The accurate prediction of the pore pressure and the in-situ stress field is becoming significantly important for deepwater drilling operations recently in the industry. From the past drilling operation experiences in the Sea of Japan, the overpressure and the borehole instability have been recognized as ones of the drilling hazards. In this study, the distribution of the pore pressure and the sediment's geomechanical parameters in the Joetsu Basin have been assessed by using well data in the area.
The Joetsu Basin is located to the southwest of the Sado Island on the eastern margin of the Sea of Japan with water depths of 800–2,000m. Two structures, named A and B, have been identified and three wells have been drilled in the basin. While Well A was drilled on Structure A, which is bounded by two major reverse faults dipping to the east and west, to a depth of 1,975 m below seabed in water depth of 1,135 m, Well B1 and B2 were drilled on Structure B to depths of 1,049m and 2,088 m below seabed in water depths of 971 m and 885 m, respectively. The Quaternary and Neogene sections were penetrated in these three wells. Pressure, temperature, rock mechanical parameters and wireline log data including micro-resistivity image have been acquired and analyzed.
Well A and B2 encountered overpressured zones with the pressure ramps. The overall regional stress orientation for this area is NW-SE, which are supported by the image logs analysis in Well B1 and B2.However, the maximum horizontal stress orientation in Well A estimated from the appearance of drilling-induced fractures are perpendicular to that, in NE-SW direction, which is somehow consistent with the orientation of the reverse faults bounding Structure A. Elsewhere in the basin, the in-situ stress regime is strike-slip. The maximum horizontal stress was constrained by frictional limits, well bore failure, shear failure along pre-existing fractures and dipole sonic logs. In Structure A, observed Sh/Sv is between 0.97–1.02 and SH/Sv is between 1.16–1.32. In Structure B, Sh/Sv is between 0.8–0.92 and SH/Sv is between 0.98–1.15. Measured pore pressure, temperature, rock mechanical parameters and constrained principal stress magnitudes were then used to build 1D geomechanical models and assess borehole stability. The 1D geomechanical model shows that thermal stress has significantly contributed to the generation of the observed drilling-induced fractures.
