A full-scale process for produced water treatment on offshore oilfield: Reduction of organic pollutants dominated by hydrocarbons

Highlights

• An industrial full-scale practice of an offshore platform was investigated.

• Organic characteristics of PW were systematically analyzed.

• Organic pollutants in PW were dominated by hydrocarbons.

• Alkanes and soluble organics can be separated simultaneously.

Abstract

Offshore oil production, which is a significant source of petroleum, has recently caused extensive concern. Produced water (PW) is the highest-volume byproduct of oil and gas production. The organic pollutants dominated by hydrocarbons, namely oil, in PW are the major concerns to researchers because of the adverse effects to the marine ecosystem. In industrial processes, the oil is needed to be separated by individual or cooperative use of separation technologies, such as gravity settlement, enhanced gravity settlement, and hydrocyclones. However, these conventional technologies cannot reach the separation requirements in a compact space under extreme conditions, such as emulsification and large volumes of PW. To improve the treatment efficiency, decrease the occupation of space, the practical use of a full-scale PW treatment process was proposed and investigated in the Bohai Sea, China. The oil content of PW after treatment with the full-scale process was less than 20 mg·L-1, alongside a reduction in the chemical dosage concentration (over 40% and 60% for reverse demulsifier and water clarifier, respectively) compared with the conventional process. Besides, an increase in the amount of produced liquid and PW of over 50% and 40% was achieved, respectively, generating a 30000 m³·a-1 growth in oil production in this single wellhead platform. The organic compounds and relative concentrations of these compounds were analyzed in this full-scale practical application. Hydrocarbons were the major organic pollutants in PW. Approximately 100 kinds of organic compounds that were detected in the influent of the process were reduced to 8 in the effluent with a separation efficiency over 90%, which made the PW cleaner to the marine environment. The simultaneous separation of alkanes and slightly soluble organics in PW can be explained by rotation and internal circulation, which provides a possible mechanism by which slightly soluble materials in major phase could be transferred from major phase to dispersed phased, in which the materials have a significantly higher solubility. This study provided an attractive strategy to answer the ecological menace caused by the hydrocarbon exploitation in offshore oilfields, to the benefit of both the economy and the environment.

Introduction

The extraction of gas and oil has been one of the most significant industrial activities in the 21st century (Igunnu and Chen, 2012). PW, which is composed of water, oil, dissolved solids, suspended solids, and other dissolved or undissolved organic and inorganic substances with hazardous or potential hazardous characteristics, is the highest-volume byproduct that is associated with the process of oil and gas production (Veil, 2011; Ottaviano et al., 2014). The treatment of the PW has produced significant technical concern with regards to the protection of global water resources and the ecological environment. The water content in the PW is usually low at the initial stages of production, increasing significantly in the middle-late stages of the process (Mccormack et al., 2001; Nasiri and Jafari, 2016). Approximately 250 million barrels of water were produced daily from the oil and gas fields in 2012 (Igunnu and Chen, 2012), with a water to oil average ratio at about 4:1 (Judd et al., 2014). To counteract the costs of investment, the life cycle of offshore oil fields is generally longer than that of onshore oil fields, leading to higher water content in the PW than in onshore fields (Kusworo et al., 2018). The water content of PW can reach as high as 98% at the final production period (Huang et al., 2015; Zhang et al., 2018). The PW from offshore platforms is generally discharged into the ocean or reinjected into the strata. The oil content that remains in the PW has been investigated rigorously and is legislated for in more than one country (Ekins et al., 2007; Jiménez et al., 2018; OSPAR Commission, 2008; GB 4914, 2008; Bader, 2007).

Because of the increasing concern for the protection of the marine environment, and the use of chemical additives such as biocides and corrosion inhibitors that are utilized during drilling, fracturing, and other operating processes (Al-Ghouti et al., 2019), the BTEX, heavy metals, deleterious organic substances, and even some of the more fundamental physicochemical properties such as pH and salinity of the PW should be scrutinized (Munirasu et al., 2016; Li et al., 2016; Arthur et al., 2011; Ahmadun et al., 2009). Most oil and gas companies and organizations around the world are dedicated to the implementation of a “zero-discharge” for pollutants released into the sea, that will ultimately lead to an increase in the amount of reinjected PW (OSPAR Commission, 2015; Pollestad, 2005). As the expense of water management can account for 5%–15% of drilling costs, this is also important from an economic perspective (Tanudjaja et al., 2019).

