Influence Of Fluid Chemistry Research Articles

Frictional characteristics of single and polycrystalline muscovite and influence of fluid chemistry

AbstractFrictional experiments were carried out on powder and single crystal specimens of muscovite. Friction coefficients for powder samples were found to be about twice as large as those for single crystals. We found that whereas in single crystal samples sliding occurs along the (001) plane, the situation is more complicated in powder samples. Experiments under wet conditions showed an ~30% reduction in friction coefficient for both powder and single crystal samples compared with dry conditions. The degree of the reduction depended on the fluid chemistry of NaCl or CsCl solutions, indicating that the presence of adsorbed water reduced the friction coefficients. Surface flatness of gauge particles and high affinity of the surface to water were crucial for realizing the effective lubrication due to adsorbed water.

Stress-Corrosion Cracking as an Alternative Time-Dependent Shale-Stability Model

Summary Wellbore stability in shale has been a crucial issue for drilling in all kinds of environments. The analysis of time-dependent wellbore stability in shales has largely concentrated on the influence of fluid chemistry and filtrate invasion into the formation to predict compressive failure using poroelasticity and continuum models. This paper presents another possible mechanism for time-dependent behavior—stress-corrosion cracking (subcritical crack growth). Using the discrete-element method (DEM) to simulate grainscale processes, we apply the concept of time-dependent cracking to hole enlargement for vertical wellbores. We use a published example from the North Sea to verify the stress-corrosion model and demonstrate the application to wellbore stability in shale. Laboratory results on rocks indicate a wide range of susceptibility to stress-corrosion cracking related to rock petrology and contact-fluid chemistry. Using laboratory calibrated rock properties, we run 2D, plane-strain simulations of vertical-wellbore stability in shale, where hole enlargement is tracked through time. As a result of stress-corrosion cracking, the numerical models show a time-dependent failure history, with an initial stable period of varying duration (influenced by mud weight, rock properties, and in-situ stress), followed by a brief period of combined shear and tensile failure, and ending with stabilization at an enlarged, elliptically shaped geometry. Time to failure increases with increasing mud weight. Enlarged-hole shape changes from elliptical to roughly circular with decreasing stress anisotropy. These behaviors simulated by the stress-corrosion model coincide with previously reported field experience. This new modeling approach for time-dependent wellbore failure can be readily constrained with straight forward fracture-mechanics tests on rock samples and has the potential to also be applied to time-dependent, intermittent sand or fines generation during production.

Permeability of illite‐bearing shale: 2. Influence of fluid chemistry on flow and functionally connected pores

Bedding‐parallel permeability of illite‐rich shale of the Wilcox formation has been investigated using distilled water and 1 M solutions of NaCl, KCl, and CaCl2 as pore fluids. Despite low modal concentrations of swelling clays, specimens expand upon fluid saturation and permeabilities depend on fluid composition. Permeabilities to flow of 1 M CaCl2 are 3–5 times greater than values measured for the other pore fluids, suggesting sensitivity to exchange of divalent cations for monovalent cations at clay mineral surfaces. Permeabilities of individual samples exhibit nonrecoverable changes with sequential changes in composition of incoming fluid. Permeabilities k at varying effective pressure Pe fit a cubic law k = k0 [1 − (Pe/P1)m]3, where m and P1 are independent of fluid composition, and k0 is greater for transport of 1 M CaCl2 than that for transport of the other pore fluids. Assuming that fluid conduits have crack‐like dimensions, the lack of sensitivity of m and P1 to fluid composition suggests that surface roughness and asperity stiffness of conduits are unaffected by cation exchange, while changes in k0 reflect changes in the clay‐fluid interfaces of the connected pore space.

Influence of fluid chemistry on shear-wave attenuation and velocity in sedimentary rocks : Dung Vo-Thanh, Geophysical Journal International, 121(3), 1995, pp 737–749

Influence of fluid chemistry on shear-wave attenuation and velocity in sedimentary rocks : Dung Vo-Thanh, Geophysical Journal International, 121(3), 1995, pp 737–749

Influence of fluid chemistry on shear-wave attenuation and velocity in sedimentary rocks

SUMMARY Measurements of shear-wave attenuation Qs−1 and velocity Vs were made in a sample of Massilon sandstone and a sample of Brauvilliers oolitic limestone in the kilohertz frequency range using a resonant bar technique. The major structural difference between these two rocks is the presence of grain contact microcracks (thin gaps) in the first and their absence in the second. Measurements were made as a function of fluid saturation Sf using a drying technique; the first and second samples were saturated successively with nine and four low-viscosity fluids, respectively, at room pressure and temperature. For water, the variations of Qs−1 and Vs are larger in Massilon sandstone than in Brauvilliers limestone. In Massilon sandstone, an attenuation peak at Sf≅ 20 per cent is observed for all fluids and the quality factor Qs and velocity Vs increase strongly at low saturations when Sf drops from 25 to 0 per cent. These variations depend on the nature of the pore fluid. They are relatively large with water, intermediate with methanol and small with other non-polar fluids (n-alcanes, aromatic hydrocarbons, carbon tetrachloride), The nature of the pore fluid has little effect at saturations greater than Sf= 25 per cent. The velocity slope is negative for water and methanol, and positive for non-polar fluids. The velocity slope is always negative in fluid-saturated Brauvilliers limestone. A model of grain contacts is used to interpret the data qualitatively. It includes two mechanisms: the gas adsorption on the solid-solid surface and the local fluid flow in the contact gaps. The first phenomenon lowers the elastic moduli while the second raises them. Calculations are qualitatively consistent with the observations and show that the saturation dependence of velocity in Massilon sandstone is principally dominated by the adsorption of volatiles at low saturations; at high saturations the velocity is affected conjointly by gas adsorption and fluid flow. The influence of fluid flow is negligible in Brauvillers limestone.