Research on the Integration of a Natural Gas-Distributed Energy System into the Oilfield Facility in China

Abstract

The oilfield facility provides a sufficient supply of self-produced natural gas and has an obvious price advantage. However, China’s oilfield facilities are supplied with electricity and heat from the external grid and natural gas boilers separately. Therefore, in this study, a natural gas distributed energy saving system is built in the oilfield facility, which can supply electricity and heating simultaneously. An oilfield facility in Changchun, China, is used as the case study in this research to design a natural gas-distributed energy system. The operational carbon emissions and the operating cost are used as evaluation criteria. Three energy supply methods of the natural gas-distributed energy system are studied. Meanwhile, the impacts of China’s distributed energy policy are also quantified to determine the capacity of the power generation units. The results reveal that under the optimized following the heating load method (FHL-restricted), where the self-electricity consumption ratio of the gas engine is kept at 50%, the natural gas-distributed energy systems can meet policy requirements while achieving optimal carbon emission reductions and minimizing operating costs. The newly built system can simultaneously achieve the goals of energy saving, carbon emission reduction, and energy cost mitigation.

1. Introduction

Natural gas accounts for a quarter of global electricity generation [1]. The combustion of natural gas does emit greenhouse gases, but it produces far fewer carbon dioxides and air pollutants than many of the fossil fuels it is replacing, especially coal. Natural gas-fired power plants can cope with seasonal and short-term fluctuations in demand and provide backup for increased use of variable renewables, such as wind and photovoltaic [2,3,4]. Natural gas-fired power generation can play an important role in transitioning to a net-zero emission system by complementing renewable energy sources [5].

China’s total natural gas consumption in 2021 was 365.4 billion cubic meters; over 46% was dependent on imports [6]. Therefore, it is crucial to enhance the utilization efficiency of natural gas and reduce losses (such as flaring or venting). Globally, about 143 billion cubic meters of natural gas was flared in 2021, almost equivalent to China’s natural gas imports in 2021. This resulted in 270 Mt of CO₂ being directly emitted into the atmosphere, plus black soot and other greenhouse gases (such as methane) [7].

An oilfield facility is a place where a mixture of various components from the oil and gas wells is commonly separated into three phases: oil, gas, and water [8,9,10]. In the three-phase separator, gas is flashed from the liquids and water is separated from the oil. The produced oil must meet the purchaser’s specifications for the content of water, salt, and other impurities. Similarly, the gas must be processed to meet the purchaser’s water vapor and hydrocarbon dewpoint specifications. The separated water can be injected into an underground reservoir or disposed of in an environmentally safe manner [11].

However, China’s oilfield facilities are supplied with electricity and heat from the external grid and natural gas boilers separately. Cheng et al. adopted the grey TOPSIS method to evaluate the energy efficiency of an oilfield heating furnace, and the results indicated that currently, the efficiency of the furnace was relatively low and needed to be updated for energy saving [12]. The heat pump was utilized by Li et al. to recover the low-temperature waste heat of oilfield sewage [13]. The economic cost and the impact of energy price fluctuation were also analyzed. According to Guo et al. [14], an abandoned oilfield can be used to extract geothermal energy using CO₂ as the working fluid. Previous studies have mainly focused on the heating load of the oilfield facility, with the use of heat pumps and high-efficiency gas boilers for energy efficiency. The energy consumption and carbon emissions of China’s oilfield facilities were relatively high, and the efficiency of natural gas utilization could have been more efficient. Zhang et al. integrated wind power into the oil and gas field distributed energy system to reduce construction costs and promote energy saving and emission reduction [15]. Li et al. explored the carbon footprint of deep-sea oil and gas production and found that 88.2% of CO₂ was released at the operational stage because of the fuel combustion process. A distributed energy system integrated with different energy resources (such as wind turbines, photovoltaics, gas engine, absorption chiller, and batteries) was developed to decrease this part of carbon emissions [16]. A distributed energy system is considered energy efficient and ecofriendly [17]. Gurbanov et al. analyzed the role of natural gas consumption in the reduction of CO₂ emissions in Azerbaijan, and the results showed that when the share of natural gas increases by 1 percent in the total energy mix, CO₂ emission per capita decreases by approximately 0.14 percent [18]. Broesicke et al. [19] found that the decentralized natural gas power generation system decreased conventional air pollutant emissions and natural gas consumption relative to the centralized system.

