Jonathan R Major1,Peter Eichhubl1,Thomas A. Dewers2

1. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, TX, United States 2. Geomechanics, Sandia National Laboratories, Albuquerque, NM, United States

Fracture mechanics and seal capacity in a natural CO2 system 2015 Carbon Storage RD Project Review MeetingPittsburgh, PA

August 18-20, 2015

1- INTRODUCTIONABSTRACT Evaluation of changes in the coupled chemical and mechanical properties of reservoir and seal rocks is critical for ensuring both short and long-term security of injected CO2. Natural analogs are advantageous for understanding these properties on longer time scales than possible using laboratory or numerical experiments. The Crystal Geyser site in Utah was studied to assess the potential for leakage via fracturing or capillary failure of reservoir and seal rocks altered by natural, long-term CO2-water-rock interactions. Fracture mechanics testing using the double torsion method was performed on a suite of naturally altered and unaltered rocks exposed at Crystal Geyser. Fracture toughness measurements demonstrate that CO2-related alteration has weakened one reservoir sandstone unit by approximately 50%, but did not affect subcritical index. In contrast, the fracture toughness measured of a weak, poorly-cemented sandstone unit was increased due to enhanced calcite cementation, and a similar effect was observed in shale samples tested. Mercury intrusion capillary pressure (MICP) analyses on shale from a fault-perpendicular transect show relatively low capillary seal capacity nearest the fault where CO2-alteration is most intense. Seal capacity increases by nearly an order of magnitude where measured bulk calcite is highest then gradually declines laterally away from the fault. SEM imaging shows matrix replacement with calcite is primarily responsible for increased seal capacity. This study demonstrates that CO2-water-rock interaction driven by changes in the geochemical environment have significantly changed rock geomechanical and flow properties over geologic time scales. Alteration and dissolution of grains and cements in reservoir and seal rock can potentially lead to leakage by favoring fracturing and/or lowering capillary seal capacity. Alternatively, where CO2-rich fluids drive significant precipitation of carbonate minerals, fracture growth may be hindered and capillary seal capacity increased.

RIGHT: Block diagram of Crystal Geyser field site, Utah. Geology constrained by mapping (Urquhart, 2011), published data, and detailed study of the Moab fault found 50 km to the SE (Eichhubl et al., 2009).

Moenkopi

Entrada

Navajo

Wingate

Chinle

Curtis

Salt WashBrushy Basin

Mancos

Summerville

Carmel

Cedar Mountain

Brines from Paleozoic aquifer+ Crustal CO2

Navajo Aquifer

Recharge AreaSan Rafael Swell

Green River

Travertine

NLittle Grand Wash Fault

Crystal Geyser

Green River

Anticline

~900 m

N

Little Grand Washfault zone

UTAH ABOVE: Stratigraphy of the Green River, Utah area with main units of study highlighted.

CRET

ACEO

US

JURA

SSIC

3350

10-30

350-400

0-30100-180

0-30240-420160-290

20-50100-400130-230410-470

220-3000-80

430-510

Tununk Member

Juana Lopez Member

Dakota SandstoneCedar Mountain Formation

Buckhorn Cg MbrBrushy Basin Mbr

Salt Wash MemberTidwell Member

Summerville FormationCurtis Formation

Entrada Sandstone

Carmel FormationPage Sandstone

Navajo Sandstone

MorrisonFm

MancosShale

Niobrara fauna

Telegraph Creekfauna

Pierre fauna

FEET

After Hintze & Kowallis, 2009

BIG QUESTION:1- How does CO2 alter the geomechanical properties of rock over long time scales, potentially leading to leakage of subsurface reservoirs? Two failure mechanisms: a. Fracturing b. Capillary failureAPPROACH:1- Characterize a range of rocks naturally altered by CO2-water-rock interactions over geologic time scales: XRD, stable isotopes, petrography2- Measure and compare fracture mechanics parameters: fracture toughness (KIC), subcritical index (SCI)3- Measure and compare seal/flow properties: mercury-intrusion capillary pressure analysis (MICP)

