prospect evaluation, resource assessment and risking - ccopccop.or.th/eppm/projects/28/docs/9...
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Prospect evaluation, resource
assessment and risking
Prospect evaluation
Knut Henrik Jakobsson
Norwegian Petroleum Directorate
Inger Fjærtoft
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Purposes of prospect evaluation
by the government
• Basis for recommendation for which blocks
should be awarded and proposals for work
commitment for licenses
• Basis for evaluation of applications for
licenses or bidding rounds
• A state participation can be decided on basis
of the evaluation carried out by the
government in addition to the
applicants/bidders
4600
4400
4500
45004700
4500
4600
4700
440045004600
4500
0 1
Km
0,5
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The Norwegian licensing round
Announcement Application Award
Negotiation
AnnouncementNomination
NPD performs own evaluations
of announced areas. This forms
an important basis for the
evaluation of applications.
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Volumetric calculation
What is the recoverable hydrocarbon quantities of this accumulation ?
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The Volumetric Function
Rvol x N/G x por x Shc x F.v.factor x Rec.f.
HCPV (hydrocarbon porevolume)
In-place resources
(HCPV at surface conditions)
Recoverable resources
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HCPV
(hydrocarbon pore volume)
Rockvol x Net/Gross x porosity x Saturationhc
trap
definition
reservoir
parameters
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The rock volume
Interpretation and mapping• Seismic interpretation
• Digitizing, map construction
• Depth conversion
Geometric descriptionl Vertical closure
l Spillpoint relations
l Trap fill
l Uncertainties in interpretation, mapping and depth
conversion
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The trap
The volume between the top surface and the HC-contact
minus
the volume between the bottom surface and the HC-contact
ROCK VOLUME OF THE TRAP =
spillpoint
top surface
bottom surface
vertical
closure
HC-contact
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The maps
Top surface
Bottom surface4600
4400
4500
45004700
4500
4600
4700
440045004600
4500
0 1
Km
0,5
A bottom surface map is not required, when:
- the reservoir thickness > vertical closure
- the reservoir thickness is constant
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Gross rock volume,
some North Sea fields
0
2000
4000
6000
8000
10000
12000
14000
Troll Frigg Heimdal E. Frigg
A.
53000G
ross
ro
ck v
olu
me,
mill
m3
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HCPV
(hydrocarbon pore volume)
Rockvol x Net/Gross x porosity x Saturationhc
trap
definition
reservoir
parameters
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Reservoir description
GR SonicSimplified litostratigraphy
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Reservoir parametres
• Reservoir thickness (constant or variable)
• Net/gross ratio (average)
• porosity (average; > cut-off value)
• HC-saturation (average)
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Gross thickness
• Should be taken care of in the mapping
procedure...
Gross thickness
top surface
bottom surface
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Net pay
a
b
c
de
f
g
h
NET PAY =
The total thickness of
all reservoir units (a-h)
with
porosity > threshold value
and
permeability > threshold value
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Porosity
• Calculated from electric
well logs
• Core measurements
Average porosity larger
than cut-off value
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Poroperm plot
Measured porosities
and permeabilities
are plotted in a XY-
diagram...
...in order to
establish the cut-off
value of efficient
porosity 0 %
5 %
10 %
15 %
20 %
25 %
30 %
0,1 1,0 10,0 100,0 1000,0
porosity
permeability (md)
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HC-saturation
• Hydrocarbon saturation (SHC) is estimated
from log analysis
• Only zones with efficient porosity are
included
Hydrocarbon saturation is the
pore volume fraction which
contains hydrocarbons
SHC = 1 - Swater
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Spread in input data
min. expected max.
rock volume x x x
net/gross ratio x x x
porosity x x x
hydrocarbon saturation x x x
= Hydrocarbon pore volume (HCPV)
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In-place resources
• When we move hydrocarbons (HCPV) from the reservoir to the
surface, physical conditions as pressure and temperature are
changed...
– the oil volume is shrinking, and
– the gas volume is expanding
Reservoir conditions (pR, TR)
Surface conditions (p0, T0)
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Oil to the surface...
