Seismic data usually has lower vertical resolution than reservoir simulation models so it is a common practice to generate maps of 4D attributes to be used as the observed data to calibrate models. In such a case, simulation results are converted to seismic attributes and a map is generated by averaging the corresponding layers. Although this seems to be a fair practice, here we show that this procedure can present some drawbacks and propose a new approach to ensure a proper data comparison.
The first step of the proposed procedure follows the traditional sequence where seismic attributes are generated by running a petro-elastic model (PEM) with reservoir simulation data, at the simulation scale. Then, instead of averaging the simulation layers, we propose to resample the simulation grid to a seismic grid and filter the seismic impedances to the seismic frequency. Lastly, we extract the map from the regular grid to be compared with the observed 4D seismic. This procedure is performed in the depth domain and allows a straight and fair comparison of the two dataset.
A synthetic dataset based on a Brazilian field produced through water injection is used to validate this procedure. This dataset is composed by a synthetic 4D seismic data (observed data) generated by a consistent seismic modeling and inversion and a set of reservoir simulation models (to be matched). We computed seismic impedance for each simulation model by applying a PEM and two maps were generated for each model: (1) by averaging impedance values throughout the corresponding layers and (2) by applying the proposed procedure. When these maps are subtracted from the observed data (error maps), as would happen in a quantitative seismic history matching, we note a relevant differences. In the dataset used, we observed that if the vertical resolution issue is not considered (Case 1) the error map presents a strong bias that would erroneously force a decrease on the water saturation to match the observed data in a seismic history matching. While the map generated in Case 2 presents the errors better balanced and related to actual water movement differences rather than being a consequence of scale and resolution issues.
The novelty of this work is a quick way to bring simulation data to seismic resolution without going through all seismic modeling process ensuring a proper data comparison, which can be promptly added in seismic history matching process.