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Removal of contaminants from polluted drilling mud using supercritical carbon dioxide extraction
J. of Supercritical Fluids 88 (2014) 1–7
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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Removal of contaminants from polluted drilling mud using supercritical carbon dioxide extraction Reza Khanpour, Mohammad Reza Sheikhi-Kouhsar, Feridun Esmaeilzadeh ∗ , Dariush Mowla School of Chemical and Petroleum Engineering, Shiraz University, Mollasadra Avenue, Shiraz, Iran
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Article history: Received 22 May 2013 Received in revised form 7 January 2014 Accepted 7 January 2014 Available online 18 January 2014 Keywords: Contamination Drilling mud Supercritical carbon dioxide Optimization Extraction
a b s t r a c t Liquid drilling fluid is often called drilling mud is heavy, viscous fluid mixtures use to carry rock cuttings to the surface and lubricate and cool the drill bit. During carrying cuttings they contaminated which not only reduce their functionality but also make them a hazardous and dangerous wastes which cannot be discharged anywhere without treatment. Due to this fact, in the present study, supercritical extraction process was used to remove contaminants from the drilling mud. Regarding this, effect of different parameters including extraction temperature (313–338 K) and pressure (100–200 bar), flow rate of CO2 (0.05–0.36 cm3 /s) and static time (20–130 min) on the removal of contaminations from drilling mud was examined using the design of experiment of changing one factor at a time. The obtained results revealed that the optimum operational conditions that lead to the highest removal degree of contaminations are temperature and pressure of 333 K and 180 bar, respectively, flow rate of lower than 0.1 cm3 /s and the static time of 110 min. In addition, to examine the effect of the supercritical extraction on the crystalline structure modification and removal contaminations X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed which confirmed the successful removal of contaminations from the drilling mud without significant crystalline modification. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Drilling fluid – mud – is usually a mixture of water, clay, and a few chemicals. Sometimes, oil may be used instead of water, or oil added to water to give mud certain desirable properties. World oil’s annual classification of fluid systems lists different categories of drilling fluids, including fresh-water systems, salt-water systems, oil-or synthetic-based systems, pneumatic (air, mist, foam, gas) “fluid” systems [1–3]. Among the above drilling fluid categories, water-based fluids (WBFs) are the most widely used systems which are less expensive than oil-based fluids (OBFs) or synthetic-based fluids (SBFs). Waterbased fluids (WBFs) are used to drill approximately 80% of all wells [4]. The base fluid may consist of fresh water, seawater, brine, saturated brine, or formation brine. The type of selected fluid depends on the anticipated well conditions or on the specific interval of the well being drilled [5,6].
∗ Corresponding author at: School of Chemical and Petroleum Engineering, Shiraz University, Mollasadra Avenue, PO Box: 71346-1719, Shiraz, Iran. Tel.: +98 711 6133710. E-mail address: [email protected] (F. Esmaeilzadeh). 0896-8446/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2014.01.004
Regardless of drilling mud categories, drilling fluids are used to raise the cuttings made by the bit and to lift them to the surface for disposal. In other words, as the drill bit grinds the rocks into drill cuttings, these cuttings will be entrained in the mud flow and then carried to the surface. In order to return the mud to the recirculation mud system and to make contaminations easier to handle, contaminations must be separated from the mud. In more details, from 1993 discharges of cuttings containing more than 1% oil were prohibited in several regions due to environmental reasons. For some period, oil-based fluid was common to be replaced by organic fluids such as esters, ethers and olefins, but the operational discharges of cuttings with residues of oil or synthetic base fluids ceased it around 1995. In practice, operational discharges in drilling mud only take place by using water-based drilling fluid. All cuttings which contain oil exceeding one percent by weight must either be re-injected or taken ashore for treatment. Generally, there are several kinds of compounds in the contaminated drilling mud which make it a dangerous waste in the case of being released into the environment without any purification and treatment [7,8]. During the past years, several treating processes have been proposed such as steam stripping, dehalogenation, chemical reduction/oxidation, ultraviolet (UV) oxidation and etc. [2,9,10]. Although these kinds of processes are efficient to some extent, all
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these techniques suffer from safety, complexity or high-energy use problems [11]. On the other hand, one of the consequences of the treatment of these wastes is the generation of residual solids or concentrated brines that are separated from the treated water. In some cases, these residual wastes consisted of high concentrations of salts and metals that are themselves landfilled, or sent for underground injection. Unfortunately, the disposal of residual waste in landfills raises the concern that landfill personnel and environmental quality may be at risk. In this regard, to overcome the limitations of current drilling waste treatment and disposal options, alternative technologies are being investigated for the treatment of oil-contaminated drill cuttings which are not only energy consuming but also are green and environmentally benign. In general, it has been reported that there are several kinds of contaminates can occupy the active sites of the drilling mud such as sulfur, chlorine and hydrocarbons mostly with carbon number of C10 –C28 [11]. In this regard, several authors have been tried to find if supercritical carbon dioxide is able to remove these contaminants from the polluted drilling mud [11–13]. For example, Goodarznia and Esmaeilzadeh [11] have been reported that using SC-CO2 by changing the temperature and pressure in the range of 328–352.5 K and 160–220 bar respectively, leads to removal of 22.4% contaminant from the polluted drilling mud. One of the most widely used and studied new techniques for the removal of pollutions from the contaminated matrixes are supercritical fluid based technologies. Supercritical fluids have several desirable properties that make them attractive for certain separation processes, e.g. the product is not contaminated with residual solvent [14–24]. The supercritical fluid extraction (SFE) processes used are environmentally friendly, inert, cheap and are widely available. Supercritical carbon dioxide (SC-CO2 ) exhibits excellent solvating characteristics which are easily manipulated to dissolve non-polar compounds like diesel and mineral oils. In details, using supercritical fluids such as carbon dioxide (CO2 ) overcomes many drawbacks in connection with the use of liquid organic solvents like liquid hexane. The commonly used solvents undergo the problems of toxicity and residual content after extraction. In addition to toxicity, there is also the dander of security during storage due to flammability [17]. In this context, using SC-CO2 enjoys many advantages: it is nontoxic, inflammable and CO2 separation is easily done by a simple depressurization. The other crucial point is that after the processes are finished (remediation, cleaning and removal of contaminates from the polluted matrixes), there is no residual organic solvent trace in the final products. Among the many investigations conducted using SC-CO2 for removal of contaminants from the dense matrixes, one can pointed out to cleaning of deactivated catalysts using supercritical carbon dioxide extraction (SCE) [25,26]. In details, Rajaei and his coworkers [25,26] used supercritical carbon dioxide extraction process to remove contaminations and pollutants from R-134 catalyst and Tonsil CO 610 G clay soil which found it successful to remove contaminants from polluted catalysts and regenerate them. Furthermore, Chen et al. [27] studied the removal of polychlorinated biphenyl (PCB) from soils in a laboratory scale using supercritical fluid extraction (SFE) unit to provide information for soil remediation. The results demonstrated the effectiveness of SFE as a promising technology for the clean-up of PCB contaminated soils/sediments. They have been reported that after 30 min of extraction at 313 K and 100 bar, more than 86% of PCBs in real world Hudson River sediment and 92% in St. Lawrence River sediment were removed. Based on the best knowledge of the authors, there are a few investigations on the removal of contaminants from the polluted drilling mud performed by co-authors [11] represented possibility of removing contaminants from the polluted drilling mud using supercritical carbon dioxide extraction process.
