Effects Of Pressure Solution Research Articles

Possible Effect of Pressure Solution on the Movement of a Canister in the Buffer of Geological Disposal System

One of the major concepts of the geological disposal of high level radioactive waste is to enclose a metallic container with bentonite buffer which is considered to be impermeable and chemically stable. Since the average density of the container is around 6 to 7 and very heavy compared to bentonite, the scenario of container sinking has been evaluated because excess sinking makes short the pathway of nuclide migration in the bentonite and is detrimental to the safety of the disposal system. Previous considerations on container sinking have been made from the viewpoint of mechanical deformation of the bentonite. In this paper, a chemical deformation process is presented as another mechanism of container sinking, which has not been previously considered for the container sinking in the field of radioactive waste disposal. The chemical deformation mentioned in this paper is the deformation through the process of pressure solution of minerals constituting the buffer, transportation by diffusion and precipitation. That such chemical deformation is a ubiquitous phenomenon occurring in various scales in the crust of the earth will be shown through the review of previous works. Then, some future research topics will be suggested which would be required in order to evaluate the container sinking in the safety case for radioactive waste disposal.

Numerical simulation for influences of pressure solution on T-H-M coupling in aggregate rock

The pressure solution model of granular aggregates was introduced into a FEM code which was developed for the analysis of thermo-hydro-mechanical (T-H-M) coupling in porous medium. Aimed at creating a hypothetical model of nuclear waste disposal in unsaturated quartz aggregate rock mass with laboratory scale, two 4-year computation cases were designed: 1) The porosity and permeability of rock mass are functions of the pressure solution; 2) The porosity and the permeability are constants. Calculation results show that the magnitude and distribution of stresses in the rock mass of these two calculation cases are roughly the same. And, the porosity and the permeability decrease to 43%–54% and 4.4%–9.1% of their original values after case 1 being accomplished; but the negative pore water pressures in cases 1 and 2 are respectively 1.0–1.25 and 1.0–1.1 times of their initial values under the action of nuclear waste. Case 1 exhibits the obvious effect of pressure solution.

FEM analyses for influences of pressure solution on thermo-hydro-mechanical coupling in porous rock mass

The model of pressure solution for granular aggregate was introduced into the FEM code for analysis of thermo-hydro-mechanical (T-H-M) coupling in porous medium. Aiming at a hypothetical nuclear waste repository in an unsaturated quartz rock mass, two computation conditions were designed: 1) the porosity and the permeability of rock mass are functions of pressure solution; 2) the porosity and the permeability are constants. Then the corresponding numerical simulations for a disposal period of 4 a were carried out, and the states of temperatures, porosities and permeabilities, pore pressures, flow velocities and stresses in the rock mass were investigated. The results show that at the end of the calculation in Case 1, pressure solution makes the porosities and the permeabilities decrease to 10%–45% and 0.05%-1.4% of their initial values, respectively. Under the action of the release heat of nuclear waste, the negative pore pressures both in Case 1 and Case 2 are 1.2–1.4 and 1.01–1.06 times of the initial values, respectively. So, the former represents an obvious effect of pressure solution. The magnitudes and distributions of stresses within the rock mass in the two calculation cases are the same.

Effects of pressure solution and fluid migration on initiation of shear zones and faults