Economic viability is a critical factor affecting the development of natural gas-distributed energy systems in China [20]. Ge et al. proposed a solar-assisted natural gas-distributed energy system with energy storage to decrease the annual total cost and increase the flexibility of the power grid [21]. The economics of the system are sensitive to the natural gas price. Similar results were illustrated by Wen et al., where the distributed energy system’s cost-saving ratio increased as the electricity price increased and showed the opposite trend as the natural gas price increased [22]. It was also observed by Sheng et al. that the impacts of international oil prices and foreign oil dependence on distributed energy resources showed U-shaped nonlinear curves and N-shaped nonlinear curves, respectively [23]. The price of natural gas produced at China’s oilfield facilities is much lower than the market price. Thus, developing natural gas-distributed energy systems within the oilfield facilities is highly economically viable.

The natural gas-distributed energy system can turn gas into other useable products, such as heating, cooling, and electrical power. Besides, the electrical power can be used on-site or sold back to an electricity grid [24,25,26,27,28]. Li et al. [29] studied the feasibility and flexibility of the distributed energy system under different grid connection modes, namely FEL (following the electric load), FHL (following the hybrid electric–thermal load), and FML (following the maximum electric–thermal load). Hu et al. considered an energy hub in which combined cooling, heating, and power units and heat pumps are integrated with renewable energy resources. The proposed energy hub provided operation flexibility when high renewable energy penetrated power systems [30]. Meanwhile, the distributed energy system needs to determine the capacity of the power generation units according to the energy demand. Mateo et al. quantified the impact of natural gas-fueled distributed generation in electricity distribution networks in Germany, Italy, and France. The power flows and the size of the network components depended strongly on the expected growth in electricity demand. When the size of the distributed system did not match the demand, it led to high energy losses and extra costs [31].

The oilfield facility provides a sufficient supply of self-produced natural gas and has an obvious price advantage. Therefore, in this study, we propose to build a natural gas-distributed energy-saving system in the oilfield facility, which can supply electricity and heating simultaneously through the cascade utilization of natural gas resources. The newly built system can achieve the goals of energy saving, carbon emission reduction, energy cost mitigation, and energy utilization efficiency improvement.

2. Methodology

2.1. The Status of Energy Consumption of the Oilfield Facility

An oilfield facility in Changchun, China, is used as the case study in this research to design a natural gas-distributed energy system. The climate of Changchun city belongs to ASHARE/C climate zone 6A, with an average temperature of −7.6 °C in winter and an average annual temperature of 5.1 °C. When determining the installation scheme of the natural gas distributed energy system, the energy utilization characteristics and load characteristics of the oilfield facility need to be analyzed.

According to the processing procedures of the oilfield facility, the industrial electric load in the facility mainly consists of the following parts: heating system, three-phase (oil, gas, and water) separation system, water supply (oil extraction) system, wastewater treatment system, and external transmission (crude oil export) system. In this study, the electrical load data are obtained through field monitoring.

According to the process flow of the oilfield facility, the industrial heating load of the facility is mainly composed of the following components: heating and dehydration of the oil from wells, heating of water for oil extraction, heating of the oil tanks, heating of the external transmission (crude oil export) pipeline, heating of the office buildings, and domestic hot water. The heating load demand of the oilfield facility is currently satisfied by hot water generated from natural gas boilers. In this study, the heating load data are obtained by monitoring the natural gas consumption of the boilers.