In this regard, the potential of supercritical carbon dioxide to extract contaminants from drilling mud was investigated by changing extraction pressure from 100 bar to 200 bar, extraction temperature from 313 K to 338 K, flow rate from 0.05 cm3 /s to 0.36 cm3 /s and static time from 20 to 130 min. Finally, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were performed for more reliable conclusion on the feasibility of supercritical carbon dioxide to remove contaminants from the polluted drilling mud considering the crystalline and morphology modification. 2. Experimental 2.1. Experimental procedure The used extraction apparatus was designed for maximum pressure and temperature of 400 bar and 373.15 K, respectively (see Fig. 1). The used procedure during this investigation was as follow in brief. Carbon dioxide supplied from a gas cylinder was liquefied through a cooling unit. Then, liquid carbon dioxide was compressed by a high-pressure air driven oil-free reciprocating pump (Haskel, USA). The liquefied high pressure carbon dioxide was dispatched into a surge vessel to dampen the pressure fluctuations generated by the operation of the pump. At the outlet of the surge tank, a bourdon gauge in the range of 0–400 bar by a division of 1 bar was placed to monitor the pressure of the system easily. Then, the pressurized supercritical carbon dioxide entered into the equilibrium cell (180 cm3 ). The surge tank and the extraction vessel were surrounded by a regulating hot water jacket to set the temperature of system up to373.15 K. The extraction temperature could be sensed easily by a PT-100 thermocouple with precision of 1 K which controls the temperature of the system using a PID control protocol. The point worthy of mentioning is that during the experiments, the extraction pressure and temperature were held constant in the range of 3% of instrumental full scale by the continuous monitoring of the system operational conditions. The contaminated drilling mud (about 147 g) which was packed by glass beads and glass wool in a stainless steel basket was placed in the extraction vessel for further processing. The glass beads were used to increase the surface area between the contaminated drilling mud particles and the SCF. Also, the wool glass was used to prevent carrying out the mud particles over the SCF flow. The basket was then placed into an extraction vessel and held in the desired conditions. In this procedure, the difference between the initial and final weight of the V-separator was the amount of the extracted contaminations from the drilling mud. The point must be mentioned is that the extracted contaminants were weighed after 24 h period which the V-separator was heated up to313 K. This heating up was performed to eliminate any possible presence of carbon dioxide and water in the extracted contaminants which may leads to errors in the calculations of extracted contaminants. In other words, before and after finishing the extraction process, the V-separator was weighted and the difference between the initial and final mass of the V-separator was considered as the amount of the extracted contamination. The mass of extracted contaminations was determined to be 0.1 mg using a Sartorius BA110S Basic series balance. The typical mass of solute for each experiment was greater than 50 mg, giving a potential error due to weighing of 0.2% wt. Finally, the vented carbon dioxide is passed through a wet test meter. 2.2. Materials Drilling mud from Nar gas field in the neighborhood of Kangan oil field, Jam, Iran was kindly supplied from South Zagros Oil &
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Fig. 1. Schematic diagram of the used extraction apparatus.
Gas Production Co. It was then used as a sample without receiving any further processes. Also, carbon dioxide (99.8% < purity) supplied from Abughadareh Industrial Gas Company (Iran) was utilized as a solvent for all the measurements.
2.3. Scanning electron microscopy (SEM) In this work, the processed and unprocessed drilling mud were examined through scanning electron microscopy (SEM) (S360CAMBRIDGE) to reliably check whether or not the drilling mud has been changed. Thus, off-line method of analysis was used and the surface structure and homogeneity of processed and unprocessed samples were compared with each other. Prior to the SEM examination, the samples were coated by a sputter-coater (SC7640-Polaron) with Pd–Pt in the presence of argon (99.9% < purity) at room temperature for 100 s under an accelerating voltage of 20 kV.
2.4. XRD analysis Verification for the crystal structures is done through XRD. The drilling mud particles, forming a weighted dispersion on a glass slide, was evaluated using an X-ray powder diffractometer (Bruker, D8 ADVANCED, Germany). The sample was irradiated using a Cu target tube, and exposed to all lines. A monochromator was used to select the K-1 line ( = 1.54056). The scanning angle ranged from 5◦ to 100◦ of the diffraction angle (2), and the counting time used was 1 s/step in steps of 2 = 0.05◦ . The scanning rate used was 3◦ /min. The excitation current used was 40 mA and the excitation voltage used was 30 kV.
3. Results and discussion In the current work, the total number of 29 different experiments was performed to examine the effect of static time, flow rate of CO2 , pressure and temperature on the removal of hydrocarbon contaminations from the drilling mud (see Table 1).
Fig. 2. Effect of static time on the extraction of contaminants from the polluted drilling mud.
