Mukul M. Sharma

Factors controlling the hydrodynamic detachment of particles from surfaces

Abstract The detachment of colloidal particles (5 to 40 μm) from surfaces has been studied. The influence of several variables such as flow rate, particle size, particle elasticity, ionic strength, pH, and gravity has been considered. Experiments were conducted to measure the critical hydrodynamic force required to detach particles from a flat glass substrate. It was demonstrated by conducting centrifuge experiments that the mechanism of detachment is rolling rather than sliding or lifting. The influence of particle size and elasticity as well as the surface chemical interactions between the particle and the substrate was included in a model that adequately explains the observed behavior. A consistent method is presented to compute the deformation of the particle and the adhesion and lift forces acting on it at equilibrium. These quantities allow us to compute the critical hydrodynamic force required for particle release. A comparison of the computed and experimentally measured forces shows good agreement, indicating that the essential physics of the problem has been captured in the model.

The effect of colloidal particles on fluid-fluid interfacial properties and emulsion stability

Abstract This paper reviews the relevant work done on the effects of colloidal particles on the rheological properties of fluid-fluid interfaces and on emulsion stability. Results are presented to show that colloidal particles stabilize emulsions primarily by providing steric hindrance to drop-drop coalescence and by modifying the rheological properties of the interfacial region. The effectiveness of colloidal particles in stabilizing emulsions depends largely on the formation of a sufficiently ‘dense layer’ of particles at the oil-water interface. The rheological properties of the interfacial region also change as the concentration of particles at the interface increases and complete surface coverage is achieved. It is shown that at sufficiently high concentrations of particles, colloid-laden interfaces exhibit viscoelastic behavior. Viscoelastic interfaces enhance emulsion stability by increasing the magnitude of steric hindrance and by retarding the rate of liquid drainage between coalescing emulsion droplets.

A generalized Maxwell-Wagner theory for membrane polarization in shaly sands

The effects of charged clay platelets on the frequency dependent electrical properties of shaly materials are analyzed using simplified models for the membrane polarization around charged spheres immersed in electrolytic solutions, under a thin double layer approximation. The polarization is defined through two possible mechanisms: (1) a surface conductivity related with a modified Stern double layer model (S‐model) according to Schurr‐Schwarz theory; (2) a coupled electro‐diffusional mechanism occurring in a Guoy‐Chapman double layer using Fixman’s approach (D‐model). By comparing the electric potential in such microscopic models with the external potentials derived for the equivalent homogeneous sphere using a Maxwell‐Wagner approach, we obtain the total current conductivity functions for these two models. The theory, therefore, provides explicit expressions relating the total conductivity functions to the model parameters. The behavior of the S‐model is described by a complex conductivity exhibiting a ...

A grain conductivity approach to shaly sandstones

The effects of clay conduction on the electrical behavior of shaly sandstones at low frequencies are analyzed by considering self-similar mixtures of conductive grains, as representative of these porous rocks. An essential step in this model is the representation of the surface conductivities of the clay platelets by an equivalent volume conductivity. Based on their pattern of occurrence, the clays are treated either as a continuous coating over the sand grains or as individual detrital grains mixed with a nonconductive mineral matrix. In the first case, the solids are modeled as layered conductive particles, taken as spheres to a first approximation. In the second case, we generalize from the results of a three-component self-similar mixture of spherical particles. For linear approximations, the models are described by expressions quite similar to the frequently used Waxman-Smits and dual-water equations, but dependent upon the clay type, concentration, and its distribution in the rock. The models are written in terms of geometric parameters readily available from well-log measurements. The derived analytical expressions are successfully applied to describe a set of available experimental core and geophysical log data. The results clearly demonstrate the applicability of the equations to well-log interpretation, as a practical procedure for computing formation factors and shaliness of argillaceous sandstones.

The influence of clay content, salinity, stress, and wettability on the dielectric properties of brine-saturated rocks; 10 Hz to 10 MHz

Complex impedance measurements have been performed on 14 shaly sand samples, Berea sandstone, and Ottawa sand-bentonite packs in a frequency range of 10 Hz to 10 MHz, using both the two- and four-electrode techniques. Measurements have been conducted at an effective radial stress varying from ambient pressure to 4000 psi for brine-saturated oil-wet and water-wet samples.The dielectric permittivity is found to correlate with the clay volume fraction, the cation exchange capacity, and electrochemical potential of the rock samples and to depend strongly on the salinity of the brine used. Stress and wettability are shown to have a small influence on the dielectric constant of fully brine-saturated rocks. A lower critical frequency is found to characterize the geometry of the pore space. Empirical correlations between the dielectric constant, frequency, permeability, cation exchange capacity, and porosity are presented for the shaly sands used in this study. These correlations provide a means of estimating important petrophysical parameters such as the permeability and the clay content from a nondestructive complex impedance sweep of shaly sands fully saturated with brine.

