Assessment of the rock fracturing degree by means of electrical resistivity measurements (original) (raw)
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Effect of stress on the hydraulic conductivity of rock pores
Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 2000
We have made a detailed study of the effect of cross-sectional shape on the hydraulic conductance of rock pores. We consider laminar flow through a single tube with an irregular cross-section; constriction effects, and interconnectedness of pores, will be studied in a future work.
Energies
This work discusses the results of a study of the influence of rates of change of confining pressure on the result of a drained compressibility tests intended to determine the modulus of compressibility of a rock skeleton Ks. A series of cyclical compressibility tests was performed on samples of sandstone soaked in kerosene, for various rates of compression and decompression of the pressure liquid filling the cell and the pore volume of the sample. The studies showed that the deformability of the tested sample was directly proportional to the rate of change of the confining pressure. As a consequence, the value of the Ks modulus and Biot coefficient α decreased with increasing sample load rate. This phenomenon should be attributed primarily to equilibration of the liquid pressure inside the high-pressure cell with the liquid pressure in the sample pore space, caused by filtration of the pore liquid. These phenomena prove that the filtration process impacts the values of the modulus ...
Effective stresses in porous rocks saturated with viscous fluid at rest
Int. J. Rock Mech. & Geomech. , 2016
The most important problems of discontinuous rocks (e.g.: knowledge of the breaking state limits for brittle rocks,-of yield strength for plastic rocks or-of triggering the hydrodynamic suffusion phenomenon in non-cohesive and poorly cohesive rocks), require accurate assessments of the effective stress. This piece of work contains computational models of the effective stresses developed by the author for discontinuous rocks-in general, and for porous rocks, saturated with fluids at rest. It is also presented an evaluation method for the effective area of porous rocks. It is only discontinuous rocks that will be analyzed, such as rocks saturated with fluids at rest, porous and crackless,-cohesive or non-cohesive. The solid rock matrix is homogenous and isotropic, and the pores are uniformly distributed as shape, size and frequency (number of pores per gross rock unit volume as well as number per surface unit in any cutting plane), therefore the rock-at macroscopic size, is also equivalent to a homogenous and isotropic rock. Generally, the rocks of the Earth's crust are solid and deformable materials. Many of them are discontinuous rocks because they include discontinuities in the form of pores and /or fissures-in their internal structure. The pores are discontinuities of the mass and density of the solid phase. The fissures or cracks are discontinuities of the structural cohesion also known as permanent or irreversible cohesion. The discontinuous rocks are saturated with fluids. These can be: viscous or non-viscous fluids,-pressurized or unpressurized,-at rest (stationary) or moving, which means that they move along the solid phase of the rock. The gross rock volume, is also known as apparent volume or total volume; it is delimited by a continuous and closed surface, called external border. For a discontinuous rock, the surfaces which settle the boundaries of pores and fissures, are the ones forming the internal border. Therefore, for discontinuous rocks, the solid phase (the matrix) has its exterior defined by the external border and-the inside, by the internal border. The forces that act on the matrix of a discontinuous rock are divided in two distinct categories: external forces (the ones that act on the external border) and internal forces-which act on the internal border. Both of the categories, create sectional efforts in the solid matrix that can be quantified by the parameters, known as stresses. Initially, the concept of mechanical stress was introduced and used in the hypothesis that claims that all solid and deformable materials are continuous, therefore the eventual discontinuities, such as the internal surface and the internal forces, will not be taken into consideration. In such situations, the value of the stress only quantifies the internal contact forces-by means of their resultant and the mass dimensions of the body (the total area corresponding to the section plane), meaning: stress= (the resultant of external contact forces) / (total area of the section).
Geophysical Research Letters, 2011
In order to investigate the role of drainage conditions in deformation and fracture behaviors of porous rocks, the authors carried out a series of rock fracture tests under triaxial compression in the laboratory. The detailed spacetime distribution of acoustic emission due to microcracking was used to examine pre-failure damage and failure behavior in Berea sandstone, which has a porosity of 20% and a permeability of 100 mD. The pore pressures or flow rates at the ends of the test sample were precisely controlled to simulate different drainage conditions. Experimental results indicate that drainage conditions play a governing role in deformation and fracture. The well-established dilatancyhardening effect can be greatly suppressed by dilatancydriven fluid flowing under good drainage conditions. Fast diffusion of pore pressure leads to a significant reduction in rock strength and stabilization of the dynamic rupture process. Furthermore, good drainage conditions have the potential to enlarge the nucleation dimension and duration, thereby improving the predictability of the final catastrophic failure. In addition, compaction bands, which were observed in porous rocks under higher confining pressure, were also observed at low confining pressure (corresponding to a depth of 1 km) in undrained tests. These results are particularly important for research fields in which fluid migration or pore pressure diffusion is expected to play a role, such as hydrocarbon reservoirs, enhanced geothermal systems, geological storage of CO 2 .
