Jeremy Sothcott

Variation in dynamic elastic shear modulus of sandstone upon fluid saturation and substitution

Experimental acoustic measurements on sandstone rocks at both sonic and ultrasonic frequencies show that fluid saturation can cause a noticeable change in both the dynamic bulk and shear elastic moduli of sandstones. We observed that the change in dynamic shear modulus upon fluid saturation is highly dependent on the type of saturant, its viscosity, rock microstructure, and applied pressures. Frequency dispersion has some influence on dynamic elastic moduli too, but its effect is limited to the ultrasonic frequency ranges and above. We propose that viscous coupling, reduction in free surface energy, and, to a limited extent, frequency dispersion due to both local and global flow are the main mechanisms responsible for the change in dynamic shear elastic modulus upon fluid saturation and substitution, and we quantify influences.

Experimental measurements of seismic attenuation in microfractured sedimentary rock

Ultrasonic compressional- and shear-wave attenuation in water-saturated Carrara Marble increase with increasing crack density and decreasing effective pressure. Between 0.4 and 1.0 MHz, empirical linear relationships between 1/Q and crack density CD were found to be:CD = 1.96 + or - 0.63 X 1/Q,for compressional waves andCD = 6.7 + or - 1.5 X 1/Q,for shear waves.

The effects of pore‐fluid salinity on ultrasonic wave propagation in sandstones

Petrophysical interpretation of increasingly refined seismic data from subsurface formations requires a more fundamental understanding of seismic wave propagation in sedimentary rocks. We consider the variation of ultrasonic wave velocity and attenuation in sandstones with pore‐fluid salinity and show that wave propagation is modified in proportion to the clay content of the rock and the salinity of the pore fluid. Using an ultrasonic pulse reflection technique (590–890 kHz), we have measured the P-wave and S-wave velocities and attenuations of 15 saturated sandstones with variable effective pressure (5–60 MPa) and pore‐fluid salinity (0.0–3.4 M). In clean sandstones, there was close agreement between experimental and Biot model values of dVP/dM, but they diverged progressively in rocks containing more than 5% clay. However, this effect is small: VP changed by only 0.6% per molar change in salinity for a rock with a clay content of 29%. The variation of VS with brine molarity exhibited Biot behavior in so...

Seismic and Petrophysical Relationships from UK Continental Shelf Rocks at Elevated Pressures

Abstract There are few published seismic (P- and S-waves) properties for seafloor bedrocks. At low pressures (1 to 10 MPa), velocities and attenuations are determined mainly by open microcracks. At higher pressures, the microcracks close, and the velocities and attenuations depend primarily on the matrix porosity. We have investigated both the relationships between the acoustic, petrophysical, and geological properties of the rocks at 40 MPa pressure and the effect of microcracks on the acoustic properties at 10 MPa pressure. In this paper we discuss the former; the latter will be discussed separately. P- and S-wave velocity and attenuation measurements were carried out on a suite of seabed sedimentary and igneous rocks at effective pressures from 10 to 40 MPa at ultrasonic frequencies. The porosities and permeabilities of the rocks ranged from 0% to 32% and 0 to 110 mDarcy, respectively. Characterization of the rocks revealed that most of the sandstones have a substantial clay content (kaolinite, illite, and chlorite) and fractures. Most of the igneous rocks are chloritized. The seismic properties of the rocks are markedly lower than those of similar continental rocks because of the microporosity formed by the alteration of feldspars, micas, and mafic minerals to clays (e.g., chloritization of pyroxenes) and the corresponding reduction of the elastic moduli. The results of this study suggest that the values of velocities and quality factors used for ocean acoustic propagation models are lower than normally assumed.

Compressional And Shear-Wave Attenuation And Velocity Measurements In Carbonate Reservoir Rocks

Labora tory u l t rasonic a t tenua t ion and velocity measurements at 0.4MHhz-l.lMHz frequency on dry and water saturated ool i t i c l imes tone core samples under pressure have been ca r r i ed ou t to s tudy the re la t ionsh ip be tween se i smic and pe t rophys ica l p rope r t i e s . Th i s pape r i s p a r t i c u l a r l y c o n c e r n e d i n t h e r e l a t i o n s h i p between compressional and shear wave attenuation (Q,) and corresponding v e l o c i t i e s V a n d V . T h e r a t i o s a r e shown to be and 0.7 for water sa tu ra ted and dry rocks respec t ive ly . The r a t i o f o r w a t e r s a t u r a t e d r o c k s i s s i g n i f i c a n t l y g r e a t e r t h a n t h e cor responding ra t io fo r sands tones (Best et al . , 1993; Marks and McCann, 1992). T h i s i m p l i e s a d i f f e r e n t l o s s mechanism for the oo l i t i c l imes tones . The ratio between the compressional and shear wave ve loc i t i e s a r e 1 .76 and 1 .94 fo r wa te r sa tu ra ted and d ry rocks r e s p e c t i v e l y , cons i s ten t wi th Wi lkens e t a l . , 1984 . To explore the relat ionship between se i smic and pe t rophys ica l p roper t i e s o f rocks the s tudy of a t tenua t ion should be conducted under simulated in situ cond i t ions o f p ressure , t empera tu re and p o r e f l u i d s a t u r a t i o n .

The change in the ultrasonic response of a water saturated sandstone with exsolved gas

Earlier static measurements of natural gas-water mixtures by Dodson and Standing, (1945); Batzle and Wang, (1992) and others indicate that a tiny amount of dissolved gas is needed to significantly lower the bulk modulus of the fluid, thereby causing a significant drop in compressional wave velocity and acoustic impedance. Recent measurements by Han and Batzle (2002) indicate that the change in elastic modulus of fluids with the introduction of gas is a function of a number of parameters, most importantly fluid pressure, gas bubble pressure and temperature.

Relationships between rock heterogeneity, attenuation and velocity dispersion at ultrasonic and sonic frequencies

Summary Compressional and shear wave velocities and quality factors have been measured on three sandstones (Berea, Tune and York) and three limestones (Portland Base, Portland Top and NH3-11), under dry and 100 % fully saturated conditions, at ultrasonic and sonic frequencies, up to 60 MPa effective pressure. These six rock types have a wide range of different textures, porosities and permeabilities. It will be shown that rock heterogeneity is related to the pattern of attenuation measured across a broad frequency range.

Modeling shale seismic attributes at effective pressures up to 60 MPa as a function of pore pressure, using a genetic annealing algorithm (GAN)

1. Summary We propose a new method of calculating the shale seismic attributes from a restricted set of experimental data. A genetic annealing algorithm (GAN) is used to predict shale velocity values at different effective pressures and the results are verified against a full data set obtained experimentally. The model is tested for a chosen wave propagation direction at a known angle t o the foliation plane. Particular emphasis is placed on the effect of pore pressure on seismic velocity. Several fully saturated North Sea shale cores were tested under very high pore pressures (up to 40 MPa) and confining pressures up to 65 MPa. Conditions of overpressure known to occur at the in-situ depth of the samples were reproduced in a pressure vessel in the laboratory. The difference between the known laboratory data and the GAN model-derived data is shown to be within acceptable error limits.