 90°C. SD-C achieved approximately24% and the reference SD-D (acetic acid based scale dissolver) approximately 41% over the same timeperiod. The results are presented in Figure 1.

FIGURE 1 - Dissolution of calcite scale at 90°C over 24 hours

The rate of dissolution of the scale dissolvers on calcite scale is presented in Figure 2. The SD-A, SD-Band SD-C proved to dissolve the calcium carbonate deposits quickly at the initial stage of tests. ProductSD-A, SD-B and SD-C almost reached their maximum scale dissolution capacity in 0.5 hour. Thereference scale dissolver, SD-D, proved less effective under this condition. Only about 20% dissolvedscale was dissolved at the initial 0.5 hour and about 41% of total dissolved scale was dissolved after 4hours test.

After 24 hours, the test solutions were transferred to an empty jar while solutions remain hot and cooledto room temperature to check the risk of jelly precipitates.

The samples were observed at regular intervals to check for jelly precipitation, with photographs takenas a record showed in Figure 3. The jelly precipitates formed in the SD-A and SD-B test solutions aftercooling to room temperature. In the SD-C and SD-D test solutions, no jelly precipitates formed aftercooling. SD-C is selected for further tests.

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FIGURE 2 - Rate of dissolution of calcite scale at 90°C over 24hours

after 24 hours at 90°C

cooling to roomtemperature

SD-A SD-B SD-C SD-D

FIGURE 3 - Observation of jelly precipitation after cooling the test solution

Thermal Stability Test At 170°C And Scale Dissolver Test At 90°C

In order to evaluate the thermal stability of SD-C under condition of field application, scale dissolverSD-C was thermal aged at 170°C for 5 days and followed by dissolving performance test at 90°C.

The appearance of SD-C before and after thermal ageing is shown in Figure 4. SD-C remained clear,with no precipitate over 5 days of thermal aged test.

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FIGURE 4 - Observation of appearance of non-aged and aged SD-C

The dissolving performance of non-aged SD-C and aged SD-C is illustrated in Figure 5 and 6. After 24hours, at 90°C, both non-aged and aged SD-C achieved about 24% dissolution. The rate of dissolutionof the non-aged SD-C and aged SD-C have similar performance and both products almost reached theirmaximum scale dissolution capacity in 0.5 hour.


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Corrosion Tests (175°C)

General corrosion

Wheel box testing results are displayed in Table 1. General corrosion rates of SD-C were low, with anaverage general corrosion rate of 1.01mpy.

TABLE 1

GENERAL CORROSION RESULTS FROM HIGH PRESSURE, HIGH TEMPERATURE

WHEEL BOX TESTING

Chemical Coupon Wt. Loss (mg) MPY

SD-C Super 13Cr 0.23 1.01

Pitting

Pitting evaluation found no pits on any of the 3 coupons tested. A representative coupon is pictured inFigure 7. It can be seen that there are no pits in the metal despite the presence of several tool marks.

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FIGURE 7 - Representative Picture of Coupon Condition at 15x

Winterized SD-C And Scale Dissolver Test At 90°C

A winterized scale dissolver SD-Cw1 has been developed based on SD-C. The antifreeze component inscale dissolvers was increased in SD-C, SD-Cw1, SD-Cw2 and SD-Cw3 respectively. The stability ofscale dissolvers at -20 °C and testing the effect of the antifreeze component on the performance of scaledissolving was carried out.

The stability of scale dissolvers SD-C and SD-CW1 were carried out at -20°C for 7 days. Samples wereobserved at regular intervals, with photographs taken as a record showed in Figure 8. SD-C was frozenat -20 °C after one day. SD-Cw1, SD-Cw2 and SD-Cw3 were still clear and not frozen after 7 days at -20 °C.