II- FRACTURE MECHANICS TESTINGGeomechanical effect of CO2-related alteration Fracture mechanics parameters, the fracture toughness (KIC) and subcritical index (SCI) of reservoir and seal units are important inputs for assessing storage capacity, leakage potential, and planned injection rates for numerical models of carbon sequestration sites. We evaluate whether CO2-related alteration affect these parameters by testing naturally altered rocks. These rocks may better represent the end-products of alteration than what can be measured under laboratory conditions and time scales. Fracture toughness describes the ability of a material containing a crack to resist fracture, or in other words, the fracture toughness is the stress intensity required to initiate new fracturing. Subcritical crack growth may be an important mechanism for and control on fracturing in reservoir and seal units at Crystal Geyser and in subsurface CO2 reservoirs. Subcritical crack growth is largely controlled by stress corrosion, which is related to chemical reactions at the fracture tip. The subcritical crack index, n, is the exponent in the power law relationship below and describes the velocity dependence of fractures propagating under subcritical conditions:

where V : fracture propagation velocity KI : mode-I stress intensity factor, KIC : mode-I critical stress intensity factor (fracture toughness) A : a pre-exponential constant (Atkinson, 1984; Swanson 1984)Together, these parameters have been shown to control the geometry of emerging fracture patterns, such as fracture lengths, spacing, and clustering, which in turn may effect connectivity and flow properties (Olson, 1993 & 2004).

18 8 100102030405060708090

SCI (

n)

SCI

5 5 60.0

0.5

1.0

1.5

2.0

2.5

3.0

MPa√m

KIC

Unaltered siltstone (1)

CO2-altered siltstone

Unaltered siltstone (2)

Summerville Fm Siltstone

SCI KIC

Unaltered shale

CO2-altered (cemented) shale (2)

CO2-altered (cemented) shale (1)

5 8 30.00.20.40.60.81.01.21.41.61.82.0

MPa√m

17 14 20

10

20

30

40

50

60

SCI (

n)

Mancos Shale (Tununk Mbr)# of tests # of tests

# of tests # of tests

5 4 60.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

MPa√m

14 16 260

10

20

30

40

50

60

70

80

SCI (

n)

KIC SCISalt Wash Sandstone

Unaltered sandstone

Intensely CO2-altered sandstoneCO2-altered (calcite-ce-mented) sandstone

5 70.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

MPa√m

14 230

10

20

30

40

50

60

70

80

SCI (

n)

KIC SCIEntrada Sandstone

Unaltered sandstone

CO2-altered (bleached) sandstone

# of tests # of tests

# of tests # of tests

Entrada Sandstone (unaltered) Bleached Entrada Sandstone

Cal

Ill-smec

Qtz

Feld

Arag

Dol

Other

Altered Salt Wash Ss (CAL -cemented)

Cal

Ill-smec

Qtz

Feld

Arag

Dol

Other

Altered Salt Wash SandstoneSalt Wash Sandstone (unaltered)

Summerville Siltstone Summerville Siltstone (II) Altered Summerville Siltstone

Cal

Ill-smec

Qtz

Feld

Arag

Dol

Other

Mancos Shale Altered Mancos Shale Altered Mancos Shale (II)

Cal

Ill-smec

Qtz

Feld

Arag

Dol

Other

CO2-related bleaching of Entrada sandstone slightly altered bulk mineralogy, yet it measurably impacts fracture toughness. Field shot on right.

CO2-related alteration characterized by slight increases in calcite, which increases fracture toughness. The most intensely altered sample is weakest.

CO2 alteration in siltstone characterized by increase in amount of clays (illite-smectite). This alteration also decreases fracture toughness.

