Toward the Exploitation of Unconventional Heavy Oils: Electrostatic Technologies for the Minimization of Dehydration Cost ^†

Abstract

The growing energy demand has created a special interest in the unconventional reserves of heavy and extra-heavy oil, despite their difficulty both in extraction and separation due to the high specific gravity, high viscosity, low mobility, and high content in asphaltenes and heteroatoms. The study is a comprehensive review of the implemented technologies and emerging techniques for the dehydration of heavy and extra-heavy oils with an °API gravity from 20 to below of 10 and a viscosity between 100 and 10,000 cP. Special focus is given to the electrostatic treatment, due to its versatility of applications and its potential for the further improvement in dehydration efficiency, thus reducing the demand for energy and chemical demulsifiers.

1. Introduction

Despite their characterization as unconventional reserves, heavy, extra-heavy crude oil, and bitumen sum up to almost the same amount as the conventional oil reserves worldwide [1]. Production of these unconventional oils, however, is much more difficult due to their high viscosities (i.e., high resistance to flow), low API gravity (high density), and the presence of pore and pipe fouling components such as asphaltenes and resins.

Heavy oils are defined by an API gravity between 10 and 20 ᵒAPI and a viscosity higher than 100 cP at reservoir conditions. Extra-heavy oils have an API gravity of less than 10 and a viscosity of less than 10,000 cP, whereas bitumens have similar API gravity values to extra-heavy oil while their viscosity is greater than 10,000 cP [2,3].

The dehydration and desalting of heavy and extra-heavy oil are possibly the most crucial treatments in the production process, and they are directly connected to the quality of the extracted oil. Since salts are soluble in the water, a reduction in the water content will reduce the salt content as well. If, however, the salt content of the produced oil remains high, a desalting stage follows the dehydration one, in which fresh water is injected into the oil to extract the remaining salt and then removed through separation.

The main challenges of both dehydration and desalination are related to the effectiveness of the oil–water separation, namely:

• The small density difference between oil and the produced water;
• The high oil viscosity where friction impedes the coalescence of water droplets;
• The high w/o emulsion viscosity, especially in the typical case of heavy oil recovery, which is characterized by high water cuts;
• The high interfacial tension between oil and water, which is mostly due to the presence of natural surfactants such as asphaltenes, prevents the coalescence of water droplets, and increases emulsion stability.

Consequently, this results in an increased use of process chemicals to remove such surfactants.

A conventional separation train includes first- and second-stage three-phase gravity separators. The outlet from the second-stage separator is introduced to a single-stage (dehydration) or dual-stage (dehydration and desalting) scheme of conventional electrocoalescers, depending on the basic sediment and water (BSW) and salinity specifications [4]. When the water content is high, a Free Water Knockout Drum (FWKD) is introduced before the first stage separator to reduce the excess water. The final oil should contain less than 0.3% of water-in-oil content and 10 lbs per thousand barrels (10 PTB) of total dissolved salts [5]. Electrostatic treatment is used in conjunction with chemical and/or heat treatment to reduce the costs of additives and energy consumption.

The technique aims at reducing the thickness of the thin film that surrounds the droplets and separates them from the continuous oil phase to a point at which it can easily rupture when the droplets come into contact. Interfacial tension is a measure of how easily this film can rupture. With the application of an electric field, the droplet surface must be energized sufficiently to overcome the interfacial tension barrier between adjacent water droplets and induce coalescence [6,7]. The high interfacial tension between water and oil makes the droplets resist deformation, and they do not coalesce, while low interfacial tension allows the droplets to deform and coalesce easily, but they can also break up as easily as they coalesce, due to the low energy barrier.

Operating conditions, flow pattern, and water/salt concentration have a significant impact on the effectiveness of the electrostatic treatment. Heavy oils may require high operation temperatures to reduce the viscosity or high pressures to avoid the formation of an excess free gas phase within the equipment. Salt and water content may also vary over a wider range than that of a typical oil field. Such conditions pose risks to the effectiveness of the electrostatic treatment with possible arch formation (voltage decay) within the produced fluids, dispersion of water droplets into smaller ones, or insufficient electric fields in some parts of the separation vessels.