It is shown in Figure 1 how the heating and electricity loads are currently supplied to the oilfield facility. The oil-gas-water mixture from the oil wells is separated by the oilfield facility and delivered to the outside for each purpose. A portion of the natural gas is burned in boilers to produce hot water (H_b), which is used to meet the heating demand (H_demand) of the oilfield facility. The electricity demand (E_demand) of the oilfield facility is served through the external grid (E_grid). This is the basic energy supply method for the oilfield facility.

The energy balance of the oilfield facility can be calculated by Equations (1) and (2) for the basic energy supply method:

E_grid = E_demand,

(1)

NG_b × LHV × η_b = H_b = H_demand,

(2)

where NG_b is the boiler natural gas consumption (m³), LHV stands for the lower heating value of natural gas (kWh/m³), and η_b refers to the natural gas boiler efficiency (%).

The operational carbon emissions of the oilfield facility are shown in Equation (3):

OCE = E_grid × F_grid + NG_b × F_b,

(3)

where OCE is the operational carbon emission (g CO₂), E_grid equals the electricity supply from the grid (kWh), F_gird represents the carbon emission factor from the grid supply (g CO₂/kWh), and F_b is the carbon emission factor per cubic meter of natural gas burned in the boiler (g CO₂/m³).

The operating cost of the oilfield facility can be derived from Equation (4):

Cost = E_grid × P_grid + NG_b × P_gas,

(4)

where Cost is the operating cost (CNY, Chinese yuan), P_grid is the price of electricity purchased from the grid (CNY/kWh), and P_gas is the price of natural gas sold to the external customers of the oilfield facility (CNY/m³).

2.2. Energy Supply Method 1: Following the Electric Load (FEL)

The schematic diagram of the energy flow of the natural gas distributed energy system under FEL is shown in Figure 2.

The power from the natural gas-distributed energy system varies with the electricity demand of the oilfield facility, and all the electricity generated is supplied to the oilfield facility. A gas engine (also known as a reciprocating internal combustion engine) is selected as the prime mover because of the high open cycle efficiency under such low electric load, suitability for hot water production, adaptability to frequent starts and stops, and flexibility to fuel types [32]. The recoverable heat from the gas engine is used to meet the heating load of the oilfield facility, and the natural gas boiler supplements the shortage. The external grid is used as a backup to ensure the safety and reliability of the oilfield facility’s energy use during gas engine failure or maintenance.

The energy balance of the oilfield facility can be calculated by Equations (5) and (6):

E_ge = E_demand,

(5)

H_ge + H_b = H_demand,

(6)

where E_ge is the electricity generation capacity of the gas engine (kWh) and H_ge is the recoverable heat from the gas engine (kWh).

The operational carbon emissions of the oilfield facility are shown in Equation (7):

OCE = NG_ge × F_ge + NG_b × F_b,

(7)

where NG_ge is the amount of natural gas consumed by the gas engine (m³) and F_ge represents the carbon emissions factor per cubic meter of natural gas burned in the gas engine (g CO₂/m³).

The operating cost of the oilfield facility can be derived from Equation (8):

Cost = NG_ge × P_gas + NG_b × P_gas,

(8)

2.3. Energy Supply Method 2: Following the Heating Load (FHL)

The schematic diagram of the energy flow of the natural gas distributed energy system under FHL is shown in Figure 3. The recoverable heat from the natural gas-distributed energy system varies with the heating demand of the oilfield facility, and all the recoverable heat is used to satisfy the heating demand of the oilfield facility. Since the oilfield facility has greater heating demand than electricity demand, part of the electricity generated by the gas engine is used to meet the electricity demand of the oilfield facility, and the surplus is sold to the grid. The external grid and the gas boiler are used as a backup to ensure the safety and reliability of the oilfield facility’s energy use in the event of gas engine failure or maintenance.

The energy balance of the oilfield facility can be calculated by Equations (9) and (10):

E_ge − E_ge,sold = E_demand,

(9)

H_ge = H_demand,

(10)

where E_ge,sold is the electricity sold to the external grid (kWh).

