Oil Removal from Oilfield Produced Water, North Rumaila by Combination Coagulation- Flocculation and Microfiltration Technique

The Southern Oil Company, which is operated in North Rumaila oilfield in Basrah/ Iraq, is one of the important companies which produced huge amounts of produced oilfield water. The aim of this study is to treat the produced water by hybrid methods: the process of coagulation-flocculation and microfiltration technique (ceramic membrane) to remove the oil content and improve the water quality to meet the allowable limit of reinjection into the reservoir. Poly-aluminum chloride (PAC), and Ferric Chloride (FeCl3.6H2O) coagulants were used separately and in combination with cationic polyelectrolyte (PE). After produced water was treated with different doses of coagulant, it was passed through the Microfiltration technique/ ceramic membrane (0.5μm) to reach the allowable limit for reinjection. It was found that the best value of oil content after passing through ceramic membrane is 0.2 mg/L at FeCl3.6H2O dose (10 mg/L) combined with PE dose (0.6 mg/L), which was less than the allowable limit for re-injection, (5) ppm. KeywordsProduced water; Coagulation; Flocculation; Ceramic Membrane, Oil Content. How to cite this article: Th.J. Mohammed, E.Sh. Awad and Th.A. Ahmed, “Oil Removal from Oilfield Produced Water, North Rumaila by Combination Coagulation-Flocculation and Microfiltration Technique,” Engineering and Technology Journal, Vol. 37, Part C, No. 2, pp. 204-208, 2019.


Introduction
During the oil and gas production, produced water brought to the surface from underground formations as an incidental byproduct. It is the largest waste stream in the oil and gas exploration and production processes [1,2]. Produced water is a mixture of organic and inorganic materials [3]. Components of produced water vary considerably from location to another according to the geological formation, geological location of the field and types of hydrocarbon product being produced [4,5]. Produced water properties and volume can even vary throughout the lifetime of a reservoir [6,7]. Produced water is injected into oil and gas reservoir for multiple purposes such as a disposal, water flooding and enhanced oil recovery, and in some cases, hydraulic fracturing. Reuse of produced water could be a stable supply of water for fracturing, drilling, and completion that would minimize freshwater used [8]. Before Produced water disposed to the environment or re-injected, it requires appropriate technological, chemical, and bacteriological processing to meet the quality requirement [9]. Reinjection of treated produced water is more frequently applied, essentially because it's the only economic way to handle large volumes of water [6]. There are four methods of treatment produce water that has been used, i.e., physical, chemical, biological, and membrane-based treatment [10]. Coagulation is one of the important technology deals with water and wastewater [11][12][13]. Oilfield produced water is treated by the application of coagulation-flocculation treatment process. Coagulation and flocculation are defined as the process by which small particles in aqueous solution are destabilized by chemical addition, and aggregate together, forming larger particles known as flocs. With a neutral suspension, the flocculation rate can be enhanced, and the concentration of the given particle distribution in the solution can be reduced [14,15]. The aim of this study is to remove oil content from produced water and meet the allowable limit for re-injection by the hybrid method: the process of coagulation-flocculation and microfiltration technique (ceramic membrane).

I. Characteristics of oilfield produced water
Oilfield produced water which used in this study was kindly provided by Petroleum Research and Development Center staff from DS1/ North Rumaila oilfield/ Southern Oil Company, which is situated in Basrah/Iraq. The location of the produced water sample is shown in Figure 1. The characteristics of produced water include pH was around (7), EC (94900 μ s/cm), TSS (333.2 mg/L), TDS (85410 mg/L, Turbidity (410.3 NTU), and oil content (36.4mg/L).