3.1. The effect of static time on the removal of contaminations from drilling mud Generally, extraction processes are divided into three different stages, namely, rapid extraction of free solute, transitional stage of surface, and internal diffusion and slow extraction mainly based on the internal diffusion. The time spent in the first extraction stage depends both on the solute solubility in SC-CO2 and on the particle size. One of the parameters affecting the internal diffusion is static time. In other words, as the static time increases, the internal diffusion increases, too since there is more time available for SC-CO2 to be penetrated into the matrix structure and to extract desired solutes. In this regard, in the first stage of this study, the effect of static time ranged between 20 and 130 min on the efficiency of contaminations removal from drilling mud was investigated while the extraction pressure, temperature, and flow rate were held constant at 100 bar, 313 K and 0.1 cm3 /s, respectively. The obtained resulted given in Fig. 2, showed that the efficiency of SC-CO2 to remove contaminants from the drilling mud increases as the static time increases from 20 min to 100 min. Of course, further increase in the static time from 100 min to 130 min leads to a slight increase in the removal of contaminants. This observed trend can be related to the fact that, with increase in the static time, the increase in contact time between contaminants and supercritical carbon dioxide
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Table 1 The obtained results of different experimental conditions. Experiment no.
Flow rate (cm3 /s)
Static time (min)
Pressure (bar)
Temperature (K)
Concentration of extracted contaminations (g/L)
Effect of flow rate 1 2 3 4 5 6 7
0.36 0.22 0.14 0.10 0.08 0.07 0.05
120 120 120 120 120 120 120
100 100 100 100 100 100 100
313 313 313 313 313 313 313
0.0595 0.0906 0.1812 0.7334 0.7617 0.7589 0.7702
Effect of static time 8 9 10 11 12 13 14 15 16 17 18 19
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
20 30 40 50 60 70 80 90 100 110 120 130
100 100 100 100 100 100 100 100 100 100 100 100
313 313 313 313 313 313 313 313 313 313 313 313
0.0110 0.0205 0.0314 0.0318 0.0371 0.0434 0.0484 0.0480 0.0523 0.0555 0.0544 0.0547
Effect of pressure 20 21 22 23 24 25
0.1 0.1 0.1 0.1 0.1 0.1
110 110 110 110 110 110
100 120 140 160 180 200
313 313 313 313 313 313
0.0555 0.0869 0.1353 0.1734 0.1791 0.1766
Effect of temperature 26 0.1 27 0.1 28 0.1 0.1 29
110 110 110 110
180 180 180 180
313 323 333 338
0.1791 0.2059 0.2200 0.2228
leads to the higher capability of the SC-CO2 to dissolve the contaminants. In other words, when the static time increases, more time is available for SC-CO2 to be penetrated into the drilling mud and to extract the pollutants. Based on these experiments, static time of 110 min was considered as the optimum one since no significant increase in the amount of extracted contaminates was observed. 3.2. Effect of flow rate on the extraction of contaminates The other important parameter in the SC-CO2 -based extraction processes is CO2 flow rate that exhibits positive and significant effects on the extraction yield of solute possibly due to a decrease in the mass transfer resistance as flow rate increases. The SFE mechanism contains three successive steps, namely, solute dissolution, intra-particle diffusion, and external diffusion. The controlling step is changing gradually and slowly, not sharply. For example, in the initial stage of extraction, the main resistance may come from the external diffusion or solute dissolution. So external diffusion limited or solute dissolution will be limited at this stage and the SC-CO2 flow rate might have a significant influence on the extraction yield. With the dissolution progressing, the dissolution front is moving from the external surface to the center of particle gradually, and the intra-particle diffusion distance is becoming bigger and bigger. As a result, the intra-particle diffusion resistance will increase [28]. So, eventually, (at the later stage) the system will convert from external diffusion controlling or dissolution controlling into intra-particle diffusion controlling. At this stage, the SC-CO2 flow rate will have no influence on the extraction yield [28]. In the second stage of this investigation, the effect of flow rate on the extraction of contaminants from the drilling mud was investigated. In this regard, the flow rate was ranged from 0.05 cm3 /s
Fig. 3. Effect of SC-CO2 flow rate on the removal of contaminants from polluted drilling mud.
to 0.36 cm3 /s while the other operating conditions including static time (110 min), extraction pressure (100 bar) and temperature (313 K) were held constant during experiments. The obtained results shown in Fig. 3 demonstrated that as the flow rate increases from 0.05 to 0.1 cm3 /s, the amount of extracted contaminants remains constant while further increase in the flow rate from 0.1 cm3 /s to 0.36 cm3 /s leads to a reduction in the extracted contaminants. It will be apparent, by closer examination of Fig. 3, that the increase in the solvent flow rate leads to an increase in the amount of extracted solutes. This observed trend can be related to the following phenomena that the analyte–matrix interactions are usually weak for analytes where extraction is solubility-controlled. Hence, the extraction rate mainly depends on the partitioning of the analytes between the matrix and the supercritical fluid and, therefore, increases by using higher flowrates [29]. However, increasing the SC-CO2 flow rate has its own
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Fig. 4. Effect of extraction temperature on the removal of contaminants from polluted drilling mud.