From an environmental and economic point of view, the treatment of the pollution dominated by hydrocarbons in PW, and particularly the development of processes and relevant devices that can achieve the high standards for emission or reinjection, is a technical challenge for PW management. The physicochemical method has been both studied and utilized due to merits such as the convenience of the procedure (Camarillo and Stringfellow, 2018). Considering the space occupation, load bearing of the offshore platforms, and the characteristics dominated by the distribution of particle sizes in the PW (Arnold and Stewart, 2008), a new type of PW treatment process, with chemical reagents injected, composed of novel equipment combined with one or more traditional separation methods, such as gravity sedimentation, compact flotation unit (CFU), hydrocyclone (HC), should be developed that can adapt to the current operating conditions (Robinson, 2013; Bagheri et al., 2018).

Coalescence technology could be considered in the PW treatment process because of the extensive application from last century. Traditional coalescence device is coalescence filter cartridge. Nonwoven single-species fine fibers of micron or nano size are used to construct filter mats with a limited thickness by electrostatic spinning (Hu et al., 2015) in this kind of devices. To achieve higher separation efficiency, two or more types of fibers could be utilized (Chase and Kulkarni, 2011; Kulkarni et al., 2012). These types of mat are more suitable for clean feeds than for treating PW from the oil extraction process because of the high content of impurities, colloids, and asphaltene in crude oil. In a broader sense, a method can be referred as coalescence when small oil drops coalesced into oil film or big droplets during the process of treatment. In this viewpoint, filtration and adsorption are also the processes that could be found associated with the coalescence process. The membrane or carbon nanotubes which separate the dispersed phase by interception (Zhu et al., 2016; Zhang et al., 2014; Wang et al., 2013), and the novel material for adsorption (Zhang et al., 2018; Kabiri et al., 2014) are studied to separate the dispersed oil phase from water phase. While most of these methods are still rest on laboratory-scale and pilot-scale when they are used for PW treatment on offshore platforms, because of the inadaptability of the complex industrial sewage.

This paper studied a full-scale process for PW that is mainly polluted by hydrocarbons, on an offshore oil field platform located in the Bohai Sea, China. This research is a practical work for PW treatment for purpose on the platform. The water content of PW from this platform has already reached over 90%, with a value of 98% achieved in some of the wells. According to the United States Department of Energy (DOE), crude oil with an API gravity from approximately 15.6 to 22 from this oil field can be classified as heavy crude oil. Conventional processes are not suitable for this kind PW unless the deck being expanded to obtain larger space for these devices. A new type of coalescence (combined fibers coalescence, CFC for short) separator endurable for the complex influent was utilized in this novel full-scale process. Two or more types of fibers woven into the structural shapes “X” and “Ω” (Yang et al., 2014; Yang et al., 2014) were used in the separator. Emulsion breaking and dispersed phase separation occur at the intersection of the heterogenic fibers. This kind of separation method cannot reach the same separation accuracy as membranes, carbon nanotubes or adsorption due to the large apertures among the fibers. However, the inferiorities induced by the apertures can be remedied by increasing the thickness of the fiber layer. Larger apertures are also advantageous for the sustainable treatment of industrial PW with complex components. Various fibers with different surface energies, such as polypropylene (PP), poly tetra fluoroethylene (PTFE), and stainless steel (AISI 316L) fibers were used in CFC separators based on the research results obtained at the laboratory (Lu et al., 2015, 2016) and via the observation of a pilot test (Liu et al., 2018). The oil content, and organic components in PW in the full-scale industrial practice were analyzed systematically. The performance on oil separation, reducing organic species, and simultaneous separation of alkanes and slightly soluble organics, of the full-scale process during long period monitoring were revealed.