stress and failure plus permeability changes induced by co2 ......stress pressure eg, ps changes...

ARI‐CoalSeq ProjectPresented at CoalSeq VIII in Pittsburgh

Predicting Stress and Failure plus Permeability Changes Induced by CO2 Injection

Ian PalmerHiggs‐Palmer TechnologiesHiggs Palmer Technologiesian@higgs‐palmer.com

24 October 2012iggs‐Palmer Technologies 1

AcknowledgementAcknowledgement

• This part of the project reflects a continuing p p j gcollaboration with BP, who have made available the mechanical properties (moduli and Poissonsratios) for anisotropic coal as well as the strain‐ratios) for anisotropic coal, as well as the strainpressure Langmuir plots for CH4 and CO2 in core from a San Juan well.

• The equations of the P‐H model for cleat anisotropy are however confidential to BP.

• Comments on this work by Bob Moore Debbie• Comments on this work by Bob Moore, Debbie Loftin, Satya Harpalani, Phil Loftin, and Nigel Higgs have been instructive.

Higgs‐Palmer Technologies 2

P‐H Model for Stress and Permeability h i h l iChanges with Depletion

• The Palmer Mansoori (P M) model (next slide)• The Palmer‐Mansoori (P‐M) model (next slide) was revised in 2007 to include cleat anisotropy in a case where the cleats were largely vertical (P l l 2007) b h(Palmer et al, 2007), as appears to be the situation in the San Juan basin.

• The cleat anisotropy manifested as a g‐factor inThe cleat anisotropy manifested as a g factor in the compaction term, which acted to suppress the perm loss expected during early depletion. A l f ~0 2 d t t h thvalue of g ~0.2 appeared to match the perm

increase data in the San Juan basin (Palmer, 2009 and Moore et al, 2011)., )


P-M ModelPressure-dependent permeability perm ↓ as depletion ↑ But g <1

Matrix shrinkage perm ↑ as depletion ↑P M Model

ppKppC ii 11

as depletion ↑ But g <1 suppresses this term

perm ↑ as depletion ↑

ppppM

Ci

i

ii

im

i

11

1fKgC

Cleat porosity Cleat-volume

3

k

1f

MMCm p y

change

Cleat

Cleat volume compressibilityis not assumed,but comes from these equations

g-factor <1 due to cleat anisotropy

iik

ppEEv 21

Cleat permeability change

qcleat anisotropy

Higgs-Palmer Technologies 7

i

iihih pp

ppp

pv

Ev

EvvppSS

)1(3)1(31

21)(

Horizontal stress


change

….more• A significant extension to the P‐M model has been made by

BP, by including a transversely‐isotropic geomechanicsanalysis (ie coal with cleat anisotropy)analysis (ie, coal with cleat anisotropy).

• The result, called the Palmer‐Higgs (P‐H) model, has a low g‐factor as before, but now two different Youngs moduli, and three different Poissons Ratios all of which are neededand three different Poissons Ratios, all of which are needed to fully characterize these properties with respect to direction in a coal core.

• The stress equation (proprietary) from the new P H model• The stress equation (proprietary) from the new P‐H model has been used to predict when failure would occur after injection of CO2 into a depleted CBM reservoir.

• For predicting the perm changes when CO2 is injected we• For predicting the perm changes when CO2 is injected, we use a proprietary approximation to the P‐H model. It is more transparent, and simpler to use when it is desired to see the effect of varying certain input parameterssee the effect of varying certain input parameters.


Concept

• Extend approach and model for predicting stress and perm changes for CH4 productionand perm changes for CH4 production (depletion) to include CO2 injection.

• The stress and perm changes are calculated usingThe stress and perm changes are calculated using the new P‐H model (unpublished): a model which includes cleat anisotropy (eg, cleats that are nominally vertical as in San Juan basin).

• The study is focused on San Juan basin, because h i f i h h iwe have more information there on the reservoir

and rock parameters. But the general conclusions may be applicable to other coal formationsmay be applicable to other coal formations


Part A: Predicting Stress Changes d C l F iland Coal Failure

• The P‐H model predicts in‐situ stress changesThe P H model predicts in situ stress changes during depletion of CH4 followed by injection of CO2of CO2.

• The stress changes are critical to coal failure. They define the stress path and whether theThey define the stress path, and whether the stress path intercepts a failure envelope, in which case coal failure would occurwhich case coal failure would occur.


