Estimation of pore pressure and oil saturation changes in the reservoir using petro-elastic modeling and 4D AVO inversion attributes in the Ravva field

Summary A 4D seismic survey was carefully planned, executed and interpreted on the Ravva Field. Geoscience and 4D seismic studies carried out on the field have provided key information that defines fault compartments, position of the current OWC and reveals potential undrained areas. The 4D seismic data has been used to optimize sub-surface targets, and underpinned Cairn India’s 2010-11 infill drilling campaign, which was instrumental in arresting the production decline in the field. The infill drilling results, as well as on-going dynamic reservoir surveillance programs are in line with 4D interpretations. All these results are being used to up-date the reservoir model for optimal reservoir management and development. The 4D response in Ravva Field is result of a combination of changes in fluid saturation and pore pressures and hence requires discrete separation of the pressure and saturation components of the 4D effect to enable quantitative interpretation. A 4D simultaneous AVO inversion was carried out using base and monitor datasets to derive the quantifiable elastic property changes in the reservoir. In this paper, we will first discuss the initial 4D interpretation results, the pitfalls of 4D interpretation based on AI – Vp/Vs domain and the importance of pressure and saturation decoupling from the 4D signal in order to identify bypassed oil areas. Next, we will discuss the methodology of decoupling of 4D signal using petro-elastic model and 4D inversion volumes and finally we will show the estimated saturation sections and maps portraying the water flooding signature in the flank side of the structure and undrained areas in the crestal portion.