Abstract

Coreflood experiments were conducted in Berea sandstone and Texas Cream limestone cores to quantify the loss in relative permeability caused by condensate accumulation. The in-situ condensate saturation was established dynamically by precise control of core inlet and outlet pressures. It is well known that retrograde condensate dropout can cause significant productivity loss in low permeability reservoirs. This paper shows that such losses can also occur in high permeability reservoirs. Gas relative permeability reductions of 91% to 97% were seen in 2–5 md limestone cores and 95% to 98% in 246–378 md sandstone cores. In light of the new data presented here, the common perception that condensate blocking around wells in high-permeability reservoirs is not significant should be re-examined. Our previous coreflood experiments showed that methanol treatments increased the gas relative permeability in low permeability carbonates, but we had not yet determined the required methanol treatment volumes as done in this study. Methanol displaces retrograde condensate and maintains improved gas relative permeability well into the post-treatment production period. Methanol also displaces water and this can also contribute to higher gas relative permeability in those cases when the initial water saturation is high enough to significantly add to the total liquid blocking of the gas. These results can be used to help reservoir engineers evaluate and treat gas-condensate wells with reduced productivity. Reservoir engineers should be especially careful to evaluate the damage done in such high-permeability reservoirs if the well's pressure drawdown is high enough to result in pressures below the dew point pressure.

Introduction

Gas production from reservoirs having a bottom hole flowing pressure below the dewpoint pressure results in an accumulation of a liquid hydrocarbon near the wells. This condensate accumulation reduces the gas relative permeability, which is known as condensate blocking, and thus the well's productivity. Condensate saturations near the well can reach as high as 50–60% under pseudo steady-state flow of gas and condensate.1 Even when the gas is very lean such as in the Arun field with a maximum liquid drop out of 1.1%, condensate blocking can cause a drastic decline in well productivity.2–4 The Cal Canal Field in California showed a very poor recovery of 10% of the original gas-in-place because of the dual effect of condensate banking and high water saturation.5

Several methods have been proposed to restore gas production rates after a decline due to condensate and/or water blocking.6,7 Gas cycling has been used to maintain reservoir pressure above the dewpoint pressure. Injection of dry gas into a retrograde gas-condensate reservoir vaporizes condensate and increases its dewpoint pressure.8 Injection of propane was experimentally found to decrease the dewpoint and vaporize condensate more efficiently than carbon dioxide.9 Hydraulic fracturing has been used to enhance gas productivity, but is not always feasible or cost-effective.5,10 Inducing hydraulic fractures into the formation can increase the bottom hole pressure. Hydraulic fracturing successfully restored the gas productivity of a well that died after the flowing bottom hole pressure dropped below the dewpoint.11

Recently, a new strategy of using solvents was developed to increase gas relative permeability reduced by condensate blocking.7 Methanol was found to be effective to remove condensate and water and restore gas productivity in low permeability limestone cores. This study is a continuation of that research to explore the effect of condensate blocking in high permeability cores and quantify the volumes of methanol required.

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