Approach: Injection into Depleted CBM ReservoirReservoir

Stage 1 1. Deplete with CH4 down to abandonment

( 200 i)

2. Use CH4 matrix strains

3. Calculate stress hpressure (eg, 200 psi) changes

Stage 2 1. Replace CH4 by CO2 at same abandonmentpressure

2. Calculate stress changes

Stage 3 1. Inject with CO2 until 2. Use CO2 3. Calculate 4. Interrogate g jres pressure rises to initial (eg, 1500 psi)

matrix strains stress changes

gfor failure

Stage 4 1. Vary abandonment pressure to see how affects results Higgs‐Palmer Technologies 8

Approximationspp• We assume pressure and stresses are constant across

reservoir i e these are static calculations and we do notreservoir, i.e. these are static calculations and we do not consider temporal changes that lead to pressure that varies with radial distance from the well.

• For example in a depleted CH4 reservoir when CO2• For example, in a depleted CH4 reservoir, when CO2 replaces CH4 we assume this takes place instantaneously.

• We assume Shmin = Shmax in all the runs here. Thi i ti h ld id fi t ti t f th• This approximation should provide a first estimate for the effects of CO2 injection, and whether the coal is likely to fail. W th CO2 di l ll f th CH4 th i• We assume the CO2 displaces all of the CH4, so there is no residual CH4 to deal with. This is justifiable because the matrix swelling of the CO2 injected will dominate that due to any residual CH4to any residual CH4.


Method• Start with a San Juan CBM reservoir at original reservoir pressure.

• Model depletion of CH4 down to a selected abandonment pressure.

• Replace all the CH4 by CO2 at this abandonment p ypressure (assume instantaneous).

• Incrementally raise the static reservoir pressure, by adding CO2 until a new sorption equilibrium has g p qbeen established.

• At each stage, calculate the changes in Shmin due to (1) pressure change, and (2) swelling of coal by CO2 ( ) p essu e c a ge, a d ( ) s e g o coa by COadsorption.

• Predict whether coal fails or not, using a generalized failure envelope obtained from the literature.


failure envelope obtained from the literature.

Results for Abandonment Pressure = 200 psiResults for Abandonment Pressure = 200 psi


Stage 1: CH4 DepletionStage 1: CH4 Depletion

• Start with a CBM reservoir at original reservoir gpressure.

• Model depletion of CH4 down to a selected b d ( )abandonment pressure (eg, 200 psi)


Input Parameters for CH4 DepletionInput Parameters for CH4 Depletion

INPUT VARIABLES for CH4 DEPLETION: P-H model for cleat anisotropyINPUT VARIABLES for CH4 DEPLETION: P-H model for cleat anisotropy

g 0.2 g-factor which suppresses cleat compaction during drawdownnu_p 0.192nu_pz 0.0865 three Poissons ratios needed to characterize cleat anisotropynu_zp 0.432Ep (cleated) 125000 Youngs modulus in a direction perpendicular to vertical cleatsbeta 1.36E-06 Grain compressibilityeps_inf 0.0168 Langmuir strain parameter for volumetric strain vs pressurePe 1434 Langmuir pressure parameter for volumetric strain vs pressurePe 1434 Langmuir pressure parameter for volumetric strain vs pressureUCS 500 Unconfined compressive strength

P0 1500 Initial reservoir pressureSv 3000 Vertical stressSh ( i ) 2100 Mi i h i t l t (0 7 i/ft)Sh (min) 2100 Minimum horizontal stress (0.7 psi/ft)SH (max) 2100 Maximum horizontal stress (0.7 psi/ft)


After Correction for Grain Compressibility

Higgs‐Palmer Technologies 14Both CH4 and CO2 strain Langmuirs are San Juan samples, and both are

unconstrained, so this data is now consistent.

Failure envelope f lit t

Ending point for stress path with CH4:

from literature

200 psi

Ending point for stress path with CH4: Sh has dropped from 2100 to 693 psi

Starting point for stress path with CH4

Stress pathstress path with CH4


CommentsComments• Initial stress state is shown by orange dot• The depletion stress path is given by the pink line• It moves toward the failure envelope because matrix shrinkage and depletion both lower thematrix shrinkage and depletion both lower the horizontal stress (while vertical stress is unchanged)

• If the pink line crosses the failure envelope then• If the pink line crosses the failure envelope, then the coal fails in shear

• This is predicted to occur at a reservoir pressure f 34 iof 34 psi

• We choose an abandonment pressure of 200 psi, ie, coal failure would not have occurred.


,

Stage 2: CO2 FillupStage 2: CO2 Fillup

• Choose abandonment pressure of 200 psi (for p p (illustration).

