Comparative Evaluation of Gas-Condensate Enhanced Recovery Methods for Deep Ukrainian Reservoirs: Synthetic Case Study

Abstract

Low value of final condensate recoveries achieved under natural depletion require implementation of enhanced gas recovery (EGR) methods to be implemented for the efficient development of gas-condensate reservoirs. The study was performed using synthetic numerical 9-component compositional simulation model that approximated the typical conditions of deep gas-condensate reservoirs of Dnieper-Donetsk Basin in Easter Ukraine. Injection of water, methane, nitrogen, carbon dioxide, mixture of methane and nitrogen, mixture of methane, ethane and propane at different concentrations were evaluated at 50% and 100% voidage replacement for reservoir fluids with 100 g/m³, 300 g/m³ and 500g/m³ potential condensate yield. Condensate recovery studied at different stages after primary depletion, when reservoir pressure reached 25, 50, 75% from dew point and at pressure of maximum liquid dropout. Results comparison was done based on the two criteria: technical efficiency – incremental condensate recovery towards the base depletion cases and economic efficiency – cumulative NPV. Status of initial depletion as well as voidage replacement have a direct impact on breakthrough time and negative economic indicators. Despite providing the highest incremental condensate recovery by injecting CO₂ at 100% voidage, it has a strong negative economic effect. Based on incremental condensate recovery EGR methods are ranked as following for all condensate potential yields and levels of primary depletion: CO₂ 100%; solvent gas mixture of C₁ 90%, C₂ 5%, C₃ 5%; solvent gas mixture C₁ 98%, C₂ 1%, C₃ 1%; C₁ 100%; mixture of C₁ 50% and N₂ 50%; N₂ 100%; water. Economically, the highest efficiency was shown for C₁ 100% injection, due to the fact, that produced re-cycled gas has a sales value as well. For the maximum incremental recovery it is advisable to start the injection as early as possible, while highest economic increments received for the cases of delayed injection, particularly when the reservoir pressure is equal to the pressure of maximum liquid condensation. The results of study can be used a guidance for rapid screening of applicable EGR method for gas-condensate fields depending on depletion stage and potential condensate yield.