Seismic time-lapse reservoir monitoring (4D) was used to assess the performance of a gas field (Cervia), located offshore Italy, in the central part ofthe gas-producing area of the Adriatic Sea. The chief objective of this work was the characterization of the field producing intervals by applying an innovative methodology based on a high level of integration of seismic data with borehole data. The work, carried out on two ‘legacy’ 3D seismic surveys and all available borehole data, consisted of three main steps.

  1. Characterization of the physical properties of the main reservoir, by extensive measurement and analysis of the core samples. This was in order to implement a petroacoustic model for calibration of seismic acoustic impedance volumes to reservoir description parameters. In addition all borehole data were employed during the implementation, calibration and testing of the petroacoustic model.

  2. Integrated time-lapse processing of the two seismic volumes to achieve high repeatability. Seismic processing parameters were determined independently and objectively for each of the two surveys using well data. Spectral equalization of wavelets in the seismic traces was performed to further improve repeatability.

  3. Integrated Multidimensional Petroacoustic Calibration (IMPAC) of the two seismic volumes. Here the microscopic properties of the reservoir rocks, obtained from the petroacoustic model outlined in step 1 above, were used, together with log data, to derive the main reservoir description parameters (i.e. lithology, porosity, saturation and pressure).

This approach produced good repeatability between the two processed seismic volumes. In addition, time-lapse analysis of the acoustic impedance volumes, calibrated to reservoir description parameters, enabled the visualization of fluid movement within the main reservoirs. This, in turn, led to the identification of areas where the gas may only have been partially depleted. These significant results will provide an important contribution for an improved and more efficient gas production from the field.

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