Positive values indicate well damage (decreased productivity). Skin is a dimensionless parameter treated mathematically as an infinitely thin damaged or stimulated zone, regardless of the actual dimensions of the altered zone. Their total effect is normally characterized with the use of a skin factor, S, which appears in the steady-state radial-flow equation as These near-wellbore effects are often very complex. Sometimes wells experience near-wellbore phenomena (e.g., fractures and mud-filtrate damage) that cause production to be different from that calculated by Darcy’s law. To concentrate on the specifics of well flow, in the remainder of the chapter the subscript e will refer to an external drainage radius, and the subscript wf will refer to pressure at the inlet sandface of a flowing well. Low-Pressure Gases (approximate) ( β units are psia 2-md-ft-D/Mscf, kPa 2 High-Pressure Gases (approximate) ( β units are psi-md-ft Real Gases ( β units are psi 2-md-ft-D/Mscf/cp, kPa 2 Liquids ( β units are psi-md-ft-D/STB, kPa-m 2-m-d/std m 3). The first line of each equation is in fundamental units, the second in oilfield units, and the third in SI units. Where β is given by the following expressions, which include the unit conversions necessary to apply Eq. First, a "generic" potential difference, Δ ψ, can be expressed for each of the fluid cases according to Table 1.Ī general radial-flow equation can then be expressed for all cases as So that one simplified set of equations can be used throughout the remainder of the chapter, some additional parameters will be defined. Note that the conversion between the real-gas potential in oilfield vs. 4 – Example graph showing that the product μ gB g is approximately constant for gases at pressures greater than approximately 2,000 psia. Also, although it is not readily apparent from the plot, at low pressures the product μz is approximately constant.įig. Note that at high pressures, p/ μz is approximately constant. Twice the area under the curve between any two pressures represents the real-gas potential difference. Where the real-gas potential Δ m is defined byįig. The exact pressure at which the oil formation volume factor and viscosity are evaluated is not critical because the product of is approximately constant.īecause this approximation is not generally valid for gases, the steady-state radial gas-flow equation is written as Where Δ p = p 2 - p 1 and is evaluated at some average pressure between p 1 and p 2. Steady-state radial horizontal liquid flowįor liquids, the product of B o μ o is approximately constant over a fairly wide pressure range so that for practical purposes, Eq.
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