Abstract
The research presented in this paper focuses on the analysis of land movements induced by underground gas storage operations in a depleted reservoir in Northern Italy with the aim of increasing the understanding of the deformation response of deep formations via a real case study. The a priori knowledge of the pseudo-elastic parameters showed a substantial discrepancy between static values from triaxial lab tests and dynamic values obtained via the interpretation of sonic data at wellbore scale. The discrepancy is not surprising for the formations under investigation: a thousand meters of a silty to shaly sequence intercalated with arenaceous banks above a reservoir formation, which is basically made up of sandstone intercalated with shale intervals and conglomerates. Information collected for over more than ten years of seasonal production/injection cycles (i.e., time and space evolution of the reservoir fluid pressure and of the induced land surface movements) was then combined in a 3D numerical geomechanical model to constrain and update the a priori knowledge on the pseudo elastic model parameters via a back analysis approach. The obtained calibrated model will then be used for reliable prediction of system safety analyses, for example in terms of induced ground movements.
Highlights
The compaction/expansion of hydrocarbon-bearing formations due to fluid withdrawal/injection is a major social concern, especially in high urbanized areas, such as the Po Plain area in NorthernItaly [1,2,3]
This paper focuses on the analysis of land movements induced by underground gas storage (UGS) operations in a depleted reservoir in Northern Italy with the aim of increasing the understanding of the deformation response of deep formations via a real case study
A common issue that arises in the analysis of stress-strain evolution induced induced by by pressure pressure variation variation in in underground underground formations formations isis the the potential potential high high range range variability variability of of pseudo-elastic parameters determined a-priori via different methods
Summary
The compaction/expansion of hydrocarbon-bearing formations due to fluid withdrawal/injection is a major social concern, especially in high urbanized areas, such as the Po Plain area in Northern. UGS basically consists of injecting gas during the summer season and producing it during the winter months This cyclical pressure fluid variation induces reservoir deformation (compaction & expansion) with a consequent seasonal subsidence and rebound of the ground surface, the so-called ‘earth breathing’. Mechanical properties are generally defined according to laboratory analysis performed on cores, usually retrieved from the reservoir and cap rock sequences, together with information from well logs and seismic surveys [14,15] As it is well known, formation stiffness is closely related to the nature and the structure of the porous media, its saturated fluids and the in situ conditions, but it is closely dependent on factors like strain amplitude and scale effects. Different information (such as pressure variation in the reservoir and monitored land movements) was combined in the numerical approach to constrain and update the a priori knowledge on model parameters, improving the geomechanical characterization of the system and, in particular, the reliability of the model prediction for future system safety analyses
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