Part of the Gullfaks field has been repeatedly surveyed seismically so that surface seismic data are available as three partial stacks for 1985, 1996, 1999 and 2003, respectively. Time-lapse, or 4-D, simultaneous amplitude variations with offset (AVO) inversion was performed in order to directly estimate acoustic impedance and Poisson's ratio for the baseline survey and to estimate change for each of the monitor surveys. The rock physics analysis of well log data and rock physics modeling for the field were applied for the interpretation of the time lapse inversion results. Hereby, production effects such as fluid and/or pressure changes were mapped and sections of potentially unswept hydrocarbons detected.
The Gullfaks field
The central part of the Gullfaks field lies in Block 34/10 in the northern part of the Norwegian North Sea. The field was discovered in 1978 and came onstream in 1986. Gullfaks is a structurally complex field with highly porous sandstone reservoir units of Middle and Early Jurassic age.
Production peaked in 1994, and the recoverable reserves are estimated to be approximately 2 billion bbl. Approximately 90% has been produced. Today there are more than 100 active production and injection wells.
Statoil is the operator with Petoro and Norsk Hydro as partners in the license.
The workflow may be divided into five main categories:
1) A well analysis was conducted focusing on the quality of both the geophysical parameters (compressional slowness, shear slowness and bulk density) and petrophysical parameters (porosity, volume of clay, saturation and pore pressure). This step also includes a cross-plot analysis in order to study the elastic behavior of the reservoir rock and what controls it.
2) Rock physics modeling was performed to predict geophysical parameters from porosity, volume of clay, saturation and pore pressure using the quality controlled well log data base. Accordingly, the model is field-specific.
3) A low-frequency model was derived directly from acoustic impedance and Poisson's ratio well logs using selected wells.
4) Independent wavelet estimations were performed for each partial stack and vintage at the well locations. Because the wavelet estimations for the different seismic volumes are independent, any variations in amplitude and phase for the different seismic volumes will be captured by the wavelets.
5) The simultaneous AVO inversion results were derived using AVO data with three partial stacks through:
Global optimization technique, where all partial stacks and all samples within each partial stack are used simultaneously through the inversion algorithm.
Inversion directly for the desired rock properties: acoustic impedance, Poisson's ratio and density.
The angle for each sample and for each partial stack is updated through the seismic inversion using the high frequency velocity model extracted continuously from the inversion result.
Advanced time-varying statistical alignment of the partial stacks pre-inversion. For time-lapse seismic data this processing sequence is applied simultaneously for all vintages and all partial stacks.
Rock physics diagnostics
Production-related effects such as changes in saturation and/or pore pressure are transformed to changes in elastic properties of the reservoir rock by application of the rock physics model. Accordingly, the dynamic reservoir changes are transformed to the same domain as the results of the simultaneous inversion.
The present rock physics model estimates the dry rock properties from porosity and volume of clay for a given effective stress datum. The variations in elastic properties with changes in effective stress (because of changes in pore pressure) were predicted from empirical equations calibrated to velocity - stress data derived in the laboratory. The variations in elastic properties with changes in fluids were predicted by the Gassmann equations.
The study of changes in acoustic impedance and Poisson's ratio-related dynamic effects may be studied using the Rock Physics Color Disc (Figure 1). This interpretational tool uses colors to illustrate how the production-related effects change the elastic properties as predicted by the rock physics model. For this study three general areas were defined on the color disc:
Blue-to-purple colors indicate decrease in pore pressure;
Red colors indicate replacement of oil by water and a decrease in pore pressure; and
Green colors indicate increase in pore pressure related to water injection.
In the Rock Physics Color Disc any changes in acoustic impedance and Poisson's ratio of less than 4% are considered as noise and are therefore blanked out. Other production effects such as gas injection can be incorporated, although this is non-relevant for the studied part of the field.
4-D results and interpretation
The output of the simultaneous inversion is given as cubes of acoustic impedance and Poisson's ratio and their respective relative changes, and can be viewed as time-slices (Figure 2). The illustrated time slice shows variations in acoustic impedance using a conventional color scale. The approximate oil-water contact (SW=1) of the reservoir simulation model for the same two-way-time is included for guidance. Within the oil-water contact, blue-to-purple colors correspond to low acoustic impedance indicating high porous reservoir rock.
In a time-lapse context, the results may be interpreted from the relative changes in elastic properties between an actual vintage (1996, 1999 or 2003) and the baseline survey (1985). A time slice of changes in both acoustic impedance and Poisson's ratio was made with the Rock Physics Color Disc (Figure 3). This type of color scale uses the rock physics driven interpretation of the observed changes in both acoustic impedance and Poisson's ratio. Figure 3 shows that most of the high porous sands have been produced and that the pore pressure has decreased, although few areas of only pore pressure decline may be observed.
A rock physics understanding of reservoir rock is a key aspect in interpreting time-lapse inversion results. From rock physics diagnostic and modeling, changes in elastic properties are related to changes in fluid and pore pressure. This knowledge is summarized in an interpretational tool, where a special color scale is related to different production effects. From 4-D simultaneous AVO inversion on the Gullfaks field, where both acoustic impedance and Poisson's ratio and their respective changes are directly derived for each of the seismic vintages, we show that one may effectively map out produced and unswept areas from changes in elastic properties using this interpretational tool.