Iranian Journal of Science & Technology, Transaction B, Engineering, Vol. 30, No. B5 Printed in The Islamic Republic of Iran, 2006 Shiraz University

TREATMENT OF OIL-CONTAMINATED DRILL CUTTINGS OF SOUTH PARS GAS FIELD IN IRAN USING

SUPERCRITICAL CARBON DIOXIDE*

I. GOODARZNIA1 AND F. ESMAEILZADEH2** 1Dept. of Petroleum and Chemical Engineering, Sharif University of Technology, Tehran, I. R. of Iran

2Dept. of Petroleum and Chemical Engineering, Shiraz University, Shiraz, I. R. of Iran Email:esmaeil@shirazu.ac.ir

Abstract New treatment technologies are currently being investigated for the treatment of contaminated drilling waste mud with oil. Supercritical fluid extraction is a promising technology that could be effectively used to extract this contaminated drilling waste mud. In this work, one step extraction efficiency of supercritical carbon dioxide to drilling waste mud of South Pars gas field has been investigated at a range of temperatures (55 to 79.5 C), and over a pressure range of 160 to 220 bar. Extraction efficiencies were calculated based on cuttings weight loss. Results show that approximately 28.45 % of cuttings weight loss in one step extraction at a pressure of 200 bar and temperature of 60 C can be extracted by supercritical carbon dioxide.

Keywords Supercritical carbon dioxide, contaminated drilling waste mud, experimental apparatus

1. INTRODUCTION

Supercritical fluid extraction is a new separation technique which has drawn much attention to the use of supercritical fluids as extraction solvents in separation processes [1-4]. Carbon dioxide is a promising solvent since it is inexpensive, non toxic, non-inflammable, environmentally acceptable and has a low critical temperature and a moderate critical pressure.

The drilling of wells for the recovery of valuable materials such as petroleum is relatively expensive, both to the equipment employed in the drilling procedures and in the corollary activities to insure that the environment of the area surrounding the well is not injured. Drilling for oil and gas wells involves the use of drilling mud. Drilling muds are fluids used to control formation pressures, lubricate and cool the bit, remove rock fragments from the drilling well, and form a consolidated wall cake on the sides of the hole prior to casing. These muds, which are highly viscous, are complex formulations and include such finely divided materials as ground ilmenite, bentonite, various clays, barite, lead ore, fibers, hulls, etc. in a liquid medium which may be aqueous (e.g., water or brine) or an oil (e.g., diesel oil). In the drilling of wells for the production of petroleum, large amounts of drilling cuttings are produced and carried by the circulating mud that passes through the drilling equipment and is then returned to the earths surface. At the earths surface, the drill cuttings are separated from the drilling mud through the use of various mechanical solids control equipment such as screens, shakers, solid separators, desanders, mud cleaners, desilters and the like.

The drill cuttings are composed of the drilling mud and solid particles. In general, three types of muds are currently in use: oil-based muds, water-based muds and synthetic-based muds. Oil base muds are composed primarily of diesel oil or mineral oil and additives. Water-based oil consists of a base of salt Received by the editors January 15, 2006; final revised form September 18, 2006. Corresponding author

