On the Use of Model Experiments for Assessing Improved Gas-Condensate Mobility Under Near-Wellbore Flow Conditions
Authors: Boom W. , Wit K. , Zeelenberg J.P.W. , Weeda H.C. , Maas J.G. 
Published: December 2007

Recently, model experiments were described to assess improved gas/condensate mobility under near-wellbore flow conditions. The improved mobility alleviates well impairment due to condensate dropout. A better assessment of this effect allows a more reliable well deliverability forecasting. The model experiments revealed a significant increase in mobility, controlled by the capillary number and not by the interfacial tension alone.

The model approach is cost- and time-effective and allows one to scan wide ranges in capillary number, interfacial tension, absolute and relative permeability and saturation. This paper aims at a validation of the model approach. It comprises of a demonstration of the equivalence of Bond and capillary number, i.e. a comparison between centrifuge and steady-state experiments, an experimental examination of the impact of connate water, an experimental evaluation of the differences between in-situ condensation, drainage and imbibition and a direct comparison between model experiments and gas/condensate experiments at elevated conditions.

Introduction

Retrograde gas will condense when the pressure of a gas/condensate system drops below its dewpoint. Even when the average pressure in a gas/condensate reservoir amply exceeds the dewpoint pressure, a condensate bank may form almost immediately in the near-wellbore region, as the producing well constitutes a pressure sink. This bank impairs the production of gas. Even at low condensate content, high condensate saturations may build up as many pore volumes of gas pass through the near-wellbore region. The degree of impairment depends on the mobility of the gas and the condensate. More specifically, depending on the gas and condensate relative permeability, a semi steady-state quickly develops, in which the (radial distribution of the) condensate saturation allows the flow of fresh gas and condensate from deep in the reservoir into the near-wellbore region to be transported to the wellbore. With declining reservoir pressure, the condensate bank slowly expands into the reservoir.

In literature only limited documented field evidence of well impairment due to condensate dropout can be found. This might be attributed to an enhancement of gas and condensate mobility under near-wellbore flow conditions, i.e. at low interfacial tension and large pressure gradient, which alleviates this kind of impairment. Experimental quantification of the enhancement in mobility. identification of the controlling parameter(s) and incorporation of this effect in the reservoir model, will eventually lead to better screening criteria for and assessments of well impairment due to condensate dropout. Hence our research focus on this phenomenon.

Recently, we have described model experiments to assess the improved gas/condensate mobility under near-wellbore flow conditions. Although the experiments used core material from actual gas/condensate reservoirs, they were model experiments in that the gas and condensate phase system were represented by a two-phase liquid-liquid system at low interfacial tension (no connate water was present) and a large centrifugal acceleration substituted for the large pressure gradient in the near-wellbore region.

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