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Collaborating Authors
Results
ABSTRACT We have developed a fiber-optic strainmeter to estimate velocities and attenuation at seismic frequencies. The two main advantages of the new system compared to strain gage techniques are the higher sensitivity to deformations (moduli) and phase lags (attenuation), and that estimates are representative of bulk values. While stress-strain measurements using strain gages or ultrasonic wave propagation sample only part of the core sample, the fiber-optic strainmeter would analyze the rock sample response to an applied stress as a whole. Still, the system is under development and the first experiment on a Plexiglas sample showed that attenuation estimates are more robust than deformation estimates due to difficulties with light intensity. Initial rock measurements are made on a dolomite sample.
ABSTRACT A set of carbonate plugs of different porosity, permeability, mineralogy and texture are measured at seismic and ultrasonic frequencies in the laboratory. For this set we show that within the uncertainty of the measured shear and bulk moduli (8%in average), Gassmann correctly predicts the observed values for brine and butane saturation. This is observed for both seismic and ultrasonic frequencies which involve independent experimental setups and procedures. Moduli dispersion over the range of acquired frequencies exists for mostly for brine saturation, and is less for dry samples. This effect should also be considered when studying if the predicted data matches the observations. However, the current data set is not statistically representative, and the validity of Gassmann''s theory on carbonate samples should still be applied with care. More samples are being analyzed to have a better representation of carbonate rock properties.
- Geology > Geological Subdiscipline (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (0.71)
ABSTRACT The conventional fluid substitution scheme using Gassmann theory is based on a set of assumptions. One of them states that the porosity is connected (as “seen” by the seismic wave) while another states that the pore fluid does not interact with the solid rock. Due to recent published observations and the presence of a generally isolated pore structure in carbonate rocks, it is questionable whether Gassmann is appropriate for carbonate rocks. Using both low and high frequency measurements (respectively modulus and velocity) saturated with varying fluid types, we have modeled the measured elastic properties using Gassmann and a set of rock models based on sound scattering principles. A central piece of information for the latter rock model set is the pore structure. In order to quantify the different pore-structures for our rock samples we have applied two different pre-defined pore forms. Actual pore form parameters were set by automatic characterization of SEM images of thin slices from the core samples. The modeled elastic properties show a strong dependency to the pre-defined pore form type and applied rock model. However, for several core samples the modeled results indicate a stronger fluid effect than predicted by Gassmann approach and compares favorably to the low frequency velocity measurements.
- Europe > Denmark > North Sea (0.29)
- North America > United States (0.28)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.88)