planning for injection safety total’s rules - start - force safety 04.12.13... · planning for...

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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)


Rules for safe injection

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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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Example of what should be avoided

Confusion of FCP with driller “FG” and calibration with LOTs

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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

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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.



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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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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



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46 Faults picked and ranked wrt risk of loss of integrity


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

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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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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

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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

planning for injection safety total’s rules - start - force safety 04.12.13... · planning for...

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