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Pyrite Scale Removal using Green Formulations for Oil and Gas Applications: Reaction Kinetics Musa Ahmed, Mohammed A. Saad, Ibnelwaleed A. Hussein, Abdulmujeeb T. Onawole, and Mohamed Mahmoud Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00444 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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130°C

150°C

8E-09 6E-09 4E-09 2E-09

0 2

4

6

8

10 12 𝒓𝒂𝒅 𝟎. 𝟓 , ( 𝟎.𝟓 𝒔𝒒𝒖𝒓𝒆 𝒓𝒐𝒐𝒕 𝒐𝒇 𝒅𝒊𝒔𝒌 𝒓𝒐𝒕𝒂𝒕𝒊𝒏𝒂𝒍 𝐬𝐩𝐞𝐞𝐝 ^ 𝝎 ) 𝒔

(a) reaction rate

Diffusion Coefficient (cm2/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Reaction rate (mole/cm2.s(

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DTPA Diffusion Coefficient at 130°C & 150 °C

1.200E-09 1.150E-09 1.100E-09 1.050E-09 1.000E-09 9.500E-10 9.000E-10

150 °C

(b) diffusion coefficient.

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130 °C

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Pyrite Scale Removal using Green Formulations for Oil and Gas Applications: Reaction Kinetics Musa E.M. Ahmeda, Mohammed A. Saad*b, Ibnelwaleed A. Husseina*, Abdulmujeeb T. Onawolea, Mohamed Mahmoudc aGas

Processing Center, College of Engineering, P.O. Box 2713, Qatar University, Doha, Qatar Engineering Department, College of Engineering, P.O. Box 2713, Qatar University, Doha, Qatar bChemical

cDepartment

of Petroleum Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261,

Saudi Arabia ABSTRACT

Pyrite is one of the toughest iron sulfide scales to remove, which causes major problems in oil and gas production by damaging production equipment. The use of inorganic acid in iron sulfide scale removal particularly pyrite is ineffective and produces toxic gases such as hydrogen sulfide. In this work, H2-S free formulation composed of Diethylene Triamine Pentaacetic acid (DTPA) combined with potassium or cesium carbonate as converters are used. The reaction kinetics of pyrite dissolution using a specially designed rotating disk apparatus (RDA) is investigated. Different characterization techniques such as SEM-EDX, XRD and XPS were used for the characterization of pyrite surface before and after chemical treatment. The effects of temperature, rotational disk speed, and converter type on the kinetics are studied. At 130 °C and 150 °C, the reaction increased linearly with the disk rotational speed representing mass transfer limited reaction and the activation energy was as 9.94 kJ/mol. The DTPA diffusion coefficients for the new formulation at 130°C and 150°C are 1.023 x10-9 cm2.s-1 and 1.177x10-9 cm2.s-1respectively. The replacement of potassium carbonate by cesium carbonate did not produce a significant effect on the reaction kinetics. Coreflooding tests were carried out using the new formulation of DTPA with K2CO3 to simulate the real dissolution of the scale in pipes and a solubility of 140 ppm/hr has been attained. The estimation of pyrite dissolution rate by DTPA is expected to support engineering design in iron sulfides removal from oil and gas wells. *Corresponding Authors: Prof. Ibnelwaleed A. Hussein ([email protected]); Dr. M.A. Saad ([email protected]). Keywords: Pyrite, Scale removal, Chelating agent, rotating disc, DTPA 1 ACS Paragon Plus Environment