Constitutive relations for an isotropic poro-power-law fluid are proposed in which the effective viscosities in both shear and dilation have the same power-law dependence on the second invariant of the deviatoric stress alone, but the viscosity in dilation is 1/ ν times the viscosity in shear. The shear rate is otherwise proportional to the deviatoric stress and the dilation rate to an effective mean stress. The behavior may either involve Darcy flow of a pore fluid whose viscosity is much less than that of the solid matrix, or involve dissolution, diffusive transport, and precipitation of a soluble phase, as in pressure-solution. Linearized relations for small additional rate-of-deformation and stress superposed on a basic-state flow are obtained. These are used to treat the deformation due to a shallow, narrow indentation at the interface between a layer and an adjacent medium when both undergo bulk uniform layer-parallel shortening or extension. If the layer, the medium, or both, are nonlinear fluids with large stress exponent, shear-band like structures, extending from the indentation, and `reflecting' from any weak, slipping interface, occur in both the instantaneous velocity field and in the strain field. Since the materials considered have constant and uniform properties — e.g., they do not exhibit strain-softening — these features leave no imprint in the medium. They persist over mean strains of 10–20%, but are, like the indentation, ephemeral. It is hypothesized that localization due to an interfacial irregularity of this sort would produce bona fide shear bands if the material were strain-softening. The development of shear bands is enhanced if the adjacent medium has a large stress exponent, or if it is a poroviscous fluid with low resistance to dilation. In a poroviscous medium, dilation is concentrated in paired positive and negative lobes at the terminations of shear bands in the adjacent layer. The development of shear bands in a poroplastic layer is mildly suppressed, relative to that in an incompressible layer. Dilation is concentrated in paired loci at shear band terminations, with weaker, paired bands of positive and negative dilation on either side of the shear zone.

Hydrocarbon Generation Potential of the Cretaceous Section from Well ALP-6, Perija Region, Venezuela

Geochemistry and sedimentology have been integrated in order to provide a better understanding of the source rock potential and depositional environments of the La Luna Formation and Machiques Member in Well ALP-6 (Perija region). These two units, the dominant source rocks in the Maracaibo Basin, are mainly shales with high to very high organic content, while thin interbeds of limestones are poor in organic matter. A detailed sedimentological study and sequence analysis indicates that both shaly units represent a period of platform infilling subsequent to drowning. Periods of progressive back stepping culminating in the deposition of organic-rich condensed intervals are recognized, based on sedimentology of cores and wireline log analysis. A succession of fining-upward sequences, 1' to 5' thick, with distinct sedimentological and geochemical signatures have been identified in the La Luna Formation. Phenomena of early diagenesis (intrashale calcite growth due to organic matter degradation; sulfur precipitated in local paleolows) to late diagenesis (pressure-solution effects with development of laterally correlatable cone-in-cone layers) are all indicators that the hydrocarbon generation potential of La Luna is not uniform and can only be assessed by detailed geological, sedimentological and geochemical investigations. Two geochemically distinct facies can be identified in both La Luna andmore » Machiques. A sulfur-rich facies is characterized by Corg/AVSul ratios averaging 1.9 and by exceptionally high concentrations of sulfur-bearing aromatic compounds. A sulfur-poor facies is characterized by Corg/AVSul ratios averaging 9.2 and by trace concentrations or absence of sulfur-bearing aromatic compounds.« less

Effects of Pressure Solution on Petroleum Migration in the Miocene Monterey Formation, Southern San Joaquin Basin, California: ABSTRACT

Pressure-solution features are common in the Monterey Formation and occur as bedding-parallel stylolites and solution cleavages at high angle to bedding. Both structures are visible in hand specimen, but they occur abundantly at a microscale. Petrographic study of siliceous rocks of the Monterey Formation shows that stylolites and solution cleavages provide horizontal (bedding parallel) and vertical avenues for petroleum migration respectively. Together with tectonic fractures, they are major migration pathways of petroleum. Pressure solution is a dissolution process that involves volume reduction of the rock and concentration of insoluble rock components, including organic matter, along pressure-solution features. Previous geochemical studies have shown the concentration of organic matter along stylolites, but no previous work has demonstrated it petrographically. Petrographic examination of siliceous rocks in the Monterey Formation under transmitted, reflected, and fluorescent light, shows the occurrence of liptinite and inertinite along stylolites. Petrographic evidence suggests that removal of rock mass by pressure solution can concentrate organic matter and help the development of a three-dimensional organic network believed to play an important role in primary migration.

