Ian Palmer HT

7/28/2019 Ian Palmer HT

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Higgs-Palmer Technologies 1

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





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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






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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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