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1. INTRODUCTION Iron sulfide is one of the persistent scales formed in oil and gas wells. It is formed by the reaction of iron and hydrogen sulfide. Iron is present in the oil and gas production system in many places including reservoir, wellbore tubular as well as surface facilities. Hydrogen sulfide can also be found as a free gas in sour gas wells, or through evolution during the degradation of organic sulfurcontaining compounds and drilling mud additives due to the presence of sulfate reducing bacteria (SRB) 1–3. The reaction of ferric ions with hydrogen sulfide gas leads to the precipitation of iron sulfide scales 4. Iron sulfides occur in different forms, which include pyrrhotite (Fe7S8), and Pyrite (FeS2). However, the latter is presumed to be the more difficult to remove as the ratio of sulfur to iron is 2 to 1 5. Hence, finding a formulation that dissolves pyrite effectively would be effective to remove other iron sulfide scales. Both mechanical and chemical treatments have been used in iron sulfide scale removal from oil and gas wells 6. However, the latter is preferred over the former except if the scales are present in the wellbore and can be simply crushed out. The mechanical method often leads to pitting which enhances corrosion 7. Hydrochloric (HCl) acid is the commonly used chemical for scale removal. However, there are many disadvantages for using HCl including the production of the toxic hydrogen sulfide (H2S) and consequently the high corrosion rate. HCl has low dissolving power for difficult iron sulfides such as pyrite and marcasite 8 which both have a 2 to 1 sulfur to iron ratio.

Another

acid

dissolver

used

to

remove

iron

sulfide

scales

is

tetrakis(hydroxymethyl)phosphonium sulfate (THPS) 9. THPS was found effecive in dissolving iron sulfide including the difficult scales like pyrite without the generation of H2S. However, this solution is still corrosive to mild steel10. Corrosion inhibitors including natural products such as proteins are often used in oilfield scale control 11,12. However, they are expensive and often cause further problems if not used minimally as they can form precipitates13. The recent review and research work of our group 14–16 developed a new green formulation using basic chelating agent, DTPA and cheap soluble salt, K2CO3, as a converter for barite (BaSO4) and mixed iron sulfide scale dissolution. This new formulation can dissolve almost 85 % of iron sulfide field scale that contain Pyrrhotite (Fe7S8), Pyrite (FeS2) & Siderite (Fe2CO3)14. Furthermore, the new formulation does not generate H2S toxic gas and it has a low corrosion rate. The proposed formulation contains Diethylene Triamine Pentaacetic Acid (DTPA) with potassium carbonate 2 ACS Paragon Plus Environment

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(K2CO3) as a converter. Furthermore, the new formulation was studied using density functional theory (DFT) 5,16 which confirmed the chelating ability of DTPA in iron sulfide scale removal. The rotating disk apparatus (RDA) is a conventional tool used in oil and gas industry to study reaction kinetics particularly in determining the dissolution rate of reservoir rocks or scales when testing the effectiveness of new formulations. Other similar tools such as spinning disk apparatus are also used for reaction kinetics studies of materials applied in the energy industry 17. In addition to the reaction rate, the diffusion coefficient, activation energy , reaction order and the reaction regimes can be predicted from RDA experiments 18. RDA was used in determining the rate of calcite and dolomite dissolution using emulsified acids19,20 and HCl

21–23

. Also, the kinetics of

formulation composed from Gelled acids24,25 and organic acids 26,27 with reservoir rock have been studied. The kinetics of pyrite present in Bazhenov Shale has also been studied 28. The theory of RDA is extensively discussed in our earlier work on barite dissolution15. White29 pointed out that the rate at which mass is transferred for Newtonian fluids whilst Lund30,31 described the surface reaction rate. Amongst the chemical treatment for scale removal, chelating agents have been introduced as environmentally friendly alternatives compared to inorganic acids32. The work aims to study the reaction kinetics of pyrite dissolution using DTPA/K2CO3 formulation in a rotating disk apparatus. Furthermore, diffusion coefficient, the effects of temperature, converter type and speed of the rotational disk reaction kinetics are investigated. This would improve the understanding of how the new formulation is effective in removing pyrite scale and provide the needed data for designing typical formulations for field applications. 2. MATERIALS AND METHODS: 2.1 Materials Pyrite rock samples, provided from international geological rock supplier, was used in this work. Pyrite disks were prepared for the rotating disk experiment (Fig. 1). Each disk had a thickness and diameter of 0.5 and 1 inch, respectively. The surface exposed to the reactive fluid was polished to ensure smoothness. The pyrite rock sample was very tight. The average measured porosity is 0.06 % while the permeability was equal zero. 1-inch diameter core samples were drilled from the raw pyrite rock samples. Then it was cut into small disks with the exposed to the fluid being highly 3 ACS Paragon Plus Environment

