One of the biggest challenges in estimating the elastic, transport and storage properties of shales has been a lack of understanding of their complete pore structure. The shale matrix is predominantly composed of micropores (pores less than 2 nm diameter) and mesopores (pores with 2–50 nm diameter). These small pores in the shale matrix are mainly associated with clay minerals and organic matter and comprehending the controls of these clays and organic matter on the pore‐size distribution is critical to understand the shale pore network. Historically, mercury intrusion techniques are used for pore‐size analysis of conventional reservoirs. However, for unconventional shale reservoirs, very high pressures (> 414 MPa (60 000 psi)) would be required for mercury to access the full pore structure, which has potential pitfalls. Current instrumental limitations do not allow reliable measurement of significant portions of the total pore volume in shales. Nitrogen gas‐adsorption techniques can be used to characterize materials dominated by micro‐ and mesopores (2–50 nm). A limitation of this technique is that it fails to measure large pores (diameter >200 nm). We use a nitrogen gas‐adsorption technique to study the micro‐ and mesopores in shales and clays and compare the results from conventional mercury porosimetry techniques. Our results on pure clay minerals and natural shales show that (i) they have a multiscale pore structure at different dimensions (ii) fine mesopores, with a characteristic 3 nm pore size obtained with N2 gas‐adsorption are associated with an illite‐smectite group of clays but not with kaolinite; (iii) compaction results in a decrease of pore volume and a reduction of pore size in the ‘inter‐aggregate’ macropores of the illite‐smectite clays while the fine ‘intra‐tachoid’ mesopores are shielded from compaction; (iv) for natural shales, mineralogy controls the pore‐size distributions for shales and the presence of micropores and fine mesopores in natural shales can be correlated with the dominance of the illite‐smectite type of clays in the rock. Our assessment of incompressible 3 nm sized pores associated with illite‐smectite clays provides an important building block for their mineral modulus.

Abstract.We offer an effective medium model for the elastic moduli of high-porosity ocean-bottom sediments. The elastic constants of the dry-sediment frame depend on porosity, elastic moduli of the solid phase, and effective pressure. The model connects two end points in the elastic-modulusporosity plane: the Hertz-Mindlin modulus of a dense elastic sphere pack at critical porosity; and zero at 100% porosity.The elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. Unlike the suspension model, our model assigns non-zero elastic constants to the dry-sediment frame and can predict the shearwave velocity. Unlike various modifications of the traveltime-average equation, it is first-principle-based and contains only physical parameters. We justify this model by matching sonic data in shallow marine sediments and in an ODP well.

SUMMARY The presence of clay minerals can alter the elastic behaviour of rocks significantly. Although clay minerals are common in sedimentary formations and seismic measurements are our main tools for studying subsurface lithologies, measurements of elastic properties of clay minerals have proven difficult. Theoretical values for the bulk modulus of clay are reported between 20 and 50 GPa. The only published experimental measurement of Young's modulus in a clay mineral using atomic force acoustic microscopy (AFAM) gave a much lower value of 6.2 GPa. This study has concentrated on using independent experimental methods to measure the elastic moduli of clay minerals as functions of pressure and saturation. First, ultrasonic P‐ and S‐wave velocities were measured as functions of hydrostatic pressure in cold‐pressed clay aggregates with porosity and grain density ranging from 4 to 43 per cent and 2.13 to 2.83 g cm−3, respectively. In the second experiment, P‐ and S‐wave velocities in clay powders were measured under uniaxial stresses compaction. In the third experiment, P‐wave velocity and attenuation in a kaolinite–water suspension with clay concentrations between 0 and 60 per cent were measured at ambient conditions. Our elastic moduli measurements of kaolinite, montmorillonite and smectite are consistent for all experiments and with reported AFAM measurements on a nanometre scale. The bulk modulus values of the solid clay phase (Ks) lie between 6 and 12 GPa and shear (μs) modulus values vary between 4 and 6 GPa. A comparison is made between the accuracy of velocity prediction in shaley sandstones and clay–water and clay–sand mixtures using the values measured in this study and those from theoretical models. Using Ks= 12 GPa and μs= 6 GPa from this study, the models give a much better prediction both of experimental velocity reduction due to increase in clay content in sandstones and velocity measurements in a kaolinite–water suspension.

