Consideration of Casing-Layout and Cementation Techniques in Very Deep Wells in the Vienna Basin
Authors: Riedl L. , Baldauf F. 
Published: December 2007

This paper was prepared for the SPE-European Spring Meeting 1974 of the Society of Petroleum Engineers of AIME, held in Amsterdam, the Netherlands, May 29–30, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
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Summary
Deep drilling in Austria is restricted to the Vienna Basin where at present time depths of 6000 meters are attained. The gas pay zone slightly overpressured lies in the pay zone slightly overpressured lies in the Triassic "Hauptdolomit" at depths between 4800 - 6000 meters. After having drilled several deep wells we came to the conclusion that the following casing layout covers our demands: 18 5/8" - 100 m, 13 3/8" - 1350 m, 9 5/8" - 3600 m, 7" liner - 5600 m, 5" liner - 6000 m. H2S environment requires careful selection of the casing grades.
High temperatures cause a considerable reduction of yield strength and excessive landing loads. The support of the cement sheath is neglected. Burst in casing design for intermediate casing strings is based on blowout conditions assuming that gas has displaced the drilling fluid in the lowest third section of the hole.
To keep the casing load at the casing head within a reasonable range the first intermediate casing string is cemented to the surface and the cement top of the casing strings to follow should be at the depth of approximately 1200 meters. Close attention is given to the design of the cementing slurries and their proper displacement based on rheological calculations.

Introduction
Deep drilling in the Vienna Basin reaches depths of 6000 meters at present time. The first deep test has been drilled in 1966/1967 to a total depth of 6009 m and discovered a gas reservoir below 4800 m. For further gas exploration we complete as an average one deep well per year.
Fig. 1 shows the geological situation of the Vienna Basin. The objective of our deep drilling is the Triassic "Hauptdolomit" within the Limestone Alps. From the surface to about 3000 m we drill through the Neogene, consisting of silty marly-clays, sands and sandstones. The remainder are conglomerates, breccias and organic limestones. The bottom of the Neogene basin - mainly Triassic dolomites - has a high relief of buried hills, formed by the downfaulted parts of the Alps. Further down there are parts of the Alps. Further down there are sections with varying thickness of Triassic Limestone and Upper Cretaceous formation, a mixture of sandstone conglomerate and shale. The slightly overpressured gas pay zone is tectonically trapped underneath the Cretaceous. After having drilled several deep wells we found by experience that the layout of casing strings showing in fig. 2 was near to the optimum.