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Figure 1. Pyrite disks used in RDA measurements

polished. Then disks were cleaned using DI water and then dried in an oven and the porosity and permeability were then measured. Finally, the disk was mounted in the reactor using heat shrinkage Teflon tubing. DTPA (Figure 2) was used as the chelating agents for the pyrite kinetics dissolution study. Though DTPA could be potassium (K)- or Sodium (Na)-based, the former was employed in this work has it been proven that it achieved better solubility results for barite scale removal33. The dissolution rates of pyrite rock samples in a fluid composed of DTPA with a convertor were determined by using the RDA. Both potassium and cesium carbonates (K2CO3 and Cs2CO3) were used as convertors. The RDA (Fig. 3) consists of a reactor where the main reaction takes place, a reservoir cell to supply the fluid at the desired reaction conditions, automatic sampling system to collect samples at a constant interval of time. The reactor, reservoir cell and the connecting piping system is made of hastelloy C276 to resist corrosion. The RDA can reach up to a maximum pressure of 162 bar (2350 psi) and a temperature range from -10°C up to 250° C. The unit was designed by our research group and manufactured by Top Industrie, France.

Figure 2. The structures of DTPA 4 ACS Paragon Plus Environment

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Figure 3. The rotating disk equipment used in this study The RDA experiments were carried out at 1000 psi. A temperature of 150C is selected to represent the highest temperatures in gas reservoirs. Lower temperatures are selected to represent different depths in the reservoir. The actual pressure in the reservoir is much higher that 1000 psi. In addition, pressure does not affect liquid phase reactions. Hence, water vapor will be kept in the liquid state. However, in our case the same pressure level will do the same job. The pyrite disk was attached to the reactor using a harsh environment heat shrinkage tube that shelters all sides except the exposed reactive surface. The concentrations of DTPA and K2CO3 concentrations used in all experiments were 20 wt.% and 9 wt.%, respectively as this was the optimum concentration that yielded the maximum iron sulfide dissolution in our previous work 14. The RDA experiments were carried out at different disk rotational speeds up to 1200 rpm. 2.2. Material Characterization

Scanning electron microscopy (SEM) was used to identify surface characteristics of pyrite before and after dissolution in DTPA/convertor formulation. The purity of the rock was confirmed with the aid of an X-ray diffraction (XRD) spectroscopic device and elemental analysis using Energydispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) analysis was employed to elucidate the changes in the Fe-S bonding both before and after treatment. The 5 ACS Paragon Plus Environment

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concentration of iron in the collected samples was measured using an ICP-OES (Inductively coupled plasma-optical emission spectrometry).

3. RESULTS AND DISCUSSIONS 3.1 Characterization Results:

Energy-dispersive X-ray (EDX) spectroscopy was used for the elemental analysis to evaluate the pyrite sample composition (Fig. 4a) while the Scanning electron microscope (SEM) provided a visual representative (Fig. 4b) of the pyrite surface. Major impurities such as Mg, Si, Na, Ca, and Al were confirmed to be present in the studied sample. Conversely, the peaks that appeared at 6.4 and 2.3 keV were attributed to Fe and S, respectively. The X-ray powder diffraction (XRD) analysis pattern (Fig. 5) confirmed that the sample is 99.9% pyrite.

Figure 4. SEM and EDX analyses of untreated pyrite.

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Figure 5. XRD analysis for pyrite samples 3.2 RDA Experimental Results The change in the surface morphology (Fig. 6) of the disk due to the dissolution of pyrite by the new formulation shows a smoother surface with some pits around the brownish area after the addition of DTPA and potassium carbonate. The brown color which shows around the pits (Fig. 6b) most likely to be iron carbonate, FeCO3 which is probably formed due to the influence of the converter, potassium carbonate. The converter provided more surface area for the dissolution as this is also evident in the SEM images (Fig. 7).

