Post on 03-Apr-2018
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Failure during CO2 Injection in
the Field
Coal-Seq VI10-11 April 2008
Houston
Ian Palmer
Higgs-Palmer Technologies
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Contents Coals are weak rocks
Injection can induce shear failure
Lab tests increase perm when coal fails
This offsets swelling of coal by CO2adsorption how affect injectivity
CO2 adsorption may weaken coalearlier failure better injectivity
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Coals are weak rocks
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Strength
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HVA MVHVA LV
Weakest coals
Carbon content (daf) = 100 volatile matter (daf)
PR
HVB
UCS of coals depends on rank
and cannot be predicted from logs
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Injection can induce
shear failure
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2. Porepressurediffuses awayfrom fractureface
Lookingdown onverticalfracture
3. Build upellipses ofstress increase
1. Fracpressureincreases pore pressure
4. Calculate at what
distance the formationfails in shear
1D pressuretransientsolution
*Palmer, Moschovidis, and Cameron, Coal Failure and Consequences forCoalbed Methane Wells, SPE 96872, ATCE, Dallas, 9-12 Oct 2005
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Failure distanc e vs . fractu re injection p ressu re
(UCS = 500 ps i)
0
50
100
150
200
250
0.8 1 1.2 1.4 1.6 1.8 2
Fractu re BH treati ng pre ssure (psi/ft)
Failure
di
stance
(ft)
Failure Distance (ft)MV, LV coals
Sometimes see fracpressures up to 2.0 psi/ft in coals
Fractreatingpressure
has the largesteffect
Shear failure
can go out100-200 ft from
fracture face
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Failure dis tance vs. fracture injection time(UCS = 500 ps i)
0
10
20
3040
50
60
70
0 1 2 3 4 5 6 7 8 9 10
Fracture in ject ion t im e (hou rs)
Failure
dist
ance
(ft)
Failure Distance (ft)
MV, LV coals
Pump timehas much less
effect
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Failu re d is tan c e v s.co al UC S
0
20
40
60
80
100
120
140
160
180
0 200 400 600 800 1000 1200 1400 1600 1800 2000
UCS (psi)
Failure
di
stance
(ft)
Failure Distance (ft)
LV & MV
c o a l s
HVA
coa ls
Coal rank nothave much
effect
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Summary
Shear failure occurs away from the faceof a hydraulic fracture: the shear zonecan extend 50-200 ft away for
abnormally high frac treating pressures(1.0-2.0 psi/ft) The shear failure may give rise to:
Microseismic bursts Fines creation/movement perm
loss Dilatancy perm increase
The last two effects can impact gas
production from the well: but which onewins?
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Creating Permeability:
Fines Movement may Destroy it
Dilatancy perm increase
Fines
creation/movement
perm loss
Shear failure
If this one wins,
we have succeededin creating
permeability
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Lab tests increase permwhen coal fails
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Shear Dilation Concept
From Chipperfield et al Shear dilation diagnostics: a new approachfor evaluating tight gas stimulation treatments , SPE 106289
Perm increase
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Perm increase
begins long
before actual
failure,
especially
at low stress
Actual
failure
Perm
increasing
This istotal dilation
of 5 cleats
in lab test
TerraTek Cavity completions for enhanced coalbed methane
recovery , GRI-95/0432, June 1996.
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Perm Increases from Shearbox Tests in Coal
Ks/Ki = 600 for Ki = 2 md
Ks/Ki = 60 for Ki = 30 md
TerraTek Cavity completions for enhanced coalbed methane
recovery , GRI-95/0432, June 1996.
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Perm Enhancement Trend
Natural fracture width
Log
(Ks/Ko)
Granite
Sandstone
Coal and
Dolostone
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This offsets swelling of coalby CO2 adsorption
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Swelling due to CO2 Injection
* Mavor et al SPE 90255, 2004
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Perm Decrease due to Swelling
* Mavor et al SPE 90255, 2004
Perm Increases by Failure
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Perm Increases by Failurefrom Shearbox Tests in Coal
Ks/Ki = 600 for Ki = 2 md
Ks/Ki = 60 for Ki = 30 md
TerraTek Cavity completions for enhanced coalbed methane
recovery , GRI-95/0432, June 1996.