• Replace all CH4 by CO2 at this fixed pressure• Calculate stress changes


Stress Changes due to CO2 FillupStress Changes due to CO2 Fillup• Sh(CO2) – Sh(CH4) = E/3(1‐v)* {e1 P/(Pe1+P) ‐ e2 P/(Pe2+P)}

where e1 and Pe1 refer to Langmuir volumetric strain parameters for CO2 And e2 and Pe2 refer to Langmuir volumetric strain parameters for CH4CH4Plus E = Ep and (1‐v) shld be (1‐nu_p)

• At abandonment pressure of 200 psi, the in‐situ horizontal h d d f 2100 i 693 i l fstress has decreased from 2100 psi to 693 psi as a result of

Stage 1 CH4 depletion.• This stress is raised from 693 psi to 937 psi after replacing

ll h CH4 b CO2 i S 2 d lliall the CH4 by CO2 in Stage 2 due to extra swelling caused by CO2.

• The stress path has moved away from the failure envelope ( d l h d)(see orange dot in plot ahead).


Stage 3: CO2 InjectionStage 3: CO2 Injection

• Start with CBM reservoir at abandonment pressure and filled with CO2 (assume no CH4).

• Model injection of CO2 up to a selected ( )maximum pressure (eg, 1700 psi)

• See if stress path crosses failure envelope.


Input Parameters for CO2 InjectionInput Parameters for CO2 Injection

INPUT VARIABLES for CO2 INJECTION: P H model for cleat anisotropyINPUT VARIABLES for CO2 INJECTION: P-H model for cleat anisotropyg 0.2 g-factor which suppresses cleat compaction during drawdownnu_p 0.192nu_pz 0.0865 three Poissons ratios needed to characterize cleat anisotropynu zp 0.432nu_zp 0.432Ep (cleated) 125000 Youngs modulus in a direction perpendicular to vertical cleatsbeta 1.36E-06 Grain compressibilityeps_inf 0.019 Langmuir strain parameter for volumetric strain vs pressurePe 360 Langmuir pressure parameter for volumetric strain vs pressureUCS 500 Unconfined compressive strength

P0 200 From Stage 2Sv 3000Sh ( i ) 937 Aft fill ith CO2Sh (min) 937 After fillup with CO2SH (max) 937 After fillup with CO3


Ending point for stress path with CH4

Starting point for stress path with CO2Starting point for

stress path with CH4


Ending point for stress path with CO2


CommentsComments

• Starting stress state is shown by orange dot (after g y g (CO2 fillup)

• The injection stress path is given by the blue line• If the blue line crosses the failure envelope, then the coal fails in shear

• However the blue line moves away from the• However, the blue line moves away from the failure envelope, meaning there is no coal failure

• This is because CO2 raises the horizontal stress by ymatrix swelling and by increased pore pressure (while vertical stress is unchanged)


Results for Other Abandonment Pressures


Ending point for stress path with CH4

Starting point for stress path with CO2Starting point for



Ending point for stress path with CO2


Conclusions

• Depletion of CBM reservoirs can move the stress path t d d i t t th f il l ( ltoward, and even intercept, the failure envelope (coal failure is observed, see SPE 146931*)

• But during CO2 injection the stress path moves awayBut during CO2 injection the stress path moves away from the failure envelope due to (1) fillup replacement of CH4 by CO2, (2) further injection of CO2 to raise reser oir press rereservoir pressure

• Shear failure of coal in a depleted CBM reservoir should not happen during CO2 injection (although itshould not happen during CO2 injection (although it might happen if the reservoir is depleted to very low pressure before CO2 injection)

* Moore, R.L., Loftin, D., and Palmer, I. “History Matching and Permeability Increases of Mature Coalbed Methane Wells in San Juan Basin” SPE 146931. APOGC, Jakarta, Indonesia, 20‐22 Sept 2011. Higgs‐Palmer Technologies 30

….more….more

• The sharp kink in the blue injection stress pathThe sharp kink in the blue injection stress path is because Sh increases, and at some point it reaches and then exceeds Sv.

• However, tensile failure should occur if reservoir pressure reaches Sv = 3000 psi (not illustrated in previous slides) this is enough to lift the overburden horizontal cracks

ld i CO2 i j i i S h hi hwould increase CO2 injectivity. Such high reservoir pressures are unlikely.


Weakening of Coal due to CO2: GoalsWeakening of Coal due to CO2: Goals

• Review Harpalani’s lab tests during CO2 injection e e a pa a s ab tests du g CO ject oto integrate into this work any changes in mechanical properties E and v. Note that coal

h ll k d lstrength UCS usually tracks Youngs modulus E.• Review lab tests by Harpalani looking for direct CO2 f ilCO2 failure.

• Assess if the previous conclusion of no CO2‐induced failure could change becauseinduced failure could change because– CO2 weakens the coal, by reducing UCS and/or changing E and v (we had assumed no changes)g g ( g )


Lab Tests by Harpalani: Mechanical Properties*Lab Tests by Harpalani: Mechanical Properties• In a coal core, changes in E (Youngs modulus) and v (Poissons ratio) are measured by high‐frequency ( ) y g q yultrasonic pulses.