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water or fresh water containing additives, while synthetic-based muds have oil-like base materials. Water-base muds are not able to perform as well as oil-base muds in deep wells with high temperature conditions. Synthetic-based muds generally perform better than water based muds, but less than oil base muds. Oil-based muds are well suited for high temperature conditions because oil-base muds are paraffinic in nature with a relatively high boiling range. In addition, it is a common practice to employ an oil based drilling fluid. The drill cuttings produced by an oil based drilling fluid are rather heavily contaminated by the oil base which is used for preparing the drilling fluid. These drilling waste/cuttings must be treated. In other words, the oily drill cuttings can not be discharged directly into a disposal site at the well, not only because of their adverse effect upon the environment, but additionally because the great value of the oil contained in them. It has been a common practice to treat the oil drill cuttings in order to produce a solid material that can be disposed into the environment surrounding the well site without injury to it. Various approaches have been attempted to remove the oil from the rock [5-6]: such as the use of a high temperature [7], liquid solvent extraction [8] and soap and water washing [9]. The chemical wash system is also described in references [10-11]. In this system, the oily drill cuttings are treated with various chemicals including detergents with relatively intense mixing. Then, this mixture is resolved into relatively oil-free solids (i.e., the drill cuttings) and a recovered liquid phase which is a mixture of water, oil and the detergents which were employed in the chemical wash system. The solids could be disposed of by burial or other means, however, they may contain sufficient oil that upon contact with bodies of water, such as surface waters, lakes or the ocean, a rainbow effect will be produced, which is unacceptable relative to preserving the environment in the best possible form. In addition, the liquid phase must be treated to separate the oil from the bulk water phase so that the water portion can be discharged or otherwise disposed of without pollution problems. The separated oil can be utilized for various uses such as fuel or be returned into the blending of additional oil based drilling muds and the like. One objection to the chemical wash system is it's relatively high cost in the amount of above several hundred dollars a day, merely from the chemical requirements. Various thermal systems for driving the oily phase from the solids of the drill cuttings through the use of thermal energy have been proposed. The greatest disadvantage in this particular procedure for treating oily drill cuttings is the danger of explosions in the system if air or other oxidizing gas enter into contact with the heated oil vapors produced by the excessive heating of the oily drill cuttings. Should the flow of inert gas be terminated through accident or inadvertence, air entering into contact with these oily vapors could produce a very serious explosion and fire. Such an arrangement is not acceptable in the area surrounding an oil well, especially while it is being drilled. Bioremediation enables the oil in the drilling waste to be biologically degraded using hydrocarbon-degrading microorganisms. One major drawback of bioremediation is the extensive time required for satisfactory remediation. Drilling waste can also be re-injected into underground oil-bearing formations, away from the groundwater. However, if re-injection of the waste is not technically or economically feasible, the waste must be treated to reduce oil concentration prior to final disposal. There is also the risk of groundwater contamination during re-injection.

All these techniques suffer from safety, complexity or high-energy use problems. 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. Supercritical fluid technology is a favorable method for treatment of oil-contaminated drill cuttings. Supercritical fluids have several desirable properties that make them attractive for certain separation processes, e.g. the product is not contaminated with residual solvent [2-3]. The SFE processes used are environmentally-friendly, inert, cheap and are widely-available. Supercritical carbon dioxide exhibits excellent solvating characteristics which are easily manipulated to dissolve non-polar compounds like diesel and mineral oils. Treatment of drill cuttings using supercritical carbon dioxide would also provide the added benefit of allowing recovery and reuse of the expensive oil-based muds. In addition, treatment by supercritical carbon dioxide can be

Treatment of oil-contaminated drill cuttings of

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performed on-site in extraction vessels, thus eliminating the need for extensive land treatment. In the case of offshore operations, supercritical fluid extraction of oil-contaminated drill cuttings would eliminate expensive transportation of offshore-generated cuttings to shore for treatment and disposal.

In this work, the extraction of oil in drill cuttings by supercritical carbon dioxide was carried out at a pressure range of 160 to 220 bar and over a temperature range of 55 to 79.5 C by a flow type apparatus.

2. EXPERIMENTAL SECTION a) Materials The carbon dioxide supplied by Roham Gas Chemical Co., Iran, had a minimum purity of 99.8 mol %. The oil-contaminated drill cuttings were donated by National Iranian Oil Company (N.I.O.C-Pars Oil Gas Company). b) Procedure

A flowtype apparatus as shown in Fig. 1 was used to extract the oil from waste drilling mud cuttings by supercritical carbon dioxide. A detailed description of the equipment and operating procedures was reported previously [8]. Cuttings weight loss was determined from the weight of extracted solutes and the weight of cuttings charge. The liquefied carbon dioxide was pressurized using a high-pressure air driven oil-free reciprocating pump. Pressurized carbon dioxide flowed into a surge vessel to dampen the fluctuations generated by the operation of the pump, a heating coil and finally to the extraction vessel. The preheater and extraction vessel were immersed in a constant-temperature water-circulating bath. The temperature inside the water bath was regulated within 0.3 K through the use of a heating element and a proportional type temperature controller using a PTC thermocouple. The vessel outlet was packed with glass wool to prevent particle entrainment. The carbon dioxide leaving the extraction vessel was then depressurized through a heated needle valve. The depressurized carbon dioxide then passed through a cold trap which was submerged in an ice bath. The system pressure was measured by a Bourdon gauge with a division of 2 bar in the range of 0-250 bar. The system pressure was constant to within 1 % of the desired value throughout the experiment. Typically, the amount of solutes collected was in the order of 1.3-2.7 g. The extraction process involved a 1 hour and 30-minute cycle: one hour static extraction (no flow of SC CO2 through the vessel) followed by a 30-minute dynamic extraction. Solvent flow rates used in this study for oil contaminated drill cuttings range from 0.124 to 0.398 standard m3/h. Within this range, the flow rate has a negligible effect on the experimental results. The reliability of the apparatus was preliminarily tested by measuring the solubility of naphthalene in supercritical carbon dioxide at 35 0 C over a pressure range of 98-200 bar as reported in reference [12].

