BAYESIAN PETROPHYSICAL-SEISMIC INVERSION TECHNIQUES FOR KEY RESERVOIR PARAMETER ESTIMATION: A CASE STUDY FROM THE OFFSHORE ABU QIR FIELD

摘要

Recent advances in seismic-constrained reservoir characterization combine statistical rock-physics and amplitude versus offset/angle (AVO/AVA) inversion in order to directly estimate petrophysical properties such as porosity, shaliness and water saturation from pre-stack seismic data. By exploiting the Bayesian inversion formalism, it is possible to propagate uncertainty from seismic to petrophysical properties, including the effect of noise on seismic data and the approximation of physical models. The results of such petrophysical-seismic inversion are spatial probability density distributions of rock and fluid properties that can be effectively integrated in the reservoir modeling workflows. This paper discusses two target-oriented Bayesian petrophysical-AVA inversion techniques: a two-stage approach and a single-stage approach, developed as part of a collaborative research project between Edison and the Earth Sciences Department of the University of Pisa. The two approaches are evaluated on the gas-bearing sands of the Pliocene interval in the Northern area of the offshore Abu Qir field where a 3D seismic survey was acquired using long-offset cables and well-control is available to validate the inversion results. The two- stage approach, is performed over the whole target-interval and is based on two cascade steps: first, seismic angle-gathers are inverted into acoustic and shear impedances using the convolutional model and a narrow-angle, time-continuous approximation of the Zoeppritz equations; then, a rock-physics model is used to transform the elastic parameters into petrophysical properties. Differently, the single-stage approach uses the rock-physics model to re- parameterize the exact Zoeppritz equations in terms of petrophysical variables; the derived equations are used to directly estimate the petrophysical property along the top-horizon of target interval by taking into account wide-angle seismic reflections. Independently from the inversion approach considered, the rock-physics model plays a crucial role in petrophysical-AVA inversion as it provides the link between elastic and petrophysical properties. In the Abu Qir field, borehole data acquired at the target-depths were exploited to derive a single rock-physics model, valid for different lithologies and for the full-ranges of shaliness and water saturation values. Despite the differences in the forward-model parameterization, the results of the two inversions are comparable and consistent with borehole data. In particular, the described inversion approaches were both able to identify the increase of porosity and the decreases of shaliness and water saturation in the target sands. It results that porosity is well resolved by both two-stage (narrow-angle) and single- stage (wide-angle) inversions. The water saturation remains poorly resolvable in both inversions due to its limited influence in determining the AVA response. Finally, wide-angle reflection inversion has demonstrated to be particularly effective in better constrain the shaliness estimations.