• Note that coal strength UCS usually tracks Youngsmodulus Emodulus E.

• No change was observed in E, v during injection of helium or methane.

l ll h b d d• Only small change in E, v was observed during injection of CO2 (6‐7%)….see next two slides.

• Weakening of coal by CO2 injection is so small it may g y j ynot be warranted to model coal failure and perm change.


* Info provided By Satya in his quarterly reports to CoalSeq

Youngs Modulus Changes 6.4 – 6.0 (6% Change)


Poissons Ratio Changes 0.368 – 0.394 (7% Change)


Lab Tests by Harpalani: Direct Coal Failure• First test: Coal core failed by injection of CO2 at 600 psi into CH4‐saturated

core (was this a San Juan core?). This was “pressure‐controlled injection” h th CO2 i b b d d th t f f th Thiwhere the CO2 is bombarded on the upstream face of the core. This

suggests that coal failure was due to local and heterogeneous (non‐equilibrated) stresses and strains induced by rapid coal swelling (next slide)slide).

• Second test: Coal core did NOT fail by injection of CO2 at 600 psi into CH4‐saturated core, due to slower injection (a series of smaller injections and stabilizations) This was “quantity‐controlled injection” where a smallstabilizations). This was quantity‐controlled injection where a small amount of CO2 was injected each time with ample time between to let the core equilibrate.

• This tells us that coal failure can be induced by CO2 injection but itThis tells us that coal failure can be induced by CO2 injection, but it depends on the rate/pressure conditions of injection (we have not tried to model and compare different conditions).

Higgs‐Palmer Technologies 36* Info provided By Satya: verbally and in his quarterly reports

FIGURE 4: Visual of polished surface showing cracks induced by differential swelling as CO2 is adsorbed in heterogeneous coal (courtesy ref. 6).


g g ( y )

Karacan, O., private communication, 2007.

ConclusionsConclusions

Ch i E d i i j ti f CO2 l 6 7%• Changes in E,v during injection of CO2 are only 6‐7%, as measured by ultrasonic. Although these are so‐called dynamic values, and static E, v are more y , ,relevant, the conclusion is unlikely to change.

• Changes in E,v caused by CO2 injection would probably t ff t i ifi tl th l f il ( bilit )not affect significantly the coal failure (or permeability)

change that we have modeled.• In the lab, coal failure can be induced by CO2 injection,In the lab, coal failure can be induced by CO2 injection, but it depends on rate/pressure conditions (we have not tried to model and compare different conditions).


Part B: Predicting Permeability Changes d I j i i d i CO2 I j iand Injectivity during CO2 Injection

• The P‐H model can predict perm changesThe P H model can predict perm changes during depletion of CH4 followed by injection of CO2.

• The perm changes are critical to CO2 injectivity. For example, if coal perm is decreased by swelling, it will become harder to inject CO2 via matrix injection.

• We do not addressing fracture stimulation by injection at a wellbore.


How does the Injectivity of CO2 Change?How does the Injectivity of CO2 Change?

• Try to model the injectivity of CO2 in theTry to model the injectivity of CO2 in the situations described above.

• During CO2 injection, the perm (and thereforeDuring CO2 injection, the perm (and therefore injectivity) will change due to:– Increasing pore pressure rise in permg p p p– Increasing swelling fall in perm– Which one wins?

• We can model the perm changes using the new P‐H model.


Approach: Injection into Depleted CBM ReservoirReservoir

Stage 1 1. Deplete with CH4 down to 2. Use CH4 matrix 3. Calculate abandonment pressure (eg, 200 psi)

strains perm change

Stage 2 1. Replace CH4 by CO2 at 2. Calculate perm g p ysame abandonment pressure

pchange

Stage 3 1. Inject CO2 until reservoir i 1500 i

2. Use CO2 matrix i

3. Calculate hpressure rises to >1500 psi strains perm change

Stage 4 1. Vary initial cleat porosity and also abandonmentand also abandonment pressure to see how affects results


Approximationspp• We assume pressure and stresses are constant across

reservoir i e these are static calculations and we do notreservoir, i.e. these are static calculations and we do not consider temporal changes that lead to pressure that varies with radial distance from the well.

• For example in a depleted CH4 reservoir when CO2• For example, in a depleted CH4 reservoir, when CO2 replaces CH4 we assume this takes place instantaneously.

• We assume Shmin = Shmax in all the runs here. Thi i ti h ld id fi t ti t f th• This approximation should provide a first estimate of the injectivity of CO2.