disadvantages like disturbing the equilibrium conditions. As the flow rate increases, SC-CO2 does not have enough time to diffuse into the deep sites of the matrix and extracts the desired compound. Consequently, the lower amount of compounds will be extracted. In other words, the flow rate will have a dual effect dictating by the net effect of these competing factors if the extraction yield increases or decreases. Similar results were reported by Rajaei et al. [26] for extracting pollutions from the deactivated catalysts named Tonsil CO 610 G. They have related their observations to the fact that the extraction rates controlled primarily by the solubility/elution process showed direct correlation with SC-CO2 flow rates. But, the extraction rates for samples being controlled primarily by the kinetics of the initial desorption step showed little or no correlation with different SCE flow rates. Finally, based on these findings, the flow rate of 0.1 cm3 /s or lower can be considered as the optimum flow rate for the contaminants removal from drilling mud. 3.3. Effect of temperature and pressure on the removal of contaminates There are two thermodynamic parameters namely extraction temperature and pressure which can significantly manipulate the extraction efficiency during supercritical based technologies. They can be used to tune the solubilizing strength of any supercritical fluids. Because of its importance, in the last section of this study, several experiments were performed to investigate the effects of extraction temperature and pressure on the removal of contaminants from drilling mud. In the first step, the extraction temperature which can affect the thermodynamic conditions was changed between 313 K and 338 K (313 K, 323 K, 333 K and 338 K) while the other operating parameters including static time (110 min), extraction pressure (180 bar) and SC-CO2 flow rate (0.1 cm3 /s) kept constant. The obtained results shown in Fig. 4 revealed that the increase of extraction temperature from 313 K to 333 K leads to an increase in the amount of removed contaminants from 0.18 g/L to 0.22 g/L while further increase in the extraction temperature shows a slight effect on the amount of extracted contaminants. Although, generally one can expect a reduction in the solute solubilization as the temperature of SCCO2 increases, contradicting results were observed in this stage. Although the examined extraction temperature range in the current study is too narrow but this observed trend can be related to the dual effect of the temperature on the solubility of solute. In more detail, it has been proved that the temperature affects supercritical extraction (SCE) recovery by changing both the thermodynamic (density) and kinetics of the process. Extraction temperature has two contradicting effects on SCE, i.e. decreasing fluid density and
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Fig. 5. Effect of extraction pressure on the removal of contaminants from polluted drilling mud.
hence, the solute recovery on the one hand and increasing solute solubility through its vapor pressure, on another hand. So, the obtained results show that the enhancement of solubility by means of changing the solute vapor pressure is the dominant effect between the lower density (higher extraction temperature) which leads to lower solvating power and higher solubility through the vapor pressure of the solute. The noteworthy point is that temperature can also affect rates of SCE through solute interactions with the matrix, as in kineticallycontrolled desorption of contaminants [29]. Therefore, the total amount of solute being able to be desorbed may sometimes depend on the temperature alone and a higher temperature can increase recovery even at lower SCF density. Contradicting results were reported by Rajaei et al. [26] for the extraction of contaminants from polluted Tonsil CO 610 G catalyst soil. They have concluded that the effect of temperature on the reduction of SC-CO2 solvating power was more dominant, in comparison with the effect of temperature on the enhancement of solute vapor pressure. So, an increase in the extraction temperature leads to a reduction in the capability of SC-CO2 to remove contaminants from polluted catalysts. Based on these findings and due to the economic issues as a result of working with higher temperatures, the optimum value of extraction temperature was held constant at 333 K for the rest of the experiments. In the next stage of this section, six other experiments were performed to examine the effect of extraction pressure on the removal of contaminants from the polluted drilling mud. To this end, the extraction pressure was ranged between 100 bar and 200 bar while the other parameters including extraction temperature (333 K), static time (110 min) and SC-CO2 flow rate (0.1 cm3 /s) were held constant (Fig. 5). The obtained results showed that there will be a direct relation between the extraction pressure and extraction yield of contaminants from the polluted drilling mud if extraction pressure changes from 100 to 180 bars. Of course, the further increase in the extraction pressure leads to no more increase in the amount of extracted contaminants. This obtained trend could be related to the density change of the supercritical carbon dioxide with the increase of extraction pressure. In other words, when the extraction pressure is increased, the density of the supercritical fluids will be increased resulting in the higher solvating power of the supercritical fluids. 3.4. SEM and XRD analysis Finally, to investigate the effect of SC-CO2 extraction on the extraction of contaminants from the polluted drilling mud, XRD and SEM analysis were performed. The obtained results of XRD analysis showed in Fig. 6 revealed that intensity of the peeks for
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4. Conclusions
Fig. 6. XRD results for processed and unprocessed drilling mud.
In the present study, the efficiency of SC-CO2 extraction process on the removal of contaminants and pollutions from the polluted drilling mud was examined. In this regard, a home-made SC-CO2 apparatus was used to investigate the influence of different parameters including extraction pressure and temperature, SC-CO2 flow rate, and static time on the removal of contaminants from polluted drilling mud. The obtained results during the experimentations revealed that the increase of the extraction pressure will lead to an increase in the removal of contaminants if extraction pressure increases from 100 to 180 bar while further increase of it up to 200 bar leads to no significant effect. In addition, the results revealed that there was a direct relation between the extraction temperature and extraction efficiency of pollutant from polluted drilling mud. Also, the results revealed that increasing the static time up to 110 min leads to higher extraction efficiency while further increase in the static time has no significant effect on the removal of contaminants from drilling mud. Finally, the SC-CO2 flow rate was optimized by means of performing several experiments. Totally, the obtained results revealed that the optimum static time of 110 min, SC-CO2 flow rate of 0.1 cm3 /s, and extraction pressure and temperature of 180 bar and 333 K, respectively, can be considered as the optimum parameter for the removal of contaminations from the polluted drilling mud. At last, for more reliable conclusions, XRD and SEM analyses were run which revealed that the SC-CO2 extraction is able to removed contaminants from the polluted drilling mud satisfactorily and make the drilling mud particle surfaces more homogenous compared with the unprocessed sample. Among all, it can be concluded that the SC-CO2 process can be considered as an applicable and feasible method to remove contaminants from the drilling mud. References
Fig. 7. SEM images of the (a) processed (magnification ×2000) and (b) unprocessed (magnification ×2000) drilling mud sample.
unprocessed particles is higher than the processed one. Based on these results there are two possibilities. The first one is modification of the crystalline structure of the drilling mud since a shifting on the degree of several plates (37.5◦ for unprocessed to 36◦ for processed and 45◦ for unprocessed to 43◦ for processed samples) has been occurred. The second one is the reduction in the intensity of the peeks (62.5◦ ) for processed samples compared with the unprocessed samples which can be due to the removal of contaminants from the processed particles. In addition, investigating the SEM images for processed and unprocessed particles of drilling mud, it can be concluded that the processed drilling mud particles are more homogenous compared with the particles of unprocessed drilling mud (Fig. 7). Totally, based on the statistical analysis, quantitative analysis and XRD and SEM investigations it can be concluded that the supercritical carbon extraction process is able to remove contaminants from the drilling mud satisfactorily.