2. New Trends in Electrostatic Technologies

Improvements to the established technologies include new frequency patterns for the AC and DC fields, new configurations and structures of the electrodes, and a homogeneous distribution of the electric field in the oil flow. Such developments have led to the construction of electrostatic coalescers that can operate in the feed pipeline.

2.1. Dual-Frequency^® AC/DC Electrostatic Treating Technology

To treat high-conductivity and low-interfacial-tension crude oils, Cameron Process Systems incorporated a variable-frequency power supply in the voltage-modulated combined AC/DC technology. The objective was to control in an effective way the electrostatic field decay and the interfacial tension of the water droplets in the oil.

In a conventional AC/DC treater, when one set of plates is being charged, the alternate experiences charge decay, which, in high-conductivity oils, can result in the loss of the DC field. To counter this phenomenon, the time between the charges is reduced by increasing the frequency of the power source. The power supply is set to a frequency close to the resonant frequency of the dispersed droplets. At the same time, the power supply frequency (base frequency) is set to a value high enough to minimize field decay and then is modulated (pulse frequency) at a rate that energizes the droplet surfaces. This combination creates a sustained high field strength, enough to coalesce the droplets. Similarly, for low interfacial tension, the voltage oscillation is minimized to a frequency below the resonant frequency of the droplets. This way, the droplets coalesce without the application of excessive electrical force that would lead to their dispersion into smaller ones [8]. The base frequency can be modulated in terms of amplitude, and the evolution of the pulse frequency modulation with time can vary [9]. A Dual-Frequency unit can both adjust the base frequency and modulate the DC field, resulting in a high electrostatic field strength and coalescing power. Therefore, Dual-Frequency treaters are smaller in size than AC treaters or conventional AC/DC treaters and require a lower demulsifier dosage [10].

The Dual-Frequency electrostatic treater also includes composite plate electrodes and the HiFlo^® distributor (Cameron Process Systems, a Schlumberger Company, Portlethen, UK). Composite plate electrodes are used instead of electrically conductive materials in high-water-cut streams to prevent chain formation and enhance coalescence. They consist of a conductive region surrounded by a nonconductive region of fiber-epoxy material. The electrical connection is made through the conductive material that distributes the charge along the electrode, and the voltage at any point on the electrode is determined by the electrical current passing through that point. This way, the bridging of the electrodes can be minimized. Composite electrodes can tolerate longer exposure times in high-voltage fields and handle high-water-cut emulsions, even twice as high compared to steel electrodes [11]. A comparison of dehydration efficiency between steel and composite plate electrodes shows an improvement in dehydration of approximately 56% [12].

The flow distribution inside the separation vessels is a very important parameter for the optimization of the dehydration process both inside the oil treater but also upstream, in the FWKD vessel. The HiFlo^® distributor is a proprietary spreader that contributes to the improvement in the treater efficiency. According to the manufacturer, the HiFlo^® distributor (spreader) can provide 35% better utilization of the vessel volume because it eliminates fluid recirculation and prevents fluid bypassing of the electrodes. This improvement has been achieved by incorporating two highly efficient perforated distribution baffles right after the inlet section and right before the oil and water outlet [13].

2.2. Vessel Internal Electrostatic Coalescer (VIEC™)

The Vessel Internal Electrostatic Coalescer (VIEC) is a technology that was originally developed by ABB Corporate Research Center in Norway in 1998–2001. Normally, most of the free water is removed in the first-stage separator, and gas is removed in both stages. However, significant amounts of emulsified water of up to 40–50% can pass to the electrostatic treater, especially in the case of heavy crude oils [14,15]. VIEC can be applied in the first-stage separation or as an inlet device of a three-phase separator [6]. The high-voltage electrodes are subjected to the harsh environment of the first-stage separator, where the water content can reach up to 90%, and significant quantities of gas are released [16]. VIEC™ includes insulated electrodes to prevent short-circuits between them [5] and combines the characteristics of a perforated distribution baffle and an electrocoalescer. The individual electrocoalescer elements have the form of tubular channels, and the emulsion flows through them. The two electrodes extend helically and continuously over the outer periphery of each one of the tubular elements and apply an electric field to the emulsion. Each tube can be characterized as a small individual separator in which the droplets settle as a liquid film in the inner walls. The flow inside the tubular channels is turbulent, and the residence time can be adjusted between one and three seconds depending on the fluid properties [6].

