Manika Prasad

Specific surface area and pore-size distribution in clays and shales

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 illitesmectite 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.

Elasticity of marine sediments: Rock physics modeling

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-modulus-porosity 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 Gassmanns equation. Unlike the suspension model, our model assigns non-zero elastic constants to the dry-sediment frame and can predict the shear-wave velocity. Unlike various modifications of the travel-time-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.

Effects of pore and differential pressure on compressional wave velocity and quality factor in Berea and Michigan sandstones

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

Acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection

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 Poissons 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...

Velocity-permeability relations within hydraulic units

Relationships between seismic velocity and permeability have been difficult to establish. I show that by grouping and sorting rocks into hydraulic units, we can establish relationships between velocity and permeability. The hydraulic units are calculated from measured porosity and permeability values. Correlation between velocity and permeability is significant within each hydraulic unit (the correlation coefficient, R2, lies in the range 0.65–0.87). This correlation is an extension of the match between porosity and permeability within a hydraulic unit. I show how the compaction and cementation history of a sediment can have effects on its physical properties such as porosity and permeability and on its seismic properties. The measured velocity data are further approximated with the Biot model. The velocity‐permeability relation and modeling results are demonstrated for a large data set of laboratory measurements. The good match between calculated and measured data demonstrates that this relation can be u...

Attenuation mechanisms in sands: Laboratory versus theoretical (Biot) data

The velocity and attenuation of compressional(Q-1P, VP respectively) and shear waves (Q-1S, VS, respectively), determined with the Pulse Transmission technique at a frequency of about 100 kHz, are compared with the grain size, shape, porosity, density, and static frame compressibility of dry and water‐saturated sands. Except for VS, all the quantities VP, Q-1P and Q-1S are dependent on grain size and are higher in coarser grains than in finer grains. Q-1S decreases significantly with increasing differential pressure in coarse‐grained sediments, but the same sediments show an anomalous increase with differential pressure in Q-1P at low pressures. We have also modeled the VP, VS, Q-1P and Q-1S of these samples to understand the mechanisms governing the observed changes. The Contact Radius model with surface force effects predicts both VP and VS to be dependent on grain size. Frictional losses in unconsolidated coarse‐grained sands must also be considered at small strains (10-7). Velocity and losses measured...

Pressure and porosity influences on VP−VS ratio in unconsolidated sands

Elevated pore pressures, commonly encountered in the shallow, unconsolidated section of the sedimentary column, present a significant hazard during the drilling and completion of offshore wells. The porosity of the ocean-floor sediments is high and a cover of low-permeability clay can prevent the underlying sediments from draining, even at very shallow depths below the seafloor. As further deposition loads the sediment, the entrapped fluids impede normal compaction by becoming pressurized. The lowered effective stress that results from the higher pore fluid pressure produces a proportional drop in the strength of non-cohesive sediments, resulting in very weak shallow sands. This weakness can result in washouts or fracturing in the shallow strata, potentially leading to the loss of the well and of neighboring wells.

Low-frequency complex conductivity of sandy and clayey materials

Low-frequency polarization of sands and sandstones seems to be dominated by the polarization of the Stern layer, the inner part of the electrical double layer coating the surface of the silica grains and clay particles. We investigate a simple model of Stern layer polarization combined with a simple complexation model of the surface of the grains immersed in a 1:1 electrolyte like NaCl. In isothermal conditions, the resulting model can be used to predict the complex conductivity of clayey materials as a function of the porosity, the cation exchange capacity of the clay fraction (alternatively the specific surface area of the material), and the salinity of the pore water. A new set of experimental data is presented. This dataset comprises low-frequency (1 mHz-45 kHz) complex conductivity measurements of saprolites and sandstones that are well characterized in terms of their petrophysical properties (porosity, permeability, specific surface area or CEC, and pore size). This dataset, together with incorporating additional data from the literature, is used to test the Stern layer polarization model. We find an excellent agreement between the predictions of this model and this experimental dataset indicating that the new model can be used to predict the complex conductivity of natural clayey materials and clay-free silica sands.

Seismic velocities of unconsolidated sands: Part 1 — Pressure trends from 0.1 to 20 MPa

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 Pwave moduli at pressures from 0.1 to 20 MPa 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 VSp 1/4 , as commonly observed at lower pressures. For the compressional-wave velocities, the exponent 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 velocitiesVp1/6 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.

Effective pressure or what is the effect of pressure

Using time-lapse seismics as a reservoir-monitoring tool, geophysics can help distinguish different reservoir production scenarios. For example, Eiken et al. (2000) successfully detected fluid-saturation changes after CO2 injection using time-lapse seismics at Sleipner Field. Over the cycle of a reservoir life, oil saturation usually decreases, reservoir pressure declines, and gas breakout may occur. These changes cause rock property changes that are detectible in time-lapse seismics. Therefore, it is important to understand the effects of pressure and saturation changes on rock properties. While the effects of saturation changes are often well described by Gassmann (1951), Brown and Korringa (1975), and Mavko (1975), the effects of pressure changes are less understood. Here we focus on understanding the effects of fluid pressure on velocities.