• We assume the CO2 displaces all of the CH4, so there is no id l CH4 t d l ith Thi i j tifi bl b thresidual CH4 to deal with. This is justifiable because the

matrix swelling of the CO2 injected will dominate that due to any residual CH4.


Method• Start with a San Juan CBM reservoir at original reservoir pressure.p

• Model depletion of CH4 down to a selected abandonment pressure.R l ll th CH4 b CO2 t thi b d t• Replace all the CH4 by CO2 at this abandonment pressure (assume instantaneous).

• Incrementally raise the static reservoir pressure, by y p , yadding CO2 until a new sorption equilibrium has been established.

• At each stage calculate the changes in cleat• At each stage, calculate the changes in cleat permeability due to (1) pressure change, and (2) swelling of coal by CO2 adsorption.


Results for Abandonment Pressure = 200 psiResults for Abandonment Pressure = 200 psi


Stage 1: CH4 DepletionStage 1: CH4 Depletion

• Start with a CBM reservoir at original reservoir gpressure.

• Model depletion of CH4 down to a selected b d ( )abandonment pressure (eg, 200 psi)


Input Parameters for CH4 DepletionInput Parameters for CH4 DepletionINPUT VARIABLES for CH4 DEPLETION: P-MAN model for cleat anisotropy

g 0.181g-factor which suppresses cleat compaction during drawdownv 0.271Ep (cleated) 113,000 Youngs modulus in a direction perpendicular to vertical cleats

γ 1.36E-06 Grain compressibilitye 0.0168 Pure Langmuir strain parameter for volumetric strain vs pressurePe 1434 Pure Langmuir pressure parameter for volumetric strain vs pressure

P0 1500 Initial reservoir pressureSv 3000 Vertical stressSh (min) 2100 Minimum horizontal stress (0.7 psi/ft)SH (max) 2100 Maximum horizontal stress (0 7 psi/ft)SH (max) 2100 Maximum horizontal stress (0.7 psi/ft)

The mechanical properties E, v, and g are slightly different from those used to predict stress changes (due to a different match by


p g ( ytheory to stress and strain observed in lab tests). This should not make a significant difference to the results

Perm increase withPerm increase with CH4 depletion

Initial reservoir pressure

Typical abandonment pressure 200 psi


CommentsComments

• Initial reservoir pressure is 1500 psi, and initialInitial reservoir pressure is 1500 psi, and initial cleat porosity is 0.15%.

• Perm increases strongly with depletion, asPerm increases strongly with depletion, as observed in San Juan basin coals.

• If we choose an abandonment pressure of 200If we choose an abandonment pressure of 200 psi, we can predict the cleat porosity and permeability at the start of CO2 injection.


Stage 2: CO2 FillupStage 2: CO2 Fillup

• Choose abandonment pressure of 200 psi (for p p (illustration).

• Replace all CH4 by CO2 at this fixed pressure• Calculate perm change due to swelling differential between CH4 and CO2.


Perm Change due to CO2 FillupPerm Change due to CO2 Fillup• To model CO2 fillup at a particular depletion pressure,

Pdep, we replace the matrix swelling of CH4 by that due to CO2, and calculate the effect on cleat porosity.

• This calculation requires the Langmuir parameters of CO2 (two slides ahead), as well as the CH4 Langmuir parameters (previous slide).

• Since CO2 swells coal more than CH4, the net effect will be a drop in cleat porosity at the depletion pressure. The equation used is:

ϕ2‐ϕ1 = [K/M‐1] [e2.Pdep/(Pe2+Pdep) ‐ e1.Pdep/(Pe1+Pdep)]ϕ ϕ [ ] [ p ( p) p ( p)]where subscript 1 refers to CH4 and subscript 2 refers to CO2. The parameters e1 and Pe1 are for CH4, while e2 and Pe2 are for CO2.


Stage 3: CO2 InjectionStage 3: CO2 Injection

• Start with CBM reservoir at abandonment• Start with CBM reservoir at abandonment pressure and filled with CO2 (assume no CH4).

• Model injection of CO2 up to a selectedModel injection of CO2 up to a selected maximum pressure (> 1500 psi)


Input Parameters for CO2 InjectionInput Parameters for CO2 Injection

INPUT VARIABLES for CO2 INJECTION: P-MAN model for cleat anisotropyINPUT VARIABLES for CO2 INJECTION: P-MAN model for cleat anisotropy

g 0.181g-factor which suppresses cleat compaction during drawdownv 0.271Ep (cleated) 113,000 Youngs modulus in a direction perpendicular to vertical cleats

γ 1.36E-06 Grain compressibilitye 0.019 Pure Langmuir strain parameter for volumetric strain vs pressurePe 360 Pure Langmuir pressure parameter for volumetric strain vs pressurePe 360 Pure Langmuir pressure parameter for volumetric strain vs pressure

P0 1500 Initial reservoir pressureSv 3000 Vertical stressSh ( i ) 2100 Mi i h i t l t (0 7 i/ft)Sh (min) 2100 Minimum horizontal stress (0.7 psi/ft)SH (max) 2100 Maximum horizontal stress (0.7 psi/ft)


Initial methane porosity is 1.0%, and starting CO2 porosity after fillup at 200 psi is 0.89%.

