Laboratory and Field-Trial Results of Condensate-Bank Removal in Retrograde-Gas Reservoirs
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 102558, "Laboratory and Field-Trial Results of Condensate-Banking Removal in Retrograde-Gas Reservoirs: Case History," by F.O. Garzon, SPE, H.A. Al-Anazi, SPE, J.A. Leal, SPE, and M.G. Al-Faifi, Saudi Aramco, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September.

Gas-condensate reservoirs experience significant productivity losses when the reservoir pressure declines below the dewpoint. This productivity loss is caused by liquid condensation near the wellbore and the subsequent decline in gas rate because of the reduced gas relative permeability. Gas-condensate wells often are fracture stimulated to reduce the drawdown and subsequent formation of a condensate bank in the vicinity of the well. The use of filtered diesel to remove the condensate bank from gas wells was investigated.

Introduction

Condensate banking can be mitigated or delayed by different methods. The most efficient and costly method is implementing a pressure-maintenance program to keep the reservoir pressure and the producing-well sandface pressure higher than the dewpoint pressure. This method prevents condensation (liquid drop-out) in the reservoir and, therefore, prevents creation of a condensate bank. A common method used to delay creation of this bank around the wellbore and keep the bottomhole flowing pressure above the dew-point pressure is to reduce the drawdown by fracture stimulating vertical wells or by drilling horizontal wells. Chemical treatments also can be used to restore well productivity by removing the condensate bank.

Chemical treatments with solvents or wettability-alteration agents have been studied with promising results. Alcohols can be used as solvents to remove condensate and/or water blockage and enhance gas relative permeability. A methanol treatment was implemented on a gas well in the Hatter Pond field that increased gas- and condensate-production rates. Altering the wettability of the reservoir from liquid-wet to intermediate-gas-wet can reduce condensate banking. Experiments performed on Berea and reservoir cores showed that some fluorinated polymeric surfactants increased gas relative permeability and altered core wettability.

In gas wells that have been fracture stimulated, the likelihood of developing a condensate bank near the wellbore is high because of a lack of pressure support and/or lower bottomhole flowing pressure in the well. The use of filtered diesel in the "X" formation was studied to investigate its effectiveness in removing the condensate bank and enhancing gas productivity. The study also ensured that the treatment had no adverse effect on reservoir permeability.

Experimental Studies

Coreflood experiments were performed to determine the effectiveness of treatments in removing condensate banking and enhancing gas relative permeability. Tests were conducted at simulated reservoir conditions of 230°F and 1,500 psig on reservoir cores recovered from different gas wells. Coreflood tests were divided into single- and two-phase tests. Single-phase tests optimized the concentration of mutual solvent (MS) and surfactants.