AAPG Middle East Region Geoscience Technology Workshop

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Using Seismic Inversion for Drilling Better Wells in a Tight Sand System

Abstract

In this challenging oil price market, it becomes not only important but also critical to use all available data for drilling better wells. This abstract would summarize a case study where seismic inversion data was used to effectively delineate the tight vs non-tight sands; this resulted in a better development strategy for the gas field in an extensional basin. The integration of seismic inversion results resulted in drilling the first well, which targeted the non-tight sands as depicted, by the inversion results and made a commercial discovery in a field, which was previously plagued by tight sands. The objective of this study was to effectively delineate the non-tight sands through seismic inversion, map these non-tight sand intervals throughout the field and recommend new well locations in the cretaceous deltaic reservoir. Previously there were 3 wells drilled in the field which penetrated the tight sand sections and were declared uneconomical. The stochastic seismic inversion (SSI) workflow was used to invert the seismic data in order to delineate the non-tight sands. This method of inverting seismic data is helping in capturing the associated uncertainty with the probability cubes by generating multiple realizations and capturing well scale heterogeneity, which is later populated on the field scale. The well data from all five (5) wells i.e. well M1A (Seirra A), M2A (Seirra B) and M2D (Seirra C), M3D (Seirra D) and M4D (Seirra E) was used to develop the initial correlations between the seismic and well data. The 125 sq.km of 3D seismic data was then inverted using correlations of Acoustic Impedance (AI) vs Volume of Clay (VCL), Porosity vs AI and Vp/Vs for obtaining the water saturation component. The workflow generated four (4) probability cubes, which are Volume of Clay, Porosity, Facies and Water Saturation cubes. These probability cubes were used to capture the uncertainty within the results by running 50 realizations of the generated cubes. The porosity and volume of shale cube correlations successfully distinguished between the “Tight and the “Non-Tight” sands. This was the key finding of this study as this has resulted in determining the spatial distribution of these two rock classes, which were critical for drilling next wells to be economically viable. The areas highlighted by the seismic inversion results, which showed the non-tight sands, distributed over the field were identified and two potential areas of new well locations were recommended. As a result, one of the new well location was then drilled by well M3E (Seirra F) and successfully encountered the non-tight sands and resulted in the first commercial discovery in the field. The successful results of well M3E (Seirra F) increased the confidence factor on the inversion results and future planning of the field is now heavily relying on the identified non-tight sand areas.