Daño A La Formacion En Pozos Gasiferos

Chapter 5
Pressure Transient Testing for Gas Wells
5.1

Introduction

In the preceding chapters, we have considered only single-phase flow of a slightly
compressible liquid. This chapter focuses on single-phase flow of real gases. For gas
wells, there are two factors that must be addressed: 1) gas fluid properties are functions of
pressure, and 2) gas wells often exhibit rate dependentor non-Darcy skin factor.
Section 5.2 introduces the use of pseudopressure, adjusted pressure, and adjusted
time to account for pressure-dependent gas properties in the flow equation for gases. The
pseudopressure and adjusted pressure integral transforms provide an exact linearization
of the gas flow equation for steady state flow. The adjusted pressure has the added
advantage that the gasflow equation has exactly the same form as that for slightly
compressible liquids. Similarly, the adjusted time, used in conjunction with adjusted
pressure, linearizes the gas material balance equation describing volumetric behavior.
Section 5.3 discusses rate-dependent, or non-Darcy, skin factor. The section
gives equations for estimating the non-Darcy flow coefficient D for open-hole andperforated completions.
Drawdown tests are primarily a flow phenomenon, thus must be described using
pseudopressure or adjusted pressure along with real time. Section 5.4 shows how to use
adjusted pressure and elapsed test time to analyze gas well pressure drawdown tests
using semilog analysis, log-log type curve analysis, and manual log-log analysis.
Unlike drawdown tests, buildup tests areprimarily a volumetric phenomenon,
thus both pseudopressure and pseudotime, or equivalently and more conveniently,
adjusted pressure and adjusted time. Section 5.5 shows how to use adjusted pressure and
adjusted time to analyze gas well pressure buildup tests, using semilog, log-log type
curve, and manual log-log methods.
After completing this chapter, the reader should be able to:
1.Calculate the real gas pseudopressure and adjusted pressure;
2. Calculate the pseudotime and adjusted time for a buildup test;
3. Estimate the non-Darcy flow coefficient for an openhole completion or a
perforated completion;
4. Analyze a gas well drawdown test using semilog, log-log type curve, or manual
log-log methods;
5. Calculate the adjusted Horner time ratio and equivalent adjusted time fora gas
well buildup test; and
6. Analyze a gas well buildup test using semilog, log-log type curve, or manual loglog methods.

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Applied Well Test Interpretation

5.2

Gas Flow Equation

In deriving the diffusivity equation, Eq. 2.20, we assumed a slightly compressible liquid
with constant viscosity µ and constant total compressibility ct. However, for gasreservoirs, formation volume factor, compressibility, and viscosity are all functions of
pressure.
For small changes in pressure, Eq. 2.20 may be used to describe flow of gases as
well as liquids. As a general rule, if the change in pressure during drawdown is less than
10% of the initial pressure, the liquid equations may be used with acceptable results.

5.2.1

Pseudopressure and AdjustedPressure

To account for pressure dependent gas properties in a more general way, we can develop
a gas flow equation having a form similar to that of the diffusivity equation by combining
the continuity equation, Eq. 2.1, Darcy’s law, Eq. 2.3, and the real gas law equation of
state, Eq. 5.1:
pV = znRT........................................................................................................(5.1)
Traditionally, the gas flow equation is written in terms of the real gas pseudopressure
function m(p), defined as (Agarwal, 1979)
p

m( p ) ≡ 2 ∫
0

p ′dp ′
.........................................................................................(5.2)
µ ( p ′)z ( p ′)

The integral in Eq. 5.2 may be evaluated using any of a number of numerical integration
methods*....