Perm increase withPerm increase with CH4 depletion


Fillup by

Perm change with CO2 injection

Fillup by CO2


CO2 injection


Comments• The perm change for methane depletion is the ratio to initial methane perm at 1500 psi (red line)initial methane perm at 1500 psi (red line)

• The perm change for CO2 injection is the ratio to initial CO2 perm at 200 psi (blue line).Th l it t t t 1 0 % d i ith• The coal porosity starts at 1.0 % and increases with methane depletion, but drops at 200 psi to 0.89% when CO2 fillup takes place.

• After this, the porosity (and perm) continues to fall because matrix swelling overwhelms the effect of pressure‐induced cleat inflation, until about 1250 psi p , pwhen the reverse happens and the porosity (and perm) begins to rise.


Initial methane porosity is 0.5%, and starting CO2 porosity after fillup at 200 psi is 0.39%.

Perm increase with CH4 depletion


Fillup

Perm change with CO2 injection

Fillupby CO2



Comments• When initial methane porosity is 0.5%, the coal perm increases more strongly with methane depletion.g y p

• But porosity drops at 200 psi to 0.39% when CO2 filluptakes place.

• After this the perm goes south quickly because matrix• After this, the perm goes south quickly because matrix swelling overwhelms the effect of pressure‐induced cleat inflation.h l • The coal porosity goes negative zero injectivityCO2 swelling has plugged the coal porosity. Continued injection would raise the pressure around the wellbore and create a hydraulic fracture, which would allow CO2 injection to resume.


Results for Other Initial Cleat Porosities

Perm changes with CO2 injectionCO2 injection

Initial coal porosity (CH4) at 1500 psi


CO2 injection starts at 200 psi Initial reservoir

pressure

Commentsl l d b d• Initial cleat porosity is varied, but perm increase due to

depletion of CH4 is not shown. Each CO2 injection starts at 200 psi abandonment pressure.

• We have normalized all the porosity (and perm) changes to the initial (methane) cleat porosity.

• In all cases, the initial coal permeability does not recover even if injection pressures are raised to 3000 psi.

• A porosity of > 0.7% in San Juan basin coals is needed for continued matrix injection.

• If cleat porosity is < 0.3% (as we find in San Juan basin*) then if injection pressure rises, CO2 injectivity will fall quickly to zero, and continued injection can only proceed after fracture j y pstimulation. This may not be desirable as far as a uniform sweep, but it will be unavoidable.


* Moore, R.L., Loftin, D., and Palmer, I. “History Matching and Permeability Increases of Mature Coalbed Methane Wells in San Juan Basin”. SPE 146931.

APOGC, Jakarta, Indonesia, 20‐22 Sept 2011.

Conclusions• When CO2 fillup replaces methane at a low depletion pressure (200 psi

here), the coal porosity and permeability are reduced due to additional matrix swelling. The percent reduction is greater when the initial coal porosity is less.

• When CO2 is then injected with increasing pressure, the coal porosity and permeability are further reduced because matrix swelling exceeds the p y geffect of pressure‐induced cleat inflation. The percent reduction is greater when the initial coal porosity is less.

• For lower initial porosities, it will be impossible to inject CO2 until tensile failure has occurred (ie fracture stimulation)failure has occurred (ie, fracture stimulation).

• For higher initial porosities, CO2 can be injected (ie, matrix injection) but the injectivity is greatly impaired as pressure increases, until some higher pressure when injectivity starts to improve.

i l i i i l l i i % i h b i *• For typical initial coal porosities <0.3% in the San Juan basin*, CO2 injection will only proceed under rising pressure conditions if tensile failure occurs at the wellbore (ie, fracture stimulation).


* Moore, R.L., Loftin, D., and Palmer, I. “History Matching and Permeability Increases of Mature CoalbedMethane Wells in San Juan Basin”. SPE 146931. APOGC, Jakarta, Indonesia, 20‐22 Sept 2011.

Results for Other Abandonment PressuresResults for Other Abandonment Pressures

• So far we focused on an abandonment pressure pof 200 psi, and varied initial reservoir porosity from 0.3% to 1.0% (cleat porosity).

