1. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Loss of injectivity and formation integrity due to pressure cycling Pierre Cerasi and Lars-Erik Walle SINTEF Petroleum Research Trondheim 1

2. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Outline ► Injectivity into storage reservoirs ► Fatigue phenomena ► Laboratory cyclic tests ► Elastoplastic model ► Discussion ► Conclusion 2 www.bigccs.no

3. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Injectivity into storage reservoirs ► Disposal of CO 2 :  Underground storage  Must guarantee containment for centuries ► This entails:  Maximum injectivity to realise full capacity potential  Avoiding potential fluid leakage mechanisms ► However:  CO 2 injection may be intermittent  Due to supply regularity  Due to shut-in (planned or not)  Can have consequences on the stability of the formation rock around a wellbore or perforation 3

4. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Fatigue phenomena ► Known from metallurgy  Fatigue cracks appearing in parts subjected to varying loads  In spite of load magnitude being below strength limit ► Also observed in rocks  Degradation in compressive strength after cyclic loading 4 After Mohd Salim et al., 2012 50% reduction in strength after 175 cycles at 50% peak strength

5. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Laboratory investigation of fatigue in sandstone ► 2 tests run on Saltwash South outcrop sandstone  Goal: Simulate aggressive sequence of injection and shut-in Use weak sandstone and many cycles for worst case scenario ► Hollow cylinder geometry  10 cm outer diameter and 10 cm height  2 cm inner borehole diameter 5

6. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Test set-up and procedure ► Plugs inserted dry into hydrostatic pressure vessel  Confining stress ramped up close to borehole failure value  Quizix pump used to deliver kerosene from the borehole Outside surface kept at atmospheric conditions ► Kerosene chosen instead of CO 2  Timescale contrast between mechanical effects and chemical reactions 6

7. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Results from first fatigue test ► Sample first ramped up 10 MPa  Value chosen to be close to known failure stress of 12 MPa from dedicated sand production tests run at SINTEF  100 injection cycles flow rate of 50 ml/min for 75 s, followed by pause in injection of same length ► No visible damage seen around borehole 7

8. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Results from first fatigue test ► Deformation measured by 2 pairs of strain gauges  Changes in external and borehole radii in 2 perpendicular directions Borehole gauge readings suggest accumulation of irreversible plastic deformation 5 mStrain accumulated during test 8

9. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Results from second fatigue test ► Again, no visible damage Borehole gauge readings suggest accumulation of irreversible plastic deformation 15 mStrain accumulated during test 9

10. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Elastoplastic analysis of tests ► Elastic hollow cylinder stress solution with pore pressure, combined with Mohr-Coulomb plastic yield  Gives calculation of plastic transition radius  Series of triaxial compression tests on Saltwash South provide needed mechanical parameters for calculation 10

11. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Discussion ► Comparing elastoplastic analysis with laboratory tests:  Deformation in conflicting directions experiments show a reversal of deformation in the borehole between confining stress ramp-up and cyclic injection analysis suggests that due to the high applied stress (10 MPa and more), HC plug expected to be in overall compression and thus any plastic deformation would be towards reducing borehole diameter  Deformation magnitude in good agreement, though ► In any case, no sign of fatigue detectable (cracks) 11

12. 8 th Trondheim Conference on CO 2 Capture, Transport and Storage TCCS-8, 19 – 18 June 2015 Conclusion ► Mohr-Coulomb analysis for low injection pressure and high confinement:  relevant for weak and depleted formations  near-well area might always be under compression, such that plastic yield is permanent  expect that each shut-in period would lead to permanent plastic deformation, thus accumulating for many cycles ► Whether this is of concern in the field remains to be investigated:  stress cycles within small limits give plastic zone with diminished shear stresses for ductile material, relatively far from failure conditions  But plastic softening might lead to eventual failure, even if each cycle only causes small plastic deformation ► It remains to be seen whether geochemical reactions caused by the CO 2 exposure might change the mechanical picture one way or another. 12

13. 13 Acknowledgment This work has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Gassco, Shell, Statoil, TOTAL, ENGIE and the Research Council of Norway (193816/S60). Thank you for your attention! Any questions?