determination of flared gas volumes offshore...
TRANSCRIPT
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