Abstract

The most crucial region affecting well productivity is the perforated region.An accurate determination of perforation performance, required in the optimum design and well productivity estimation, could be made by fine grid simulation, but it would be costly with many technical limitations.The alternative practical approach is the use of a skin factor in open hole calculations resulting in the same flow performance as that of the perforated well.However most of the skin calculations done today are based on single-phase flow conditions.

In this work, we have developed a method to determine the effect of perforation on gas condensate well productivity by defining a skin factor to be used in the pseudo pressure application.The main problem is that pseudo pressure in such systems is a function of velocity and the conventional skin factor, which is calculated for a perforated well, cannot be used directly in open hole calculations.This is due to the fact that the gas and condensate relative permeability (k[r]) can increase significantly by increasing the flow rate, contrary to the common understanding.This effect, known as positive coupling, complicates the flow of gas and condensate near the wellbore even further when it competes with inertial losses at higher velocities typical of those around perforation tips.

The above complications, which require information on velocity-location in the perforation region, have been tackled by proposing new techniques with an emphasis on their practicality.Sensitivity studies, comparing the results of developed methods with those using finite element methods, indicated the reliability of the proposed techniques.The methods can be used to estimate variations of skin due to long-term condensate banking and the need for any intervention and remedial action.

Introduction

The significance of perforation characteristics on productivity of perforated wells has been studied by several investigators [1–3], focusing mostly on single-phase flow in a perforated region.The limited literature available on multi-phase flow in a perforated region usually involves major oversimplifications in representing the actual complex flow behaviour [4–5].

In gas-condensate systems, at relatively low interfacial tension (IFT) values, the process of condensation around the wellbore, when the pressure falls below the dew point, creates a region in which both gas and condensate phases flow.The flow behaviour in this region is controlled by the viscous, capillary and inertial forces.This along with the presence of condensate in all the pores dictates a flow mechanism that is different to that of gas-oil and also gas-condensate in the reservoir bulk.The improvement of relative permeability of condensing systems due to an increase in velocity, as well as that caused by a reduction in interfacial tension, is a well established finding both experimentally [6–7] and theoretically [8–9].This flow behaviour, referred to as the "positive coupling", has been shown to be in strong competition with negative inertia at very high velocities [10].Jamiolahmady et al. [11] have recently developed a new generalised fractional flow based correlation, which expresses the combined effects.

In perforated well studies, the effect of perforation on well productivity is usually expressed by a skin factor [12].This skin factor is then used in open hole simulations giving the same flow performance as that of perforated well.In the case of single-phase incompressible flow, skin is introduced in the form of an additional pressure drop at the wellbore face.For the case of compressible gas and that of two-phase flow the concept of pseudo-pressure has been introduced [13] to preserve similarities with that of incompressible flow by representing the skin in the form of an additional pseudo-pressure drop, "For the former" accounts for the variation of fluid properties with pressure whilst for the latter it also includes the relative permeability.

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