1. COMPARTMENTALIZATION EFFECT IN GEOLOGIC CO 2 SEQUESTRATION. A CASE STUDY IN AN OFF-SHORE RESERVOIR IN ITALY N. Castelletto, M. Ferronato, G. Gambolati, C. Janna, P. Teatini Department of Civil, Environmental and Architectural Engineering email: pietro.teatini@dmsa.unipd.it, http://www.dmsa.unipd.it/~teatini

2. OUTLINE ►INTRODUCTION ►SITE CHARACTERIZATION ►MATHEMATICAL MODELLING  Modelling approach  Geomechanics of the medium  Geomechanics of the discontinuities ►RESULTS  Failure of the injected formation  Failure of the caproch  Fault activation and induced micro-seismicity  Surface displacement ►CONCLUSION

3. INTRODUCTION Modelling CO 2 sequestration into comparted geological formations: ►Multi-phase flow and transport processes in porous media ►Chemical interactions among gaseous CO 2, in situ fluids and solid grains ►Poro-mechanical issues due to the coupling between flow and deformation (continuous medium and discontinuity surfaces) Is the sequestration safe enough? Are there any possible impacts on the ground surface?

4. Main poro-mechanical issues: 1)Activation of existing faults: ►possible CO 2 escape ►micro-seismic phenomena 2)Analysis of the induced stress: ►shear or tensile failure in the injected formation with variation of the medium properties ►shear or tensile failure in the caprock with generation of cracks and leakage paths for the CO 2 3)Land surface motion: ►environmental impact of the ground surface heave ►structural instability for possible differential displacements INTRODUCTION

5. Within the European Energy Programme for Recovery (EEPR), ENEL (the largest Italian energy provider) is planning a pilot project to inject underground 1Mt/yr (10% of the plant production) for 10 years Several partners:  ENI : geological data end injection knowledgment  OGS-National Geophysical Institute : specific geophysical interpretation  IFP-Institute Francais du Petrole : flow model (COORES)  Univ. Padova : geomechanical model (GEPS3D)  SAIPEM (Eni) : geochemical issues SITE LOCATION & PROJECT DESCRIPTION

6. Photo of the DELTA power plant The Po river delta with the location of the DELTA power plant ► power production capacity: 2640 MW (8% of the Italian need) ► fuel: oil ► CO 2 production: 10 Mt/yr to be stored into the subsurface by 2 injection wells SITE LOCATION & PROJECT DESCRIPTION

7. Map of the zone of interest for CO 2 injection of ZEPT: existing wells in Northern-Central Adriatic Sea and location of DELTA power plant and of the RIMINI injection area Constraints from the government and ENI:  off-shore (to avoid problems with the population)  within structures hydraulically disconnected from CH 4 fields  less than 100 km far from the power plant SITE LOCATION & PROJECT DESCRIPTION

8. Cross Section with the main formations involved in the Rimini structure modeling 3D reconstruction by seismic survey of the complex structure of the Rimini reservoir Surfaces of the top/bottom of the main regional geologic formations GEOLOGICAL SETTING

9. Several info on the geomechanical setting of the basin are available due to the pluri- decadal activities of hydrocarbon exploration and production Soil compressibility vs vertical effective stress in the Northern Adriatic basin (loading stage) from RMT c M = 0.01241× σ z -1.1342 c M,loading = 3.5 c M,unloading GEOMECHANICAL CHARACTERIZATION Overburden / fracturing gradients (  ) and hydrostatic / measured pressure (  ) from well logs

10. MODELLING APPROACH  one-way coupling approach (no feedback from geomechanics to fluid dynamics)  the failure analysis is performed in two steps:  a basic geomechanical analysis to calculate injection-induced changes in the stress field  a-posteriori failure analysis using the stress field previously calculated The complexity of the geological system has suggested the use of a simplified modeling approach:

11. MATHEMATICAL MODELLING Geomechanics of the medium Mechanical model of the continuum: ►Terzaghi-Bishop relationship: ►Cauchy equations with a constitutive model: with ►Constitutive mechanical model:  Hypo-plastic law with hysteresis: solved by FE

12. Mohr-Coulomb failure criterion as yield surface φ : friction angle c : cohesion F  slippage  K s =0  τ s no longer transferred F  opening  K n =0   n no longer transferred slippage  K s =0  τ s no longer transferred Interface Elements : special zero or finite thickness elements able to describe slippage or opening of contact surfaces Goodman et al. [1978] MATHEMATICAL MODELLING Geomechanics of the discontinuities Sketch of the RMT technology

13. Hypo-plastic model with a post-processing failure analysis. Easily understandable with the aid of the schematic Mohr representation of the stress state Shear failure safety factor: Tensile failure safety factor:  = 30º, c = 0 bar [Fjaer et al. (2008)] MATHEMATICAL MODELLING Failure analysis

14. FE – IE 2D GRID Fault implementation in the geomechanical model : triangulated surfaces 2D Mesh # nodes: 4315 # triangles: 8559