Fig. 1. Schematic diagram of experimental apparatus

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3. RESULTS AND DISCUSSION Extractions of raw drill cuttings of South Pars gas field in Iran were carried out at various conditions of temperature and pressures as shown in Table 1.

Table 1. Experimental test conditions for supercritical fluid of carbon dioxide

Sample no.

Extraction conditions

(bar, C)

Cuttings

charge (g)

Weight of

extracted oil (g)

Cuttings weight

loss (%)

CO2 flow rate at standard conditions

(ft3 min-1) 1 160 bar, 60 C 5.8 0 0 0.0702 2 3

180 bar, 60 C 200 bar, 55 C

5.8 4.2

1.3 1.1

22.4 26.2

0.0727 0.2456

4 200 bar, 60 C 5.7 1.7 29.8 0.1686 5 200 bar, 60 C 5.4 1.6 29.6 0.2065 6 200 bar, 60 C 5.2 1.4 26.9 0.2341 7 200 bar, 60 C 5.1 1.4 27.5 0.1736 8 200 bar, 79.5 C 5.5 2.7 49.1 0.1187 9 220 bar, 60 C 5.8 1.8 31.0 0.0812

The data seem to indicate that the cutting weight loss may increase with increasing temperature and

pressure. The extraction efficiency is more sensible by changing temperature rather than pressure. The experimental results also show that, the supercritical carbon dioxide cannot extract oil below the pressure range of 160 bar. For a pressure of 200 bar and a temperature of 60 C, the average extraction efficiency is 28.45 %.

The weight minute space velocity (WMSV) is defined as the mass flow rate of the solvent divided by the mass of cuttings and oil initially in the extractor. The weight minute space velocity (WMSV) at 200 bar pressure and 60 C for extraction of oil in waste drilling mud by supercritical fluid of carbon dioxide versus residence time is given in Table 2 and Fig. 2.

Table 2. Experimental data of weight minute space velocity (WMSV) versus residence time at 200 bar and 60 C

Sample no. Residence time (min) WMSV (min-1)

4 36 1.66 5 27 2.146 6 22 2.527 7 29.4 1.9105

26.5

27

27.5

28

28.5

29

29.5

30

55 56 57 58 59 60

WMSV.Residence Time

Cut

tings

Wei

ght l

oss

(%)

Fig. 2. Carbon dioxide extraction results at 200 bar pressure and 60 C

WMSV Residence Time

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Results show that the (WMSV Residence time) values greater than 58, similar to hydrocarbon removal were obtained.

Visual observations were made before and after extraction. The raw cuttings prior to extraction by supercritical carbon dioxide appeared extremely viscous and dark in color. Once the extraction was completed, the color of cuttings changed to light gray and the extracted oil appeared yellow in color.

4. CONCLUSION The experimental data of oil extraction from waste drill cuttings have been presented in supercritical carbon dioxide at a pressure and temperature range of 160 to 200 bar and 55 to 79.5 oC respectively. A minimal extraction efficiency of 22.4 % was obtained with extractions at 180 bar and 60 oC. The experimental results show that at a high constant pressure, the extraction efficiency is increased by increasing temperature. Also, for each condition of extraction with supercritical fluid, there is an optimum value of (WMSV residence time) to extract the oil from waste drilling mud. Acknowledgement- The authors are grateful to the Sharif and Shiraz Universities for supporting this research. This study was part of a research project sponsored by the R & D N.I.O.C-Pars Oil Gas Company, which is gratefully acknowledged.

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