Novel approach of recovering energy from high-pressure multiphase streams

 SAQIB NAZIR and AHMED TAHA AL-GAZI, Saudi Aramco 

The oil and gas industry is characterized by the production of high-pressure multiphase streams that consist of gas, liquid, and occasionally solid phases. Managing these streams efficiently is crucial for both operational performance and environmental sustainability. Traditionally, excess pressure energy in these streams is dissipated through choke valves, leading to significant energy waste. The inherent complexity of multiphase flows, coupled with the mechanical stresses they impose on conventional energy recovery systems, poses substantial challenges to effective energy recovery​​​​. 

High-pressure multiphase wells are a common feature in oil and gas extraction, especially in fields with significant reservoir pressure or during the early stages of production. In mature fields, the pressure might decline, but the management of multiphase flows remains crucial for maintaining production efficiency. The pressure energy contained in these streams represents a significant potential energy resource that, if harnessed effectively, can improve the overall energy efficiency of production operations. Traditional approaches to energy recovery often fail to address the complexities of multiphase flows, which can vary widely in composition and behavior​​​​. 

This article introduces a novel method that integrates a turbo-expander with a jet ejector (Fig. 1) to recover energy from high-pressure multiphase wells. The turbo-expander facilitates the conversion of pressure energy from the gas phase into mechanical work, which is then converted into electricity. Simultaneously, the jet ejector utilizes the pressure energy from the liquid phase to extract additional energy from gas phase by connecting to the low-pressure nozzle of the jet ejector. This synergistic approach maximizes energy recovery by efficiently utilizing both gas and liquid phases in one recovery system. 

Fig. 1. Typical illustration of an ejector.

The concept of combining a turbo-expander (Fig. 2) and a jet ejector is innovative in its application to multiphase streams. Turbo-expanders are well-established in gas processing for their ability to efficiently convert pressure energy into mechanical work. However, their application in multiphase wells is limited, due to the challenges posed by liquid phases, which can cause damage and reduce efficiency. The jet ejector, on the other hand, is adept at handling liquid flows but typically operates with lower efficiency when used alone. By integrating these two technologies, the proposed system leverages their strengths while mitigating their individual limitations​​​​. 

Fig. 2. Typical illustration of a turbo-expander.

There are several benefits of this system. First, it offers a cost-effective solution by utilizing existing infrastructure, making it feasible for retrofitting in current oil and gas wells sites. Second, by converting wasted pressure energy into electricity, it significantly reduces operational costs and enhances the sustainability of production processes. Moreover, the system's flexibility allows it to be tailored to specific site conditions, ensuring optimal performance across a variety of scenarios. 

Environmental impact is another critical consideration. The proposed method not only improves energy efficiency but also contributes to reducing the carbon footprint of oil and gas operations. By capturing and utilizing energy that would otherwise be wasted, the system supports the industry's broader goals of sustainability and environmental stewardship.  

SYSTEM DESIGN AND CONFIGURATION 

This section describes the components of the system and demonstrates its operation. Fig. 3 provides an illustration of the proposed system within a typical upstream oil and gas manifold setting.  

Fig. 3. Proposed system for multiphase energy recovery (Saudi Arabia Patent No. 18/115523 – Filed application no. – 2024).

System Components and Operation:  

• Two-phase separator. The high-pressure multiphase wells are directed into a two-phase separator, which segregates the gas and liquid phases. This separation is crucial for stable performance of the subsequent components. 
• Turbo-expander. The gas phase is routed through a turbo-expander. In the turbo-expander, the high-pressure gas expands, reducing its pressure and temperature. This expansion process drives a generator, producing electricity. The exhaust gas from the turbo-expander connects to the low-pressure nozzle of the ejector driven by the liquid stream.  
• Jet ejector. The liquid phase, still under high pressure, is introduced into a jet ejector. Low pressure generated in the ejector is utilized to pull the turbo-expander exhaust gas connected to the low-pressure nozzle. Therefore, the jet ejector uses the pressure energy from the liquid phase to extract additional energy from the gas phase. Liquid/multiphase ejectors traditionally exhibit low efficiencies. In order to improve the efficiency, injection of drag-reducing agent is proposed in the motive stream, upstream of the ejector. Drag-reducing agents (DRAs) are known to reduce frictional losses in pipelines. Since frictional losses are a major component of ejector inefficiencies, adding drag-reducing agent is expected to improve the energy recovery performance of the system. The improvement in ejector efficiencies, through the use of DRA, has been tested and proven through lab studies ​(Abdelsalam AlSarkhi, 2024)​  
• Combined flow. The outputs from the turbo-expander and ejector are recombined and directed for further processing or reinjection.  

ILLUSTRATION OF BENEFITS 

To showcase the potential of this system, we examine a field case study. Explanations of the assumptions, a step-by-step breakdown of the calculations, and a discussion on potential uncertainties or limitations are provided below. 

Thorough analysis and detailed design calculations are required for integrated design of the whole system. These calculations will be iterative in nature, as turbo-expander gas outlet operating conditions are dependent on the design of the ejector, which will have an impact on performance of the turbo-expander.  

Following simplified calculations is an attempt to understand the impact of vital operating parameters on the performance of the system. These types of high-level calculations can be performed to establish the performance of the system in order to ascertain the feasibility. These case studies depict typical, real operating conditions in upstream crude oil production pipelines. 

Assumptions used in the calculations: 

• Steady-state operation. The system is assumed to operate under steady-state conditions, where the flow rates and pressures remain constant over time. 
• Isentropic efficiency. The efficiency of the turbo-expander is assumed to be 85%, which is a typical value for such devices. 
• Heat losses. Heat losses to the environment are considered negligible, meaning all energy changes are accounted for within the system. 

