New Approach Towards Understanding of Near Well Bore Behaviour of Perforated Completions
Authors: Saleh Amer M. , Stewart George 
Published: December 2007

This paper considers the steady-state flow behaviour of dry gas and gas condensate fluid flow towards perforated completions and the possibility of obtaining an analytical solution to the problem of flow convergence into the individual perforations and estimating the perforation skin factor. An analytical solution of the skin factor for both steady-state Darcy and non-Darcy flow is presented and all the interacting perforation parameters as well as formation anistropy have been considered. The skin model is based on the observation that the non-Darcy pressure drop of high rate gas wells is located at the tips of perforations. Accordingly a spherical - like flow into perforation has been modeled. The single phase results have been extended to account for two-phase condensate-gas flow. In case of two-phase flow it was assumed that the liquid drop-out will contribute to the total skin factor by an extra pressure drop. Accordingly the liquid drop-out behaviour and the liquid drop-out contribution to the skin, which is of importance in the understanding of gas condensate reservoirs, have been examined. In gas condensate systems the problem of non-Darcy flow skin factor is both pressure and rate dependent. A comprehensive understanding of pressure and rate dependency of the skin factor of two-phase flow applied to gas condensate reservoirs has been achieved. Our understanding of gas condensate flow behaviour was approached through the application of the two-phase pseudo-pressure function (2PPP) based on the steady-state flow. This paper also addresses the problem of evaluating gas condensate inflow performance (GCIPR) diagrams and provides new ideas to understand them. The influence of the liquid drop-out on the shape of the GCIPR has also been investigated and documented.

Introduction

The concept of a well skin factor, S, was first introduced by Van Everdingen and Hurst to explain the effect of formation damage on well productivity. For an open hole completion with a cylindrical altered region of radius, ra, and reduced permeability, ka, the theoretical skin is given by the familiar Hawkins equation:

(1)

Here ps is the incremental pressure drop occurring in the wellbore region due to the changed permeability. The well skin factor can be determined in the field from transient pressure testing but this does not allow the independent calculations of ka and ra from Eq. 1. Presuming ra to be associated with the depth of invasion of mud filtrate then an estimate of this quantity is possible via resistivity logs and hence ka - the damage zone permeability - can be computed in principle from Eq. 1 if S has been determined from a well test. When a well is cased and perforated the skin factor represents the combined effect of perforation and formation damage.

Obviously there is a complicated three dimensional flow distribution in the vicinity of the wellbore with localised flow concentration into the individual perforations. It is interesting that the first study of the detailed nature of the flow into a perforated well was carried out by Muskat in 1950 using an electrolytic tank analogue model. In more recent investigations sophisticated finite element numerical simulations have been employed to model the flow behavior. An approximate analytical model will be developed here which can be used to correlate the results of such studies and provide physical insight into the flow mechanism. In general the inflow behaviour of a perforated completion depends on the following key factors:

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