Effect of fluid substitution on ultrasonic velocities in chalk plugs, South Arne field, North Sea

Ultrasonic P- and S-velocities were measured for 34 chalk samples from the Ekofisk and Tor formations in the South Arne field, North Sea. V _P and V _S correlate strongly with porosity for both dry and saturated samples. Outlying values are found for three samples of the Ekofisk formation that regained a high water content after drying, and this may be indicative of a high clay content. The mean change in V _P and V _S from saturated to dry samples is a drop of 0.15 km/s and an increase of 0.21 km/s, respectively, for 31 of the samples. These changes correspond to 62% of the drop in V _P predicted by Gassmann's relations, whereas the increase in V _S is 154% of that predicted by Gassmann. The main reason for these deviations from Gassmann-theory is that the shear modulus in average is 0.5 GPa higher for the dry plugs than for the saturated plugs whereas Gassmann-theory predicts the shear modulus to be unchanged by fluid content. However, 94% of the change in the bulk modulus is in average predicted by Gassmann's relations. The shear modulus is found to be constant when measured for two samples at saturated and partly saturated conditions. The factors that cause the increased moduli of the dry samples is not understood, but further laboratory investigations are under way. The acoustic properties of the partly saturated chalk can, however, be estimated by applying Gassmann's relations to data for the saturated samples.