1. Monitoring and modelling 3-D ground movements induced by seasonal gas storage in deep reservoirs P. Teatini, G. Gambolati, N. Castelletto, M. Ferronato, C. Janna – Uni. Padova, Italy E. Cairo, D. Marzorati – Stogit S.p.A., Crema, Italy D. Colombo, A. Ferretti, F. Rocca – T.R.E. S.r.l., Milan, Italy A. Bagliani, F. Bottazzi – Eni E&P, Milan, Italy DMMMSA Department of Mathematical Methods and Models for Scientific Applications

2. Outline  Introduction  The Lombardia field: history, location, and geology  Analysis and management tools: fluid-dynamic and geomechanical modelling, PSInSAR monitoring  UGS activity in the Lombardia field: geomechanical response  Future UGS scenarios: land displacement prediction  Summary and conclusions

3. Introduction  Underground Gas Storage projects are increasingly popular worldwide: about 600 UGS plants currently active with an overall working gas volume of about 350 billions of Sm 3  Hazards: safety related to public perception, economic risk and environmental impact  Geomechanical issues:  Need for a comprehensive modelling and monitoring effort subsurface: sealing caprock integrity surface: land displacements

4. The Lombardia field Location and geological setting Northern Italy, Po River plain: normally consolidated and pressurized basin Three gas-bearing pools between 1050 and 1350 m below m.s.l. The reservoir is of stratigraphic type and consists of Pliocene sandy sediments

5. The Lombardia field Production life and UGS program  Primary production started from pool C in 1981 with 2.7 GSm 3 withdrawn until 1986 and 35 bar of pore pressure decline  UGS started soon after superposing to the natural pressure recovery  From 2002, UGS program has pushed the maximum pressure up to 103% p i with 30-35 bar cycles and 1.2 GSm 3 of working gas  Objective: pushing the maximum pressure up to 107% and 120% p i

6. Analysis and management tools PSInSAR data over the Lombardia field PSInSAR is a methodology for monitoring the land displacements through the analysis of Synthetic Aperture Radar (SAR) scenes acquired from satellites over a number of benchmark points known as Permanent Scatterers (PS) Seasonal behavior of two PS compared to the stored gas volume: phase accordance The ground surface “breathes” as the gas is pumped in and out

7. Analysis and management tools PSInSAR data over the Lombardia field PSInSAR analysis of data acquired from:  November 2005 to April 2006 (left)  April 2006 to November 2007 (right) Seasonal movements of 8-10 mm (vertically) and 6-8 mm (horizontally)

8. Analysis and management tools Fluid-dynamic and geomechanical computational models Water dynamics in the connected aquifer is predicted by coupling a 3D FE groundwater flow simulator calibrated over the primary production period 1981-1986 Gas/water dynamics in the reservoir is predicted by the multi-phase flow simulator Eclipse calibrated over the 1981-2007 period

9. Analysis and management tools Fluid-dynamic and geomechanical computational models  Geomechanics is predicted with the aid of the FE code GEPS3D developed by UniPD  Non-linear hysteretic hypo-plastic law for the soil compressibility  Anisotropic constitutive model: calibrated using both vertical and horizontal PSInSAR displacements  Five independent parameters:  =E h /E v,  =G h /G v, h, v and s=c M,I /c M,II

10. UGS activity in the Lombardia field Geomechanical response Calibration of the anisotropic model using the 2003-2007 PSInSAR measurements  Selected parameters:  Very good match between measures and computation  The parameters are consistent with literature data in the same basin  = 3.0 and  = 1.0 h = 0.15 and v = 0.25 s = 4.0

11. UGS activity in the Lombardia field Geomechanical response Measured (from PSInSAR) and simulated vertical and horizontal displacements from April 2006 to November 2006 Maximum vertical displacement simulated over the reservoir

12. Future UGS scenarios Pore pressure prediction Two future UGS scenarios: 107% p i and 120% p i Increase of the stored gas volume by 65% and 180% relative to the current maximum storage  Pore pressure variation in a cycle and its propagation in the connected aquifer: 50 bar and 70 bar  No problems are expected for the caprock integrity

13. Future UGS scenarios Land displacement prediction Absolute value of the expected vertical displacement (mm) during a seasonal injection/extraction cycle  From the current 10 mm at 103% p i to 17 mm at 107% p i and 27 mm to 120% p i  No problems are expected for the stability and integrity of surface structures because of the small differential displacements (< 10 -5 m/m)

14. Summary and conclusions Major points:  UGS at the Lombardia field is responsible for a 10-mm excursion over the last five years which can increase to about 15 mm and 25 mm at 107% and 120% p i with no expected consequences on surface structures  The model calibration provides geomechanical parameters consistent with available studies on the same basin in the framework of a transversally isotropic constitutive model  This study can be viewed as a prototype application of a promising interdisciplinary approach integrating petroleum engineering, remote sensing and computer modeling for a complete geomechanical characterization and management of a UGS program

15. DMMMSA Department of Mathematical Methods and Models for Scientific Applications Thank you for your attention