The operational carbon emissions of the oilfield facility are shown in Equation (11):

OCE = NG_ge × F_ge,

(11)

The operating cost of the oilfield facility can be derived from Equation (12):

Cost = NG_ge × P_gas − E_ge,sold × P_grid,sold,

(12)

where P_grid,sold is the price of electricity sold to the external grid (CNY/kWh).

2.4. Natural Gas Components

Natural gas, after dehydration and de-oiling, is used as the gas source for the oilfield facility. The components are shown in

Table 1. Natural gas components.

No.	Item	Unit	Value
1	Methane	%	85.5
2	Ethane	%	7.23
3	Propane	%	4.51
4	Isobutane	%	0.20
5	n-Butane	%	0.19
6	isopentane	%	0.02
7	pentane	%	0.01
8	hydrogen	%	0
9	Oxygen	%	0.10
10	Nitrogen	%	0.92
11	carbon monoxide	%	0
12	carbon dioxide	%	1.32

. The lower heating value (LHV) of the natural gas is 37.2 MJ/m³ and the density is 0.7923 kg/m³.

3. Results and Discussion

3.1. Basic Energy Supply Method

The current electrical load of the oilfield facility is shown in Figure 4. In Figure 4, the electricity consumption of the oilfield facility is measured for each day of the year, and the hourly average electrical load is calculated based on Equation (13). The oilfield facility’s maximum and minimum electrical loads are 939 kW and 700 kW, with an average electrical load of 780.14 kW. The crude oil production process mainly influences the electrical load of the oilfield facility. The increase in electricity consumption due to heating and insulation in winter is insignificant.

$$\mathrm{hourly} \mathrm{average} \mathrm{electrical} \mathrm{load} =\frac{\mathrm{daily} \mathrm{electricity} \mathrm{consumption}}{24},$$

(13)

The current heating load of the oilfield facility is shown in Figure 5. In Figure 5, the daily gas consumption of the oilfield facility during a year is counted, and the hourly average heating load of the oilfield facility are calculated based on Equation (14). The natural gas boiler efficiency is 90% and the LHV of natural gas is 37.2 MJ/m³. The maximum heating load occurs in winter at 9.3 MW, and the minimum heating load occurs in summer at 6.975 MW. The average heating load is 8.04 MW. Changchun is located in ASHARE/C climate zone 6A, and the heating load is high in winter because of the great demand for heating and insulation of tanks, pipelines, and other equipment.

$$\mathrm{hourly} \mathrm{average} \mathrm{heating} \mathrm{load} =\frac{\mathrm{daily} \mathrm{natural} \mathrm{gas} \mathrm{consumption} \times \mathrm{LHV}\times {\eta }_b}{24\times 3600},$$

(14)

The fundamental data required to calculate the operational carbon emissions and operating costs of the oilfield facility are listed in

Table 2. The fundamental data.

Fgrid	Fb	Fge	Pgrid	Pgrid,sold	Pgas	ηb
g CO2/kWh	g CO2/m3	g CO2/m3	CNY/kWh	CNY/kWh	CNY/m3	%
565	2066.09	2066.09	0.5866	0.374	0.76	90

. The value of F_grid (the carbon emission factor from the grid supply) is taken from the Annual Development Report of China Electricity Industry 2021 [33]. F_b and F_ge are calculated from natural gas components. P_grid is the current price of electricity purchased from the external grid for the oilfield facility. P_grid,sold exhibits the feed-in tariff for the sale of natural gas distributed energy generation in Changchun. P_gas represents the price of natural gas sold externally from the oilfield facility. η_b is the natural gas boiler efficiency.

The operational carbon emissions and operating costs of the oilfield facility under the basic energy supply method are shown in

Table 3. Operating data under different energy supply methods.