CO2-alteration characterized by large increases in calcite content, which helps increase fracture toughness.

subcriticalcrack

growth

K*I = stress corrosion limit

KIC = fracture toughness

criticalcrack

growth

0 2 4 6 8Pore Volume (%)

Pore Aperture Size Distribution

CG 5-23-12 #6Jacketed File 5-911

0 1 2 3 4 5Pore Volume (%)

Pore Aperture Size Distribution

CG 5-23-12 #1Jacketed File 5-909

0.001

0.01

0.1

1

10

100

0 1 2 3 4 5

Pore

Ape

rtur

e D

iam

eter

(mic

rons

)

Pore Volume (%)

Pore Aperture Size Distribution

CG 10-20-12 #3Jacketed File 5-711

Smaller pore throats, narrower distributionBimodal pore

aperture size distribution

1

10

100

1000

10000

100000

0 20 40 60 80 100

Capi

llary

Pre

ssur

e (p

sia)

Wetting Phase Saturation (%)

Mercury Injection Capillary Pressure (Pore Volume)

CG 10-20-12 #3CG 10-20-12 #3 JacketedCG 10-19-12 #3ACG 5-23-12 #1CG 5-23-12 #1 JacketedCG 5-23-12 #6CG 5-23-12 #6 JacketedCG 5-23-12 #7CG 5-23-12 #7 JacketedCG 7-24-11 #7

At fault

Halo

Unaltered

III- MERCURY INTRUSION CAPILLARY PRESSURE ANALYSIS (MICP)Effect of CO2-alteration on seal capacity

The flow properties, permeability, and seal capacity of seal rocks such as shales may change due to CO2-water-rock interactions. Coupled chemical-mechanical effects have implications for long term storage and injection scenarios. Decreases in seal capacity over time due to alteration may lead to leakage. Conversely, alteration that leads to precipitation of new minerals may increase seal capacity, enhancing storage security. MICP analysis is a standard approach for measuring capillary entry pressure (Pe), permeability (k), grain density (ρ), and pore aperture size distributions of fine-grained rocks such as shale. EQ: Pe = (2 γ cos r)/θ where Pe is capillary entry pressure (in Pa), γ is surface tension (in N/m), θ is wetting angle (in degrees), and r is average pore throat radius (in m) Samples were taken from a fault-perpendicular transect in the Mancos Shale and bulk composition measured using XRD.

Strong spatial trends in seal capacity correlate with bulk calcite content. Seal capacity is:• lowest near the fault where alteration is presumably most intense• highest 5-25 m from the fault where bulk calcite content is highest• decreasing further away from the fault, following the bulk calcite trendSeal capacity is reduced where alteration is most intense and higher where CO2-alteration has driven precipitation of calcite. SEM images (BELOW) confirms that mineral dissolution and precipitation has altered pore networks in the shale, changing its flow properties.

Summary

01020304050607080

0

1000

2000

3000

4000

5000

6000

0 25 50 75 100 125 150

Calc

ite w

t%

-Hg

-air

Capi

llary

Ent

ry P

ress

ure

(psi

a)

Distance from Fault (m)

Mancos Shale TransectPc vs. distance from fault (CO2 alteration intensity)

Shale(jacketed)

Shale(unjacketed)

Shale"background"(jacketed)Shale"background"(unjacketed)CalciteBack-ground

Qz

Cal

Py

Dol

Ab

100 μm

At fault: CG 5-23-12 #2 (CO2-altered Mancos Shale)

Py

QzCal

clay

10 μm

15 m from fault: CG 5-23-12 #6 (CO2-altered Mancos Shale)

Qz

Kfs

Ab

clays

100 μm

150 m from fault: CG 5-29-13 #4 (unaltered Mancos Shale)

Dol

Py

Qz

Clay

Cal

25 μm

“Background”: CG 5-28-13 #1 (unaltered Mancos Shale)

SE

M- E

DS

imag

es o

f ion

-mill

ed

shal

e sa

mpl

es. S

ee c

ente

r plo

t for

lo

catio

n on

tran

sect

.

-calcite + clay matrix-evidence of dissolution-small fractures

-calcite replaced clay matrix-few, small pores

-detrital quartz, feldspar, carbonate grains-open pores- clay matrix

-small, open pores visible-detrital and authigenic grains (e.g. pyrite)-clay matrix-littlecalcite

Capillary entry pressures (α to seal capacity) and bulk calcite content plotted against distance (alteration intensity)

MICP Analysis DataRIGHT:Capillary entry pressure (Pe) calculated at rollover point

RIGHT: Altered/ unaltered shale ample pore aperture size distributions. See center plot for transect location.

