Olav-Magnar Nes

In-situ stress dependence of wave velocities in reservoir and overburden rocks

Seismic waves propagate through a stressed earth. Sonic waves recorded by a borehole sonic logging tool propagate in the near-well environment—which is also stressed, albeit under different stress conditions than the untouched formation far away. Cores excavated from within the earth undergo stress release, and are then reloaded in the laboratory—but hardly ever to the complete, fully anisotropic in-situ state of stress and pore pressure. Based on this, we easily recognize that stress sensitivity affects our ability to compare and correlate geophysical and petrophysical measurements of wave velocities. On the other hand, it also provides an avenue to extract information about rock stresses. As an example, the presence of abnormally low velocities in seismic or log data is evidence of high pore pressure.

The Impact of Heterogeneity on the Anisotropic Strength of an Outcrop Shale

Properly accounting for the mechanical anisotropy of shales can be critical for successful drilling of high inclination wells, because shales are known to be weak along bedding planes. To optimize the drilling parameters in such cases, a sufficiently representative, anisotropic rock mechanical model is therefore required. This paper presents such a model developed to better match results from a dedicated, extensive set of uniaxial and triaxial compression tests performed on plugs of Mancos outcrop shale with different orientations relative to the bedding plane. Post failure inspection of the plugs shows that the failure planes are to some extent affected by the orientation of the applied stress relative to the bedding planes, indicating that the bedding planes may represent weak planes which tend to fail before intrinsic failure occurs, whenever the orientation of these planes is suitable. The simple “plane of weakness” model is commonly used to predict strength as function of orientation for such a rock. A comparison of this model to the experimental data shows, however, that the weak planes seem to have an impact on strength even outside the range of orientations where the model predicts such impact. An extension of this model allowing the weak planes to be heterogeneous in terms of patchy weakness was therefore developed. In this model, local shear sliding may occur prior to macroscopic failure, leading to enhanced local stresses and corresponding reduction in strength. The model is found to give better match with strength data at intermediate orientations. The model is also able to partly predict the qualitatively different variation of Young’s modulus with orientation for this data set.

Finite-difference Modeling of Viscoelastic Wave Propagation in Core Samples

Summary We present a numerical study of wave propagation in a core sample, using viscoelastic finite-difference modeling. The rheological model is a standard linear solid. P- and S-wave source transducers are simulated by single couple body force equivalents. Viscoelastic P- and S- wave velocities computed from simulated pulse transmission experiments using the traditional pulse arrival definitions are neither phase nor group velocity, but typically lie between these values. First break times give results closer to the group velocity and zero crossing and peak amplitude times closer to the phase velocity.

Shale Porosities As Determined By NMR

We present NMR measurements on three different shales from the North Sea. The motivation was to investigate whether one may employ NMR, which is a fast, non-destructive analytical tool, to measure the effective porosity of lowpermeable shales. The latter porosity is relevant for possible correlations to permeability. Presently, log-derived porosities (neutron, density) tend to yield closer to the total porosity which may differ significantly from in shales. Core porosities may be determined by means of timeconsuming, destructive drying-up techniques, restricting the access to the cores. Our measured NMR-porosities seem to agree with those attained by the loss of water. However, there may occasionally arise an extra NMRcontribution from the “dry” sample. This part may tentatively stem from bound or confined water within the shale. Even though more work remains to be done, NMR has proven its potential as a tool for petrophysical measurements on shale.

Anisotropic Kirchhoff Migration with Input from Wireline Logs

Summary Skew angle ultrasonic velocity measurements have been performed on plugs from a field in the North Sea. The plugs were subject to estimated effective in situ conditions during measurements. Using an inversion procedure on the data, the Thomsen parameters e and δ were estimated. The degree of anisotropy appeared to correlate with the amount of mica/illite, also enabling a continuous estimate of e and δ vs. depth from clay indicator wireline logs from the same wells where the cores stemmed from. This log-result was subsequently used as an input to migration of walk-away VSP data from the field, employing a multi-layer model with intrinsically anisotropic layers. Inclusion of this anisotropy gave a significantly better fit to the traveltime data, as well as improved imaging of the reflectors when compared to an isotropic migration.