The analysis of well-testing pressure data obtained at a well producing a gas condensate reservoir is considered. It is assumed that a liquid bank forms in the vicinity of the well. Only radial flow problems are considered; thus, our objective is to find semilog analysis procedures that can be applied to our two-phase flow problem.
The contributions and results of this research work can be summarized as follows:
We present theoretical results for drawdown and buildup semilog analysis equations based on the single-phase real gas pseudopressure of Al-Hussainy.
A theoretical equation for the apparent skin factor observed during buildup is derived. The apparent skin factor is a linear combination of the mechanical skin factor and a pseudoskin factor due to the existence of a two-phase zone near the wellbore. The value of the pseudoskin factor depends on the size of the liquid bank and the near-wellbore value of effective gas permeability with the two-phase zone.
It is shown that by combining results of drawdown and buildup analysis or by analyzing buildup data from a modified isochronal test, we can separate the components of the skin factor, i.e., to determine the mechanical skin factor, the radial extent of the liquid zone and the average effective gas permeability within the liquid zone.
The determination of the individual components is based on the theoretical equation for the buildup apparent skin factor.
In this work, we present simplified models for analyzing both drawdown and buildup data from retrograde gas condensate systems in terms of the real gas pseudopressure function, m(p). Jones and coworkers showed that drawdown or buildup pressure responses from retrograde gas condensate systems could be correlated with the classical liquid solutions, if the pressures were transformed to appropriate two-phase pseudopressure functions. In particular, Jones applied "reservoir integral" pseudopressure functions for analyzing pressure transient tests. For drawdown, the reservoir integral, mR, was defined as:
For drawdown, Jones showed that the following approximation is valid:
Similarly, reservoir integral forms were also defined for buildup, (see Eq. B-1, Appendix B). Unfortunately, since construction of mR(t) requires knowledge of the reservoir saturation and pressure profiles, along with a priori knowledge of the relative permeability-saturation relationship, the "reservoir integral" (although a useful theoretical tool) has not found practical application as a tool for analysis.
Jones also used the Boe et al. "sandface integral" two-phase pseudopressure function, mS(t), defined as:
Unlike the reservoir integral, the sandface integral requires measurement of properties only at the sandface; however, a priori knowledge of the relative permeability-saturation relationship is assumed. Jones showed that the reservoir and sandface integrals were equivalent for no skin damage; i.e., Eq. 2 holds with mR replaced by mS provided S = 0. However, more generally, for non-zero skin cases, the sandface integral exhibits the correct slope at late times, but the intercept (i.e., apparent skin factor) is much larger than the mechanical skin factor.