Estimation of gas hydrates saturation and pore pressure prediction from offshore well log data

Abstract

Estimation of pore pressure and in-situ vertical stress play a vital role in understanding the geomechanical behaviour of deep sedimentary formations especially those bearing gas-hydrates. Often such formations are not hydrostatic, instead the pore pressure is quite elevated. In order to carry out drilling with minimum risk, these abnormal pressures must be predicted precisely. This study has been carried out using conventional well-log data from the site 19 of the Mahanadi basin conducted under the Indian national gas hydrate programme (NGHP). The pore pressure and fracture pressures in the subsurface formation is derived by Bower’s method. The vertical stress has been computed using formation density log. The coefficients of best fit curve have been computed from velocity-effective stress plot for the well under study and applied on the seismic velocity to transform into the effective stress. By using resistivity log data and Archie’s empirical relation at concerned site, the saturation of gas hydrates is calculated.The Zhang’s porosity method of pore pressure prediction has also been used to validate the results obtained from Bower’s method. Since, gas hydrate bearing sediment has higher electrical resistivity than that of the host sediments, two gas hydrate zones have been found in the well in the depth interval between 174 m to 192 m and between 195 m to 213 m with saturations up to 26% and less than 20% respectively. Since LOT (Leak off test) data is not available, Bower’s method has been used to compute the formation fracture pressure. The probable mud weight window has been estimated from calculated pore pressure and fracture pressure. © 2019 1st Indian Near Surface Geophysics Conference and Exhibition. All rights reserved.