• Now we fix reservoir porosity at 0 3% and vary• Now we fix reservoir porosity at 0.3% and vary abandonment pressure when CO2 injection is started.

• Results are summarized in the next slide.• This aspect of the work also provides an

i b h k l iopportunity to benchmark our results against modeling work associated with extensive Canadian field injections of CO2. j


ϕo = 0.3% in all cases here (typical of San Juan Basin)


Comments• Injecting at a lower abandonment pressure will mean a higher injectivity

(injectivity is proportional to permeability if injection rate is very slow). For example 100 psi allows much greater injectivity than 200 psiexample, 100 psi allows much greater injectivity than 200 psi.

• But if injection rate raises reservoir pressure, perm to CO2 and injectivitywill fall rapidly fracture stimulation will be needed to continue injection.

• Very slow (or intermittent) injection rate is desired to try to keep reservoir pressure close to 100 psi, so that perm to CO2 and injectivity remain high (previous slide).

• Fixed flowrate controlled injection was tried in 2005 at theFixed flowrate controlled injection was tried in 2005 at the RECOPOL/MOVECBM site. It never worked since the pressure required to achieve the flowrate was way too high, and the system tripped three times*. The operators were probably aiming for a predetermined injection rate which was high enough to be economical. This may turn out to be arate which was high enough to be economical. This may turn out to be a serious dilemma for commercial injection of CO2.


* Courtesy of Satya Harpalani, 2011

Strategy for CO2 InjectionStrategy for CO2 Injection

• If the goal is matrix injection of CO2, toIf the goal is matrix injection of CO2, to maintain better sweep unaffected by fracture stimulation at the wellbore…….

• …..the best strategy is to inject CO2 at a low abandonment pressure, and at a very slow rate (two slides back).

• This is based on SJB coals where initial cleat porosity is < 0.3% and perm compaction is suppressed (ie, cleats are vertical).


Courtesy Nino Ripepi, Virg. Polytechnic Univ

Injection pressure

Polytechnic. Univ.

Injectivity Q/ΔP

h

Phase 1

Phase 2

ϕo = 0.3% in all cases here (typical of San Juan Basin)

Ph 1Phase 1Phase 2


Benchmark: CO2 Injections by ARCBenchmark: CO2 Injections by ARC

• Well tests done in two adjacent wells inWell tests done in two adjacent wells in Alberta.

• Manville coal seam at ~4 100 ft and ~13 ft• Manville coal seam at ~4,100 ft, and ~13 ftthick.O ll d d fi d d• One well used to define pressure‐dependent permeability. Second well to define sorption

istrain components.


Mavor and Gunter, “Secondary Porosity and Permeability of Coal vs. Gas Composition and Pressure”, SPE 90255, ACTE, Houston, Sept 2004.

Results of Well Test Interpretation

E = 353,200 psi v = 0.21 ϕo = 0.002


ARC Modeling of CH4 Depletion + CO2 Injection

Higgs‐Palmer Technologies

Mavor and Gunter, “Secondary Porosity and Permeability of Coal vs. Gas Composition and Pressure”, SPE 90255, ACTE, Houston, Sept 2004. 72

P‐MAN Modeling of CH4 Depletion + CO2 Injection


All input parameters are in next slide

Input Parameters for P‐H ModelInput variables for CH4 depletion and CO2 injection P‐MAN model for cleat anisotropyg 0.9g 0.9v 0.21 Poissons ratioEp 353,200 Cleated Youngs modulus (psi)f 0.5 Distortion parameterγ 1 30E 06 Grain compressibility (psi 1)γ 1.30E‐06 Grain compressibility (psi‐1)

e1 0.013 CH4 infinite strainPe1 600 CH4 pressure at half infinite strain (psi)1P 1 7 8 CH4e1Pe1 7.8 CH4

e2 0.01593 CO2 infinite strainPe2 550 CO2 pressure at half infinite strain (psi)e2Pe2 8.76 CO2

Po 1146 Initial reservoir pressure (psi)Pdep 0 Abandonment pressure (psi)


ϕo 0.002 Initial cleat porosity (at Po)

Summary• The perm profiles for CH4 and CO2 (ours and those from Mavor and

Gunter, 2004) match quite closely. This adds support to our modeling.W d th i t d t M d G t t th t• We used the same input data as Mavor and Gunter, except that we used g = 0.9 instead of g = 1.0 that they would have used.

• This cleat anisotropy parameter g ≈ 1 causes the large period where K/Ko <1 in the CH4 profileK/Ko <1 in the CH4 profile.

• The “compaction” effect of depletion holds full sway when g ≈ 1. This lies in contrast to our current interpretation of continual perm increases with depletion (in San Juan basin), where g ≈ 0.2, and compaction effects are suppressed.

