Pressure Drawdown and Buildup Analysis in Gas Condensate Reservoirs

Abstract

Abstract This paper introduces a new method to analyze pressure drawdown and buildup tests in gas condensate reservoirs producing under two-phase flow conditions. Integral method of pseudopressure is used to handle the physical properties and the phase change with pressure. The phase change is the result of the retrograde behavior of condensate fluids that occurs within the reservoir when conditions of pressure and temperature are right for it. Resulting phase change causes change in the liquid saturation thereby changing the relative permeability of that phase. To use the relative permeability as function saturation, saturation has to be known at all the times. This method, however, shows how to estimate the effective permeability of the liquid and gas phase as a function of pressure, skin factor, and the arial extent of the different regions at a given stage of the depletion. Effective permeability then can be used to project the well performance during the pseudosteady state, reserves, and well flow potential (AOF). During pressure test period, which is very short as compared with pseudostaedy state depletion, producing gas oil ratio is assumed constant. In other words oil and gas rates are held constant. In reality it might be hard to do so in a phase changing environment. So, a sensitivity analysis is performed to see the effect of the change in the producing gas oil ratio on the estimation of pseudopressure. We see steady increase in percent error in pseudopressure with increasing pressure at certain percent increase in Rp. At 8,200 psi, 10% percent increase in Rp introduces 3.96% error in pseudopressure. However, the error is 1.49% at 4,000 psi and 0.27% at 1,000 psi respectively. The error at 5% increase in Rp at 8,200, 4,000, and 1,000 is 2.13, 0.79, and 0.14% respectively. It is also observed that the amount of error in the effective gas permeability is 3% and 9% in oil effective permeability. This paper is divided in four parts. First the pseudopressure is simplified to separate the effective permeability terms. Second, well testing equations are modified in order to provide correlation for effective permeability of each phase. Third, the error in pseudopressure is determined in case over or under estimated GOR value is used. Finally, a field example is solved in a step-by-step manner to show the use of the technique.