Deformation behavior of a regional shale formation from integrated laboratory and well data analysis: Insights for underground fluid storage in northern Italy

The Po Plain in northern Italy is one of the most populated and industrialized European regions and hosts numerous 12 gas fields, which have been in production for decades. Several reservoirs were subsequently converted into underground 13 storage sites for natural gas (UGS) while others are currently candidates for future storage of CO2 or H2. In all cases, the 14 reservoir caprock confining the fluids underground is the Argille del Santerno formation. The Argille del Santerno is a 15 good-quality shale; it has a thickness ranging from several tens to hundreds of meters and it extends across a large part of 16 the subsurface of the Po Plain. The part of the formation that acts as a caprock for the main reservoirs lies at an average 17 depth of 1200-1500 m. 18 The scope of the present work was to obtain a sound evaluation of Young’s modulus of the Argille del Santerno based 19 on experimental data so as to describe the deformation behavior of the formation at current or prospective storage sites. 20 To this end, we collected and analyzed a wide range of data from triaxial tests, ultrasonic laboratory analysis, and well 21 logs from 6 different locations (corresponding to 6 reservoirs) in the Po Plain. Our dataset was enriched by publicly 22 available log data, which we used for stratigraphic correlation purposes and identification of the Argille del Santerno 23 throughout the Po Plain. An empirical relation between Young’s modulus from log data and depth was inferred. 24 Discrepancies among the results of the analyses are presented and discussed. We investigated the effects of alteration of 25 cementation, diagenetic and aging behavior due to sampling operations, and the impact of the investigation techniques at 26 the specimen scale (triaxial test and sonic wave propagation data) on Young’s modulus values. This was done to explain 27 the differences between lab and well values. 28 This paper provides a unique characterization of Young’s modulus of the Argille del Santerno at a regional scale, to 29 serve as a reference for reliably predicting ground-level movements induced by underground storage activities.