15. Rimini FE – IE 3D GRID 3D Mesh # nodes: 463’783 # FE: 2’868’292 # IE: 50’789 (d)

16. Various scenarios have been addressed by the geomechanical model: 1.petrophysical model (porosity, horizontal and vertical permeability) 2.only one injection well 3.pervious lateral boundaries 4.natural stress regime (compressive vs oedometric) 5.pressure distribution (hydrostatic vs overpressured) 6.yield surface Investigated issues:  Formation integrity  Sealing capacity of the caprock  Activation of the existing faults  Micro-seismicity  Sea bottom/ground surface motion RESULTS Scenarios and issues

17. Flow model outcome (i.e. the strength source for the geomechanical simulations): FLOW MODEL OUTCOME Pore overpressure Overpressure profiles at different time steps at injection well W2 vs fracturing overpressure Pore overpressure (left) and CO 2 saturation (right) distributions after 2 (a, e), 3 (b, f), 4 (c, g), and 10 (d, h) years in a cross section through the two injection wells

18. (a) Portion of the FE grid where the stress field generated by the CO 2 injection is investigated. The thrusts and faults are shown with different colors. (b) Safety factor ψ in the selected portion of the Serena structure after 7 years of CO 2 injection. (c) FE where ψ  0, i.e. tensile failure is likely to occur. tensile failure RESULTS Reservoir integrity Tensile failure in the injected multi-compartment formation 7 years

19. shear failure RESULTS Reservoir integrity (a) portion of the FE grid where the stress field generated by the CO 2 injection is investigated. The thrusts and faults are shown with different colors. (b) Safety factor χ in the selected portion of the Serena structure after 1 year of CO 2 injection. (c) FE where χ  0, i.e. shear failure is likely to occur. 1 year Shear failure in the injected multi-compartment formation

20. RESULTS Reservoir integrity (a) portion of the FE grid where the stress field generated by the CO 2 injection is investigated. The thrusts and faults are shown with different colors. (b) Safety factor χ in the selected portion of the Serena structure after 2 years of CO 2 injection. (c) FE where χ  0, i.e. shear failure is likely to occur. 2 years Shear failure in the injected multi-compartment formation

21. RESULTS Reservoir integrity (a) portion of the FE grid where the stress field generated by the CO 2 injection is investigated. The thrusts and faults are shown with different colors. (b) Safety factor χ in the selected portion of the Serena structure after 3 years of CO 2 injection. (c) FE where χ  0, i.e. shear failure is likely to occur. 3 years Shear failure in the injected multi-compartment formation

22. RESULTS Reservoir integrity (a) portion of the FE grid where the stress field generated by the CO 2 injection is investigated. The thrusts and faults are shown with different colors. (b) Safety factor χ in the selected portion of the Serena structure after 4 years of CO 2 injection. (c) FE where χ  0, i.e. shear failure is likely to occur. 4 years Shear failure in the injected multi-compartment formation

23. RESULTS Reservoir integrity (a) Pore overpressure versus time, (b) Mohr circle evolution, and (c) stress path on the t-s plane for a FE located in the vicinity of an injection well. Shear failure in the injected multi-compartment formation

24. Horizontal view of the safety factors χ (left) and ψ (right) at the bottom of the caprock sealing the injected formation after 7 years of CO 2 injection RESULTS Caprock integrity Evaluation of possible failure conditions in the caprock bounding the reservoir

25. (above) Modulus of the stress τ s tangential to the fault and thrust surfaces after 7 years of CO 2 injection. (below) Active (red) and inactive (blue) IE: red elements slip due to the stress change induced by the injection RESULTS Fault activation / Induced seismicity The methodology advanced by Mazzoldi et al. (IJGGC, 2012) is followed:  the seismic moment ( M 0 ) of an earthquake for a rupture patch on a fault is defined by:  M 0 is then used to evaluate the magnitude ( M ) by the equation: G : shear modulus of the host rock  s : average slip A : area of the activated fault G = 1.5×10 9 Nm 2  s : ~ 1-2 mm A = 10 4 m 2 M ≈ 1

26. Uplift of the sea bottom/land after 1 year of CO 2 injection RESULTS Displacement of the ground surface / sea bottom

27. Uplift of the sea bottom/land after 2 years of CO 2 injection RESULTS Displacement of the ground surface / sea bottom

28. Uplift of the sea bottom/land after 3 years of CO 2 injection RESULTS Displacement of the ground surface / sea bottom

29. Uplift of the sea bottom/land after 4 years of CO 2 injection RESULTS Displacement of the ground surface / sea bottom 0.12 m  12 cm : quite negligible on account of the 30 m depth of the Adriatic Sea in the area above the reservoir  displacement gradient 9 ×10 –5 : approximately 50 times smaller than the limiting bound acceptable for steel structures

30. ► the FE-IE mesh accurately reproduce the regional geometry of the geological formations, the horizons bounding the structure and the orientation and extent of the discontinuity surfaces (faults and thrusts) ► the geomechanical issues can play a most important role in relation to a safe geological disposal of carbon dioxide in multi-compartment reservoirs ► the geomechanical simulations indicate that shear failure in large portion of the reservoir is likely to occur well in advance to tensile failure (independently of the various scenarios investigated in the study) Conclusions a new larger reservoir is under investigation

31. DICEA Department of Civil, Emvironmental and Architectural Engineering Thank you for your attention