Effect of Lean Gas Injection on the Enhancement of Condensate Recovery in Gas Condensate Reservoirs

One of the problems currently in the oil and Gas industry is the difficulty for reaching a high condensate recovery in gas condensate reservoirs. Liquid dropout usually occurs in gas condensate reservoirs, when pressure decreases below the dew point, especially around the well bore. The liquid that is formed during the condensation is trapped by capillary forces or is left behind due to the low liquid relative permeability[1] Since saturation of this liquid is lower than critical saturation, so the liquid which is economically valuable cannot move in the reservoir and be produced.Gas cycling/injection is a common practice used in the oil and gas fields to alleviate this problem and enhance condensate recovery [2] by preventing condensate liquid loss and to help re-vaporizing retrograde liquid. This paper investigates the situation of one of the reservoirs located in southern Iran in Zagros area. First, based on the reservoir composition, the phase diagram has been plotted. Peng-Robinson equation of state forthe equilibrium calculations and Lee-Kesler characterization of heavy fractions are used in this software. After plotting the phase diagram and making clear the situation of the reservoir, the effect of nitrogen and pure methane and a composition of ethane and methane injections on reservoir productivity and recovery has been investigated and compared to natural depletion scheme. The full system including two separators and a stock tank are simulated simultaneously and the effect of each type of injection on the liquid and gas production is investigated. Full mixing has been assumed in all of the injections studied. By comparing between the results it is concluded that in injection process, required injection rate tomaintain reservoir pressure above dew point pressure and avoid liquid formation in the reservoir for pure methane and a composition of ethane and methane and pure nitrogen and Liquid recovery in all cases are investigated. According to the results, with increasing ethane mole percent in the injecting gas we need lower injecting rate for the same liquid recovery. Because with increasing heavy components mole percent in injecting gas, average molecular weight of injecting gas and reservoir gas becomes closer and there will be a better mixing between them and therefore liquid recovery will be improved. So, a composition of ethane and methane with more ethane mole percent is better than others. According to the results, nitrogen cannot prevent the reservoir conditions to go in two phase region. In addition, nitrogen can contaminate the reservoir gas and increase the separation process cost. Therefore, by comparing these injection plans, it is concluded that a composition of ethane and methane with more ethane mole percent is better than nitrogen, because the equipment required to N2 production and separation from production gas is highly expensive. So, N2 cannot be used for injection. Observed results show, a composition of 20% ethane and 80% methane have the maximum liquid recovery at the lowest injection rate respectively it seems that a composition of 20% ethane and 80% methane is the best option for injecting to the reservoir. The results show that liquid recovery by pure methane injection is higher than nitrogen injection and nitrogen injection cannot prevent liquid formation. All the computer programs used for this study were written using MATLAB software. To simplify the calculations, we assume that only one production and one injection well exist in the reservoir and there is a full miscibility between gases.

کلیدواژه ها:

recycling ، gas condensate recovery ، N2 injection ، methane injection