	Egrid (GWh)	Ege,sold (GWh)	NGb (106 m3)	NGge (106 m3)	OCE (tons)	Cost (106 CNY)
Basic method	6.88	0	7.58	0	19,541.13	9.79
FEL	0	0	6.59	1.76	17,241.79	6.34
FHL	0	69.38	0	15.41	31,821.38	−14.24
FHL-restricted	0	6.88	6.16	3.02	18,975.62	4.41

. The oilfield facility purchases 6.88 GWh of electricity from the external grid and burns 7.58 × 10⁶ m³ of natural gas in the natural gas boiler annually. It emits 19,541.13 tons of CO₂ per year. The annual operating cost is 9.79 × 10⁶ CNY.

3.2. FEL

Under the FEL (following the electric load) method, the total power consumption of the oilfield facility is provided by the gas engine. As illustrated in Figure 4, the oilfield facility’s maximum and minimum electrical loads are 939 kW and 700 kW, with an average electrical load of 780.14 kW. Therefore, the selected gas engine and its data at different loading rates are displayed in

Table 4. The data of the selected gas engine at different loading rates under FEL.

Item	Loading Rate (%)
	100%	75%	50%
power generation (kW)	1053	787	519
power generation efficiency (%)	39%	37.90%	35.80%
recoverable heat (kW)	1324	1054	773
thermal efficiency (%)	49.07%	50.77%	53.31%

. At a 100% loading rate, the power generated by the gas engine is 1053 kW, and the recoverable heat available is 1324 kW. The efficiency of the gas engine decreases severely and cannot be appropriately operated below a 50% loading rate. The power and efficiency of the gas engine at different electrical loads were obtained by the interpolation method. The amount of natural gas consumed by the gas engine per day is depicted in Figure 6.

The operational carbon emissions and operating costs of the oilfield facility under FEL are shown in Table 3. The gas engine provides the oilfield facility’s entire electricity consumption, so the electricity purchased from the external grid is 0 GWh. Since the gas engine only meets the electrical load of the oilfield facility and does not sell electricity to the outside, the E_ge,sold is 0 GWh. The gas engine consumes 1.76 × 10⁶ m³ of natural gas per year. The natural gas boiler is used as a supplemental heating source and consumes 6.59 × 10⁶ m³ of natural gas per year, as shown in Figure 6. Although the total natural gas consumption increases by 1.47 × 10⁶ m³ under energy supply method 1 compared to the basic method, the CO₂ emissions decrease by 11.77% (from 19,541.13 tons to 17,241.79 tons) and the operating costs decrease by 35.24% (from 9.79 × 10⁶ CNY to 6.34 × 10⁶ CNY).

According to Table 2, the carbon dioxide emission per cubic meter of natural gas after complete combustion is 2066.09 g. By utilizing the LHV and gas engine’s power generation efficiencies, it can be calculated that the carbon emissions of the power supply by the gas engine at 50% loading rate are 558.61 g CO₂/kWh and 512.11 g CO₂/kWh at 100% loading rate, both of which are lower than the carbon emission from the grid supply (565 g CO₂/kWh).

The price of natural gas provided by the oilfield facility is significantly lower than that of natural gas sold in the market, resulting in a lower price for power from gas engines than from E_grid.

Therefore, natural gas-distributed energy can effectively reduce the operational carbon emissions and operating costs of the oilfield facility.

3.3. FHL

Under FHL (following the heating load) method, the entire heating demand of the oilfield facility is provided by recoverable heat from the gas engine. As illustrated in Figure 5, the maximum heating load occurs in winter at 9.3 MW, and the minimum heating load occurs in summer at 6.975 MW. Therefore, the selected gas engine and its data at different loading rates are displayed in

Table 5. The data of the selected gas engine at different loading rates under FHL.

