Improvement of non-CO₂ greenhouse gas emission estimates for oil and gas operations in Russia

Abstract

Oil and gas is a key sector for greenhouse gas inventory of the Russian Federation represented by non-CO₂ greenhouse gases methane (CH₄) and nitrous oxide (N₂O). The CH₄ and N₂O emission estimates are mainly performed with the use of default method (IPCC 2000). As a result of continuous inventory improvement, the national parameters for oil and gas sector were derived and IPCC methodology was adjusted. The parameters were developed on the basis of specific features of producing industry and the properties of oil and gas produced. These developments enabled a shift to tier 2 estimation approach (IPCC 2006). The aim of the study was to highlight the differences between both estimation methods. For this purpose, the CH₄ and N₂O emissions were estimated from 1990 to 2009 with the use of default (tier 1) and a combination of tier 1 and tier 2 methods. The results of the estimates were compared and their uncertainty assessed. The results of comparison showed insignificant differences in the emission profile. The uncertainty of the combined tier 1 and tier 2 estimates was lower than of those performed with the tier 1 (15% and 23%, respectively). Thus, the use of higher tier increases the reliability of the inventory for oil and gas sector.

Keywords:

• greenhouse gases
• emissions
• methane
• nitrous oxide
• inventory uncertainty
• oil and gas sector

1. Introduction

According to the IPCC Fourth Assessment Report (IPCC 2007), the increased concentration of anthropogenic greenhouse gases in the atmosphere caused dramatic temperature increase and resulted in global climate change. With the Kyoto Protocol entry into force in February 2005, the developed countries committed themselves to undertake joint effort to reduce greenhouse gas emissions to the atmosphere. The developed countries are included at the Annex 1 to the UNFCCC. The implementation of the commitments under the Kyoto Protocol is assessed and reviewed through the national inventory submissions, including the common reporting format (CRF) data tables and a national inventory report, that include greenhouse gas emission estimates from the base year up to two years before the year of any specific submission. The national greenhouse gas emission estimates are represented in a set of predefined tables (CRF) providing an emission profile, which shows their composition and contribution to the national totals (UNFCCC 2006). The profile is assessed in relation to the base year and the judgement on the implementation of the national commitments is made. The added value of assessments is the identification of priorities in emission mitigation strategies including inter alia deployment of climate friendly technologies in the economic sectors.

Following the requirements of the UNFCCC reporting guidelines for Annex I Parties (UNFCCC 2006), Russian Federation prepares, publishes and regularly updates its greenhouse gas emission inventories. At the same time, a special consideration is given to reliability of emission estimates of the key source categories. According to the IPCC Good Practice Guidance, a key source category is one that is prioritised within the national inventory system because its estimate has a significant influence on a country's total inventory of direct greenhouse gases in terms of the absolute level of emissions, the trend in emissions or both (IPCC 2000). Oil and gas sector is a key category in the national inventory of Russia, making the contribution about 15% to the total emission profile of the country (NIR 2011).

Reliable estimates are essential for greenhouse gas emission mitigation, particularly for key categories. The aim of this work was to improve the greenhouse gas estimates for oil and gas sector by means of elaboration of country-specific emission parameters and the adjustment of conventional for national conditions of the country.

2. Materials and methods

2.1. Production-based method

The non-CO₂ greenhouse gas emissions from oil and gas operations are currently estimated with the use of production-based method, which is also known as a default method. It corresponds to IPCC tier 1 and is generalised in the equation below (IPCC 2000):

where E is the emission of greenhouse gas in question (CH₄, N₂O), Gg; AD are the activity data on total oil and natural gas production, transportation etc., dimension depending on activity; EF is the emission factor for greenhouse gas in question (CH₄, N₂O), dimension depending on activity.

As it follows from Equation (1), the default method is a product of activity data by relevant emission factors that represent portion released during the operation to the atmosphere as a greenhouse gas. The activity data are taken from national statistics. The emission factors are country-specific and those recommended by the IPCC guidelines (IPCC 2000). Non-CO₂ emission estimates from oil and gas operations in common based on the Equation (1).

