Inverted cross- sections of oil saturation for different seismic surveys. In this example, volumes of remaining oil can be clearly detected in the 2005 example in the
Lower Middle Jurassic Brent Group and the uppermost Tarbert Formation.

Statoil’s novel 4D seismic workflow shows great promise for better identifying remaining reserves in complex geological areas and making more accurate assessments of the uncertainties involved.

Another invaluable seismic technique is standard 4D seismic reservoir monitoring. It is done by comparing the results of offshore, high fidelity 3D seismic reflection surveys carried out over the same area several months or years apart.

Time is the fourth dimension. Any differences noted between consecutive surveys are attributed to fluid or pressure changes caused by reservoir production. Areas showing little or no change may point to the locations of untapped (remaining) reserves.    

This technology has contributed to increased hydrocarbon production at the Statoil-operated Gullfaks field valued at USD 950 million NPV (net present value).

Since the remaining targets at the field are becoming smaller and more difficult to identify, it is important to devise a more quantitative approach to improve the vertical resolution of the seismic data and provide better estimates of remaining oil saturations in different reservoir segments.

Furthermore, by integrating the seismic data with complementary information (e.g. from well logs, production and injection wells, as well as a priori geological knowledge) the technique can be indirectly used to assign values to changes in hydrocarbon saturations and pore fluid pressures.

The quantitative 4D seismic workflow currently used at the Gullfaks field includes two main steps:

The first step – 4D simultaneous seismic inversion – involves a novel Statoil technique for extracting quantitative information on reservoirs rock types (lithologies) and fluid parameters from seismic data, where changes in reflectivity can be related to changes in elastic parameters (e.g. pressure and shear wave velocities and density). The term ‘simultaneous’ denotes that the technique simultaneously inverts seismic data of different vintages. 

The second step – 4D rock physics inversion and uncertainty analysis – is largely based on the same principles, but this time generates petrophysical parameters from seismic data and assigns uncertainties.

Statoil's latest rock physics inversion technique is based on:

• Gassmann equations to evaluate saturation effects
• relationships between seismic-calibrated sonic well logs and pressure measurements to evaluate pressure effects. In this way, a non-linear relationship is obtained between changes in elastic properties and dynamic reservoir properties, relative to those prior to production

A statistical approach, in which various parameters are simulated non-independently, is then used to determine uncertainties arising from 4D seismic noise, variations in rock and fluid properties, and spreads of sonic well log and pressure measurements etc.
 
Hydrocarbon saturation and pore pressure data cubes and their associated uncertainties are then generated to assess various stages in the field’s production history.

The combined results from the workflow are very promising as they enable remaining oil targets to be better identified in complex (heterogeneous) parts of the reservoirs and more accurate assessments of the inherent uncertainties.

The method also allows the monitoring of gas injected into the reservoir to improve oil recovery, because it provides a clearer picture of where the injected gas is going.