SEISMIC PROPERTIES INVESTIGATION OF FLUID FILLED CARBONATE ROCKS

Abstract

this dissertation work, I have discussed the effects of heterogeneity, fluid substitution, differential pressures and frequency on the seismic properties of carbonates. In this study, six carbonate core samples having different fabric heterogeneity were selected for the elastic property measurement in the seismic frequency range (2 - 2000 Hz) and at the ultrasonic frequency 0.8 MHz. Measurements were taken dry (humidified) and in substituted - with polar (brine & CO2) and non-polar (butane) fluids - conditions. The samples showed the usual trend of decreasing modulus with increasing porosity. It is shown in the thesis that when a heterogeneous formation is saturated with brine, it exhibits more modulus dispersion, compared to the case when it is saturated with butane. It is also shown that the bulk modulus of the carbonate rocks is nearly the same when they are saturated with butane as compared to the case when they are measured dry at all frequencies, whereas all samples consistently showed higher bulk modulus values when saturated with brine. Also, the shear modulus decreases significantly when the heterogeneous rocks are saturated with brine & CO2 (about 10% for brine saturation and about 70% for CO2 saturation) whereas for the homogenous samples, the decrease in shear modulus was comparatively less. I used the Gassmann’s fluid substitution equation (Gassmann, 1951) to predict the elastic properties of rocks when they are saturated with various fluids. I evaluated the Gassmann’s rock physics model and the underlying assumptions for its application to the carbonate rocks and found that in can be applied to homogenous rocks having uniform saturation. I also checked the applicability of different velocity models on carbonates and found that both Wyllie’s and Raymer’s equation overestimated the velocity values even more than the Gassmann’s model. This work is an effort to infer some valuable insights regarding the fluid substitution models when they are applied to carbonate rocks as not many studies have been done in this field. This type of study can help in understanding the carbonate rocks undergoing enhanced oil recovery.