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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Removal of contaminants from polluted drilling mud using supercritical carbon dioxide extraction Reza Khanpour, Mohammad Reza Sheikhi-Kouhsar, Feridun Esmaeilzadeh ∗ , Dariush Mowla School of Chemical and Petroleum Engineering, Shiraz University, Mollasadra Avenue, Shiraz, Iran
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Article history: Received 22 May 2013 Received in revised form 7 January 2014 Accepted 7 January 2014 Available online 18 January 2014 Keywords: Contamination Drilling mud Supercritical carbon dioxide Optimization Extraction
a b s t r a c t Liquid drilling fluid is often called drilling mud is heavy, viscous fluid mixtures use to carry rock cuttings to the surface and lubricate and cool the drill bit. During carrying cuttings they contaminated which not only reduce their functionality but also make them a hazardous and dangerous wastes which cannot be discharged anywhere without treatment. Due to this fact, in the present study, supercritical extraction process was used to remove contaminants from the drilling mud. Regarding this, effect of different parameters including extraction temperature (313–338 K) and pressure (100–200 bar), flow rate of CO2 (0.05–0.36 cm3 /s) and static time (20–130 min) on the removal of contaminations from drilling mud was examined using the design of experiment of changing one factor at a time. The obtained results revealed that the optimum operational conditions that lead to the highest removal degree of contaminations are temperature and pressure of 333 K and 180 bar, respectively, flow rate of lower than 0.1 cm3 /s and the static time of 110 min. In addition, to examine the effect of the supercritical extraction on the crystalline structure modification and removal contaminations X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed which confirmed the successful removal of contaminations from the drilling mud without significant crystalline modification. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Drilling fluid – mud – is usually a mixture of water, clay, and a few chemicals. Sometimes, oil may be used instead of water, or oil added to water to give mud certain desirable properties. World oil’s annual classification of fluid systems lists different categories of drilling fluids, including fresh-water systems, salt-water systems, oil-or synthetic-based systems, pneumatic (air, mist, foam, gas) “fluid” systems [1–3]. Among the above drilling fluid categories, water-based fluids (WBFs) are the most widely used systems which are less expensive than oil-based fluids (OBFs) or synthetic-based fluids (SBFs). Waterbased fluids (WBFs) are used to drill approximately 80% of all wells [4]. The base fluid may consist of fresh water, seawater, brine, saturated brine, or formation brine. The type of selected fluid depends on the anticipated well conditions or on the specific interval of the well being drilled [5,6].
∗ Corresponding author at: School of Chemical and Petroleum Engineering, Shiraz University, Mollasadra Avenue, PO Box: 71346-1719, Shiraz, Iran. Tel.: +98 711 6133710. E-mail address: [email protected] (F. Esmaeilzadeh). 0896-8446/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2014.01.004
Regardless of drilling mud categories, drilling fluids are used to raise the cuttings made by the bit and to lift them to the surface for disposal. In other words, as the drill bit grinds the rocks into drill cuttings, these cuttings will be entrained in the mud flow and then carried to the surface. In order to return the mud to the recirculation mud system and to make contaminations easier to handle, contaminations must be separated from the mud. In more details, from 1993 discharges of cuttings containing more than 1% oil were prohibited in several regions due to environmental reasons. For some period, oil-based fluid was common to be replaced by organic fluids such as esters, ethers and olefins, but the operational discharges of cuttings with residues of oil or synthetic base fluids ceased it around 1995. In practice, operational discharges in drilling mud only take place by using water-based drilling fluid. All cuttings which contain oil exceeding one percent by weight must either be re-injected or taken ashore for treatment. Generally, there are several kinds of compounds in the contaminated drilling mud which make it a dangerous waste in the case of being released into the environment without any purification and treatment [7,8]. During the past years, several treating processes have been proposed such as steam stripping, dehalogenation, chemical reduction/oxidation, ultraviolet (UV) oxidation and etc. [2,9,10]. Although these kinds of processes are efficient to some extent, all
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these techniques suffer from safety, complexity or high-energy use problems [11]. On the other hand, one of the consequences of the treatment of these wastes is the generation of residual solids or concentrated brines that are separated from the treated water. In some cases, these residual wastes consisted of high concentrations of salts and metals that are themselves landfilled, or sent for underground injection. Unfortunately, the disposal of residual waste in landfills raises the concern that landfill personnel and environmental quality may be at risk. In this regard, to overcome the limitations of current drilling waste treatment and disposal options, alternative technologies are being investigated for the treatment of oil-contaminated drill cuttings which are not only energy consuming but also are green and environmentally benign. In general, it has been reported that there are several kinds of contaminates can occupy the active sites of the drilling mud such as sulfur, chlorine and hydrocarbons mostly with carbon number of C10 –C28 [11]. In this regard, several authors have been tried to find if supercritical carbon dioxide is able to remove these contaminants from the polluted drilling mud [11–13]. For example, Goodarznia and Esmaeilzadeh [11] have been reported that using SC-CO2 by changing the temperature and pressure in the range of 328–352.5 K and 160–220 bar respectively, leads to removal of 22.4% contaminant from the polluted drilling mud. One of the most widely used and studied new techniques for the removal of pollutions from the contaminated matrixes are supercritical fluid based technologies. Supercritical fluids have several desirable properties that make them attractive for certain separation processes, e.g. the product is not contaminated with residual solvent [14–24]. The supercritical fluid extraction (SFE) processes used are environmentally friendly, inert, cheap and are widely available. Supercritical carbon dioxide (SC-CO2 ) exhibits excellent solvating characteristics which are easily manipulated to dissolve non-polar compounds like diesel and mineral oils. In details, using supercritical fluids such as carbon dioxide (CO2 ) overcomes many drawbacks in connection with the use of liquid organic solvents like liquid hexane. The commonly used solvents undergo the problems of toxicity and residual content after extraction. In addition to toxicity, there is also the dander of security during storage due to flammability [17]. In this context, using SC-CO2 enjoys many advantages: it is nontoxic, inflammable and CO2 separation is easily done by a simple depressurization. The other crucial point is that after the processes are finished (remediation, cleaning and removal of contaminates from the polluted matrixes), there is no residual organic solvent trace in the final products. Among the many investigations conducted using SC-CO2 for removal of contaminants from the dense matrixes, one can pointed out to cleaning of deactivated catalysts using supercritical carbon dioxide extraction (SCE) [25,26]. In details, Rajaei and his coworkers [25,26] used supercritical carbon dioxide extraction process to remove contaminations and pollutants from R-134 catalyst and Tonsil CO 610 G clay soil which found it successful to remove contaminants from polluted catalysts and regenerate them. Furthermore, Chen et al. [27] studied the removal of polychlorinated biphenyl (PCB) from soils in a laboratory scale using supercritical fluid extraction (SFE) unit to provide information for soil remediation. The results demonstrated the effectiveness of SFE as a promising technology for the clean-up of PCB contaminated soils/sediments. They have been reported that after 30 min of extraction at 313 K and 100 bar, more than 86% of PCBs in real world Hudson River sediment and 92% in St. Lawrence River sediment were removed. Based on the best knowledge of the authors, there are a few investigations on the removal of contaminants from the polluted drilling mud performed by co-authors [11] represented possibility of removing contaminants from the polluted drilling mud using supercritical carbon dioxide extraction process.