Item	Loading Rate (%)
	100%	75%	50%
power generation (kW)	10,400	7800	5200
power generation efficiency (%)	48.70%	47.30%	45.10%
recoverable heat (kW)	9674	6992	5016
thermal efficiency (%)	45.30%	42.40%	43.50%

. At a 100% loading rate, the power generated by the gas engine is 10,400 kW, and the recoverable heat available is 9674 kW. The amount of natural gas consumed by the gas engine per day is depicted in Figure 7.

The operational carbon emissions and operating costs of the oilfield facility under FHL are shown in Table 3. Part of the electricity generated by the gas engine is used to meet the electricity demand of the oilfield facility, and the surplus is sold to the grid. Thus, E_grid is zero and E_ge,sold is 69.38 GWh. The amount of surplus electricity generated by the gas engine per day is depicted in Figure 7. The gas engine consumes 15.41 × 10⁶ m³ of natural gas per year, and the natural gas boiler is used as a backup heating source. Although the total natural gas consumption and the operational CO₂ emissions under energy supply method 2 become 2.03 times and 1.85 times higher than under the basic method, the operating costs decline by 245.45% (from 9.79 × 10⁶ CNY to −14.24 × 10⁶ CNY). A negative value (−14.24 × 10⁶ CNY) indicates that the operating cost of the oilfield facility is zero and that the oilfield facility can profit by selling electricity to the grid.

The price of natural gas provided by the oilfield facility is significantly lower than that of natural gas sold in the market, resulting in a lower price for power from gas engines than from E_grid. At the same time, additional revenue can be obtained by using the gas engine to generate surplus electricity. Therefore, the operating costs of the oilfield facility are significantly reduced.

The increase in carbon emissions from the oilfield facility is mainly due to the gas engine’s sales of electricity to the external grid. The carbon emissions of the power supply by the gas engine under energy supply method 2 at a 50% loading rate are 443.33 g CO₂/kWh and 410.56 g CO₂/kWh at a 100% loading rate, both of which are lower than the carbon emission from the grid supply (565 g CO₂/kWh). Therefore, although the operational carbon emissions of the oilfield facility increase, it can effectively reduce the carbon emissions of the grid supply.

3.4. FHL-Restricted

Under the FHL method, the more electricity sold to the grid, the lower the operating cost of the oilfield facility and the lower the carbon emissions of the grid supply. However, according to China’s distributed energy policy, the total amount of gas engine external power sales is limited. In the case of external power sales, the installed capacity of the gas engine is mainly constrained by the following two equations based on “Grid connection specification of distributed energy”, published by State Grid Corporation of China in 2014 [34]:

$$\frac{E_{demand}}{E_{ge}}\times 100\%\ge 50\%$$

(15)

$$\frac{E_{ge}+H_{ge}}{N{G}_{ge} \times LHV} \times 100\%\ge 70\%,$$

(16)

Equation (15) represents the self-electricity consumption ratio, which means that the oilfield facility must use at least 50% of the electricity generated by the gas engine before the remainder can be sold to the grid. Equation (16) equals the primary energy consumption ratio, which indicates that at least 70% of the energy in each cubic meter of natural gas must be converted by the gas engine into electrical and thermal energy that the oilfield facility can use. The main purpose of this policy is not to disallow distributed energy projects to sell electricity to the grid, but to improve the efficiency of energy use on the customer side.

Under FHL, the primary energy consumption ratio is 92.19%, which can meet the requirements of Equation (14). In comparison, the self-electricity consumption ratio is only 9.02%, which does not meet the requirements of Equation (13). Therefore, it is necessary to develop a new “FHL-restricted” energy supply mode to meet the demands of China’s distributed energy policy.

The schematic diagram of the energy flow of the natural gas distributed energy system under the FHL-restricted mode is shown in Figure 8. The power from the gas engine varies with the electricity demand of the oilfield facility. In total, 50% of the electricity generated by the gas engine is used to meet the electricity demand of the oilfield facility, and the surplus is sold to the grid. The recoverable heat from the gas engine is used to meet the heating load of the oilfield facility, and the natural gas boiler supplements the shortage. The external grid is used as a backup to ensure the safety and reliability of the combined station’s energy use during gas engine failure or maintenance.

