Partner Reported Opportunities (PROs) PRO Fact Sheet No. 503 for Reducing Methane Emissions 

Convert Water Tank Blanket from 

Natural Gas to Produced CO2 Gas 

Technology/Practice Overview 

Description 

Crude oil production often contains 

water, which is separated at the 

wellhead. The produced water is 

saturated with methane and light

hydrocarbons at the pressure of the gas/

oil/water separator. This water is 

normally transferred to a fixed roof 

storage tank where a drop in pressure

releases gas from the solution. This gas

can also mix with the air in the tank to 

form an explosive mixture. To exclude 

oxygen from the water tank and prevent

a hazardous situation, some operators

blanket the vapor space in the tank with

natural gas. As the tank fills and 

empties of water, the produced gas and 

blanket gas are emitted into the 

atmosphere through the roof vent. 

One Partner reported switching the 

water tank blanket from natural gas to 

CO2-rich produced gas since some gas 

production is rich in CO2. This acid gas

is typically handled in one of three ways: 

A) separated in gas processing plants, 

B) vented to the atmosphere, or 

C) injected into a crude oil reservoir for

enhanced oil recovery. This CO2-rich gas 

may be a better choice for tank blanket 

gas than methane-rich natural gas. 

Operating Requirements 

Acid is formed when the CO2 dissolves in 

the produced water. As a result, the 

water tank must be internally coated to 

protect against corrosion. 

Applicability 

This practice can be implemented where 

there is a source of CO2-rich produced

gas or a nearby gas processing plant

with acid gas removal. 

Estimated 

Gas Price 

Annual 

Methane 

Savings 

Value of 

Annual

 Gas Savings* 

Estimated 

Implementation 

Cost 

Incremental 

Operating Cost 

Payback 

(months) 

$7.00/Mcf 2,000 Mcf $14,900 $3,000 $0 3 Months 

$5.00/Mcf 2,000 Mcf $10,600 $3,000 $0 4 Months 

$3.00/Mcf 2,000 Mcf $6,400 $3,000 $0 6 Months 

Economic and Environmental Benefits 

Economic Evaluation 

Additional Benefits 

 Useful outlet for produced gas with high CO2 content 

Estimated annual methane emission reductions 2,000 Mcf per tank 

Methane Savings 

* Whole gas savings are calculated using a conversion factor of 94% methane in pipeline quality natural gas. 

Compressors/Engines 

Dehydrators 

Directed Inspection & 

Maintenance 

Pipelines 

Pneumatics/Controls 

Tanks 

Valves 

Wells 

Other 

Applicable Sector(s) 

Production 

Processing 

Transmission 

Distribution 

Other Related Documents: 

Install Downhole Separator 

Pumps, PRO No. 705 

Installing Vapor Recovery Units 

on Storage Tanks, Lessons 

Learned 
 

 

 

PRO Fact Sheet No. 503 Continued 

Convert Water Tank Blanket from Natural Gas to Produced CO2 

Gas (Cont’d) 

Methane Emissions 

Methane emissions savings assume the Partner reported 

compositions of the natural gas blanket (90 percent 

methane) and the CO2-rich replacement gas (5 percent 

methane). The Partner reported saving 32,600 Mcf per

year of methane by converting the water tank blankets 

on 9 units at a water treatment station from fuel gas to

CO2-rich produced gas. 

Economic Analysis 

Basis for Costs and Emissions Savings 

Methane emissions savings of 2,000 Mcf per year is based

on engineering estimations for blanketing a 4,000-barrel 

water tank that is emptied twice a week. The value of

fuel gas saved by this practice generally has a good 

payback. Required capital costs are estimated to be 

$3,000 and include design and installation of piping to

direct CO2-rich gas to the produced water tank. Minor 

additional O&M costs are associated with operating the 

rich CO2 line. Costs assume that the water tanks are 

already internally coated for corrosion protection. 

Discussion 

The payback for this project is favorable and is often less

than one year. Economic implementation relies on the

availability of CO2-rich produced gas. Using produced gas 

as the blanket gas, operators can avoid purchasing

natural gas and/or loss of a valuable product. 

The average methane content of natural gas varies by natural gas 

industry sector. The Natural Gas STAR Program assumes the 

following methane content of natural gas when estimating 

methane savings for Partner Reported Opportunities. 

Production 79 % 

Processing 87 % 

Transmission and Distribution 94 % 

Methane Content of Natural Gas 

EPA provides the suggested methane emissions estimating methods contained 

in this document as a tool to develop basic methane emissions estimates only. 

As regulatory reporting demands a higher-level of accuracy, the methane emission estimating methods and terminology contained in this document may not 

conform to the Greenhouse Gas Reporting Rule, 40 CFR Part 98, Subpart W 

methods or those in other EPA regulations. 

2011 2