2.2. Combination of production-based and mass-balance methods

According to the IPCC 2006 Guidelines for National Greenhouse Gas Inventories, tier 2 could be described by the several equations shown below (IPCC 2006):

where E _venting is the emission of greenhouse gas in question due to production, Gg y⁻¹; E _{CH4,N2Oflaring} is the emission of the of greenhouse gas in question during due to associated gas flaring at oil production facilities, Gg y⁻¹; GOR is the average gas-oil ratio referenced at 15°C and 101.325 kPa, m³ m⁻³; Q _oil is annual oil production; M _CH4 is the molecular weight of methane, g mol⁻¹; y _CH4 is mol or volume fraction of CH₄ in the associated gas; CE is gas conservation efficiency factor, dimensionless; X _flared is fraction of the waste gas that is flared rather than vented; FE is flaring destruction efficiency, dimensionless; EF_N2O is emission factor for N₂O flared, Gg 10⁻³ m⁻³ of associated gas flared; 42.3 × 10⁻⁶ is the number of kmol per m³ at 15°C and 101.325 kPa (i.e. 42.3 × 10⁻⁶ kmol m⁻³) times a unit conversion factor of 10⁻³ Gg Mg⁻¹ (brings the results to Gg y⁻¹).

The Equations (2) and (3) correspond to mass balance method, whereas the Equation (4) is enhanced production-based method. Production-based method is applied for N₂O emission estimates from associated gas flaring due to complexity of chemical processes relevant to it (Hayhurst and Lawrence 1992).

With the aim at obtaining more accurate emission estimates, an enhanced combination of production-based and mass-balance methods was used. For this purpose, national emission parameters were derived along with adaptation of default methodologies to national conditions. National conditions mean standard conditions under 20°C and 101.325 kPa (instead of 15°C and 101.325 kPa) specifics of national statistic data and the composition of oil and natural gas.

Production-based method (Equation (1)) was applied to following operations with oil and natural gas as:

•	production (non-combustible emissions, i.e. venting and production leakages), flaring, transport and storage of natural gas;
•	storage and transport of oil.

The CH₄ emissions from natural gas transport and storage were estimated with the use of dimensionless country-specific emission factors 9 × 10⁻³ and 3.2 × 10⁻⁴, respectively, derived at Gazprom JSC industrial facilities (Dedikov et al. 1999; IEA 2006). The other emission factors were taken from the IPCC guidelines (IPCC 2000). To account for country-specific conditions of natural gas transport and storage, the additional parameters were introduced in the calculations: CH₄ content in the natural gas (reduction factor) and volume to mass conversion factor for CH₄. The CH₄ content in natural gas (RF value) was that derived at Gazprom JSC industrial facilities (Dedikov et al. 1999). Volume to mass conversion factor for CH₄ is a constant value which was recalculated for standard conditions accepted in the country (20°C and 101.325 kPa).

The Equations (2)–(4) correspond to IPCC tier 2 (IPCC 2006). Tier 2 was applied to estimate emissions from oil production (non-combustible emissions, i.e. venting and production leakages), and flaring, which provide the major contribution to emission profile. To comply with specific conditions of the Russian Federation, the parameters in the Equations (2)–(4) were recalculated. The invariable value of 42.3 × 10⁻⁶ kmol m⁻³, which is the number of kmol per m³ (i.e. 42.3 × 10⁻⁶) times a unit conversion factor of 10⁻³ Gg Mg⁻¹ (the inverse of the Molar volume, i.e. Vm⁻¹), was recalculated to standard conditions 101.325 kPa and 20°C with the use of molar volume and the Mendeleev-Clapeyron ideal gas equation. The recalculated invariable is 41.57656 × 10⁻⁶ kmol m⁻³ (Atkins 1998). The product GOR • Q _OIL means the volume of associated gas produced referenced at the standard conditions of the country. The activity data collected by national statistics include the data on volume of associated gas produced as the sum of the volumes of associated gas used and its losses (including associated gas flared). To accommodate the national data on associated gas, GOR • Q _OIL was replaced with , which is of the same meaning. The fraction of waste gas that is flared rather than vented (X _flared) was estimated based on data for operations with associated gas: . The calculations were performed with the use of chemical composition of West-Siberian oil, which makes up to 66% of oil production in Russia (Andreykina 2005). The amounts of associated gas produced, utilised and flared were taken from the national statistics (Rosstat 2007, 2010). Table 1 summarises country-specific parameters for estimates of non-CO₂ emissions from operations with oil.

Table 1. The country-specific parameters for estimation greenhouse gas emissions from operations with oil in the Russian Federation.

