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p.1 F
orce
Sem
inar
on
Inje
ctio
n S
afet
y –
Sta
vang
er 4
Dec
201
3
Atef ONAISI
Planning for injection safety
Total’s Rules
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RULE 1 : DEFINITION OF THE MINIMUM IN-SITU STRESS
The minimum stress is defined as the fracture closure pressure that can only be measured during a minifrac or an ELOT.
It should not be confused with drilling data such as LOTs/FITs or derived from simplistic models or sonic logs
The ELOT is subject to derogation based on risk for MWW of next drilling section
Geomechanics models calibrated with measurements at wells positions might be necessary to determine stresses at field scale in case of complex structures (large depth difference between flank and crest wells)
Planning for injection safety
Rules for safe injection
p.3 F
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Planning for injection safety
Example of ELOT
FBP test 1 : 0.249 bar/m
FROP test 2 : 0.152 bar/m
Interpreted FCP : 0.135 bar/m (1.38 sg)
LOP
FIP or FBP
FROP
FCP
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Planning for injection safety
Example of what should be avoided
Confusion of FCP with driller “FG” and calibration with LOTs
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Planning for injection safety
Rules for safe injection
RULE 2 : RISK OF FRACTURE
PROPAGATION AT RESERVOIR CREST
The pressure anywhere at top
reservoir, especially at its crest, must
be less than the minimum stress in
the overlying seal so that there is no
risk of propagating existing fractures
Pmax (Top reservoir) ≤ hmin cap rock
RULE 3 : LINK BETWEEN BHIP AND
PRESSURE AT CREST
In highly permeable or fractured
reservoirs and zero skin, the
pressure profile between injection
depth and reservoir crest is
controlled by reservoir fluid densities
BHIPmax = hmin (@crest) + i g Hi
Top OP Layer 1
Top OP Layer 2
Top OP Layer 3
p.6 F
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Dec
201
3
Planning for injection safety
Field example : safety margin definition
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RULE 4 : CHANGE OF STRESSES IN THE CAP ROCK DUE TO INCREASE OF PRESSURE AROUND INJECTORS
In case of thin seal and when the injection pressure is felt over several grid cells around the injector, there is a risk of uplift and subsequent stretching of the overburden.
2D or 3D models must be developed to calculate the reduction of minimum stress in the overburden. This reduction must be included in the maximum pressure calculations based on rules 2 and 3.
Planning for injection safety
Rules for safe injection
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Planning for injection safety
Example of reservoir pressure change around injectors
DELTA P > 0 PRESSURE ABOVE INITIAL FROM ECLIPSE
Step 1: 01/07/2018 Step 7:1/01/2043 Step 4:1/01/2024
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Planning for injection safety
Example of reservoir pressure change around injectors
Step 0: 01/07/2018 Step 4:1/01/2024 Step 7:1/01/2043
Total horizontal stress change in the cap rock from 3D coupled model
Pressure increase in the reservoir induce horizontal stress
reduction in the cap rock
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RULE 5: IDENTIFICATION OF INITIALLY LEAKING FAULTS AND ASSESSMENT OF RE-ACTIVATION OF SEALING FAULTS It is critical to identify, with the help of geologists and geophysicists, if any faults are already leaking before injection and the vertical reach of such faults For faults that are initially sealing, shear slip risk or elastic opening due to change of stress in the cap rock must be considered and its impact on fault conductivity evaluated
Planning for injection safety
Rules for safe injection
n
P h
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p.11 F
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46 Faults picked and ranked wrt risk of loss of integrity
Planning for injection safety
Example of faults identification and ranking
Nb.
Faults Risk
Border Faults up to
seabed 3 4
Faults crossing gas pool 25 3
Border faults 3 2
Overburden Faults, not
connected to the
carbonate
15 1
Total = 46
Risk Low High
Main risk from border faults
p.12 F
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Planning for injection safety
Rules for safe injection
RULE 6 : MARGIN FOR INCREASING THE BHIP IN CASE OF INCREASED
SKIN OR FORMATION DAMAGE AROUND INJECTOR
In case of injectivity impairment (PWRI, raw SW, etc..) it is allowed to raise the
BHIP to maintain the same injection rate as in the case of zero skin. The
reason is that this pressure is confined within the damaged zone around the
well and does not affect the whole reservoir.
The decision to increase the pressure is subject to derogation on a well by well
basis to assess the risks associated with uneven injection impairment along the
well versus reservoir quality/thickness/distance between the well and the cap-
rock, quality of the cap rock, etc..
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Sem
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Planning for injection safety
Maximum safe injection pressure during PWRI
Overpressure in
damage zone
during PWRI
P ≤ “FG”
BHIP
P ≤ h_CR
Pressure radial
profile W/O damage
Pressure radial
profile with damage
Cap Rock
Reservoir
p.14 F
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Planning safe injection is a transverse issue involving geologists, geophysicists, reservoir engineers and geomechanics
A clear, precisely defined terminology regarding stresses and other mechanical parameters is very important
A comprehensive workflow has been developed for safe pressure management at both reservoir and well scales
Planning for injection safety: Total’s approach
Conclusions