The severe breakouts encountered in the deep borehole at Gravberg starting at about 1500 m depth are presented. The state of stress at depth have been estimated from; shallow stress measurements, focal plane solutions from deep earthquakes and results of deep stress measurements in compressional regimes of the Eurasian plate. There is a positive correlation between mud weight and breakout. Based on rock strength, stress magnitudes and borehole deviation the proper mud weight for stable borehole walls has been calculated. Computation is too optimistic for determining compressive/shear failure for small deviations. Loss of drilling fluid at a depth of 6590 m, borehole deviation of 25° and mud weight of 1.5 g/cm3 confirm the failure limit for tensile failure. The distinct element program MUDEC was used to study stability of deep boreholes with various diameter in jointed rocks. The borehole wall remains stable for any studied depth, borehole radius and mud weight. Maximum tangential stress at the borehole wall is 2/3 to 1/2 of the calculated stresses assuming isotropic elastic material. The stress relieve by the joints is the major reason for more stable borehole walls at the fracture zones in the well.
In an attempt to explore deep earth gas from the earth's mantle a prospecting venture has been conducted in the area of the lake Siljan in Central-North Sweden. The basic idea suggested by Gold (1987) is the methan gas from deep seated rocks in the mantle will migrate to the earth's surface through fractures and weak zones and like conventional oil and gas resources should be kept underneath a cap-rock. The Siljan Ring structure was formed by a meteoritic impact that occurred in Devonian time about 360 million years ago. As a result of the impact the bedrock became fractured to great depth, both as micro fractures in the rock matrix as large- scale fault and fracture zones. The Gravberg well is located just outside the meteorite crater and close to the outer rim syncline. The bedrock at the site is of Precambrian age and consists in principal of two types of granites (quartzmonzonite) dated to about 1700 m Ys. Aside from the granites, several sub- horizontal dolerite intrusions occur. Their thickness vary between 5 and 50 m and they gave very strong seismic reflectors during the seismic survey, Juhlin (1987). These fracture zones typically extend over 2–20 m of borehole and occur at a frequency of about every 200–300 m, Juhlin (op. cit.). Many of the fracture zones are associated with basic sills of dolerites.
The following changes in the data are characteristic for fracture zones below 1500 m.
no breakouts
drilling rate increases
sonic travel times increases
deep laterlog restivity decreases
increase in fracture associatedminerals
fracturing of cuttings
The Gravberg well was spud in the first of July 1986 and drilled down to 6600 m till 1987. Rotary drilling with 30–60 rpm and 8.5 inches roller cone bits are used.