The selected gas engine and its data at different loading rates are displayed in

Table 6. The data of the selected gas engine under the FHL-restricted method.

Item	Loading Rate (%)
	100%	75%	50%
power generation (kW)	2002	1501.5	1001
power generation efficiency (%)	45.10%	43.80%	41.60%
Recoverable heat (kW)	1847	1450	1042
thermal efficiency (%)	41.60%	42.30%	43.30%

. At a 100% loading rate, the power generated by the gas engine is 2002 kW, and the recoverable heat available is 1847 kW. The amount of natural gas consumed by the gas engine per day is depicted in Figure 9.

The operational carbon emissions and operating costs of the oilfield facility under FHL-restricted mode are shown in Table 3. Part of the electricity generated by the gas engine is used to meet the electricity demand of the oilfield facility, and the surplus is sold to the grid. Thus, E_grid is zero and E_ge,sold is 6.88 GWh. The amount of surplus electricity generated by the gas engine per day is depicted in Figure 9. The gas engine consumes 3.02 × 10⁶ m³ of natural gas per year. The natural gas boiler is used as a supplemental heating source and consumes 6.16 × 10⁶ m³ of natural gas per year, as shown in Figure 9. Although the total natural gas consumption increases by 1.6 × 10⁶ m³ under the FHL-restricted mode compared to the basic method, the CO₂ emissions decrease by 2.89% (from 19,541.13 tons to 18,975.62 tons), and the operating costs decline by 54.95% (from 9.79 × 10⁶ CNY to 4.41 × 10⁶ CNY).

Under the FHL-restricted mode, both the primary energy consumption ratio and the self-electricity consumption ratio can meet the Chinese distributed energy policy requirements, as shown in Figure 10.

It can be seen from Figure 10 that the self-electricity consumption ratio of the gas engine has been kept at 50%. This is because, under the FHL-restricted mode, the more the gas engine sells electricity to the grid, the smaller the operational carbon emissions and operating costs of the oilfield facility. However, the self-electricity consumption ratio requires that the gas engine cannot sell more than 50% of its electricity to the grid. Therefore, when the self-electricity consumption ratio of the gas engine is kept at 50%, the oilfield facility’s operating carbon emissions and operating costs are most reasonable under the FHL-restricted mode.

4. Conclusions

China’s oilfield facilities are supplied with electricity and heat from the external grid and natural gas boilers separately. Compared with the existing energy supply method, the use of a natural gas-distributed energy system is conducive to achieving the goals of energy saving and emission reduction, reducing energy costs, and improving energy efficiency. Meanwhile, the safety and economic performance of the oilfield facility can also be improved. An oilfield facility in Changchun, China, is used as the case study in this research to design a natural gas-distributed energy system. The operational carbon emissions and the operating cost are used as evaluation criteria. Three energy supply methods of the natural gas-distributed energy system are studied. The results reveal that:

(1)

The natural gas distributed energy system can significantly reduce the operating cost because the oilfield facility has a prominent price advantage over self-produced natural gas.

(2)

The economic performance of distributed energy systems is optimal when electricity can be sold to the external grid (FHL). However, distributed energy systems in China are subject to policy constraints and must simultaneously meet the requirements of the self-electricity and primary energy consumption ratios. Therefore, under the FHL-restricted mode, when the self-electricity consumption ratio of the gas engine is kept at 50%, the natural gas-distributed energy systems can meet policy requirements while achieving optimal carbon emission reductions and minimizing operating costs.

(3)

The increase in carbon emissions from the oilfield facility is mainly due to the gas engine’s sales of electricity to the external grid. The carbon emissions of the power supply by the gas engine are lower than the carbon emission from the grid supply. Therefore, although the operational carbon emissions of the oilfield facility increase, it can effectively reduce the carbon emissions of the grid supply.