Surface
conditions
Reservoir
conditions
P (reservoir)
T (reservoir)
V (reservoir)OIL
OIL
ass. GAS
P (surface)
T (surface)
V (surface)
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In-place resources
• “GOIP” - Gas Originally in-place
• “STOOIP” - Stock tank Oil originally in-place
In-place resources =
HCPV x Formation volume factor
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Recoverable resources
• between 50 and 80 % for gas
• between 25 and 70 % for oil
Recoverable resources =
In-place resources x recovery factor
Depending on drive mechanisms and
production strategy, the recovery factor in
general varies:
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Recovery factors for some
Norwegian oil fields
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
76 80 82 85 91 94
StatfjordGullfaksOseberg
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
73 76 80 82 85 91 94
EkofiskEldfiskValhall
Middle Jurassic sandstones Cretaceous chalk
(carbonates)
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Recoverable and in-place
resources
0
200
400
600
800
1000
1200
mill Sm 3 o.e.
resources remaining in res.
recoverable resources
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HCPV - prognosis vs result
• There is clearly a tendency to overestimate HCPV
• The same conclusion can be made for BRV, HCCOL, and reservoir thickness
• The wider result distribution as compared to prognosis distribution indicates that industry estimates a too narrow range of most likely outcomes
106 108 109
30
107
Prognosis
Result
25
20
15
10
5
0
No. of P
rospects
Resu
lts (S
m3)
106 107 108 109
Prognosis (Sm3)
106
107
108
109
HCPV
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Conclusions,
- volume assessments• As explorers, we find less than we predict.
• Explorationists put too narrow ranges on possible
outcomes for field sizes and volumetric parameters.
• The above statements are generally valid for any
play and trap type, phase, pre-drill probability of
discovery and distance to nearest well, however
there is a tendency to do better in regions of longer
exploration history.
• Bulk Rock Volume (and behind that hydrocarbon
column) is clearly the parameter explaining most of
the differences between pre- and post-drill
hydrocarbon pore volume.
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Recommendations,
- volume assessments
• We need to improve our volume and parameter
estimations!
• Expand ranges of possible outcomes for the volumetric
parameters. Our prediction capabilities are poor.
– Uncertainties related to seismic interpretation, depth
conversion, and petrophysical parameter prediction are
larger than what is generally perceived
• Spend the time evaluating a prospect according to the
importance of the parameters.
– Bulk Rock Volume should get the highest attention
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Risking resources
- geological risk
analysis
Petrad course: Policy and management of petroleum sector development,
Stavanger
September 2006
Inger Fjærtoft
Norwegian Petroleum Directorate
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Risk analysis
What is the chance of finding the
minimum amount of recoverable
hydrocarbons as estimated in the
prospect assessment ?
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Some Definitions
“There is a RISK that I
am going to fall off this
cliff and I am
UNCERTAIN how far it
is to the bottom!”
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Risk - Probability
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.1
probability
risk
Probability = 1 - Risk
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Success rate
Success rate =no. of hits
no. of trials = 8/14 = 0.57
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Probability categories
Stochastic probabilities- measured values
- success rates, etc
Objective probabilities- logical arguments,
- analogue events, etc
Subjective probabilities- beliefs,
- “guts feeling”, etc
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The independent risk factors- NPD’s risk factors
Probability of discovery:
P = P1 x P2 x P3 x P4
...where:
P1 - probability of efficient reservoir
P2 - probability of efficient trap
P3 - probability of efficient source &
migration
P4 - probability of efficient retention after
accumulation
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Probability of discovery
The estimated prospect probability is
not the probability of making a
discovery, but:
The probability of finding at least the
minimum quantity of hydrocarbons we
estimated in the resource assessment.
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burial
time
P1:
deposition
of reservoir
P2:
trap
formation
P3:
generation,
migration and
accumulation
of hydrocarbons P4:
retention of
hydrocarbons
after accumulation
Reconstruction of the
hydrocarbon accumulation process
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Sum up - Main
principlesIndependent risk factors for:
The probability of finding at least the
minimum quantity of hydrocarbons we
estimated in the resource assessment.
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Probability of discovery
– High risk prospects
are risked too low
– Low risk prospects are
risked too high
0 %
20 %
40 %
60 %
80 %
100 %
(0-19%) (20-39%) (40-59%) (60-79%) (80-99%)
FUNNSANNSYNLIGHET
FU
NN
FR
EK
VE
NS
SUCCESS RATE
PROBABILITY OF DISCOVERY
OPTIMISTIC
PESSIMISTIC
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Prospect prognosis and drilling results:Analysis of discoveries from 1990-2002
In place (Mill. b o.e.)
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
Total Oil Gas
Prognosis prior to drilling Status 2002
50%
35% 85%
50%