Author: haroonkhan
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Pseudocritical Properties from Gas Gravity
In the absence of detailed composition of a natural gas, Figure 4-2 can be used to relate the gas gravity (to air) with the pseudocritical properties of gas mixtures. Using the results of Example 4-2, the calculated molecular weight is 18.92, leading to γg = 18.92/28.97 = 0.65. From Figure 4-2, ppc = 670 psi and Tpc = 375°R, which compare with 671 psi and…
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Correlations and Useful Calculations for Natural Gases
Several important works have presented correlations for natural gas properties. Following is a summary of these, with brief descriptions of the use of these correlations.
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Real Gas Law
The behavior of natural gas mixtures can be approximated by the real gas law where Z is the compressibility factor, also called the gas deviation factor in the petroleum engineering literature. The universal gas constant, R, is equal to 10.73 psi ft3/lb-mol-°R. Equation (4-2) is a general equation of state for gases. The gas compressibility factor for mixtures of…
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Gas Gravity
Gas gravity, as used in natural gas production and reservoir engineering, is the ratio of the molecular weight of a natural gas mixture to that of air, itself a mixture of gases. Gas gravity is perhaps the most important defining property of a natural gas because almost all properties and, in fact, the actual description…
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Introduction
Natural gas reservoirs produce hydrocarbons that exist primarily in the gaseous phase at reservoir conditions. To predict the gas production rate from these reservoirs, there is a need to review some of the fundamental properties of hydrocarbon gases. This is particularly important (more so than in the case of oil reservoirs) because certain physical properties…
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Fetkovich’s Approximation
Vogel’s correlation, normalizing qo by qo,max, is frequently not in close accordance with field data. Fetkovich (1973) suggested a normalization with , and in a flow equation of the form the relationship becomes Equation (3-63) requires the determination of two unknowns, the absolute open flow potential, qo,max, and the exponent n. Both of them are characteristic of a specific well and…
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Generalized Vogel Inflow Performance
If the reservoir pressure is above the bubble point and yet the flowing bottomhole pressure is below, a generalized inflow performance can be written. The following approach enables generation of an IPR that has a straight portion for pwf ≥ pb, and follows the Vogel equation for pwf < pb adapted to the straightforward logic found in Standing (1971). This can be…
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Oil Inflow Performance for a Two-Phase Reservoir
Vogel (1968) introduced an empirical relationship for qo based on a number of history-matching simulations. The relationship, normalized for the absolute open flow potential, qo,max is where, for pseudosteady state, where ko is the effective permeability to oil that might be estimated from a pressure buildup test. Therefore, The convenience of the Vogel correlation is that it allows the use of…
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Two-Phase Flow in a Reservoir
Although a rigorous treatment of two-phase flow in a reservoir is outside the scope. It is necessary to understand the impact of competing phases on the flow of a fluid through the porous medium. If there are two or three fluids flowing at the same time in a porous medium, the absolute reservoir permeability, k, is…
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Accounting for the Presence of Water
When water is produced, the liquid flow properties are generally taken to be averages of the oil and water properties. If there is no slip between the oil and water phases, the liquid density is the volume fraction-weighted average of the oil and water densities. The volume fraction-weighted averages will be used to estimate liquid…