Determination of Flared Gas Volumes Offshore Norway

By Stig Arvid Knutsson

Principal Engineer

Fiscal Metering

Norwegian Petroleum Directorate

Presentation Outline

Regulatory requirements

Flaring of Associated Gas

Flow Rates and Measurement Ranges

Technical Challenges

Chosen Technology

Uncertainty Requirements

Back-up Methods

Calculation Methods

Closing Remarks

Petroleum Act:No flaring of associated gas for other reasons than safety

Measurement Regulation/ Norwegian Industry Standard (Norsok):Requirements to metering technology, uncertainty and reportingRequirements to back-up quantification

The CO2 Tax Act (1 Jan 1991)CO2 tax shall be paid on the burning of petroleum and discharge of natural gas. The tax is the same for both burned and unburned gas

The Greenhouse gas Emission Trading Act (EU Directive Oct 2004)(National legislation 1 Oct 2007)

Determination of CO2 emissions (from measured/calculated volumes) for trading of CO2 quotas. Introduce compositional analysis of flare and fuel quantities.

Regulatory Requirements

Gas flaring and venting regulations (Petr. Act 4-4)

“Flaring in excess of the quantities needed for normal operational safety shall not be allowed unless approved by the Ministry” :

- Associated gas utilisation required to get authority approval of Plan for Development and Operation

- Annual gas flaring permits

- Avoid waste of petroleum and reservoir energy

- Cold venting is not in accordance with the principle of environmentally prudent petroleum production

Regulatory Requirements

The first oil production in Norway was done by

Philips Petroleum Company Norway using the revamped drilling rig Gulftide from the summer 1971 until spring 1974, i.e.

during development of the Ekofisk field.

This is the only time flaring of associated gas

has been approved in Norway for other reason

than safety. I.e. for regular production.

Oil production was approximately 40,000

bbl/day. Gas rates derived from test separator GOR.

Flaring of Associated Gas

Flow Rates & Ranges

This creates a situation whereby generally the flow rate is very close

to zero.

The exemptions are normally planned and unplanned

shutdowns, with associated blow

downs.

In this case it has been three shut-

downs over a period of three

months.

Note the high turndown ratio

-1000

1000

3000

5000

7000

9000

11000

13000

15000

17000

19000

2005

-11-17

2005

-11-24

2005

-12-01

2005

-12-08

2005

-12-15

2005

-12-22

2005

-12-29

2006

-01-05

2006

-01-12

2006

-01-19

2006

-01-26

2006

-02-02

2006

-02-09

2006

-02-16

Tid

Strø

mni

ngsr

ate

(Sm

3/h)

Fakkel (Sm3/h)N2 (Sm3/h)

Flow Rates & Ranges

-500

0

500

1000

1500

2000

2500

3000

2005-12-04 2005-12-14 2005-12-24 2006-01-03 2006-01-13 2006-01-23 2006-02-02

Tid

Strø

mni

ngsr

ate

(Sm

3/h)

HP (Sm3/h)LP (Sm3/h)Atm. (Sm3/h)N2 (Sm3/h)

The figure shows three flare streams

and the N2 measurement in a

period with frequent

production interruptions.

Turndown ratio in excess of 100:1 (≈ 0 – 100+ m/s)

Composition vary from dry gas to gas with high percentage of HC liquid

Difficulties in calculating/estimating (small) flare/vent volumes accurately by material balance

Technical Challenges

Chosen Measurement Technology -USM

tA-BtB-A

Upstream transducer

Downstream transducerWhen there is no gas flowing the transit times tA-B and tB-A will be equal.

As the flow increases, the downstream transit time (tA-B) will decrease, while the upstream transit time (tB-A) will increase.

The difference (Δt) will give the flow velocity, while the average will give us the velocity of sound.

Operating Principle

Uncertainty Requirements

NPD Requirement: ± 5 % of standard volume

NPCA by EU directive: ± 7.5 % of emitted CO2 mass

± 2.5 % on emission factor

Back-up Method Using Pressure Measurement

PT

In this case the maximum pressure for a high pressure

flare (HP) is 8.00 Barg, and zero at no flow

Alternative/back-up measurement done by measuring the pressure

immediately upstream of the flare stack tip.

0-8.00 Barg

Back-up Method Using Pressure Measurement

Approximate range for USM

A correlation between mass flow rate is

worked out. Here with three different

molecular weights.

As indicated the USM works in normal flow range but is limited in

maximum velocity. Then pressure

measurements from the flare tip takes over.

Back-up Method Using Pressure Measurement

At the LP flare the correlation between

flow rate and pressure becomes

more complex.

Note that the low pressure may create

problems with changing

atmospheric pressure at low flow

rates.

Four different molecular weight

values are used here.

Back-up Method Using DP Measurement on an Elbow

The differential pressure between the

two tappings, created by the centrifugal force of

the flow is used to measure the rate of

flow.

HP side

LP side

Estimation by Material Balance

FUEL

<2%

VENT

<2%

INJECTION

<2%

EXPORT

<1%

Well Streams

Uncertainty

+/- 5-15%

FLARE

N2

<3%

Vflare=Well Stream gas – (Export+fuel+vent+N2+Injection)

This method may be possible, but:

with high uncertainty on large input values, low flare rates will be calculated with very high uncertainty.

Use of GOR to Calculate Flare Volumes

Failure in flare gas measurements are not uncommon, for reasons like too high flow velocity or liquid collection in/at the meter.

If no back-up measurement is done on the flare stack it is possible to estimate the flared volumes. Two methods are commonly used:

Method A (used when production from individual wells has been stable):

VFLARE A = (VOIL X GOR PREVIOUS DAY)–VFUEL–VEXPORT–VINJECTION–VVENT–VTAX FREE GAS

Method B (used when wells are closed in/opened or wells with high GOR replace wells with low GOR or the opposite):

VFLARE B = Sum(VOi X GORi)–VFUEL-VEXPORT-VINJECTION-VVENT- vTAX FREE GAS

The gas to oil ratio (GOR) can be used to verify measured flare, vent, fuel and injection gas. This by comparing Daily GOR with Historical GOR

Daily GOR = (Daily Fuel+flare+vent+gas export+injected gas)/daily dry oil production

If Daily GOR differs with more than e.g. 2% from Historical GOR then:Verify actual well production configurationVerify all measured volumes and correct if necessary

Use of GOR to Verify measured quantities

Closing Remarks

Flaring of hydrocarbons in Norway is:

Low

Taxed

Highly governed by national and international regulations

Technically challenging, measurement wise

•It will be even more challenging as pollution restrictions are

become heavier enforced

Not easy to determine by calculations