Validating the Integrity of the Amposta Structure for Gas Storage Offshore Spain
Authors: Batchelor J. A. , Sherley B. W. , Klepacki D. , Lewis R. E. , Jensen R. E. 
Published: December 2007

Abstract
Spain's consumption of natural gas is expected to double by 2011. Within the Castor Underground Gas Storage Project, the Amposta structure has the capacity to meet 25% of Spain's future requirements for gas storage by providing a dedicated supply of high-deliverability natural gas during seasonal periods of peak demand. The location, shape and excellent petrophysical nature of the reservoir, combined with an active water drive, will provide a storage capacity of 1.3 billion cubic metres of working gas at sustained rates of up to 25 million cubic metres of gas per day. To date, analysis of the geological, petrophysical and geophysical data indicates the Amposta structure will provide all the necessary criteria to be an excellent gas storage facility.
In 1995, Escal UGS S.L. and Eurogas Corporation geophysical data indicates the Amposta structure will provide all the necessary criteria to be an excellent gas storage reservoir.
In 1995, Escal UGS S.L., a subsidiary of Eurogas Corporation, obtained the hydrocarbon rights to the Castor Permit which included the abandoned Amposta oilfield. The intention was to use the Amposta structure as a gas storage facility for the Mediterranean region of Spain. An analysis of existing data determined that two reservoir criteria remained unresolved: the framework geometry of the reservoir, and the sealing capability of the cap rock for gas. To answer these questions, and to further determine the suitability of the Amposta structure for gas storage, Escal undertook an evaluation program in late 2004 and 2005 drilling and testing the Castor-1 well and acquiring 150 km2 of 3-D seismic data. Analysis of the geological and geophysical data indicates the Amposta structure is an excellent candidate for gas storage. The structure consists of a tilted fault block of Lower Cretaceous limestone of the Montsia Formation. Well data indicates the limestone is intensely karstified, creating a complex and chaotic distribution of porosity within the structure.
Schlumberger provided an advanced logging suite and data interpretation services for the Castor-1 well to assist in determining the reservoir capacity of the Montsia Formation and evaluate the sealing potential of the cap rock. The log evaluation confirms the Montsia reservoir consists of limestone with several highly porous intervals that provide most of the reservoir storage capacity. Total porosity for these intervals can be as high as 25 pu (porosity units), based on the sonic log. The remainder of the limestone has very low porosity with negligible hydrocarbon storage capacity. Each pay zone in the well is marked by significant washout associated with the karsted intervals, making conventional log interpretation difficult. Outcrop analogs in the Tarragona region of Spain suggest these karsted pay zones are fractured, highly porous and permeable, which in part was confirmed by the Castor-1 integrated log set. The top seal for the Montsia reservoir is provided by thinly interbedded calcareous shales and tightly cemented, very fine-grained argillaceous sandstones and siltstones of the Lower Miocene aged, Lower Castellon Formation. A high-vertical resolution lithofacies log was generated using advanced algorithms which show the Lower Castellon exhibits significant vertical heterogeneity, inhibiting vertical permeability.
Additional work is being undertaken to assess the karstic porosity distribution within the structure using the recently acquired "state-of-the-art" 3-D seismic data set. Pre-Stack Depth Migration (PSDM) has greatly improved the image of the reservoir structure and its interior, which was poorly imaged on older seismic data. The new PSDM seismic volume will be used for reservoir modeling and selection of optimal drilling locations that will target areas of the field combining high structure with good karstic reservoir development.