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Figure 6. Pyrite surface before and after treatment with DTPA and K2CO3 at 1200 rpm, 150 °C and 1000 psi after 30 minutes. 3.3 Effect of Temperature and Rotational disk speed

Two sets of RDA experiments were conducted at 130°C and 150°C to study the effects of temperature on pyrite with DTPA and K2CO3. All experiments were conducted for 30 minutes at 1000 psi, which ensures that all gases will be in solution. Effluent samples were collected periodically during experiment every 5 minutes. An increase in temperature from 130 to 150°C led to an increase in the rate of reaction. For example, at rpm of 1200 the measured reaction rate was 2.38x10-9 and 7.3x10-9 (mole/cm2.s) at 130o and 150°C, respectively. This indicates that the reaction rate at 150°C is almost 3 times that at 130°C. The SEM images (Fig. 8) confirmed more surface area were involved in the reaction with a rise in temperature. Hence, temperature effect is a strong influence on the reaction rate.

Figure 7. SEM analysis showing pyrite surface before and after treatment with DTPA/K2CO3

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Figure 8. SEM images showing disk surface at different temperatures For the rotational disk rotational speed effect, the speed was varied between 300 to 1200 rpm at 1000 psi at the two different temperatures used earlier (130°C and 150°C). The graphs (Fig. 9) show that the reaction rate linearly increased as the disk speed increased. The increase in the dissolution rate with increasing rpm at 130oC was not observed exactly at 150C. Similar dissolution rates were observed up to 20 minutes, moving forward, only 1200 rpm shows higher dissolution rate, while 300 and 900 rpm gave the same exact results. The DTPA diffusion coefficients for the new formulation at 130°C and 150°C were 1.023 x10-9 cm2.s-1 and 1.177x10-9 cm2.s-1, respectively (Fig. 10). The activation energy, 𝐸𝑎, was derived using the well-known Arrhenius equation: ―𝐸𝑎

𝑘 = 𝐴𝑒 𝑅𝑇 where: k = rate constant, T = temperature (in kelvins), A = pre-exponential factor, a constant for each chemical reaction, Ea = the activation energy for the reaction and R is the universal gas constant. The activation energy, 𝐸𝑎, has been calculated as 9.94 kJmol-1. A positive activation energy confirms that the reaction rate increases as the temperature increases.

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Figure 9. Effect of disk rotational speed on pyrite dissolution at (a) 130oC (b) 150oC

(a) reaction rate

(b) diffusion coefficient.

Figure 10. Reaction rate at different disk rotational speeds and diffusion coefficients for DTPA + K2CO3 with pyrite sample. 3.4 Effect of converter type

The addition of converters to chelating agents proved to have a synergistic effect on the dissolution rate of oilfield scales 34–36. The effect of the convertor type with DTPA was studied at 150 °C, 1000 psi, and 1200 rpm by using equimolar concentrations of K2CO3 and Cs2CO3. Overall, there was no significant difference between K and Cs in the dissolution of pyrite (Fig.

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11). However, the SEM images of the treated surfaces (Fig. 12) suggest that K2CO3 is similar to CS2CO3 as reflected in the surface area that has undergone reaction.

