Gas prediction using an improved seismic dispersion attribute inversion for tight sandstone gas reservoirs in the Ordos Basin, China

Abstract

Predicting gas saturation is critical for identifying gas zones in tight sandstones, but poroelastic behaviours of tight sandstones with low porosity and permeability lead to complex relationships between gas saturation and elastic attributes. Thus, gas prediction based on conventional seismic methods tends to be a challenging task. However, the frequency-dependence of elastic properties associated with fluid flow in tight sandstones allows the utilisation of seismic dispersion attributes for gas prediction. This study proposes a new inversion method to compute the bulk-modulus-related dispersion attribute DK for improved gas prediction by using the sensitivity of the bulk modulus to fluids in tight sandstones. Rock physical modelling and synthetic tests indicate that the proposed DK exhibits increased sensitivity to gas saturation compared to the conventional P-wave velocity dispersion attribute DP and the commonly utilised VP/VS ratio. Real data applications demonstrate that DK represents a preferable gas identification factor for the tight sandstone gas reservoir in the Ordos Basin. Results indicate that DK shows improved apparent anomalies to gas zones and better correspondence with the actual production status of drilled wells than DP and the VP/VS ratio. A combined factor, CF, is further proposed to improve the accuracy and robustness of gas prediction by considering the sensitivity of DK while simultaneously incorporating the VP/VS ratio as an essential constraint. A close correlation between the proposed CF and gas saturation measured in boreholes justifies the applicability of the CF for the reliable identification of gas-bearing tight sandstones. Gas zones identified by the CF provide essential information for gas exploration in the region of interest. The method proposed in this study extends the existing seismic dispersion inversion for improved gas prediction. The methodology to obtain the gas identification factor can inspire the development of other practical fluid indicators.