ELASTICITY OF HIGH-POROSITY SANDSTONES - THEORY FOR 2 NORTH-SEA DATA SETS

We have analyzed two laboratory data sets obtained on high-porosity ro ck samples from the North Sea. The velocities observed are unusual in that they seem to disagree with some simple models based on porosity. On the other hand, the rocks are unusually poorly-cemented (for labora tory studies, at least), and we investigate the likelihood that this i s the cause of the disagreement. One set of rocks, from the Oseberg Fi eld, is made of slightly cemented quartz sands. We find that we can mo del their dry-rock velocities using a cementation theory where the gra ins mechanically interact through cement at the grain boundaries. This model does not allow for pressure dependence. The other set of rocks, from the Troll Field, is almost completely uncemented. The grains are held together by the applied confining pressure. In this case, a lowe r bound for the velocities can be found by using the Hertz-Mindlin con tact theory (interaction of uncemented spheres) to predict velocities at a critical porosity, combined with the modified Hashin-Strikman low er bound for other porosities. This model, which allows for pressure-d ependence, also predicts fairly large Poisson's ratios for saturated r ocks, such as those observed in the measurements. The usefulness of th ese theories may be in estimating the nature of cement in rocks from m easurements such as sonic logs. The theories could help indicate sand strength in poorly consolidated formations and predict the likelihood of sand production. Both theoretical methods have analytical expressio ns and are ready for practical use.