• However g ≈ 1 also explains why CO2 injection suffers a large perm loss before it recovers, but it does not go negative as we have found it does when using g ≈ 0 2it does when using g ≈ 0.2.

• This cleat anisotropy factor g ≈ 0.2, plus very small initial cleat porosity, are the main reasons why CO2 injectivity is so difficult in San Juan basin (ie, why CO2 perm falls so quickly after injection).( , y p q y j )


ConclusionsConclusions• When at a low depletion pressure CO2 fillup replaces methane, and then CO2 is injected while raising reservoir pressure, the coal porosity and permeability are seriously reduced due to additional matrix swelling y gwhich exceeds the effect of pressure‐induced cleat inflation.

• For higher initial porosities, CO2 can be injected (ie,For higher initial porosities, CO2 can be injected (ie, matrix injection) but the injectivity is greatly impaired until some higher pressure when injectivity starts to improveimprove.

• For typical initial coal porosities <0.3% in the San Juan basin, CO2 injection will only proceed if tensile failure occurs at the wellbore (ie fracture stimulation)occurs at the wellbore (ie, fracture stimulation).


….more• A cleat anisotropy factor g ≈ 0.2, plus very small initial cleat

porosity, are the main reasons why CO2 injectivity is predicted to be so difficult in San Juan basin (ie, why CO2 perm falls so quickly after injection). The exact same reasons are why strong perm increases with depletion are observed in the San Juan basin.

• This work leads to a strategy for CO2 injection. If the goal is matrix injection of CO2, to maintain better sweep unaffected by fracture stimulation at the wellbore, the ideal strategy is to i j t CO2 t th l t b d t ibl dinject CO2 at the lowest abandonment pressure possible, and at a rate slow enough that reservoir pressure barely rises. Note: although this is based on SJB coals where initial cleat porosity is < 0 3% the guidelines still apply (but not soporosity is < 0.3%, the guidelines still apply (but not so strictly) for higher initial cleat porosities.

• The perm profiles from our model for CH4 and CO2 match quite closely those from an independent study This addsquite closely those from an independent study. This adds support to our modeling.


Possible Coal Failure ModesPossible Coal Failure Modes• Shear failure at low pressure during methane depletion (although not been observed in lab tests using pure CH4).

• Shear failure during CO2 injection: No, it should notShear failure during CO2 injection: No, it should not happen.

• Tensile failure if coal permeability becomes very low and CO2 injection pressure becomes high enoughand CO2 injection pressure becomes high enough.

• Tensile failure if injection pressure becomes very high and reaches Sv = 3000 psi. This pressure is enough to lif h b d d hi ld h i llift the overburden, and this would create horizontal cracks along bedding planes. This would act to increase CO2 injectivity. However, such high reservoir pressure i lik lis unlikely.


….more….more

• Harpalani has observed coal failure when CO2Harpalani has observed coal failure when CO2 is injected into a core at 600 psi, but this depends on rate/pressure conditions (we havedepends on rate/pressure conditions (we have not tried to model and compare different conditions)conditions).

• The failure is attributed to differential swelling in components of heterogeneous coal ifin components of heterogeneous coal, if insufficient time is allowed for equilibration.


What Happens to Permeability in the Field f l il ?after Coal Failure?

• After failure, we have found a wide range of permeability , g p yslopes – from increasing to flat to decreasing*.

• The general behavior is a flattening of the pronounced ( ti l) i ith d l ti(exponential) perm increase with depletion.

• The flattening of the perm increase has been interpreted as a loss of permeability due to fines creation and movement*. p yNew data from lab tests supports this contention^.

• Some permeability increases which resume after failure ld i di h i h i k h d bwould indicate that matrix shrinkage has resumed, but

perhaps under different conditions. •* Moore R L Loftin D and Palmer I “History Matching and Permeability Increases of Mature Coalbed

Higgs‐Palmer Technologies 80^ Duan et al, 2011.

Moore, R.L., Loftin, D., and Palmer, I. History Matching and Permeability Increases of Mature CoalbedMethane Wells in San Juan Basin”. SPE 146931. APOGC, Jakarta, Indonesia, 20‐22 Sept 2011.

….more….more

• The flattening of the exponential permThe flattening of the exponential perm increase at failure amounts to a reduction of injectivity and this has implications forinjectivity, and this has implications for injectivity of gases into coal.

• During injection of CO2 if the coal fails by any• During injection of CO2, if the coal fails by any mechanism we would expect the injectivity to decrease (by analogy with CH4 behaviordecrease (by analogy with CH4 behavior during depletion).


THE ENDTHE END


stress and failure plus permeability changes induced by co2 ......stress pressure eg, ps changes...

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