Pressure measurements

A widely used technique for measuring the surface tension is known as the method of maximal pressure of a bubble. Few people are aware that one of its pioneers was a young physicist named Erwin Schrodinger, back in 1915. The principle is quite simple. A capillary tube is partially immersed in a liquid bath down to a depth h much larger than its radius. Air is blown into the tube while the pressure is being measured when the bubble is forming. 

FIGURE 2.23. Measuring the maximal pressure of a bubble. When air is blown into a tube immersed into a liquid, the pressure at the end of the tube goes through a maximum when the bubble is hemispherical. 

Gas phase reactions at constant volume and temperature are routinely monitored by pressure measurements when the gas is assumed to behave ideally and a direct proportionality between pressure and concentration can be assumed, but the technique is much less commonly used for reactions in solution. If a gas is evolved or consumed in a reaction, it can be monitored continuously by the change in pressure when the reaction is investigated in a closed reactor at constant volume. An early example is the thermal decomposition of arenediazonium salts with evolution of N2 [61]  

The technique has drawbacks associated with the establishment of equilibrium between the gas and the other phase, typically a liquid. If the gas is a reactant, its rate of dissolving in the liquid phase should be faster than the chemical rate-determining step. If it is a product, its release should be sufficiently fast that supersaturation in the liquid phase does not become a problem. (The relationships between rates of transport between phases and chemical processes are explored in Chapter 5.) 

Even though hot-cathode gauges are preferred because of their better accuracy, they have disadvantages over cold-cathode gauges because the hot filament causes light, excess electrons, outgassing, and loss of performance over time. 

It should be pointed out that ion gauges measure only the integral pressure. In order to determine the composition of a gas, it is necessary to use an extra 

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SDY(I) cols 1-10 standard deviation of pressure measurement SDX(I,l)cols 11-20 standard deviation of temperature measurement SDX(l,2)cols 21-30 standard deviation of liquid composition measurement... 

The gradients may be caloulated from surface fluid densities, or may be directly measured by downhole pressure measurements using the repeat formation testing tool (RFT). The interfaces predicted can be used to confirm wireline measurements of fluid contact. 

If a pressure measuring device were run inside the capillary, an oil gradient would be measured in the oil column. A pressure discontinuity would be apparent across the interface (the difference being the capillary pressure), and a water gradient would be measured below the interface. If the device also measured resistivity, a contact would be determined at this interface, and would be described as the oil-water contact (OWC). Note that if oil and water pressure measurements alone were used to construct a pressure-depth plot, and the gradient intercept technigue was used to determine an interface, it is the free water level which would be determined, not the OWC. 

Keywords d rec methods, indirect methods, rock properties, coring, core barrel, standard core analysis, special core analysis, slabbed core, sidewall samples, direct indications, microfossils, sonde, logging unit, invasion, mudcake, formation pressure measurement, fluid sampling, measurement while drilling, formation evaluation while drilling. 

The pressure measurment and fluid sampling procedures can be repealed at multiple depths in the reservoir. 

A static bottom hole pressure survey (SBHP) is useful for determining the reservoir pressure near the well, undisturbed by the effects of production. This often cannot be achieved by simply correcting a surface pressure measurement, because the tubing contents may be unknown, or the tubing contains a compressible fluid whose density varies with pressure (which itself has an unknown profile). 

Heydemann P L M 1997 The Bi l-ll transition pressure measured with a dead-weight piston gauge J. Appi. Phys. 38 2640... 

Zhao Y, Barvosa-Carter W, Theiss S D, Mitha S, Aziz M J and Schiferl D 1998 Pressure measurement at high temperature using ten Sm YAG fluorescence peaks J. Appi. Phys. 84 4049... 

Pressure measurements are not often quoted in the text but the unit of Torr, where 1 Torr= 1 mmHg = 133.322 387 Pa, is a convenient practical unit and appears occasionally. 

COPPERALLOYS-WROUGHTCOPPERANDWROUGHTCOPPERALLOYS] (Vol7) -for pressure gauge elements [PRESSURE MEASUREMENT] (Vol 20)... 

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