In reservoirs at high initial pressures, the gas compressibility is much lower, in the same order of magnitude as the formation. Rafiqul Islam, in Reservoir Development, 2022 6.2.1 Overpressured reservoirĪt typical reservoir conditions, gas compressibility is orders of magnitude greater than that of the formation rock or residual fluids. For our CBM gas composition, all three methods are close. For our sour gas composition, up to about 1300 psia (8900 kPaa), the GPSA is closer to the EOS than corresponding states is. For our sweet gas composition, both EOS and corresponding states show a distinct upturn in the compressibility value above 2000 psia (13,800 KPaa) that the GPSA equation dos not match. GPSA has a pair of equations that do a good job for many gases. This “equation” is a complex multistage program that requires upward of 30 steps in an application like MathCad. Some later researchers have developed adjustments that improve performance in acid gases. Hall–Yarborough provides good results for hydrocarbon gases, but the so-called acid gases (e.g., CO 2 and H 2S) cause it to provide numbers that deviate too much from the EOS methods. The most commonly used method is the Hall–Yarborough equation. This method uses critical behavior to predict deviation from ideal behavior. Programs like REFPROP.EXE from the US National Institute of Standards and Technology (NIST), which is inexpensive and quite capable, and the HYSYS from Aspen Technologies, which is quite expensive and able to evaluate very complex chemical and flow relationships, are required to use the EOS method. These values tend to be consistent and reliable, but generally require very complex programming. The main techniques to develop compressibility relationships are: Gas compressibility is not an intrinsic characteristic of a gas mixture and methods of determining it range from very difficult to quite easy and the results range from an excellent representation of reality to a poor representation of reality, and there is not much correlation between ease of calculation and quality of results. Ignoring deviation from ideal behavior is one of the most common sources of engineering failure in Oil & Gas. Sweet gas at standard temperature and pressure (STP) has Z=0.99 and at 2000 psia it is Z=0.59. At 1000 psig (6700 kPag) and 60☏ (15.6☌) sweet gas (see Section 0.6.6.7 for the description of “sweet gas,” “sour gas,” and “CBM”) has Z=0.727, sour gas has Z=0.791, and CBM has Z=0.863-a range of 17% relative to the average. Gas compressibility is clearly a function of gas composition. Methane and CO 2 exhibit distinctly nonlinear (and often nontrivial) response to applied force. Air is very nearly an ideal gas where Z=1.0. The primary nonliner mechanism is called “compressibility” (symbol “ Z”) which is fundamentally a measure of the amount that a gas deviates from ideal gas behavior. Gas is quite compressible, and when you stack it vertically you find that the pressure exerted by the gas stacked above it changes via both linear and nonlinear mechanisms. Simpson P.E., in Practical Onshore Gas Field Engineering, 2017 0.6.3 Gas Compressibility
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