Discrimination of fizz water and gas reservoir by AVO analysis: a modified approach

Amplitude versus offset response is analogous to variation in P-wave velocity re- sulting from different pore fluid saturations. However, the input parameters of fluid mixtures such as fluid modulus and density are often estimated using volume average method, and the resulting estimates of fluid effects can be overestimated. In seismic frequency band, the vol- ume average method ignores the heat and mass transfers between the liquid and gas phases, which are caused by pore pressure perturbations. These effects need to be accounted for the interpretation of the seismic events and forward modeling of fizz water reservoirs. The con- ventional model is corrected in present study by considering the thermodynamic properties of the fluid phases. This corrected model is then successfully applied on a gas producing field in the North Sea. AVO response, based on the corrected model is highly affected by pressure related variations in bulk modulus of multi-phase formation fluid. Velocity push down effect appears, as the free gas saturation generates stronger AVO response than ob- tained by a conventional AVO model. The, present research reveals that such response is helpful to discriminate fizz water from commercial gas, to detect primary leakage of gas (CO 2 or CH 4 ) from geological structures and to model free gas effects on seismic attributes