ABSTRACT:

Wellbore stability of the KTB main well (Hauptbohrung), and its technical importance are described. The well is drilled in NE Bavaria, near the town of Windischeschenbach (Germany). Its diameter is 17 1/2"" down to 3,000 m, 14 ¾" to actual depth of nearly 4,000 m. Observed instabilities can be attributed in part to the drilling technique applied (mud composition, rotation of drill string from the surface). Countermeasures are dealt with, and open questions are discussed.

RESUME:

Ce rapport presente l"etude de la stabilite des parois du puits principal KTB situe à Windischeschenbach dans le Nord Est de la Bavière, Allemagne. Le forage a lieu au sein de roches metamorphiques (gneiss et amphibolites) en diamètre de 17 ½" jusqu" à la profondeur de 3 000 m, puis en 14 ¾". Le rôle des techniques de forage (composition de la bone, moteur de fond ou en rotary) comme causes d" instabilites a ete analyse. Le KTB presente ici les mesures qui ont ete decidees afin d"assumer la stabilite des parois du puits, des questions restent cependant ouvertes et devront faire l"objet de la poursuite des travaux.

ZUSAMMENFASSUNG:

Es wird ueber Stabilitatsverhalten der KTB- Hauptbohrung (Windischeschenbach, NE-Bayern, Deutschland) berichtet. Die Bohrung wird in metamorphen Gesteinen (Gneisen ump Amphiboliten) niedergebracht, mit einem nurctJnesser von 17 ½" bis 3 000 m Teufe, danach mit 14 ¾". Es lassen sich eindeutig bohrtechnisch bedingte Ursachen fuer Instabilitaten angeben (Spuelungszusammensetzung, Art des Meiβelantriebes). Die hiergegen getroffenen Maβnahmen werden dargestellt und offene Fragen diskutiert.

INTRODUCTION

The KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland, continental Scientific Deep Drilling Program of the Federal Republic of Germany) is a non-commercial project of basic geo-scientific research. It came into existence in 1985, after some years of intensive site selection, both on a local and a regional scale. In 1986, it was decided to drill in the Oberpfalz region, NE Bavaria (Fig. 1). It was clear from the beginning that two boreholes would be drilled, the pilot hole or core hole (Vorbohrung), and the main well (Hauptbohrung). The reasons for this strategy are stated by Rischmueller (1990), and it has to be regarded as highly successful as its scientific and technical as well as economic objectives were met. The pilot hole was drilled to a depth of 4,000 m, target depth of the main well is 10,000 m. The main well was spudded in September 1990 and has reached nearly 4,000 m at the time of writing (August 1991). As one of the major scientific objectives of the KTB is to elucidate the geologic and tectonic structure of parts of the Variscan orogen of Central Europe, especially the nature of the tectonic borders between the subunits of the Moldanubicum and Saxothuringicum (Behr & Emmermann1987), a sequence of folded tectonically disturbed, altered and lithologically varying metamorphic rocks forms the host rock for the two wells. The main types of rock encountered are gneiss, preferentially biotite-sillimanite- gneiss, and amphibolite. The foliation of the formation dips in all angles from the vertical to the almost horizontal.

THE MAIN WELL

The ultradeep KTB main well with its target depth of 10 km, with an option to go for 12 km, if circumstances are favorable, is the main effort of the entire KTB program. Experiences gained while drilling the pilot hole were used planning the main well. The casing scheme of the main well (Fig. 2) had to be revised from an earlier version, in order to avoid longer open hole sections (Sperber.1990). Also, the mud system of the pilot well is used in the main well as well (Ellins & Tran Viet 1990).

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