Abstract

Due to "condensate blocking" near well-bore, well deliverability in the gas condensate reservoir often decreases from initial value. This deliverability loss has been investigated by conducting a series of compositional simulations with an actual reservoir data. The well gas productivity index (PI) at initial reservoir pressure of 4750 psia for a single-phase gas flow was found to drop rapidly approximately 10% to 35% due to single phase transient effects. As the reservoir pressure drops below the dew point pressure, this PI drop slows down but continually losses for 20% till movable oil saturation reaches in reservoir. Then another further rapid loss of about 20% of the initial gas PI occurs due to the oil flow and later a continued loss come in PI as reservoir pressure declines. The gas PI at a reservoir pressure of 2000 psia is only 10–20% of the initial value (at reservoir pressure 4750 psia).

We also found that both fluids PVT and absolute permeability of the reservoir have significant influences on the well productivity.

Introduction

In gas condensate reservoirs, when the flowing bottomhole pressure falls below the dew point pressure of the reservoir fluid, condensate liquid starts to drop out from gas and a relatively high liquid saturation builds up near the wellbore will occur. This liquid saturation build-up leads to a reduction in gas relative permeabilities and thus a loss in gas well productivity. This effect is referred to as "condensate blocking" in the literature (1, 2, 3). Due to this "condensate blocking" near well-bore, well deliverability in the gas condensate reservoir often decreases from initial value. This deliverability loss has been investigated by conducting a series of compositional simulations with an actual reservoir data in this paper.

Gas Condensate Flow Behavior A model that describes the subsurface gas condensate flow recognizes the existence of three regions, as presented in Figure 1. Whitson (3) divided the flow in the condensate reservoir into following three regions.

Region 1: An inner near-wellbore region where both gas and oil flow simultaneously at different velocities. This region has a constant flowing Gas-Oil ratio (GOR) equal to the producing GOR. The size of region 1 increases with time and stabilizes when the single phase gas entering this region has sufficient mobility for it to flow without any net accumulation.

Region 2: This region defines the net accumulation of condensate in sections of the reservoir where the pressure is below the gas dew point pressure. There is only gas flow (no liquid) in this region as the condensate is immobile. This is a region of liquid saturation build-up.

Region 3: This region consists of single phase gas as the pressure in this region is above the dew point pressure. There is no hydrocarbon liquid in this region.

Model Description

In order to investigate factors contributing to gas-flow impairment due to condensate buildup, single-well radial compositional reservoir simulation models were extracted from the earth model with an out boundary of 3000ft.

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