FIGURE 8 - Thermal stability at -20°C after 7 days

The dissolving performance of SD-C, SD-Cw1, SD-Cw2 and SD-Cw3 is illustrated in Figure 9 and 10.After 24 hours, at 90°C, SD-C SD-Cw1, SD-Cw2 and SD-Cw3 achieved about 24%, 23% 18.5% and17.5% dissolution respectively. The product ranking based on the rate of dissolution is: SD-C>SD-Cw1>SD-Cw2>SD-Cw3. The quantity of antifreeze component in the scale dissolvers has a negative

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effect on the dissolving performance on calcium carbonate dissolution. With the increase of antifreezecomponent, both the dissolving capacity and dissolving rate decreased.



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Scale Dissolver Tests (170°C)

Static tests were performed to assess the dissolution characteristics of scale dissolvers, SD-C and SD-CW1, on calcium carbonate scale at 170°C. The dissolution performance has been assessed over 24hours. The weight loss and risk of jelly precipitate can be approached.

After 24 hours, at 170°C, both of SD-C and SD-Cw1 achieved approximately 27% dissolution after 24hours. The results are presented in Figure 11.


After 24 hours, the test solutions were cooled to less than 90°C and transferred to an empty jar whilesolutions remain hot. Then this solution was further cooled to room temperature to check the risk of jellyprecipitates.

Samples were observed at regular intervals to check for jelly precipitation, with photographs taken as arecord showed in Figure 12. No jelly precipitates formed after cooling in both SD-C and SD-CW1 testsolutions.

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TABLE 2

SUMMARY OF SCALE DISSOLVER SELECTION

SD-A SD-B SD-C SD-Cw1 SD-D

Performance

Non-aged pass pass pass pass pass

Aged N/A N/A pass pass N/A

Winterized N/A N/A N/A pass N/A

Corrosion N/A N/A pass pass fail

Environmental N/A N/A pass pass fail

Reprecipitation after cooling fail fail pass pass pass

Stable at -20oC fail fail fail pass fail

CONCLUSIONS

A low corrosive environmental friendly calcium carbonate scale dissolver (SD-C) has beendeveloped to provide a treatment to remove calcium carbonate scale formed in HTHP wells up to170°C. The characteristics of this product are low corrosiveness at temperatures up to 170°C; lowcorrosion rate and no pitting corrosion; environmentally acceptable chemical in Norway; both theaged (170°C) and non-aged samples showed similar CaCO3 dissolving capacity; a high dissolvingrate in the initial stages of scale dissolution; no re-precipitation risk after cooling.

A winterized scale dissolver (SD-Cw1) has been developed based on SD-C. The quantity ofantifreeze component in scale dissolvers has a negative effect on the dissolving performance oncalcium carbonate dissolution. With the increase of antifreeze component, both the dissolvingcapacity and dissolving rate decreased.

ACKNOWLEDGMENTS

The authors would like thank Champion Technologies and the NACE committee for the permission topublish this paper.

REFERENCE

1. M.H. Alkhaldi, H.A. Nasr-El-Din, H. Sarma, Kinetics of the Reaction of Citric Acid With Calcite,SPE International Symposium on Oilfield Chemistry, 20-22 April 2009, The Woodlands. Texas,SPE 118724-MS

2.

S.M. Proctor, Scale Dissolver Development and Testing for HP/HT Systems, InternationalSymposium on Oilfield Scale, 26-27 January 2000, Aberdeen, United Kingdom, paper 60221-MS

3. Kjellin P., Holmberg K., Nyden M., 2001, A: Physicochemical and Engineering Aspects, 194, 49-55

4. Oddo J. E., Thomson M. B., 1982, Simplified calculations of CaCO3 saturation at high

temperatures and pressures in brine solutions, Journal of Petroleum Technology, July, 1583-1590

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5. B. Senthilmurugan and B. Ghosh, Copolymer for Calcium Scale Inhibition at High Temperature,SPE International Symposium on Oilfield Chemistry, 20-22 April 2009, The Woodlands. Texas,SPE 121784-MS

6. Zhang, Y., Shaw, H., Farquhar, R., Dawe, R., 2001. The kinetics of carbonate scaling-applicationfor the prediction of downhole carbonate scaling. Journal of Petroleum Science and Engineering29, 85–95

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10136 Development of Low Corrosive Environmental Friendly Calcium Carbonate Scale Dissolver for HTHP Wells (51300-10136-SG)