Design Methods of Setting Depth of Protecting Casing String in Hungary
Authors: Alliquander O. 
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.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Netherland Section of the Society of Petroleum Engineers, P. O. Box 228, The Hague, the Netherlands. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Hydrocarbon-bearing formations in the Carpathian basin frequently experience a normal or hydrostatic fluid pressure. However, occasionally formations are encountered that are overpressured. Most reservoir rocks are situated between, or covered by, a tectonically relaxed overburden. The combination of either normal or overpressured reservoirs with a relaxed overburden causes a high potential hazard of lost circulation with subsequent blowout danger if the setting depth of the casing shoe is not properly determined.

Formations crossed by an open-hole section should, under all circumstances, be prevented from being fractured by the pressure gradient in the borehole resulting from either the maximum backpressure produced during a killing operation or by the pressure inversion due to a shutdown following a kick.

Graphical methods to determine the minimum setting depth of the protecting casing string which still offer a sufficient safety margin against calamities are described.

Introduction

The central part of the Carpathian basin in Hungary is composed of and filled up by, respectively, a series of sand, clay, and shale layers of Middle Pliocene and Lower Pliocene ages. The thickness of the Middle Pliocene formations varies from a few hundred meters to 2,500 m. In these tectonically relaxed formations, the pore pressure is, according to the experience gained with several thousands of exploratory and exploitation wells drilled up to now, always normal, i.e., hydrostatic.

As demonstrated by cracks and craters formed at the surface as a result of a number of blowouts, for these relaxed, locally very thick sand-clay intervals of highly varying nature, the theory of Hubbert and Willis is basically valid. (1) Fractures induced by fluid pressure in holes drilled in these relaxed zones obtain, in general, a vertical direction. (2) The magnitude of the pressure that causes formation fracturing is, in general, smaller than the overburden pressure. (3) The value of the fracturing pressure gradient or its equivalent mud density is independent of depth.

For these conditions, the setting depth of a protecting casing string in the Middle Pliocene formations should be determined very Pliocene formations should be determined very carefully.