II. Materials and Methods
The hybrid processes for treating produced water consist of the coagulation-flocculation process by Jar-test (Model: Flocculator/ SW6) and filtration system (Nutshell filter and microfiltration technique/ceramic membrane filtration (Model: HK-U-M, Module: Tubular, porosity: 0.5 Micron). Initially, a series of experiments were conducted with produced water in the laboratory using: The Flocculator/ SW6 jar tester, coagulationflocculation process with variety doses of polyaluminum chloride (PAC) (Al 2 (OH) 3 Cl 3 ), Ferric Chloride (FeCl3.6H2O) as primary coagulant and cationic poly-electrolyte (PE) as a coagulant aid to achieve maximum removal of oil content. The coagulants which are used in this study were provided by Al-Dura Refinery (Iraq). Coagulant dosing was added to produce water and mixed 1 min under rapid mixing condition "flash mix" (150 rpm). Reduce the speed to the minimum required (50 rpm) to retain floc particles uniformly suspended during the "slow mix" of 20 min period. The oil content of supernatant liquor was measured and other required analyses after settling for 20 min. After that, the optimum doses of the sample of produced water were filtered by Nutshell filter and then passed through ceramic membrane filtration (0.5 Micron) to achieve the allowable concentration range of the produced water constituents to facilitate the reinjection to the reservoir [12,16]. A schematic diagram of the filtration system used in this study is shown in Figure 2. Oil Content was determined by oil content analyzer (HORIBA/OCMA-350).

III. Experimental Procedure of the Membrane System
The general experimental filtration system procedure is as shown below: 1. Measurement of the oil content value of the produced water treated by the coagulationflocculation process before filtration. 2. Filtration by nutshell filter to reduce the oil content value to less than or equal to 5 mg/L. 3. After filtration, the effluents are treated with ceramic membrane filtration (0.5 Micron) at maximum output pressure =1 bar and flow rate= 1.35 L/min to reduce the oil content value in order to reach allowable requirement range. 4. Measurement of the permeate water flux after the filtration experiment. 5. Measurement of the oil content after the filtration experiment. The ceramic membrane is cleaned to be used for another run.  Figure 6.
The results show that the requirement of PAC dose is higher than FeCl 3 .6H 2 O dose, but doses are lowered when combined with Polyelectrolyte (PE), Polyelectrolyte performs very well as a coagulant aid, particularly with FeCl 3 .6H 2 O. The oil content decreases with increasing dose of PAC and FeCl3.6H2O until reaching the optimum dose, increasing doses cause an increase in oil content and reduction the removal efficiency, the over-dose causes destabilization that causes a weak attraction between droplets of oil and then causes a reduction in particles' settling velocity according to Stock law. Therefore the removal is decreasing [12].

II. Oil Removal by Ceramic Membrane filtration
After produced water treated with different doses of coagulant-alone and combination-it would filter by Nutshell filtration to reduce the oil content and then passed through the ceramic membrane (0.5 Micron) to reach within allowable requirement range. Table 1 shows the best optimum doses selected to pass through the ceramic membrane and their symbols.  Figure 7 shows the oil content removal by the ceramic membrane. The 1 st column shows the oil content after the coagulation-flocculation process and then filtered by Nutshell filter which notices in the 2 nd column after that passes through ceramic membrane filtration (0.5 Micron) to reach the allowable rang for reinjection to the reservoir, which shows in the 3 rd column. The best value after passing through ceramic membrane (gives oil content= 0.2 mg/L) at FeCl 3 .6H 2 O Dose=15 mg/L combined with PE Dose=0.6 mg/L [17].

Conclusions
 When using Cationic polyelectrolyte as a coagulant aid will reduce the PAC dosage required and increase the efficiency of removal.  The coagulation power of FeCl 3 .6H 2 O, when used as a primary coagulant, seems to be more effective than PAC.  The oil content removal in coagulationflocculation treatment is equal to (5.4, 3 and 5.3 mg/L) respectively with optimum (PAC, PE and FeCl 3 .6H 2 O) doses (80, 2 and 20) mg/L.  The best optimum combination of coagulant doses for oil removal efficiency is (20 + 0.6) mg/L and (10 + 0.6) mg/L for PAC and FeCl 3 .6H 2 O, respectively combined with PE.  For ceramic membrane filtration method, which is used after the coagulation-flocculation treatment, the oil content could decrease to (3, 0.2, 2.6, 5, 4.3) mg/L with different optimum doses.