In this regard, the potential of supercritical carbon dioxide to extract contaminants from drilling mud was investigated by changing extraction pressure from 100 bar to 200 bar, extraction temperature from 313 K to 338 K, flow rate from 0.05 cm3 /s to 0.36 cm3 /s and static time from 20 to 130 min. Finally, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were performed for more reliable conclusion on the feasibility of supercritical carbon dioxide to remove contaminants from the polluted drilling mud considering the crystalline and morphology modification. 2. Experimental 2.1. Experimental procedure The used extraction apparatus was designed for maximum pressure and temperature of 400 bar and 373.15 K, respectively (see Fig. 1). The used procedure during this investigation was as follow in brief. Carbon dioxide supplied from a gas cylinder was liquefied through a cooling unit. Then, liquid carbon dioxide was compressed by a high-pressure air driven oil-free reciprocating pump (Haskel, USA). The liquefied high pressure carbon dioxide was dispatched into a surge vessel to dampen the pressure fluctuations generated by the operation of the pump. At the outlet of the surge tank, a bourdon gauge in the range of 0–400 bar by a division of 1 bar was placed to monitor the pressure of the system easily. Then, the pressurized supercritical carbon dioxide entered into the equilibrium cell (180 cm3 ). The surge tank and the extraction vessel were surrounded by a regulating hot water jacket to set the temperature of system up to373.15 K. The extraction temperature could be sensed easily by a PT-100 thermocouple with precision of 1 K which controls the temperature of the system using a PID control protocol. The point worthy of mentioning is that during the experiments, the extraction pressure and temperature were held constant in the range of 3% of instrumental full scale by the continuous monitoring of the system operational conditions. The contaminated drilling mud (about 147 g) which was packed by glass beads and glass wool in a stainless steel basket was placed in the extraction vessel for further processing. The glass beads were used to increase the surface area between the contaminated drilling mud particles and the SCF. Also, the wool glass was used to prevent carrying out the mud particles over the SCF flow. The basket was then placed into an extraction vessel and held in the desired conditions. In this procedure, the difference between the initial and final weight of the V-separator was the amount of the extracted contaminations from the drilling mud. The point must be mentioned is that the extracted contaminants were weighed after 24 h period which the V-separator was heated up to313 K. This heating up was performed to eliminate any possible presence of carbon dioxide and water in the extracted contaminants which may leads to errors in the calculations of extracted contaminants. In other words, before and after finishing the extraction process, the V-separator was weighted and the difference between the initial and final mass of the V-separator was considered as the amount of the extracted contamination. The mass of extracted contaminations was determined to be 0.1 mg using a Sartorius BA110S Basic series balance. The typical mass of solute for each experiment was greater than 50 mg, giving a potential error due to weighing of 0.2% wt. Finally, the vented carbon dioxide is passed through a wet test meter. 2.2. Materials Drilling mud from Nar gas field in the neighborhood of Kangan oil field, Jam, Iran was kindly supplied from South Zagros Oil &
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Fig. 1. Schematic diagram of the used extraction apparatus.
Gas Production Co. It was then used as a sample without receiving any further processes. Also, carbon dioxide (99.8% < purity) supplied from Abughadareh Industrial Gas Company (Iran) was utilized as a solvent for all the measurements.
2.3. Scanning electron microscopy (SEM) In this work, the processed and unprocessed drilling mud were examined through scanning electron microscopy (SEM) (S360CAMBRIDGE) to reliably check whether or not the drilling mud has been changed. Thus, off-line method of analysis was used and the surface structure and homogeneity of processed and unprocessed samples were compared with each other. Prior to the SEM examination, the samples were coated by a sputter-coater (SC7640-Polaron) with Pd–Pt in the presence of argon (99.9% < purity) at room temperature for 100 s under an accelerating voltage of 20 kV.
2.4. XRD analysis Verification for the crystal structures is done through XRD. The drilling mud particles, forming a weighted dispersion on a glass slide, was evaluated using an X-ray powder diffractometer (Bruker, D8 ADVANCED, Germany). The sample was irradiated using a Cu target tube, and exposed to all lines. A monochromator was used to select the K-1 line ( = 1.54056). The scanning angle ranged from 5◦ to 100◦ of the diffraction angle (2), and the counting time used was 1 s/step in steps of 2 = 0.05◦ . The scanning rate used was 3◦ /min. The excitation current used was 40 mA and the excitation voltage used was 30 kV.
3. Results and discussion In the current work, the total number of 29 different experiments was performed to examine the effect of static time, flow rate of CO2 , pressure and temperature on the removal of hydrocarbon contaminations from the drilling mud (see Table 1).
Fig. 2. Effect of static time on the extraction of contaminants from the polluted drilling mud.
