Gas gravity

Reservoir fluids are broadly cafegorised using those properties which are easy to measure in the field, namely oil and gas gravity, and the producing gasioil ratio (GOR) which is the volumetric ratio of the gas produced at standard condition of femperature and pressure (STP) fo fhe oil produced at STP. The commonly used units are shown in the following table. 

When hquids discharge vertically downward from orifices into gas, gravity increases the discharge coefficient. Figure 6-19 shows this effect, giving the discharge coefficient in terms of a modified Fronde number, Fr = pgAp/D. ... 

If the gas composition is not known, this procedure cannot be used to develop the hydrate formation point. Figure 4-5 gives approximate hydrate formation temperatures as a function of gas gravity and pressure. For example, for the 0.67 specific gravity gas of our example field (Table 2-10), Figure 4-5 predicts a hydrate formation temperature at 4,000 psia at 76T-... 

Given An engine-driven compressor to handle 244 cfm natural gas (gravity = 0.658) at intake conditions of 90 psig and SOT, to discharge at 150 psig k = 1.27. 

The Reynolds number for gases can be calculated directly in terms of flowrate and gas gravity as, , ... 

TT( is nondimensionalized by the flame transit hme T( = 3/Sl. Here, 7 is the normalized gas expansion ratio. Because of the difference in density between the fresh and burnt gas, gravity will also influence the d5mamics of the flame front. The effect of gravity has been included through a Froude number, Fr = Sl/ Sg), positive for a flame propagating downward. Similar expressions have been derived by Matalon and Matkowski [13], and by Frankel and Sivashinsky [14]. 

Often, specific gravity is called gravity or gas gravity however, specific gravity is the correct term. 

Pressure PSIG R d Solution Gas/Oil Ratio l] Differential Vaporization at Bod Bio Relative Relative Oil Oil Total Density Vo lumaVolume(3] gm/cc 220 BF. Deviation Factor Z Gas Formation Volume Factor(4] Incremental Gas Gravity... 

Pressure psig Gas removed, cc Gas removed, scf Oil volume, cc Incremental gas gravity... 

Figure 7-1, page 204, Ratio of Reservoir Gas Gravity to Surface Gas... 

Hammerschmidt discovers hydrates as pipeline plugs provides Hammerschmidt equation discovers thermodynamic inhibitors 1941 Katz et al. begin -values and gas gravity methods to predict hydrate mixtures... 

FIGURE 1.4 Gas gravity chart for prediction of three-phase (Lyy-H-V) pressure and temperature. (Reproduced from Katz, D.L., Transactions AIME, 160, 140 (1945). With permission.)... 

For accurate mixture formation pressure estimation, a simple phase diagram or Antoine-like equation does not exist, but the engineer may use the gas gravity chart (Figure 1.4) to approximate the phase... 

Section 4.2 deals with the most useful hydrate equilibria—calculations of temperatures and pressures at which hydrates form from gas and free water. In this section, two historical methods, namely, the gas gravity method (Section 4.2.1) and the Kvs, value method (Section 4.2.2), for calculating the pressure-temperature equilibrium of three phases (liquid water-hydrate-vapor, Lw-H-V)1 are discussed. With the gas gravity method in Section 4.2.1.1, a method is given for limits to expansion, as for flow through a valve. In Section 4.2.2 a distribution coefficient (KVSi) method is provided to determine whether a component prefers residing in the hydrate or the vapor phase. These methods provide initial estimates for the calculation and provide a qualitative understanding of the equilibria. A statistical... 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

The simplest method of determining the temperature and pressure of a gas mixture three-phase (Lw-H-V) conditions is available through the gas gravity charts of Katz (1945). Gas gravity is defined as the molecular mass of the gas divided by that of air. In order to use this chart, reproduced as Figure 4.5 from Figure 1.4,... 

Example 4.2 Calculating Hydrate Formation Using Gas Gravity Chart... 

Figure 4.6 Gas gravity chart error bars. (Reproduced from Sloan, E.D., in Proc. 63rd Annual Convention of Gas Processors Association, 63, 163 (1984). With permission from the Gas Processors Association.)...

The inaccuracies listed in the previous section for the gas gravity chart are inherent in the expansion charts of Figures 4.7 through 4.9 due to their method of derivation. Accuracy limits to these expansion curves have been determined by Loh et al. (1983) who found, for example, that the allowable 0.6 gravity gas expansion from 339 K and 24 MPa was 2.8 MPa rather than the value of 4.8 MPa, given in Figure 4.7. 

Brown, and the statistical thermodynamic method of van der Waals and Platteeuw (1959a) was substituted for the three-phase hydrate line prediction by the gas gravity chart of Katz. 

It should be noted that the use of the KVSj charts implies that both the gas phase and the hydrate phase can be represented as ideal solutions. This means that the Kvsi of a given component is independent of the other components present, with no interaction between molecules. While the ideal solution model is approximately acceptable for hydrocarbons in the hydrate phase (perhaps because of a shielding effect by the host water cages), the ideal solution assumption is not accurate for a dense gas phase. Mann et al. (1989) indicated that gas gravity may be a viable way of including gas nonidealities as a composition variable. 

Both the gas gravity method and the Kysi-value method enable the estimation of three-phase (Lw-H-V) equilibrium between quadruple points Qi and Q2 for mixtures as well as for simple natural gas hydrate formers such as those in Table 4.2. 

Ahydrate blockage in the export line from Shell s Bullwinkle platform in the Green Canyon Block 65 to the Boxe platform was reported in DeepStar Report A208-1 (Mentor Subsea, 1996, p. 52). The 12 in., 39,000 ft line was not insulated. The seawater temperature was 50°F at the base of the platform in 1,400 ft of water. Gas gravity was 0.7 and the flow rate was 140 MMscf/d at an inlet pressure of 800 psi. 

The first of these is based on the gas gravity. A simple chart is provided that plots the temperature-pressure locus with the gas gravity as the third parameter. 

In addition to these weighing methods of determining gas gravity, a method based on the effusion of gases through a small orifice can he employed. This method is based on Graham s Law which may be stated The rate of effusion of a gas throu an orifice is inversely proportional to the square root of the gas density. Consequently, the... 

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