The operating frequency is in the kHz region to prevent the high-voltage drop across the insulated electrodes and allows frequency and amplitude adjustments to the optimal operating conditions. The power consumption of each module is typically 250 W, and the operating temperature is up to 90 °C. The standard design for VIEC™ is to cover the entire oil/emulsion phase and force the emulsion to flow through it [6].

The High-Temperature VIEC (HT VIEC) is a version of the VIEC™ design that is developed to enhance the processing of fields that operate at high temperatures and pressures up to 150 °C and 150 bar, respectively, and experience separation difficulties.

The Low-Water-Content Coalescer (LOWACC or VIEC LW from Hamworthy Technology and Products AS) is a post-treatment technology that was developed by Vetco Aibel to operate downstream the VIEC™ or HT VIEC and polish the crude oil to export specifications. VIEC LW applies dielectrophoretic forces [15] imposed by the non-uniform field gradient, which are 2–5 times higher than the gravity force. This way, the droplets are guided to predetermined sections of the separator with the highest field strength and they are forced to merge [6]. VIEC LW complements the process of VIEC by removing the remaining water down to the export oil quality of 0.5% BS&W. VIEC LW can be applied in cases where there are intense emulsion separation problems and in subsea separation processes, where the water content in oil should be reduced at the first stages of the separation train [15]. According to Hamworthy, the VIEC LW reaches the BS&W of export quality (0.5%) after the first- and/or second-stage separation without the need of traditional electrostatic treaters [17].

2.3. Compact Electrostatic Coalescer CEC^TM

The CEC is an inline vertical vessel that can be placed upstream of an existing gravity separator to enhance its performance. It is considered a relatively small and lightweight electrocoalescer, since a typical unit that operates with flow rates of 130,000 BOPD has a height of 5.5 m, a diameter of 1.2 m, and an approximate dry weight of 7 tonnes [5]. The basic operating principle of CEC is the application of a strong AC electrostatic field on the water-in-oil emulsion, which flows vertically in the vessel in turbulent conditions [18]. The CEC consists of annular concentric, fully insulated electrodes placed one inside the other. Voltages of the order of a thousand volts create a strong electrostatic field that forces the water droplets to merge within a matter of seconds. The result is water droplets increased by ten times in diameter than before entering the CEC vessel.

CEC is qualified for crude oil of 14 to 40 API. It is tolerant to any water cut, but the high performance is between 2 and 40% water cut. The CEC can operate up to 160 °C with high-viscosity crudes showing efficiency in breaking heavy crude oil emulsions and emulsions created from ESP pumps [19]. An important benefit of CEC is the very low energy consumption and the undisturbed production during maintenance. Additionally, CEC has a very quick-response power modulation and an automatic control that protects from short-circuiting.

The CEC is tolerant to vessel motions (level fluctuations) and water slugs, but it is limited to a maximum gas volume content of <10%. Considering the 2–40% water cut for which the device demonstrates the higher performance, the typical installation for CEC is after the second-stage separator.

2.4. Three-Dimensional Horizontal Flow Electrostatic Coalescer

Fjords Processing (currently NOV) has commercialized the 3D Horizontal Flow Electrostatic treater that utilizes horizontal flow instead of the vertical flow used in the conventional electrocoalescers. The horizontal flow direction minimizes water carry-over with respect to the vertical flow electrostatic treaters, where the flow direction of the bulk phase is upward and the coalesced water droplets should flow countercurrent to the vertical oil flow. In the case of heavy oils, the countercurrent flow creates additional problems for the separation of the phases. Consequently, in heavy oil dehydration applications, horizontal flow is preferable to vertical or countercurrent flow. The horizontal flow configuration is also beneficial to crudes that exhibit a foaming tendency.

