Free gas

Discussion of sampling techniques must involve both free and bound gases. To facilitate this discussion the collection, measurement and analysis of light (C1-C4) hydrocarbons will be broken into two main categories each with two subcategories (1) free gas, which can be vapour or dissolved gas and (2) bound gas, which can be adsorbed gas or chemi-adsorbed gas. 

Sampling of vapour can be extended to any depth above the water table by analysing the exhaust air from an air-drilled well. Complications occur because of dilution effects by the air injected for drilling and by the additional fact that the drill bit disaggregates and liberates rock or matrix gas in the process of drilling the hole. 

Dissolved gases must be extracted from the aqueous system before analysis. This is usually accomplished by a simple gas-water partition into a vapour phase followed by standard headspace measurement techniques (McAuliffe, 1966). Alternatively a so-called stripper continuously partitions the dissolved gases into a carrier gas which is then sent to a gas chromatograph for analysis (Mousseau and Williams, 1979 Aldridge and Jones, 1987). These separations are aided by the very low solubility of the light hydrocarbon gases. 

Standard mud gas logging is one variant of dissolved gas analysis conducted on deeper drill holes. A gas trap is deployed in the return mud system for extracting the dissolved and free gases. Compositional information obtained from mud logging gas is useful for predicting the composition of a potential reservoir (Pixler, 1969). These same ratios have been found to be indicative of oil versus gas potential from surface seeps observed from 4 m (12 feet) deep soil-gas measurements or from analysis of gases dissolved in the shallow groundwater (Jones and Drozd, 1983). 

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An oil reservoir which exists at initial conditions with an overlying gas cap must by definition be at the bubble point pressure at the interface between the gas and the oil, the gas-oil-contact (GOC). Gas existing in an initial gas cap is called free gas, while the gas in solution in the oil is called dissolved or solution gas. 

Gas has a much higher compressibility than oil or water, and therefore expands by a relatively large amount for a given pressure drop. As underground fluids are withdrawn (i.e. production occurs), any free gas present expands readily to replace the voidage, with only a small drop in reservoir pressure. If only oil and water were present in the reservoir system, a much greater reduction in reservoir pressure would be experienced for the same amount of production. 

The relatively concentrated hydrochloric acid is employed so that with ordinary use of the apparatus, spent liquor does not accumulate very rapidly the concentrated acid also ensures a brisk and delicately controlled flow of gas. When the generator is replenished with acid, marble or both, the de-aeration procedure detailed above is repeated until a sufficiently air-free gas supply is obtained. 

In a combined power cycle operation, clean (sulfur- and particulate-free) gas is burned with air in the combustor at elevated pressure. The gas is either low or medium heat-value, depending on the method of gasification. 

An unstabilized high surface area alumina siaters severely upon exposure to temperatures over 900°C. Sintering is a process by which the small internal pores ia the particles coalesce and lose large fractions of the total surface area. This process is to be avoided because it occludes some of the precious metal catalyst sites. The network of small pores and passages for gas transfer collapses and restricts free gas exchange iato and out of the activated catalyst layer resulting ia thermal deactivation of the catalyst. 

Compared to Ras, Gq subunits are extended at their N-termini by about 30 residues that in free Ga are disordered but, as we will see, obtain a helical conformation in association with Gpy. 

Zero count rate The number of counts recorded in unit time by an optical particle counter when a particle-free gas is passed through the measuring chamber. 

Schildknecht, M., W. Geiger, and M. Stock. 1984. Flame propagation and pressure buildup in a free gas-air mixture due to jet ignition. Progress in Astronautics and Aeronautics. 94 474-490. 

Free gas (or vapor) capacity of ralve = (free air capacity) (temp. corr. factor)... 

At the same pressure and temperature, the free gas capacity of a valve varies inversely as the square root of the specific gravity of the vapor, with air = 1.0. 

Size, Thickness, Length, Number per Weight per Surface Area, Free Gas Spher. Diani. 

Nominal Size, In. WaU Thickness, In. O.D. and Leng[th, In. Approx. Avg. Number per Ft Approx. Avg. Weight per Ft , Lb Approx. Avg. Siuface Area, F /Ft Percent Free Gas Space Equivalent Spher. Diam. Dp, In. 

Length, Thickness, Rings per Ft, Ft Free Gas Surface Area Diameter ... 

Nominal Size, In. Approximate Number per Ft Approximate Weight per Ft , Lb Approximate Surface Area FtVFt Percent Free Gas Space Equivalent Spherical Diameter Dp, In. 

Out- side Diam. In. Out- side Length In. WaU Thick- ness In. Number of Rings per Ft Weight of Rings per Ft Percent Free Gas Space Ft of Surface Area per Ft of Packing Net Cross-Section Area of Packing in Ft2/Ft2 ... 

No. Units Weight per Surface Area Free Gas per Ft Ft, Lb Ft /Ft Space... 

Size No. Designation Approximate Number/Ft We ht, Lb/Ft Free Gas Space... 

Lobo et al. [47] proposed the packing factor, F, and experimentally determined that it better represented the data than the calculated a/e term. Values calculated using surface area per cubic foot and percent free gas space from manufacturer s tables can be as much as 40% off. The ralues are dependent upon the method of packing the tower, i.e., dry dumped, wet dumped, or wet dumped and shaken. The latter condition may approximate the situation after a tower has been running a while and the packing settled. 

Dissolved Gas. Gas which enters the borehole when penetrating a high pressure zone may not dissolve immediately in the mud. The free gas considered here is the gas entering the borehole minus the dissolved gas. Table 4-128 indicates the maximum volume of dissolved gas at bottomhole conditions expressed in percent of annulus mud volume. Thus, when entering a high pressure permeable formation this much gas will dissolve first before free gas appears in the mud. 

Note that from Table 4-128 the very large volumes that can dissolve in oil-base muds. For the water-base muds, 0.6 to 0.9% of gas will dissolve and not appreciably change the density or compressibility of the mud. It will be difficult to detect these low concentrations with downhole physical measurements. Free gas will be easily detected as shown hereafter. For the oil-base muds we will assume no free gas is present at bottomhole and the mud properties are changed only due to the dissolved gas. The detection will be more difficult than with free gas. 

A sharp velocity decrease is seen for the water-base muds. Assuming a threshold detection of 500 ft/s, the alarm could be given for 0.5% of free gas or 1.1 to 1.4% of total gas (dissolved and free). 

The oil-base muds having no free gas behave differently and the 500-ft/s threshold is not reached before approximately 5% of gas is dissolved. Then the velocity decrease is almost as fast as with the water-base mud. 

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