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Knowledge of the pressure dependences of seismic velocities in unconsolidated sands is necessary for the remote prediction of effective pressures and for the projection of velocities to unsampled locations within shallow sand layers. We have measured the compressional- and shear-wave velocities and bulk, shear, and P-wave moduli at pressures from [Formula: see text] in a series of unconsolidated granular samples including dry and water-saturated natural sands and dry synthetic sand and glass-bead samples. The shear-wave velocities in these samples demonstrate an average pressure dependence approximately proportional to the fourth root of the effective pressure [Formula: see text], as commonly observed at lower pressures. For the compressional-wave velocities, theexponent in the pressure dependence of individual dry samples is consistently less than the exponent for the shear-wave velocity of the same sample, averaging 0.23 for the dry sands and 0.20 for the glass-bead samples. These pressure dependences are generally consistent over the entire pressure range measured. A comparison of the empirical results to theoretical predictions based on Hertz-Mindlin effective-medium models demonstrates that the theoretical models vastly overpredict the shear moduli of the dry granular frame unless the contacts are assumed to have no tangential stiffness. The models also predict a lower pressure exponent for the moduli and velocities [Formula: see text] than is generally observed in the data. We attribute this discrepancy in part to the inability of the models to account for decreases in the amount of slip or grain rotation occurring at grain-to-grain contacts with increasing pressure.

Compressional‐wave velocity [Formula: see text] and quality factor [Formula: see text] have been measured in Berea and Michigan sandstones as a function of confining pressure [Formula: see text] to 55 MPa and pore pressure [Formula: see text] to 35 MPa. [Formula: see text] values are lower in the poorly cemented, finer grained, and microcracked Berea sandstone. [Formula: see text] values are affected to a lesser extent by the microstructural differences. A directional dependence of [Formula: see text] is observed in both sandstones and can be related to pore alignment with pressure. [Formula: see text] anisotropy is observed only in Berea sandstone. [Formula: see text] and [Formula: see text] increase with both increasing differential pressure [Formula: see text] and increasing [Formula: see text]. The effect of [Formula: see text] on [Formula: see text] is greater at higher [Formula: see text]. The results suggest that the effective stress coefficient, a measure of pore space deformation, for both [Formula: see text] and [Formula: see text] is less than 1 and decreases with increasing [Formula: see text].

Shallow water flows and over‐pressured zones are a major hazard in deepwater drilling projects. Their detection prior to drilling would save millions of dollars in lost drilling costs. I have investigated the sensitivity of seismic methods for this purpose. Using P‐wave information alone can be ambiguous, because a drop in P‐wave velocity (Vp) can be caused both by overpressure and by presence of gas. The ratio of P‐wave velocity to S‐wave velocity (Vp/Vs), which increases with overpressure and decreases with gas saturation, can help differentiate between the two cases. Since P‐wave velocity in a suspension is slightly below that of the suspending fluid and Vs=0, Vp/Vs and Poisson's ratio must increase exponentially as a load‐bearing sediment approaches a state of suspension. On the other hand, presence of gas will also decrease Vp but Vs will remain unaffected and Vp/Vs will decrease. Analyses of ultrasonic P‐ and S‐wave velocities in sands show that the Vp/Vs ratio, especially at low effective pressures, decreases rapidly with pressure. At very low pressures, Vp/Vs values can be as large as 100 and higher. Above pressures greater than 2 MPa, it plateaus and does not change much with pressure. There is significant change in signal amplitudes and frequency of shear waves below 1 MPa. The current ultrasonic data shows that Vp/Vs values can be invaluable indicators of low differential pressures.

We documented the porosity, permeability, pore geometry, pore type, textural anisotropy, and capillary pressure of carbonate rock samples collected along basin-bounding normal faults in central Italy. The study samples consist of one Mesozoic platform carbonate host rock with low porosity and permeability, four fractured host rocks of the damage zones, and four fault rocks of the fault cores. The four fractured samples have high secondary porosity, due to elongated, connected, soft pores that provide fluid pathways in the damage zone. We modeled this zone as an elastic cracked medium, and used the Budiansky-O'Connell correlation to compute its permeability from the measured elastic moduli. This correlation can be applied only to fractured rocks with large secondary porosity and high-aspect ratio pores. The four fault rock samples are made up of survivor clasts embedded in fine carbonate matrices and cements with sub-spherical, stiff pores. The low porosity and permeability of these rocks, and their high values of capillary pressure, are consistent with the fault core sealing as much as 77 and 140 m of gas and oil columns, respectively. We modeled the fault core as a granular medium, and used the Kozeny-Carmen correlation, assigning the value of 5 to the Kozeny constant, to compute its permeability from the measured porosities and pore radii. The permeability structure of the normal faults is composed of two main units with unique hydraulic characteristics: a granular fault core that acts as a seal to cross-fault fluid flow, and an elastic cracked damage zone that surrounds the core and forms a conduit for fluid flow. Transient pathways for alongfault fluid flow may form in the fault core during seismic faulting due to the formation of opening-mode fractures within the cemented fault rocks.