Determine HTHP reservoir depth with advanced acoustic impedance inversion in an offshore well at South China Sea

The high temperature high pressure (HTHP) of reservoir brings significant challenges for offshore drilling, which requires to optimize casing depth and drilling mud weight. Well-A as the first exploration well locates in Lingshui T block Southeast Qiong Basin. It is of great importance to safely drill through the target layer in Mesozoic buried hill granite HTHP formation, so that the hydrocarbon potential could be evaluated. In order to determine HTHP formation depth and formation pressure, zero offset vertical seismic profile (VSP) was acquired to obtain corridor stack which contains reflection information from deeper layers below current total depth (TD) and to conduct Bayesian acoustic impedance inversion. The Bayesian acoustic impedance inversion offers a time-depth-velocity relation ahead of the bit. Combining with corridor stack and surface seismic section, it predicts the depth of HTHP formation, which helps drilling team to determine the depth to set casing. The formation velocity is used in a geomechanics model to calculate the pore pressure and fracture gradient, which defined a safe mud window, as well as the equivalent circulation density (ECD) while drilling. With this technique, the HTHP formation depth was accurately determined. The difference between the predicted and actual drill depth of the target granite formation was only 3m, at a depth of 3600m. The pore pressure and fracture gradient were calculated, and mud weight was optimized, ensuring safe drilling into the reservoir formation. This technique is proved effective and ensured the first exploration well was successfully drilled and completed in this new area.