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Perm drop
by factor 8
Perm rise by
factor 200
Combine the Two Effects
Swellingeffect
Failure
effect
Perm increasemore than
offsets permloss due toswelling
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Implications for CO2 Injectivity
In last slide perm increase due to failure
exceeds perm loss due to swelling CO2 injectivity will increase
But the result depends on several factors:
Whether the coal reaches shear failure (ie, coalstrength, in-situ stress, etc)
The magnitude of the perm increase (ie, initial coal
perm, in-situ stress, etc)
More geomechanics modeling work needs to bedone to achieve predictability
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CO2 adsorption
may weaken coal better injectivity
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Evidence for CO2 Weakening Coal
Studies have been made of coal samplessubject to CO2 sorption, and examined under X-
ray CT. Cracks developed between coal andnon-coal units, and hairline cracks appear in thecoal units after CO2 cycles. A hairline crack is
all that is needed at a localized failure site,before many such sites are linked up by stressincrease needed to maintain a uniaxial strain
condition.* Karacan, O. Heterogeneous sorption and swelling in a confined and stressed
coal during CO2 injection, Energy and Fuels, 17, 1595-1608, 2003.
Karacan, O. Swelling-induced volumetric strains internal to a stressed coalassociated with CO2 sorption, Intl J . Coal Geology, in press, 2007.
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.more
Two Australian papers give interesting results onCO2-induced failure and permeability changes.The papers conclude that weakening by CO2,
failure by in-situ stresses, and permeabilityincreases will not be an issue for sufficientlydeep coal seams. However, this is based on labtests of very weak lignite, and its risky to applythe same conclusion to much strongerbituminous coals, such as HVB, HVA, or MV.Further study of how CO2 weakening depends
on coal rank and in-situ stress will be important.
Viete, D.R. and Ranjith, P.G. The effect of CO2 on the geomechanical andpermeability behavior of brown coal: implications for coal seam CO2
sequestration, Int J . of Coal Geology, 66, 204-216, 2006.Viete, D.R. and Ranjith, P.G. The mechanical behavior of coal with respect
to CO2 sequestration in deep coal seams, Fuel, in press, 2007.
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.more
Cracking by thermal cycling in granite, and
weakening of this very strong rock,appears to be an analog to the situation ofCO2 adsorption/cycling in coal and the
weakening of coal*
* Higgs, N., private communication, 2007.
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.more It has been reported* that huff-and-puff cycles of CO2
injection caused coal cores to fail in the lab, and thatmore cycles gave more breakage (each cycle was a fewdays). Some experiments were done to explore if CO2
huff-n-puff could be used to enhance near-wellborepermeability, by inducing coal to fracture. Some coalsare fragile enough that the swelling and shrinking of coalby CO2 adsorption and desorption can induce sufficient
stress to cause the coal to fall apart. Each cycle was afew days: injection followed by time to reach equilibrium.The failure seemed to be progressive, and became moreapparent after several cycles. None of this waspublished, but the idea was transmuted into a patent, byRaj Puri (see below).
* Unpublished Amoco workPuri, R., private communication, 2007.
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.more
In a micropilot in China, a well produced
better after a huff-and-puff treatment withCO2, and this may indicate failure of coal*
In a CO2 sequestration project in J apan,there is evidence for an injectivity increaseover time**
*Gunter et al, Report on CO2 micropilot in China, 2004 or 2005?
**Fujioka, M. Yubari project, Coal-Seq V, Houston, November 2006.
Th P i f I j ti it
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The Promise of Injectivity
Loss of injectivity may hurt commercial schemesof CO2 sequestration
Earlier coal failure should increase CO2injectivity
CO2 injections should be designed to maximizecoal failure:
Huff-and-puff cycles of CO2 injection may causemore failure
Injecting liquid CO2 may cause more failure bythermal cooling (opposite of matrix swelling)
Future work should include geomechanicsmodeling of shear failure and permeabilityincrease, as an input to reservoir modeling of
CO2 injectivity
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The End