ABSTRACT

Snam-Stogit, among the European leaders for underground gas storage (UGS) development, manages nine depleted fields in silicoclastic geological systems, ensuring safety conditions and compliance with environmental constraints.

Multiple solutions have been deployed to improve technical efficiency and to optimize related costs, adopting the latest technology to perform a green formation evaluation and geosteer horizontal wells in complex tectonic frameworks. Snam-Stogit tested and routinely applied new technologies such as high-resolution resistivity images in slim holes recordings, sourceless formation evaluation platforms, multilayer bed boundary mapping, and reservoir mapping services. This method allowed for improving the storage capabilities while decreasing the number of wells needed for evaluation and to better understand the geological-structural setting.

The logging-while-drilling (LWD) solutions provided the opportunity to acquire data at any well deviation, minimizing the target formation alteration and invasion, often occurring in unconsolidated sands. The possibility for acquiring and interpreting data in real time was effectively used to minimize critical issues related to drilling operations, such as entering a depleted reservoir with inadequate mud weight.

A high number of wells have been drilled, including more than 30 horizontal wells, yielding the possibility to build a solid set of solutions to meet the challenges of drilling in mature gas fields, often with limited predrill data. This paper presents an overview of the evolution and state-of-the art LWD solutions applied to UGS projects. Case studies are also presented to complement the technology description and to demonstrate the impact from applying the solutions.

INTRODUCTION

UGS activity began in North America in 1916 (Marzorati et al. 2012)1 and is presently a commonly used practice. Today, they are more than 600 active storage sites worldwide, with approximately 70% located in the USA and the remaining sites mainly concentrated in Europe. UGS was initially tested in Italy during 1964, converting the depleted gas reservoir of Cortemaggiore (Emilia-Romagna Region) from production to storage. Snam-Stogit, currently the major UGS Italian company, manages nine storage facilities, including Brugherio, Bordolano, Cortemaggiore, Fiume Treste, Minerbio, Ripalta, Sabbioncello, Sergnano, and Settala, (Figure 1), which operates in synergy with the transport and regasification infrastructures of the company, ensuring energy security for Italy. The UGS facilities are located in depleted gas reservoirs with average depths between 1000 and 1700 m. The geology is therefore suitable for UGS, as it worked for ensured containment in the existing natural gas reservoirs existing for millions of years. The fields are mainly located in the Po Plain, while the Fiume Treste field is situated in the Abruzzo region (central Italy). Miocene to Pliocene poorly cemented sands and gravels, which make up the target reservoir, are covered by several hundred-m thick massive shale, acting as very efficient seal. The geological traps, both structural and stratigraphic, are associated with buried thrusts in medium-complexity geological context. Selecting the storage sites is driven by specialized studies, 3D modelling, and technical-economical evaluations. Continuous monitoring of the field performance is improved by introducing new technology and solutions, and acquiring surface and downhole data. At the well, scale, the information is obtained by wireline reported by Cantini et al. (2010)2, LWD, coring, and continuous acquisition of dynamic records of pressure, temperature, and fluid volumes. Storage performance optimization and future reservoir behaviour used updated numerical models.

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