A study of wellbore stability in shales including poroelastic, chemical, and thermal effects

Abstract This paper presents the development of a model for determining wellbore stability for oil and gas drilling operations. The effects of mechanical forces and poroelasticity on shale behavior are included, as well as chemical and thermal effects. Chemical effects are caused by the imbalance between the water activity in the drilling mud and the shale water activity. The magnitude of this contribution depends on the effectiveness of the mud/shale system to perform as a semipermeable membrane. Experimental results show that osmotic pressures develop inside shales when they are exposed to different drilling fluids. This osmotic pressure is treated as an equivalent hydraulic potential, and is then added to the hydraulic wellbore and pore pressure as time progresses. Thermal diffusion inside the drilled formation induces additional pore pressure and rock stress changes and consequently affects shale stability. Thermal effects are important because thermal diffusion into shale formations occurs more quickly than hydraulic diffusion and thereby dominates pore pressure changes during early time. Rock temperature and pore pressure are coupled for most porous media studies; however, we have found that they can be partially decoupled for shale formations by assuming that convective heat transfer is negligible. The partially decoupled temperature and pore pressure effects can therefore be solved analytically under appropriate initial and boundary conditions. Experimental data for shale strength alteration, which occurs when shales are exposed to different fluids, are also included for the determination of cohesion strength decay. Pore pressure, collapse stress, and critical mud weights are variables investigated for determining poroelastic, chemical, and thermal effects on shale stability. The most important factors, which affect wellbore stability, are clearly identified.

Reversible and irreversible surface charge modification of bacteria for facilitating transport through porous media

Abstract Reversible and irreversible surface charge modification of Bacillus subtilis (a gram-positive aerobe), Pseudomonas fluorescens (a gram-negative species) and polymeric microspheres was attempted using common chemicals. This study was undertaken to see if such chemicals could be used to alter significantly the transport of various bacterial species through porous media. Significant surface charge alterations were observed in the irreversible treatments as shown by zeta potential measurements. The changes were shown to be partly reversible as evidenced by much larger shifts in zeta potential in the presence of the chemicals. Sodium pyrophosphate was found to alter the surface charge most significantly. All three species showed similar trends. Experiments on the transport of these species through sandpacks showed that their transport was significantly enhanced by the presence of the chemicals. The excellent correlation between high surface charge and transportability clearly suggests that electrostatic interactions between bacteria and sand grains are a dominant factor in their retention. Other bacterial retention mechanisms such as polymer adhesion seem to be relatively unimportant, at least for the species studied here. The various mechanisms responsible for the improved transportability and the surface charge modification are discussed. The applications of the study to filtration and microbial-enhanced oil recovery are pointed out.

Modeling Relative Permeability Effects in Gas-Condensate Reservoirs With a New Trapping Model

Many gas-condensate wells show a significant decrease in productivity once the pressure falls below the dew point pressure. A widely accepted cause of this decrease in productivity index is the decrease in the gas relative permeability due to a buildup of condensate in the near wellbore region. Predictions of well inflow performance require accurate models for the gas relative permeability. Since these relative permeabilities depend on fluid composition and pressure as well as on condensate and water saturations, a model is essential for both interpretation of laboratory data and for predictive field simulations as illustrated in this article.

Nonlinear viscoelastic behavior of sedimentary rocks, Part I: Effect of frequency and strain amplitude

Sedimentary rocks display nonlinear elastic behavior. This nonlinearity is a strong function of frequency, strain amplitude, and the properties of the saturating fluid. Experimental observations and potential mechanisms that cause these nonlinearities are presented in this and a companion paper. Young’s moduli and Poisson’s ratios obtained from ultrasonic laboratory measurements (50 kHz, 100 kHz, 180kHz and 1 MHz), low‐frequency measurements (1–2000 Hz) and static measurements (0.001–0.05 Hz) show significant differences under identical stress conditions. A comparison of the laboratory‐measured quantities with log‐derived moduli measured at 20 kHz indicates that Eultrasonic>Elog>Elowfreq>Estatic. This shows clearly that a wide variety of sandstones demonstrate frequency‐dependent elastic behavior (viscoelastic behavior) over a range of frequencies. Differences between static (low‐frequency, high‐strain amplitude) velocities and ultrasonic velocities can be explained partially by differences in frequency a...

An experimental investigation of factors influencing compressional‐ and shear‐wave velocities and attenuations in tight gas sandstones

Results are presented for compressional and shear velocities and attenuations in fully brine‐saturated tight gas cores with porosities from 3 to 11.9 percent and clay contents from 1 to 38 percent. The influence of porosity, clay content, frequency, and stress on velocities and attenuations were examined using the amplitude spectra of P‐ and S‐waves in the frequency domain. Attenuations of samples were obtained using the spectral ratio method. For a few selected samples the attenuations were also measured using the length correlation method and these results were compared with the spectral ratio results. In tight gas sandstones, the attenuations obtained were 2 to 5 times greater than the attenuation obtained for Berea sandstone. In general, the presence of clay softens the rock grain contacts causing smaller values of compressional (VP) and shear (VS) velocities as the clay content increases. However, the VP/VS ratio was found to increase with clay content. Compressional‐and shear‐wave amplitude spectra ...