233 7. Seismic inversion and characterization applied to geothermal energy In practice, P-impedance and S-impedance properties are computed from well logs and compared to the petrophysical properties. In a multi-disciplinary project, geologists may identify dozens of lithologies from core or well logs. The number of lithologies must be limited, as the number of independent attributes is limited to P- and S-impedance only. Some guidelines are as follows: • The problem can be split into several ones by studying the intervals separately. • Only the most typical lithologies should be kept, such as clean sand. Porous or tight sands are not lithologies, but sub-groups of the sand lithology that can be detected afterward by identifying the corresponding impedance values in the predicted sand. • The facies can be grouped (one versus all) or to apply a nested approach. • An upscaling analysis must be performed to identify lithologies/properties detectable from the seismic data. Figure 7.8 illustrates the lithology and property changes with the upscaling, removing the information with a frequency content greater than various limits, representing the expected quality of seismic data: • For a discrete lithology column, a “most of” algorithm is used, assigning to the cell the most represented lithology. The size of the cell is computed depending on the resolution formula. • For a continuous property (like a volume of shale for instance), a frequency filter (low-pass) is used, adjusting the frequency limit to the resolution. In this example, it is interesting to assess the critical frequencies from which each sand layer is no longer detected or merged with another one. It allows the interpreter to predict what to expect from a seismic characterization study. Figure 7.8 Upscaling of lithology (left) or porosity (right) considering different seismic frequencies.
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