Crack closure pressures inferred from ultrasonic drill-core measurements to 8 km depth in the KTB wells

Abstract

Ultrasonic P-wave traveltimes were examined in 73 directions of cylindrical core specimens, measuring 30 mm in length and in diameter, at room temperature in a 400 MPa pressure vessel. The wave-velocity analysis was carried out on 66 crystalline drill cores taken at depths of between 127 and 3888 m in the KTB-VB well and on 18 drill cores taken at depths of between 4195 and 8080 m in the KTS-HB well. Based on omnidirectional wave velocities versus pressure, a method is developed to separate crack-caused and textural velocity anisotropies. The crack-caused part of the velocity anisotropy is used to infer crack closure pressures. The ratio of horizontal to vertical differential pressure obtained from textural-reduced crack closure curves decreased from four near the surface to one at depths greater than 5 km. The difference in horizontal crack closure pressures was higher than the overburden pressure only for specimens from shallow depths (<1 km); it reached a maximum value of 40 MPa at 2 km depth and decreased to 15 MPa in the depth range from 4 to 6 km. For 45 cores we observed a rotation in velocity azimuth under pressure. Assuming the velocity azimuth of nine cores with weak fabric and the crack velocity azimuth of 23 anisotropic cores with rotating azimuths to be parallel to the SH-direction, our data imply a significant rotation of the stress field in the depth range from 1 to 4 km in the KTB pilot hole. Our core results support the previously determined average SH-direction of N162°E at the drill site only in the suite of metabasites at depths ranging from 3629 to 3746 m.