Factors Controlling Pressure Solution

Influence of Quartz Grain Size. Correlations observed reveal that there is a clear statistical correlation between mean grain size and volume of quartz dissolved by pressure solution, i.e. the dissolution of finer grains is more intense than that of coarse grains (Weyl 1959 Renton et al. 1969 Sprunt and Nur 1977). This results from the decrease of the distance required for diffusion along the grain contours caused by the decrease of their size. However, sometimes there is a certain dispersion of the regression lines which results from the influence of the other factors to be described below. 

Influence of Clay Mineral Content. A number of authors like Thompson (1959), Heald and Renton (1966) and Sibley and Blatt (1976) have shown quantitatively that thin isolated sand intercalations or continuous sandstones possessing abundant clay bridges on their quartz grains have suffered more pressure solution and eventual removal of SiOj than sandstones with a smaller amount or total absence of clay bridges in the form of a film. This was also observed in the Saharan sandstones (Plate 13). It is, however, difficult to establish a quantitative statistical correlation between the amount of clay components and pressure solution. There is at the same time in the Saharan sandstones a clear inverse statistical correlation between the amount of clay minerals in the form of a film and the quartz cement (Fig. 4.12, Plates 13,14). This is in agreement with observations showing that the clay cement impedes in the sandstones the formation of overgrowth rims of quartz (Millot 1964 Heald and Larese 1974). 

Influence of Sandstone Matrix Composition. Some of the sandstones investigated contain a notable quantity of feldspar. It has been confirmed that there is a clear correlation between the composition of the sandstone matrix and pressme solution. The 

Plate 13. Intergranular pressure solution of quartz. The contact zones between the quartz grains are clearly discernible. The silica dissolved in the contact zones appears to be exported . Despite the obvious dissolution of quartz the amount of quartz cement is negligible. Photo h clearly shows dissolution features on quartz. The films enveloping the quartz grains are made up of authigenic chlorite 

Chapter 4 Reservoir Decompaction and Formation of Accumulation Capacity 

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Its function is to expand the pressurized solution to separate the "solvent gas" from dissolved extracted components. If a fixed restrictor is used, the mass flow rate of the fluid changes as a function of pressure (density) mass flow can increase by a factor of 25 as pressure is increased from 80 to 400 bar (24). Not only are the pressure and flow coupled, the coupling is via a static conduit whose dimensions are imprecisely controlled during an extraction (partial plugging by particulates and precipitated components, temperature) and from component to component during maintenance replacements. This results in a lack of control in operating parameters (density) and timed sequences (via flow rate and time). A variable restrictor whose dimensions are set and adjusted by an electronic feedback control loop is an alternative solution. 

The timing of the dolomitization of carbonate rock bodies and emplacement of dolomite cements has been one of the more controversial aspects of the "dolomite problem." Most of the basic factors controlling dolomite formation, where were discussed in Chapters 6 and 7, also apply to dolomite formation during the later stages of diagenesis. However, the extended periods of time, the solution compositions likely to be encountered, and the elevated temperature and pressure that occur during deep burial provide highly favorable conditions for dolomite formation. 

As the distribution of samples of a certain succession is more or less even over the export and import fields we may assume that silica has migrated locally from the exporters , i.e. fine-grained sandstones, towards importers , i.e. coarser-grained sandstones, and in this way on the local scale an approximate mass balance was maintained. Nevertheless, the overall data show that an important transfer of silica has taken place on a much wider scale and that this is closely related to temperature as the controlling factor for intergranular pressure solution. In zones of higher thermal maturity pressure solution probably was so efficient that it could act simultaneously as the source and driving force for the mass transfer of silica. In zones of low thermal maturity pressure solution was not particularly effective for the mass transfer of silica and the reduction of the primary porosity. 

It is clear that the major factor controlling the particle diameter will be the separation ratio (a), which reflects the difficulty of the separation. The more difficult the separation, the more theoretical plates are needed, and thus the column must be longer. However, to use a longer column, the particle diameter must be increased to allow the optimum velocity to be realized without exceeding the maximum system pressure. The effect of the capacity ratio of the first solute of the critical pair on the optimum particle diameter is complex. Extracting the function of the capacity ratio (f(k )) from equation (1),... 

At the limit of Knudsen diffusion control it is not reasonable to expect that any of the proposed approximation methods will perform well since, as we know, percentage variations in pressure are quite large. Nevertheless it is interesting to examine their results, which are shown in Figure 11 4 At this limit it is easy to check algebraically that equations (11.54) and (11.55) become the same, while (11.60) differs from the other two. Correspondingly the values of the effectiveness factor calculated using the approximation of Kehoe and Aris coincide with the results of Apecetche et al., and with the exact solution, ile Hite and Jackson s effectiveness factors differ substantially. 

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

Solid alkalis Solid alkalis may be used, in principle, for the corrosion control of drum boilers at all pressures but other factors, e.g. carryover or hideout a (reversible disappearance from solution on-load), may preclude them in some cases. However, they are used for feed-line treatment only in lower pressure plant where the boiler has increased tolerance to the higher solids burden which their use entails. Sodium hydroxide or, at very low pressures, sodium carbonate, (which is hydrolysed to the hydroxide at boiler temperatures) have been used, as have potassium and lithium hydroxides and various phosphate mixtures. (For a comparison of various alkalis for this purpose see References.)... 

The diffusion current Id depends upon several factors, such as temperature, the viscosity of the medium, the composition of the base electrolyte, the molecular or ionic state of the electro-active species, the dimensions of the capillary, and the pressure on the dropping mercury. The temperature coefficient is about 1.5-2 per cent °C 1 precise measurements of the diffusion current require temperature control to about 0.2 °C, which is generally achieved by immersing the cell in a water thermostat (preferably at 25 °C). A metal ion complex usually yields a different diffusion current from the simple (hydrated) metal ion. The drop time t depends largely upon the pressure on the dropping mercury and to a smaller extent upon the interfacial tension at the mercury-solution interface the latter is dependent upon the potential of the electrode. Fortunately t appears only as the sixth root in the Ilkovib equation, so that variation in this quantity will have a relatively small effect upon the diffusion current. The product m2/3 t1/6 is important because it permits results with different capillaries under otherwise identical conditions to be compared the ratio of the diffusion currents is simply the ratio of the m2/3 r1/6 values. 

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