Figure 11. Effect of converter type on pyrite dissolution (P, 1000 psi; T, 150°C, rpm 1200 and the reaction time, 30 min)

Figure 12. SEM analysis after reaction with the chelating agent with different converters (disk rotational speed, 1200 rpm; P, 1000 psi; T, 150°C and reaction time, 30 min)

3.5 Coreflooding studies

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The core-flooding studies for DTPA/K2CO3 was evaluated to simulate the real dissolution of the scale in a tubular system. A hole was drilled into the center of the Pyrite core sample (Fig. 13). The Coreflooding experiment was performed using a constant rate of 0.1 cm3/min at 150°C and 500 psi for 48 hours. The overburden pressure was 3000 psi and the back pressure is 500 psi. Effluent samples effluent was collected periodically on hourly basis. The concentration of iron in the collected samples was measured using an ICP-OES. The iron concentration increased in the first three hours of the injection of DTPA/K2CO3 indicating high dissolution. This was attributed to the higher surface area due to the irregular edges from drilled hole. After, the surface had become smoothened, the iron concentration was stabilized at almost 140 ppm for remainder of the 40 hours. The results indicate that DTPA/K2CO3 possess good ability for pyrite dissolution at typical physical conditions (temperature and pressure) found in oil wells where iron sulfides scales are formed. In comparison with the dissolution rate in coreflood experiment we observed that the rate in the coreflood is orders of magnetude higher and this is mainly due to differences in Re numbers in the two cases. The radius of the driiled hole was 5 mm with 2 in length. Therefore, ReRDA/Recoreflood for the case of 1200 rpm=

𝜔𝑟2 𝑣𝐷

= 7.39x106 which suggets that the flow in the two cases is in different flow regimes.

Figure 13. Iron concentration for effluent samples collected periodically every hour (inset: pyrite sample used for coreflooding)

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3.6 XPS Analysis The X-ray photoelectron spectroscopy (XPS) was used to further analyze the pyrite scale sample before and after DTPA/K2CO3 experiments. The samples to be analyzed were first rinsed with deionized water and dried at a temperature of 105 °C. The binding energies of sulfur compounds occurs within the range of 160-178 eV. Sulfides occur within the 162 to 164 eV range (Fig. 14a) of which FeS occurs at 162.5eV while FeS2 occurs at 163.7 eV. The peak at 163.7 for treated pyrite with DTPA/K2CO3 is almost non-existent as compared to the untreated pyrite. Hence confirming dissolution of the pyrite. The compounds formed by iron are usually observed within the range of 706-713 eV. The binding energies of FeS2 (pyrite) and FeS occurs at 707 eV and 712 eV, respectively (Fig. 14b). Upon treating the sample with DTPA/K2CO3. The FeS2 peak decreased while the FeS peak increased which affirmed that the pyrite underwent dissolution and produced FeS, which is less difficult to remove compared to FeS2. 4. CONCLUSIONS

In this paper, a new environmentally friendly formulation consisting of DTPA chelating agent and potassium carbonate as a converter is used in dissolving iron sulfide hard scale (pyrite). The formulation was evaluated using rotating disk apparatus, RDA and coreflooding. The effect of temperature, converter type, and disk rotational speed are addressed. The conclusions of this study are as follows:

(a)

(b)

Figure 14. XPS spectra of pyrite sample (a) before and (b) after reaction with formulations. 13 ACS Paragon Plus Environment

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

The reaction of pyrite with DTPA-K2CO3 formulation at 130°C and 150 °C found to be increasing with increasing disk rotational speed up to 1200 indicating mass transfer limited reaction regime.



The DTPA diffusion coefficients for the new formulation at 130°C and 150°C have been obtained as 1.023 x10-9cm2.s-1 and 1.177x10-9cm2.s-1, respectively



The activation energy has been calculated to be 9.94 kJmol-1.



Overall, the convertor type (K2CO3 vs Cs2CO3) did not yield a significant influence on the dissolution rate.



The new formulation is tested in a coreflooding system using hollow cylinder of iron sulfide rock sample to simulate the real dissolution of the scale in pipes and a dissolution of 140 ppm/hr has been achieved.



The estimation of the pyrite dissolution rate by DTPA is expected to support designing formulations for removing pyrite scale from oil and gas wells.

ACKNOWLEDGMENT This publication was made possible by NPRP Grant # 9-084-2-041 from Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors. Qatar University and the Gas Processing Center are acknowledged for their support. Analysis of iron was accomplished in the Central Laboratories unit, Qatar University.

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