Rock physics of impure chalk sequences
We have estimated the elastic properties of pure and impure chalk at three different scales on the South Arne field, North Sea, by analysing ultrasonic core data, downhole log-readings and results of AVO-inversion based on near- and far-offset stack seismic data (Amplitude Versus Offset).
We intend to develop the basis for interpreting porosity and fluid content of chalk from seismic data by determining how stratigraphic differences in the composition of chalk affect its acoustic properties. Some chalk consists of nearly pure calcite, as in the Tor Formation, whose acoustic properties were investigated in the EFP-98 project 'Rock Physics of Chalk'. Other chalks, e.g. the Ekofisk Formation, contain variable amounts of silica and clay, whose acoustic properties are the subject of this project. We expect that small amounts of clay will be present in the pores between chalk grains, thus reducing porosity without affecting rigidity, whereas larger amounts of clay will exist partly between the chalk grains and effect the rigidity of the rock. A comprehensive programme of core analysis will investigate how differences in mineralogy affects chalk's texture and acoustic properties. The affect on velocity of fluid content, porosity and lithology will be investigated using velocity logs. The petrophysical properties of chalk that can be detected on seismic data will be investigated using AVO-analysis and seismic modelling. The problem of comparing micro-texture data with seismic data will be investigated using measurements on plugs and through seismic inversion. The project will be carried out as a multi-disciplinary Danish co-operation, with the participation of an international expert on rock physics
In impure chalk the elastic moduli are not only controlled by porosity, but also by cementation resulting in relatively large moduli and by admixtures of clay and fine silica which results in relatively small moduli. Based on a concept of framebuilding contra suspended solids, we model P-wave and S-wave moduli of dry and wet plug samples by an effective medium model using chemical, mineralogical and textural input. We have found that Gassmann's relations can be used to understand the variations of ultrasonic velocity with saturation in chalk samples. Our results suggest that fine-scale mixing is dominant at logging frequencies in chalks. We find that the velocity-porosity relation of the pure chalk plugs are in agreement with an empirical, modified upper Hashin-Shtrikman model for chalk. Due to higher porosities in the South Arne field we extend the range of the model to 45% porosity. Variations of the bulk modulus as a function of water saturation are predicted by the model combined with Gassmann's equations, and we find that the sonic log data for the Rigs-2 well represent chalk where the oil has been partly flushed by invasion of mud filtrate. A characteristic depth-wise pattern of the Poisson ratio is derived from the forward modeling of the acoustic properties of the virgin zone. This pattern agrees with the inverted seismic data. We have thus found AVO-inversion to provide direct evidence for presence of light oil in the high-porous chalk of the South Arne field. We have investigated changes in seismic response caused by changes in degree of compaction and fluid content in North Sea Chalk reservoirs away from a well bore by forward modelling. The three main modelling tools used for the modelling are 1) rock physics, 2) saturation modelling and 3) compaction/de-compaction modelling. Based on three assumptions we find that reflectivity is correlating with porosity, acoustic impedance is more susceptible to porosity variation than to hydrocarbon saturation, and the poisson ratio may be rather sensitive to hydrocarbon saturation
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Danmarks Tekniske Universitet (DTU) | ||
| Ødegaard og Danneskiold-Samsø S/S | ||
| HESS ENERGI ApS | ||
| Ørsted A/S |
Contact
Øster Voldgade 10
DK-1350 København K, Denmark
Japsen, Peter , 38142000, geus@geus.dk
Øvr. Partnere: Danmarks Tekniske Universitet; Ødegård A/S; Stanford Universitet (US)