3.1. The effect of static time on the removal of contaminations from drilling mud Generally, extraction processes are divided into three different stages, namely, rapid extraction of free solute, transitional stage of surface, and internal diffusion and slow extraction mainly based on the internal diffusion. The time spent in the first extraction stage depends both on the solute solubility in SC-CO2 and on the particle size. One of the parameters affecting the internal diffusion is static time. In other words, as the static time increases, the internal diffusion increases, too since there is more time available for SC-CO2 to be penetrated into the matrix structure and to extract desired solutes. In this regard, in the first stage of this study, the effect of static time ranged between 20 and 130 min on the efficiency of contaminations removal from drilling mud was investigated while the extraction pressure, temperature, and flow rate were held constant at 100 bar, 313 K and 0.1 cm3 /s, respectively. The obtained resulted given in Fig. 2, showed that the efficiency of SC-CO2 to remove contaminants from the drilling mud increases as the static time increases from 20 min to 100 min. Of course, further increase in the static time from 100 min to 130 min leads to a slight increase in the removal of contaminants. This observed trend can be related to the fact that, with increase in the static time, the increase in contact time between contaminants and supercritical carbon dioxide
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Table 1 The obtained results of different experimental conditions. Experiment no.
Flow rate (cm3 /s)
Static time (min)
Pressure (bar)
Temperature (K)
Concentration of extracted contaminations (g/L)
Effect of flow rate 1 2 3 4 5 6 7
0.36 0.22 0.14 0.10 0.08 0.07 0.05
120 120 120 120 120 120 120
100 100 100 100 100 100 100
313 313 313 313 313 313 313
0.0595 0.0906 0.1812 0.7334 0.7617 0.7589 0.7702
Effect of static time 8 9 10 11 12 13 14 15 16 17 18 19
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
20 30 40 50 60 70 80 90 100 110 120 130
100 100 100 100 100 100 100 100 100 100 100 100
313 313 313 313 313 313 313 313 313 313 313 313
0.0110 0.0205 0.0314 0.0318 0.0371 0.0434 0.0484 0.0480 0.0523 0.0555 0.0544 0.0547
Effect of pressure 20 21 22 23 24 25
0.1 0.1 0.1 0.1 0.1 0.1
110 110 110 110 110 110
100 120 140 160 180 200
313 313 313 313 313 313
0.0555 0.0869 0.1353 0.1734 0.1791 0.1766
Effect of temperature 26 0.1 27 0.1 28 0.1 0.1 29
110 110 110 110
180 180 180 180
313 323 333 338
0.1791 0.2059 0.2200 0.2228
leads to the higher capability of the SC-CO2 to dissolve the contaminants. In other words, when the static time increases, more time is available for SC-CO2 to be penetrated into the drilling mud and to extract the pollutants. Based on these experiments, static time of 110 min was considered as the optimum one since no significant increase in the amount of extracted contaminates was observed. 3.2. Effect of flow rate on the extraction of contaminates The other important parameter in the SC-CO2 -based extraction processes is CO2 flow rate that exhibits positive and significant effects on the extraction yield of solute possibly due to a decrease in the mass transfer resistance as flow rate increases. The SFE mechanism contains three successive steps, namely, solute dissolution, intra-particle diffusion, and external diffusion. The controlling step is changing gradually and slowly, not sharply. For example, in the initial stage of extraction, the main resistance may come from the external diffusion or solute dissolution. So external diffusion limited or solute dissolution will be limited at this stage and the SC-CO2 flow rate might have a significant influence on the extraction yield. With the dissolution progressing, the dissolution front is moving from the external surface to the center of particle gradually, and the intra-particle diffusion distance is becoming bigger and bigger. As a result, the intra-particle diffusion resistance will increase [28]. So, eventually, (at the later stage) the system will convert from external diffusion controlling or dissolution controlling into intra-particle diffusion controlling. At this stage, the SC-CO2 flow rate will have no influence on the extraction yield [28]. In the second stage of this investigation, the effect of flow rate on the extraction of contaminants from the drilling mud was investigated. In this regard, the flow rate was ranged from 0.05 cm3 /s
Fig. 3. Effect of SC-CO2 flow rate on the removal of contaminants from polluted drilling mud.
to 0.36 cm3 /s while the other operating conditions including static time (110 min), extraction pressure (100 bar) and temperature (313 K) were held constant during experiments. The obtained results shown in Fig. 3 demonstrated that as the flow rate increases from 0.05 to 0.1 cm3 /s, the amount of extracted contaminants remains constant while further increase in the flow rate from 0.1 cm3 /s to 0.36 cm3 /s leads to a reduction in the extracted contaminants. It will be apparent, by closer examination of Fig. 3, that the increase in the solvent flow rate leads to an increase in the amount of extracted solutes. This observed trend can be related to the following phenomena that the analyte–matrix interactions are usually weak for analytes where extraction is solubility-controlled. Hence, the extraction rate mainly depends on the partitioning of the analytes between the matrix and the supercritical fluid and, therefore, increases by using higher flowrates [29]. However, increasing the SC-CO2 flow rate has its own
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Fig. 4. Effect of extraction temperature on the removal of contaminants from polluted drilling mud.
