Shear wave velocity estimation using seismic attributes in one of the sandstone reservoirs of southern Iran

Shear wave velocity is a key factor to estimate the elastic and petrophysical parameters of the hydrocarbon reservoir. However, shear wave velocity is rarely logged at wells due to the imposition of high costs. Therefore, it is usually attempted to estimate this parameter by different methods from the available and related data. Describing the elastic parameters of reservoir rock, including shear modulus, bulk modulus and Poisson's ratio, requires the measurement of density and compressional and shear wave velocities of the reservoir formations. Direct measurement of the shear wave velocity is done by drilling cores and DSI (Dipole Shear Sonic imager) tools, which are unfortunately very time-consuming and expensive. In this study, a practical method for estimating shear wave velocity in a sandstone oil reservoir is presented. In the studied reservoir, from seven existing wells, the shear wave velocity has been measured by DSI tools in only one of them (well #7). The shear wave velocity log in the location of the other wells was estimated using a petrophysical equation, defined for the location of well #7. The correlation of other logs (i.e. acoustic, density, porosity, resistivity, gamma ray, dolomite volume, quartz volume, and water saturation logs) with the shear wave velocity was investigated in well #7. We found that the compressional wave velocity, density, porosity, dolomite volume and quartz volume logs were more correlated with the shear wave velocity log in well #7. Thus, these logs were selected as input for estimating shear wave velocity log and the experimental equation using the multivariable linear regression method was calculated. The estimated shear wave velocity log using the obtained relationship has a 90% correlation with the measured shear wave velocity log in well #7. Using this petrophysical relationship, the shear wave velocity were estimated in the other wells (blind wells). The main goal in this study, was to produce the volume of the shear wave velocity information at the sandstone reservoir. To obtain 3D volume of shear wave velocity distribution in the reservoir, the seismic and well data are integrated. To achieve this goal, the model-based seismic inversion technique has been performed to obtain the acoustic impedance volume for the sandstone reservoir. The calculated acoustic impedance volume using model-based algorithm has an average of 99% correlation and 15% error with the real acoustic impedance log. The results of the seismic inversion were fed into the cross validation method to derive the optimal number of seismic attributes relevant to shear wave velocity information. The cross validation method shows that the attributes of the filter 20/25-30/45, the cosine instantaneous phase, the acoustic impedance and the instantaneous frequency have the reasonable correlation with the shear wave velocity information respectively, and are selected as the input attributes for the estimation of shear wave velocity volume in the sandstone reservoir. Our results show a good agreement between the real shear velosity log and the predicted shear velocity from the seismic attributes in the place of well #7. The obtained shear wave velocity volume accompanied by the compressional wave velocity information can be used to infer more robust petrophysical parameters in the reservoir.