Porosity Prediction in Quartz-Rich Sandstones: Middle Jurassic, Haltenbanken Area, Offshore Central Norway: ABSTRACT

Reduction of primary porosity by mechanical compaction and pressure solution is the main control of reservoir quality in the upper member of Middle Jurassic, quartz-rich (Q > 75%) sandstones in the Haltenbanken area. The extent of secondary porosity and permeability generation is limited to volumetrically minor alteration or dissolution of feldspars. The effects of pressure solution, exhibited by petrographic textures, vary widely. The extent of significant porosity reduction by pressure solution was determined to be a function of the residence time of the sandstones within the pressure-solution domain, a region roughly below 1500-m burial depth in the Haltenbanken area. Porosity prediction curves were developed from plots of measured porosity and an integration of the burial history within the pressure-solution domain (i.e., the product of burial time and depth between 1500 m and present-day depth). The curves, both for compacted and decompacted profiles, were constructed using data from six wells in the Haltenbanken area. Porosities of three wells drilled after the porosity prediction approach was developed, closely correspond to the predicted porosity values. Average permeability also can be predicted relatively accurately because of the good correlation between porosity and permeability in these quartz-rich sandstones. The porosity-burial history relationship can be applied more » to porosity predictions prior to drilling in the Haltenbanken and related areas using burial histories interpreted from seismic-stratigraphic analysis. « less

Development of cleavage in lapilli-bearing volcaniclastic rock

The development of cleavage in a suite of lithic and accretionary lapilli-bearing volcaniclastic rocks of known strain ( \\ ̄ ge s = 0.14−2.4 ) has been studied. Analysis of these specimens reveals that various mechanisms of cleavage formation have been active during the deformation. In both lithic and accretionary lapilli-bearing rock, the cleavage in the matrix appears to have formed directly by recrystallization of devitrified and/or altered volcanic glass (ash). Preferred alignment of the neoblasts in the matrix with increasing strain results from some combination of neocrystallization parallel to cleavage and at an angle to the cleavage followed by rotation into that plane as the deformation progresses. Fragments in lithic lapilli-bearing rocks show two stages of rotation during cleavage development. The first stage ( \\ ̄ ge s = 0 to ∼ 0.60; + \\ ̄ ge 1 = 0 to ∼ 40%; − \\ ̄ ge 3 = 0 to ∼ 35% , where \\ ̄ ge 1 ⩾ \\ ̄ ge 2 ⩾ \\ ̄ ge 3 , elongations) is dominated by rigid body rotation of lapilli. Recrystallization and shape change of the lapilli accompanying the rotation are slight at first, but increases with strain. In the second stage ( \\ ̄ ge s> 0.60 ), recrystallization becomes pronounced, and is accompanied by a ductile flattening of lapilli facilitated by inter- and intragranular movements. These movements allow the lapilli to rotate by migration of the vertex along the lapilli perimeter, and is accompanied by increasing axial ratios. In the accretionary lapilli-bearing rocks, lapilli show one continuous stage of rotation up to values of strain where \\ ̄ ge s ∼ 1.25 (+ \\ ̄ ge 1 ∼ 110%, − \\ ̄ ge 3 − 65%) . Recrystallization is active from the start, rendering the lapilli ductile even in the early stages of deformation. Lapilli rotate and increase their axial ratios by the flattening process active in the second stage described for lithic lapilli. Once the values of strain exceed \\ ̄ ge s > 1.25 , rotation of lapilli is essentially complete, although their axial ratios continue to increase with strain. Primary crystals in both rock types also show two stages of rotation. In the first stage ( \\ ̄ ge s = 0 to ∼ 0.70; + \\ ̄ ge 1 = 0 to ∼ 50%, − \\ ̄ ge s = 0 to ∼ 40% ), the primary crystals undergo essentially all their rotation which is accompanied by some plastic deformation, fracturing, and minor quartz-mica beard formation. In the second stage ( \\ ̄ ge s>0.70 ), further rotation is statistically minor, the mechanisms just mentioned become more common, and recrystallization becomes an important response of the crystal to the deformation. At strains where \\ ̄ ge s > 1.25 ( + \\ ̄ ge 1 ⩾ 110%, − \\ ̄ ge 3 ⩾ 60% ), the metamorphic products of deformed and recrystallized primary crystals tend to become smeared out in the cleavage plane. The effects of pressure solution can be seen in the deformation of primary crystals (beard formation, etc.) and in the response of some of the more dense lithic lapilli. It is also relatively well-developed in a few specimens which are crystal-rich. In general, however, it plays a minor role relative to the other mechanisms described.