disadvantages like disturbing the equilibrium conditions. As the flow rate increases, SC-CO2 does not have enough time to diffuse into the deep sites of the matrix and extracts the desired compound. Consequently, the lower amount of compounds will be extracted. In other words, the flow rate will have a dual effect dictating by the net effect of these competing factors if the extraction yield increases or decreases. Similar results were reported by Rajaei et al. [26] for extracting pollutions from the deactivated catalysts named Tonsil CO 610 G. They have related their observations to the fact that the extraction rates controlled primarily by the solubility/elution process showed direct correlation with SC-CO2 flow rates. But, the extraction rates for samples being controlled primarily by the kinetics of the initial desorption step showed little or no correlation with different SCE flow rates. Finally, based on these findings, the flow rate of 0.1 cm3 /s or lower can be considered as the optimum flow rate for the contaminants removal from drilling mud. 3.3. Effect of temperature and pressure on the removal of contaminates There are two thermodynamic parameters namely extraction temperature and pressure which can significantly manipulate the extraction efficiency during supercritical based technologies. They can be used to tune the solubilizing strength of any supercritical fluids. Because of its importance, in the last section of this study, several experiments were performed to investigate the effects of extraction temperature and pressure on the removal of contaminants from drilling mud. In the first step, the extraction temperature which can affect the thermodynamic conditions was changed between 313 K and 338 K (313 K, 323 K, 333 K and 338 K) while the other operating parameters including static time (110 min), extraction pressure (180 bar) and SC-CO2 flow rate (0.1 cm3 /s) kept constant. The obtained results shown in Fig. 4 revealed that the increase of extraction temperature from 313 K to 333 K leads to an increase in the amount of removed contaminants from 0.18 g/L to 0.22 g/L while further increase in the extraction temperature shows a slight effect on the amount of extracted contaminants. Although, generally one can expect a reduction in the solute solubilization as the temperature of SCCO2 increases, contradicting results were observed in this stage. Although the examined extraction temperature range in the current study is too narrow but this observed trend can be related to the dual effect of the temperature on the solubility of solute. In more detail, it has been proved that the temperature affects supercritical extraction (SCE) recovery by changing both the thermodynamic (density) and kinetics of the process. Extraction temperature has two contradicting effects on SCE, i.e. decreasing fluid density and
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Fig. 5. Effect of extraction pressure on the removal of contaminants from polluted drilling mud.
hence, the solute recovery on the one hand and increasing solute solubility through its vapor pressure, on another hand. So, the obtained results show that the enhancement of solubility by means of changing the solute vapor pressure is the dominant effect between the lower density (higher extraction temperature) which leads to lower solvating power and higher solubility through the vapor pressure of the solute. The noteworthy point is that temperature can also affect rates of SCE through solute interactions with the matrix, as in kineticallycontrolled desorption of contaminants [29]. Therefore, the total amount of solute being able to be desorbed may sometimes depend on the temperature alone and a higher temperature can increase recovery even at lower SCF density. Contradicting results were reported by Rajaei et al. [26] for the extraction of contaminants from polluted Tonsil CO 610 G catalyst soil. They have concluded that the effect of temperature on the reduction of SC-CO2 solvating power was more dominant, in comparison with the effect of temperature on the enhancement of solute vapor pressure. So, an increase in the extraction temperature leads to a reduction in the capability of SC-CO2 to remove contaminants from polluted catalysts. Based on these findings and due to the economic issues as a result of working with higher temperatures, the optimum value of extraction temperature was held constant at 333 K for the rest of the experiments. In the next stage of this section, six other experiments were performed to examine the effect of extraction pressure on the removal of contaminants from the polluted drilling mud. To this end, the extraction pressure was ranged between 100 bar and 200 bar while the other parameters including extraction temperature (333 K), static time (110 min) and SC-CO2 flow rate (0.1 cm3 /s) were held constant (Fig. 5). The obtained results showed that there will be a direct relation between the extraction pressure and extraction yield of contaminants from the polluted drilling mud if extraction pressure changes from 100 to 180 bars. Of course, the further increase in the extraction pressure leads to no more increase in the amount of extracted contaminants. This obtained trend could be related to the density change of the supercritical carbon dioxide with the increase of extraction pressure. In other words, when the extraction pressure is increased, the density of the supercritical fluids will be increased resulting in the higher solvating power of the supercritical fluids. 3.4. SEM and XRD analysis Finally, to investigate the effect of SC-CO2 extraction on the extraction of contaminants from the polluted drilling mud, XRD and SEM analysis were performed. The obtained results of XRD analysis showed in Fig. 6 revealed that intensity of the peeks for
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4. Conclusions
Fig. 6. XRD results for processed and unprocessed drilling mud.
In the present study, the efficiency of SC-CO2 extraction process on the removal of contaminants and pollutions from the polluted drilling mud was examined. In this regard, a home-made SC-CO2 apparatus was used to investigate the influence of different parameters including extraction pressure and temperature, SC-CO2 flow rate, and static time on the removal of contaminants from polluted drilling mud. The obtained results during the experimentations revealed that the increase of the extraction pressure will lead to an increase in the removal of contaminants if extraction pressure increases from 100 to 180 bar while further increase of it up to 200 bar leads to no significant effect. In addition, the results revealed that there was a direct relation between the extraction temperature and extraction efficiency of pollutant from polluted drilling mud. Also, the results revealed that increasing the static time up to 110 min leads to higher extraction efficiency while further increase in the static time has no significant effect on the removal of contaminants from drilling mud. Finally, the SC-CO2 flow rate was optimized by means of performing several experiments. Totally, the obtained results revealed that the optimum static time of 110 min, SC-CO2 flow rate of 0.1 cm3 /s, and extraction pressure and temperature of 180 bar and 333 K, respectively, can be considered as the optimum parameter for the removal of contaminations from the polluted drilling mud. At last, for more reliable conclusions, XRD and SEM analyses were run which revealed that the SC-CO2 extraction is able to removed contaminants from the polluted drilling mud satisfactorily and make the drilling mud particle surfaces more homogenous compared with the unprocessed sample. Among all, it can be concluded that the SC-CO2 process can be considered as an applicable and feasible method to remove contaminants from the drilling mud. References
Fig. 7. SEM images of the (a) processed (magnification ×2000) and (b) unprocessed (magnification ×2000) drilling mud sample.
unprocessed particles is higher than the processed one. Based on these results there are two possibilities. The first one is modification of the crystalline structure of the drilling mud since a shifting on the degree of several plates (37.5◦ for unprocessed to 36◦ for processed and 45◦ for unprocessed to 43◦ for processed samples) has been occurred. The second one is the reduction in the intensity of the peeks (62.5◦ ) for processed samples compared with the unprocessed samples which can be due to the removal of contaminants from the processed particles. In addition, investigating the SEM images for processed and unprocessed particles of drilling mud, it can be concluded that the processed drilling mud particles are more homogenous compared with the particles of unprocessed drilling mud (Fig. 7). Totally, based on the statistical analysis, quantitative analysis and XRD and SEM investigations it can be concluded that the supercritical carbon extraction process is able to remove contaminants from the drilling mud satisfactorily.
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