The cyclonic inlet device of the horizontal flow electrostatic treater prevents foaming and ensures a very good liquid/gas bulk separation. A subsequent series of mounted externally adjustable louver plates create a baffle section that can be constantly adjusted to match the flow conditions, and with a slight pressure drop, they provide a uniform flow distribution and additional surface for the water droplets to coalesce. Then, electrostatic grids are placed to prevent short-circuiting caused by production upsets and high inlet water cuts by favorably adjusting the distance between the charged and the ground electrode rather than reducing the grid voltage with a voltage transformer. The adjustable spacing between the grids reduces the field density and eliminates short-circuiting. The grids have extensions on the bottom and sides, which create an additional electrostatic field between the vessel wall and the water phase. The combination of adjustable louver baffles and grids renders the electrostatic coalescer effective in a wide range of operating conditions, such as variable flow rates, inlet water cuts, and emulsion levels.

The 3D Horizontal Flow Coalescer operates in a range of 2 to 35% water cut and produces lower than 0.5% BS&W crude oils of over 14 °API [19]. It can efficiently handle water, oil, and gas, and thus a separate upstream degasser is not required. The design ensures that 100% of the water-in-oil emulsion is exposed to the electrostatic field, so it results in a smaller size or increased capacity compared to the vertical flow electrocoalescers.

2.5. Inline Electrostatic Coalescer

In 2014, ExxonMobile Upstream Research Company (EMURC), while working in a subsea separation technology program, developed and performed laboratory-scale tests on an inline electrocoalescer supplied by FMC Technologies. The inline electrocoalescer was tested with medium and heavy crude oil in varying operating temperatures and a range of oil viscosities [20]. Currently, this inline electrostatic coalescer is being commercialized by FMC Technologies.

The inline electrocoalescence device enhances the efficiency of the downstream separator. The inline electrocoalescer includes insulated electrodes in a ceramic housing. An intense electrical field is formed by a high-voltage AC current applied to the electrodes. The dispersed droplets become polarized and collide in a turbulent flow regime. The polarization of the droplets with a short-range attraction force enables the breaking of the interfacial film between the droplets. Additionally, due to the rapid change in the direction of the AC field, the droplets deform because they constantly follow the direction of the field at a relatively high frequency [21]. The turbulent flow regime in the inline electrocoalescer increases the collision rate between the droplets and, thus, their chances to coalesce. Because of its compact design, it is suitable for subsea and offshore facilities where space and weight restrictions exist and has the potential to improve medium and heavy oil dehydration processes.

3. Conclusions

In the past few years, there have been significant improvements in the capacity and performance of the electrostatic treaters through continuous improvements in their mechanism and research over the electric field development. The significance of the well-distributed flow inside the electrocoalescers has been highlighted, together with the use of variable electric fields and turbulent flow conditions to increase the collision rate of the water droplets.

Each one of the presented techniques exhibits different characteristics, advantages, and disadvantages that render them suitable for certain industrial applications. Even though these techniques cannot fully replace thermal and chemical demulsification, they have significantly reduced the respective requirements.

• VIEC, HT VIEC, and LOWACC technologies are designed to provide efficient dewatering in the first- and/or second-stage separation, where the high gas and water content in the feed prohibits other separation solutions.
• Dual-Frequency crude dehydration utilizes modulated voltage/frequency, and it has been applied over the last 15 years in second- and third-stage separation processes. Dual-Frequency crude dehydration results in the further reduction in the treater size and heating demand, while it increases the treatment capacity.
• The 3D Horizontal Flow Electrocoalescer is the preferred dehydration method for heavy oils and crudes that exhibit foaming tendencies.
• CEC is an inline vertical lightweight coalescer that is tolerant to high water cuts, water slugs, and level fluctuations (vessel motions), but non-tolerant to a gas content higher than 10% per volume. The inline electrostatic coalescer is suitable for subsea and offshore facilities that face weight and space limitations.