Input data for case study:   

The following input data were used for the evaluations: 

Initial conditions: 

• Inlet temperature: 183.3°F
• Inlet pressure: 820.6 psig
• Ejector outlet pressure: 135.3 psig (Required pipeline pressure)
• Discharge pressure after turbo-expander: 85.3 psig (Obtained from ejector vendor)
• Flowrates: Oil (5,523 bpd), Water (5,695 bpd), Gas (4.0 MMscfd), Gas-to-liquid ratio : 0.357 MMscfd/Thousand bpd
• Oil density: 46.6 lb/ft³
• Oil viscosity: 0.82 cP 

Power recovery calculations: 

• The operating data for the separated liquid were provided to an ejector vendor, who took the inefficiencies in the multiphase ejector into consideration, to suggest the minimum possible operating pressure of the low-pressure nozzle, where the gas outlet from the turbo-expander connects. 
• Using the pressure at the low-pressure nozzle of the ejector, and separated gas operating data, energy recovery is calculated by using the Expander model in Hysys, using an isentropic efficiency of 85% to be 300 hp. 

The power generated by the turbo-expander, alone, just with the expansion of gas (without assistance of the ejector) was estimated to be 248 hp. Therefore, an additional 52 hp (17%) power generation is contributed by the ejector.   

The power output estimated for a GLR of 0.154 (gas flow of 2 MMscfd, assuming a turbo-expander outlet pressure of 85.3 psig) is around 152 hp. 

The designer needs to determine the minimum and maximum GLR for each application, to ensure stable operation: 

• Minimum GLR: Below a certain gas flowrate, the ejector's low-pressure nozzle can approach vacuum conditions, making stable operation difficult. This threshold defines the minimum GLR. 
• Maximum GLR: Conversely, if the liquid flow drops below a specific limit, the ejector does not contribute enough, and the system loses effectiveness. This threshold defines the maximum GLR. 

Establishing these boundaries allows the system to operate within an optimal range, maintaining performance and stability. This is illustrated in Fig. 4. 

Fig. 4. Impact of GLR and its limits.

The slope of the line in Fig. 4 depends on the efficiency contrast between the turbo-expander and the liquid jet ejector. Since the turbo-expander is typically more efficient than the ejector, energy recovery generally increases with the GLR. This means that as GLR increases, the turbo-expander's efficiency has a greater impact on overall system performance, creating an upward slope.  

If the ejector's efficiency improves—such as by adding drag-reducing chemicals—the difference in efficiency between the turbo-expander and ejector decreases. This could change the slope of the line in Fig. 4, potentially making it less steep. Improved ejector efficiency would mean that energy recovery becomes less dependent on the turbo-expander alone, resulting in a more balanced contribution from both components across different GLR values. 

Optimization: 

Detailed optimization studies are necessary to tailor the system for specific operational conditions. These studies involve: 

• Flowrate and GLR optimization. Maximizing the performance of the system by designing according to flows and physical characteristics of each phase. 
• Pressure optimization. Fine-tuning the pressure at the inlet of the low-pressure nozzle of the ejector to maximize power recovery. 
• Component integration: Ensuring seamless integration of the turbo-expander and ejector to minimize energy losses and enhance overall system performance. 

LIMITATIONS 

Several limitations of the turbo-expander and liquid jet ejector combination are described below.  

• Due to the low efficiency of the ejector, some energy in the liquid stream is wasted, which can be recovered. In order to mitigate this deficiency, it is proposed to inject drag-reducing chemical in the motive stream upstream of the ejector. Injection of the drag-reducing agent acts to reduce the friction losses and thus improve the energy recovery performance of the ejector. 
• Component performance variability. The actual performance of the turbo-expander and jet ejector may vary, based on operational conditions and equipment specifications. Field testing is necessary to account for these variabilities. 
• Environmental factors. Factors, such as temperature fluctuations, impurities in the multiphase stream, and equipment wear and tear can affect the system's efficiency. These factors should be considered during field testing and system optimization. 

CONCLUSION  

The integration of a turbo-expander and a jet ejector offers a promising and innovative solution for energy recovery from high-pressure multiphase streams in oil and gas production, providing a sustainable and cost-effective method for converting wasted pressure energy into electricity. Compared to traditional methods, the proposed system significantly reduces operational costs, enhances energy efficiency, and lowers carbon emissions, contributing to environmental sustainability. Future work will focus on detailed field testing, component integration, economic analysis, and environmental assessment to facilitate the successful implementation and widespread adoption of this technology, setting a new standard for energy efficiency in the oil and gas industry. 

​​REFERENCES 

• ​​Abdelsalam AlSarkhi, A. K., "Optimizing jet pump efficiency via drag reducing polymers and enhanced efficiency definitions," Springer nature Ltd., 2024. 
• ​Saqib Nazir, A. T., Saudi Arabia Patent No. 18/115523 (Filed application no.), 2024.

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About the authors

SAQIB NAZIR is a lead process engineer at Saudi, with expertise in process engineering, energy efficiency, and produced water management. He brings over 24 years of experience in process engineering and design, acquired through roles in both EPC organizations and operating oil and gas plants. He holds a Bachelor’s degree in chemical engineering from NED University in Pakistan and a Master’s degree in chemical engineering from the University of Auckland in New Zealand. 

AHMED T. AL-GAZI is a lead engineer at Saudi Aramco, proficient at leading technical improvements and supporting water management initiatives within the oil and gas sectors. With 16 years of experience at Aramco and a master's degree in mechanical engineering from the University of Tulsa, his commitment extends beyond engineering solutions, focusing on developing personnel and promoting a collaborative and supportive workplace culture. With a track record of impactful process optimization and effective communication of achievements to leadership, Mr. Al-Gazi is dedicated to fostering talents and leading strategic efforts towards sustainability and organizational growth.