Archives of Mining Sciences, 2012
This paper presents the results of laboratory tests carried out in order to formulate effective stress law. The law was sought for two different cases: first - when rock was treated as a porous Biot medium (Biot, 1941; Nur & Byerlee, 1971) and second - when the law was formulated according to definition of Robin (1973) developed by Gustkiewicz (1990) and Nowakowski (2007). In the first case coefficents (4) and (5) of the Biot equation (3) were were determined on the basis of compressibility test, in the second one effective pressure equation (9) and effective pressure value (11) were found on the basis of results of so called individual triaxial compression test (see Kovari et al., 1983) according to the methodology given by Nowakowski (2007). On the basis of Biot coefficients set of values was found that volumetric strain of the pore space described by a coefficient (5) was not dependent on the type of pore fluid and the pore pressure of only, while in case of volumetric strain of ...
Journal of Geophysical Research: Solid Earth, 2017
Over the last decades, a large understanding has been gained on the elastic properties of rocks. Rocks are, however, porous materials, which properties depend on both response of the bulk material and of the pores. Because in that case both the applied external pressure and the fluid pressure play a role, different poroelasticity coefficients exist. While theoretical relations exist, measuring precisely those different coefficients remains an experimental challenge. Accounting for the different experimental complexities, a new methodology is designed that allows attaining accurately a large set of compressibility and poroelasticity coefficients in porous and permeable rocks. This new method relies on the use of forced confining or pore fluid pressure oscillations. In total, seven independent coefficients have been measured using three different boundary conditions. Because the usual theories predict only four independent coefficients, this overdetermined set of data can be checked against existing thermodynamic relations. Measurements have been performed on a Bentheim sandstone under, water-and glycerine-saturated conditions for different values of confining and pore fluid pressure. Consistently with the poroelasticity theory, the effect of the fluid bulk modulus is observed under undrained conditions but not under drained ones. Using thermodynamic relations, (i) the unjacketed, quartz, and skeleton (Zimmerman's relation) bulk moduli fit, (ii) the drained and undrained properties fit, and (iii) it is directly inferred from the measurements that the pore skeleton compressibility C is expected to be constant with pressure and to be exceedingly near the bulk skeleton C s and mineral C m compressibility coefficients. Plain Language Summary Poroelastic properties of rocks are a major information for a large variety of applications linked to rupture processes in saturated media. Theories exist to link the properties to measurable parameters. Yet only little measurements exist to confront those theories. This article aims at providing a method for accurately measuring those properties. Moreover, for one particular rock, a large amount of compressibility and poroelastic coefficients is measured and used to confront those theories.
The effect on rock swelling due to the salinity difference between rock pore fluid and ambient fluid
2015
A consideration of swelling characteristics of shaley rock is essential for underground structures such as tunnels. The swelling characteristic known as time dependent deformation is mainly due to a mechanism of osmosis and diffusion between pore fluid in rock and ambient fluid. An extensive experimental program has been carried out to investigate the effect on rock swelling due to the salinity difference between rock pore fluid and ambient fluid having different salt concentrations. The test program includes free swell tests, semi-confined swell tests and null swell tests with measurements of salinity of rock pore fluid and calcite content on three rock formations including Georgian Bay, Queenston and Shaftesbury. It is observed that, for a given salinity difference between pore fluid of rock and the ambient fluid, a significant effect on swelling potentials exists both in vertical and horizontal directions during the free swell tests and semi-confined tests on Queenston, Georgian ...
International Journal of Rock Mechanics and Mining Sciences, 2007
Closure pressures measured during injection tests such as mini-fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude. This paper presents stress, strength and image log data from the Australian Cooper Basin, which suggests that in reservoirs with high in situ stress, high tensile strength and weak geological fabrics, interpreted closure pressures may be significantly greater than the minimum principal stress. Closure pressures interpreted from mini-frac injection tests in the Cooper Basin, suggest the minimum principal stress varies from 12.4-27.2 MPa/km (0.55-1.2 psi/ft). To better understand the reasons for this variation in closure pressure, image logs and mini-frac data from 13 treatment zones, and core from seven of these treatment zones, were analysed. The analysis revealed that treatment zones with high measured closure pressures (X18.1 MPa/km; 0.8 psi/ft), high treating pressures (431.6 MPa/km; 1.4 psi/ft) and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength (47 MPa; 1015 psi) and geological fabrics (planes of weakness) including natural fractures. Conversely, treatment zones with lower measured closure stress (p19 MPa/km; 0.84 psi/ft) and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and/or no geological fabrics. We suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the Cooper Basin where they are associated with hydraulic fracture complexity. Rather, they reflect the normal stress incident on pre-existing weaknesses that are exploited by hydraulic fluid during the mini-frac injection.