17.1 Introduction

Various types of skin factor evaluation techniques and impact on gas well performance with examples are described in this chapter; more discussion including equations is given, and the accuracy, data requirements, and results of these techniques are presented in detail.

17.2 Rate-Dependent Skin Factor

In many gas wells, the observed flow rate is different from that calculated theoretically. As noted in Chapter 1, the concept of skin factor was developed to account for the deviation from the theoretical rate. For example a gas well located centrally in the drainage plane, during pseudo-steady-state flow, the gas flow rate can be calculated as follows.

In term of pressure-squared treatment:

In term of pseudopressure treatment:

All parameters in Eqs. 17 1 and 17 2 are in U.S. gas field units and s _t is total skin factor, which includes the effects of partial penetration, perforation density, mechanical skin damage due to drilling and completion, etc. Positive skin effects can be created by mechanical causes such as partial completion (i.e., a perforated height that is less than the reservoir height) and inadequate number of perforations (again, causing a distortion of flow lines), by phase changes (relative permeability reduction to the main fluid), by turbulence, and, of course, by damage to the natural reservoir permeability.

Negative skin effects would result in a flow enhancement. The pressure drop in the near-wellbore zone is less than it would have been from the normal, undisturbed, reservoir flow mechanisms. Such negative skin effects, or a negative contribution...