This paper presents a regional scale 3D geomechanical modeling study of the HB large produced gas field located in the northwest of China, which was converted to UGS facility in June 2013. The reservoir is approximately 3500m deep and there are three faults completely penetrate the caprock. The potential of the caprock failure and reservoir-bounding faults leakage after 14 years depleted production and their dynamic containment during UGS operations must be comprehensively evaluated.

In this study, a multi-disciplinary approach was used to build the static regional-scale 3D geomechanical model through interagrated analysis of geologic, seismic, well drilling, logging and rock experimental data. Gas-bearing reservoir, primary and secondary caprock, three main faults as well as underburden were all covered in this model. Comparing to the reservoir model, the regional scale geomechanical model is expanded laterally by about 5 times. Reservoir pressure during gas production and subsequent annual gas injection/withdrawal was passed to the model using Eclipse software.

The 3D fluid-dynamical geomechanical model was calibrated using the surface vertical displacements measured by InSAR interferometry over the second operation cycle. Simulation indicated that the change of reservoir pressure has a significant impact on the state of the regional-scale stress. Significant differences of in-situ stress have occurred on the both sides of the faults after gas production. It was found that the caprock failure and fault slip had not occur during past gas production, thus ensuring the safety of the UGS conversion. However, distribution of the in-situ stress became more heterogeneous due to high rate gas injection and withdrawal during the UGS operations that may have a significant impact on well integrity. Shear failure and hydraulic fracture of the caprock will not occur during the long term gas storage operations. But the model predicted a maximum shear slip up to 5 cm associated with cyclic gas injection and extraction and the slip tendency of the Southern fault is increasing with the UGS operations which will be weak spot of the UGS integrity. Consequently, it is strongly recommended to strengthen the dynamic monitoring of the Southern fault.

This paper provides a comprehensive geomechanical simulation method for assessment of the risk of caprock integrity failure and fault leakage of large UGS in produced gas filed during long term operations. This work is particularly important in the selection and design of UGS in complex faulted depleted gas filed in China.

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