Parameter in the Equations (3)–(5)	Replaced with country-specific parameter	Explanatory comment
GOR • Q oil		Provided in national statistics
42.3 × 10^−6	41.6 × 10^−6	Oil density recalculated to the temperature 20°C and pressure 101,325 kPa
y gas	y CH4 = 0.583	Derived from the national data on composition of the associated gas
X flared		Recalculated based on national statistics on associated gas operations

The CH₄ and N₂O emissions for the described above operations with oil and natural gas were estimated from 1990 to 2009 with the use of default (tier 1) and a combination of tier 1 and tier 2 methods. The estimates were further recalculated into CO₂ equivalent on the basis of IPCC global warming potentials given in the IPCC Second Assessment Report (IPCC 1996).

2.3. Uncertainty assessment method

The uncertainty of both estimates assessed made in accordance with the recommendations of the IPCC guidelines (IPCC 2000):

where σ ₁ and σ ₂ are standard deviations of probability density functions of the emissions in year t ₁ and t ₂. U _TOTAL is the percentage uncertainty in the sum of the quantities (half the 95% confidence interval divided by the total and expressed as a percentage). The x_i and U_i parameters, respectively, are the uncertain quantities and the percentage uncertainties associated with them.

The errors of the default emission factors are about 25% (IPCC 2000). The activity data on operations with oil and natural gas in the national statistics have the higher accuracy with errors being no more than 5%. The standard deviations for non-CO₂ greenhouse gases and category sources were obtained based on the Equation (5) and the above error values. Then they were included in the Equation (6) along with the contribution of individual non-CO₂ greenhouse gases and source categories arriving to the overall uncertainty estimate for oil and natural gas operations.

The uncertainty analysis for the combination of tier 1 and tier 2 methods implied the assessment of larger number of parameters included in the calculations. Gas compositions are usually estimated accurately with errors to be within ±2.5% for the individual components (Ministry of Oil and Gas Industry, 1991). The emission factors were chosen in accordance with the IPCC guidelines. So, their errors should be ±25% (IPCC 2000, 2006).

3. Results and discussion

The Equations (3)–(5) merge specific oil operations inscribed in step-by-step tier 1 calculations. This combination adequately represents activities in oil and gas sector of the Russian Federation. The non-CO₂ greenhouse gas emissions from operations with oil and gas such as oil and natural gas production, transport, storage and flaring of associated and natural gas were calculated for the time series from 1990 to 2009.

As a result of the performed estimates, new non-CO₂ emission profile was obtained. The new profile was compared with the tier 1 profile for the same operations (Figure 1). The results of the estimates were compared and their uncertainty assessed. The results of the uncertainty assessment both of the estimates are shown in Table 2.

Figure 1. Comparison of the tier 1 and the combination of tier 1 and tier 2 calculations of non-CO₂ emissions from oil and natural gas production, transport, storage and flaring of associated and natural gas in Russia.

Table 2. The results of uncertainty assessment of the estimates made with the use of two approaches.

Method of emission estimation	Combined uncertainty, %	Uncertainty introduced into the trend in total national emissions, %
Tier 1 method	23	32
Combination of tier 1 and tier 2 methods	15	22

Comparison of non-CO₂ emission profiles showed that the combined tier 1 and tier 2 estimates are somewhat higher than those for tier 1: 160.6 Tg (tear 1) and 196.7 Tg (combination of tier 1 and tier 2). The difference is about 23% (average value for the time series from 1990 to 2009). The reason of the difference is that the combination of tier 1 and tier 2 methods involves additional activity data, which results in increasing of NO₂ emission contribution to the new profile.

The average difference about 23% fits in the combined uncertainty as shown in Table 2. In that case, one could assume that both profiles are of the same level. As it follows from the Table 2, uncertainty of the estimates based on combination of tier 1 and tier 2 methods is less than tier 1 estimates uncertainty. The lower values of uncertainty indicate of the higher quality of the non-CO₂ emission estimates.

4. Conclusion

Thus, production-based and mass-balance methods were analysed along with the national conditions such as specifics of national statistic data, standard conditions and national composition of oil and natural gas. As a result, the national parameters were derived and default methods adjusted. It allows the application of combination of tier 1 and tier 2 methods to estimate non-CO₂ emissions from operations of oil and gas sector:

•	oil and natural gas production, transport and storage;
•	associated and natural gas flaring.

As a result, more accurate emission profile was obtained and its uncertainty decreased (from 23% to 15%). Consequently, combination of tier 1 and tier 2 allows more accurate estimate non-CO₂ emissions than tier 1 and adequately represents activities in oil and gas sector of the Russian Federation. The further improvements of the emission estimates can be achieved by the development of the national emission factors and other parameters (particularly for N₂O emission assessment).