Porous Underground Gas Storage Can Rapidly Provide Multiple Services to the Grid

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 220051, “Cornegliano, or How a New Porous UGS Asset Can Rapidly Provide Multiple Services to the Grid,” by Giuseppe Gervasi, Alberto Ton, and Stefano Lombardi, IGS, et al. The paper has not been peer reviewed. _ An underground gas storage (UGS) project has been developed from the Cornegliano depleted gas field in the Po Valley of Italy. The gas field was converted in December 2018. The complete paper describes how ramp-up operations and commercialization were combined in an optimal manner. These were conducted successfully in parallel with continuous development of the seasonal product and increasing flexible short-term activities. The authors write that diversified and increasing services can be provided to the Italian gas system with 100% reliability. Field History and Description The Cornegliano field consists of a typical anticline, bounded both to the north and to the south by reverse faults. The producing reservoir is composed of sands and shaly sands of the Sabbie di Caviaga formation of the Lower Pliocene at a depth range of 1350–1450 m. The field was discovered in the 1950s and produced approximately 2.4 billion std m3 of gas from 13 wells until 1997, although 95% of total production occurred before 1983. Original gas in place was on the order of 3.2 billion std m3; the field reached a recovery factor of 74%. Twenty wells were drilled until 1968, all plugged and abandoned by Eni before IGS operations in the field began in 2016. The current operator began the conversion of the field by drilling 14 new wells: 12 storage wells (six for each of two clusters) and two additional wells (Wells A04 and B03) used as microseismic monitoring wells. In 2017, after the first four wells had been drilled, a 3D seismic survey was performed. The last two wells (Wells B02 and B07) were completed in 2019. Except for the two vertical wells (Wells A01 and B01), all other wells were drilled as slanted holes, with inclinations of up to 90° at target depth. With commercial operations beginning in December 2018, the operator, contrary to common sector practice, had to manage in parallel the ramp-up of the reservoir, the injection of cushion gas, and the delivery of commercial service in the context of a limited and progressive knowledge of reservoir dynamic behavior. The plant was designed to facilitate fast switching (i.e., in half an hour) between injection and production operation modes, thus allowing operation of short-term products in addition to the conventional seasonal product. Reservoir Geology and Well Coverage: Effect on Storage Operation Flexibility The overall reservoir sequence is mud-rich and composed by intercalation of sandstones, claystones, and hemipelagic marlstones representing a deepwater system sedimentation. The stratigraphic sequence was interpreted from mud-logging data in conjunction with logging-while-drilling data. The Cornegliano reservoir is composed of four main zones: B, C1, C2, and C3, developed over a depth range of 1300–1700 m and overlain by a 300-m-thick shale unit (caprock). The major structural trend is oriented northwest/southeast to east/west.