Orientation and magnitude of in situ stress to 6.5 km depth in the Baltic Shield

Abstract

Abstract Understanding the state of stress in the earth is important for a broad range of engineering and geological problems. To obtain the state of stress in boreholes where conditions are such that conventional stress measurement techniques are impossible, we have used recent developments in the analysis of compressive and tensile wellbore failure in an integrated stress measurement strategy, involving also direct measurement of the least principal stress. The analysis is carried out in the two deep boreholes in the Siljan Ring area of the Baltic Shield. The Gravberg-1 borehole reached 6779 m true vertical depth (TVD) in the Siljan region, central Sweden, and the Stenberg-1 borehole, drilled 10 km to the south of Gravberg-1, was completed at 6529 m TVD. Analysis of vertical, drilling-induced tensile fractures in the nondeviating part of the Gravberg-1 well indicated that one principal stress is vertical and thus could be calculated from density estimates. Borehole breakouts and tensile fractures indicated that the average direction of the maximum horizontal stress, S H , is N72°W±7° in Gravberg-1 and N53°W±9° in the Stenberg-1 well. The direction of S H is on average very stable in both wells. Lower bound limits on the magnitude of the minimum horizontal stress, S h , in the Gravberg-1 well were obtained from controlled and uncontrolled hydraulic fracturing and formation integrity tests. At 5 km depth in the Gravberg-1 borehole the minimum horizontal stress is approximately two-thirds of the vertical stress. We estimated the magnitude of the maximum horizontal stress in Gravberg-1 on the basis of drilling-induced tensile fractures identified in the borehole. S H was estimated by calculating the stress at the borehole wall necessary to cause tensile failure of the formation, incorporating our lower bound S h estimates, corrections for the cooling of the wellbore by drilling fluids and differential fluid pressures. Our results indicate a strike-slip faulting regime in the Siljan area and that the state of stress is in frictional equilibrium with a coefficient of friction in the range 0.5 to 0.6.