1

2

3

4

1 2 3 4

I II III

I

II

III

unaltered shale

Stable isotope data of shale samples taken from a fault-perpendicular transect at the Crystal Geyser field site. Elevated δ13C values are indicative of CO2-related alteration. Background samples plot near 0‰ (PDB), as expected for a marine shale. Scatter is greater in δ18O values, but shows similar trends. Light isotopes preferentially fractionate with degassing CO2, and heavier isotopes tend to remain in calcite and other precipitated minerals.

IV- IDENTIFY AND MEASURE EXTENT OF ALTERATIONStable isotope data

XRD data strongly suggests that the alteration at Crystal Geyser is related to interaction with CO2-rich fluids, but weathering reactions can also effect rock mineralogy in similar ways. Stable isotope data can help confirm that the rocks at Crystal Geyser are effected by CO2-water-rock interactions.

V- CONCLUSIONS AND IMPLICATIONSImpact of CO2 -related alteration on reservoir & seal rocks:• Geomechanical effects: o CO2 alteration which is characterized by mineral dissolution and precipitation of secondary clays can weaken rocks to fracturing by up to 50% in sandstone o CO2 alteration which is characterized by precipitation of carbonate cement can increases KIC in shale samples by ~30-50%• Permeability & seal capacity : o Intense alteration lowers seal capacity of shale by nearly half an order of magnitude through dissolution of grains and cement o Enhanced precipitation of carbonate cements in shale increases seal capacity by 3-5X o CO2 alteration significantly impacts permeability in all lithologies

IMPLICATIONS1. Subsurface CO2 injection and storage involves inherently coupled chemical-mechanical processes 2. Diagenetic reactions affect fracture mechanics properties and matrix flow of reservoir and seal rocks3. Comparable chemical reactions can also affect reservoir and seal rocks in EOR operations, wastewater and CO2 injection because fluids are out of chemical equilibrium with the host formation

ACKNOWLEDGEMENTS Jon Olson, Jon Holder, and Iona Williams have assisted with double torsion testing at the University of Texas Center for Petroleum and Geosystems Engineering. MICP analyses were performed by George Bolger and Petrotech Associates, Houston, TX. Toti Larsen (JSG-DGS) has helped with stable isotope analyses. This material is based upon work supported as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Additional funding provided by the Jackson School of Geosciences at The University of Texas at Austin. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

WORKS CITEDAtkinson BK. Subcritical crack growth in geological materials. Journal of Geophysical Research. 1984; 89: 4077-114.Eichhubl, P., N. C. Davatzes, and S. P. Becker (2009), Structural and diagenetic control of fluid migration and cementation along the Moab fault, Utah, AAPG Bull, 93(5), 653-681.Hintze, L. F., and B. J. Kowallis, 2009, Geologic history of Utah, United States, Brigham Young University Printing Services : Provo, UT, United States.Holder, J., J. E. Olson, and Z. Philip (2001), Experimental determination of subcritical crack growth parameters in sedimentary rock, Geophysical Research Letters, 28(4), 599-602.Olson JE. Joint Pattern Development: Effects of Subcritical Crack Growth and Mechanical Crack Interaction. J GeophysRes. 1993; 98: 12251-65 Olson, J. E., 2004, Predicting fracture swarms — the influence of subcritical crack growth and the crack-tip process zone on joint spacing in rock: Geological Society, London, Special Publications, v. 231, no. 1, p. 73-88.Swanson PL. Subcritical crack growth and other time- and environment-dependent behavior in crustal rocks. Journal of Geophysical Research. 1984;89:4137-52.Urquhart, A. S. M., 2011, Structural controls on CO2 leakage and diagenesis in a natural long-term carbon sequestration analogue: Little Grand Wash fault, Utah: Thesis, University of Texas at Austin, Austin, xxiv, 437 p

-16-14-12-10

-8-6-4-2024

0 20 40 60 80 100 120 140 160

‰ P

DB

Distance from fault (m)

Mancos (hanging wall) transect: stable isotopes

δ13C

calcite slickenside in Mancos

δ18O

calcite slickenside in Mancos

"Background" δ13C

"Background" δ18 OBackground

Marin

e shales