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 CO
2 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 CO
2 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 CO
2 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 CO
2 emissions in Azerbaijan, and the results showed that when the share of natural gas increases by 1 percent in the total energy mix, CO
2 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.
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.
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/m
3. 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.
The fundamental data required to calculate the operational carbon emissions and operating costs of the oilfield facility are listed in
Table 2. 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. The oilfield facility purchases 6.88 GWh of electricity from the external grid and burns 7.58 × 10
6 m
3 of natural gas in the natural gas boiler annually. It emits 19,541.13 tons of CO
2 per year. The annual operating cost is 9.79 × 10
6 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. 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 × 10
6 m
3 of natural gas per year. The natural gas boiler is used as a supplemental heating source and consumes 6.59 × 10
6 m
3 of natural gas per year, as shown in
Figure 6. Although the total natural gas consumption increases by 1.47 × 10
6 m
3 under energy supply method 1 compared to the basic method, the CO
2 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 × 10
6 CNY to 6.34 × 10
6 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 CO
2/kWh and 512.11 g CO
2/kWh at 100% loading rate, both of which are lower than the carbon emission from the grid supply (565 g CO
2/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 Egrid.
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. 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 × 10
6 m
3 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 CO
2 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 × 10
6 CNY to −14.24 × 10
6 CNY). A negative value (−14.24 × 10
6 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 Egrid. 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]:
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. 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 × 10
6 m
3 of natural gas per year. The natural gas boiler is used as a supplemental heating source and consumes 6.16 × 10
6 m
3 of natural gas per year, as shown in
Figure 9. Although the total natural gas consumption increases by 1.6 × 10
6 m
3 under the FHL-restricted mode compared to the basic method, the CO
2 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 × 10
6 CNY to 4.41 × 10
6 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.