RPM-Based Petrophysical Characterization of a Depleted Gas Reservoir Using the Adaptive Stiff Sand Model (ASSM) for Possible CCUS Applications

The Adapted Stiff Sand model (ASSM), a new rock physical model that was recently proposed at the Vietnam Petroleum Institute (VPI), to aid construction of a site-adapted or adaptive rock physics template (ARPT) for seismo-petrophysical characterization of a strongly cemented sand. It is modified from the known Stiff Sand Model (SSM) by Stanford rock physics group, with a key change in determination of the dry modulus of the sand based on the in-situ geological and well log data using a modified Gassmann’s equation. It is known that the success of a rock physics template (RPT) depends very much on the cementation type of the study sand and the rock physics model (RPM) used for its construction. This study dealt with a Middle Miocene depleted gas sand at the Hai Thach-Moc Tinh (HT-MT) gas field, Nam Con Son basin, offshore southern Vietnam, which has cementation type varying at different depth levels. As the results, we could identify: (i) four lithological types, water sand, gas clayey-cemented gas sand, gas calcite-cemented gas sand, and shale; (ii) two main cementation types, i.e., clayey and calcite based on the Adaptive Rock Physics Template (ARPT); and different permeability subzones within the target gas sand using the permeability-linked Adaptive Rock Physics Template (KARPT), i.e., less than 0.5 mD, <0.5 mD, from 0.5 to10 mD, from 10 to 200 mD), and >200 mD. The permeable zones of K higher than 10 mD can be suitable target for carbon injection and storage in a future CCUS application.