Pressure solution: Theory and experiments

Parts of a solid under stress show a higher solubility than stress-free parts. In geological formations, this effect causes a reaction which is called ‘pressure solution’. During this process grain contacts dissolve and the grain boundaries in contact with the pore solution grow. This leads to a loss of porosity and an increase in formation strength. Various attempts have been made to describe this process thermodynamically. Neither Weyl's (1959) nor Riecke's (1894) formula — both predicting only minute solubility differences —is appropriate to describe the process. The following equation —equivalent to an equation proposed by Bridgman (1916) describes the solubility increase caused by stresses a p = a o exp { ΔvP n − Δw RT } where: a p = the solubility of part of a grain under normal stress P n a o = the solubility under hydrostatic pressure p. This equation can be used to describe the pressure solution phenomenon and to predict preferred lattice orientation of anisotropic crystals. There is a tendency for crystals to dissolve in places where the stress component normal to the crystal face is high and tc precipitate simultaneously in places where this stress component is low. It has been found that this tendency is many orders of magnitude greater than any possible preference for a particular lattice orientation to grow or disappear. Preferred lattice orientation will therefore probably be determined by the dissolution and precipitation rates of the different crystal faces, and not by the highest thermodynamic stability. Experimental data on pressure solution are very scarce, since experiments are very time-consuming, and are usually carried out under circumstances that are widely different from those occurring in nature. In the Shell laboratories at Rijswijk (The Netherlands), experiments have been carried out on quartz sands compacted in a 1M NaCl-solution at pressure up to 700 atm. The temperatures in the experiments are between 200 and 330°C. At the highest temperature, very distinct indications of pressure solution have been observed in the sand pack after one year of compaction. At lower temperatures, the effect of pressure solution could only be deduced from the compaction behaviour of the sands.

Diagenesis of the Mt. Simon and Rose Run Sandstones in Western West Virginia and Southern Ohio

ABSTRACT Samples from wells in western West Virginia and southern Ohio indicate that the Mt. Simon (Cambrian) and Rose Run (Cambro-Ordovician) sandstones are well sorted and well rounded. Most of the sands are feldspathic with some units being arkosic. Carbonate generally in the form of dolomite is common in some beds. Most of the sands were porous initially, but porosity has been reduced by a number of postdepositional processes. Feldspar cement typically ranged from 3 to 9% and significantly reduced porosity in many samples. Quartz cement was much less common ranging generally from I to 3%, although a few samples contained as much as 18% secondary quartz. In the Rose Run sandstone quartz cementation was inhibited by coatings of illite, but feldspar was able to grow mainly by pushing the illite aside. The highly calcareous sands typically had low porosity. Anhydrite was a minor pore filling in many sands but locally was abundant and reduced porosity considerably. In spite of the considerable depth of burial of some of these sands (16,000 ft.), pressure solution effects are not abnormally high. Illite promoted pressure solution and caused the elimination of porosity in some layers. Dolomite fills some pores, but much of the dolomite replaced other minerals so was not a factor in pore reduction in these cases. Secondary porosity resulted from the dolomitization of original calcite in some places. Porosity was also increased by the solution of feldspar in the feldspathic beds of the Mt. Simon Sandstone. In future exploration, the best porosity in the Mt. Simon Sandstone is to be expected in the quartzose and arkosic sands which are low in carbonate. In the Rose Run sandstone, the highest porosity is likely in the low carbonate sands where illite is sufficient to block quartz growth but not abundant enough to promote pressure solution. Solution of feldspar and dolomitization of original calcite are also favorable factors.