Vertical composition gradient effects on original hydrocarbon in place volumes and liquid recovery for volatile oil and gas condensate reservoirs

Abstract

Around the world, volatile oil and retrograde gas reservoirs are considered as complex thermodynamic systems and even more when they exhibit vertical composition variations. Those systems must be characterized by an equation of state (EOS) to properly describe the thermodynamic behavior of the fluid in any compositional simulation. Thus, one can obtain accurate values of hydrocarbon fluids in place, reservoir performance and ultimate liquid recovery. Vertical compositional gradients are present in all type of reservoir fluids, from black oils to wet gases. However, the compositional gradient effect has been neglected for black oil and for wet and dry gases, but it cannot be ignored in volatile oil and gas condensate systems. This can be demonstrated by analyzing its effect upon the original gas and oil in place calculations and liquid recovery for volatile and gas condensate reservoirs. In this study we evaluated the hydrocarbon in place and liquid recovery when the compositional gradient was used or neglected during a depletion process. To accomplish this, we analyzed several z-grid sizes and well completion cases, for volatile oil, gas condensate and two-phase reservoirs. The studied reservoir fluid belongs to the Cusiana gas condensate field in Colombia. The experimental PVT tests used consisted of constant composition expansion (CCE) and constant volume depletion (CVD) and were reported by Brunal. This study was accomplished by tuning an equation of state, EOS, to the available CCE and CVD test data, generating tables of composition versus depth, and simulating a depletion process using several scenarios. The software used to calibrate the EOS and to generate the composition-depth tables is PVT-i and for the compositional simulation we used Eclipse 300. The proposed approach provided guidelines on the proper way to calculate properly the volume of oil and gas in place and liquid recovery for three types of reservoirs: a volatile oil, a gas condensate, and a two-phase reservoir. The last reservoir was considered when liquid and gas phases coexist at the initial reservoir conditions. The result of this study addressed the discrepancies that take place when the compositional gradient is not considered.