Integrating PVT Properties for the Description of Well Responses in Gas Condensate Reservoirs

Abstract

A gas condensate reservoir exhibits complex behaviors when the bottomhole pressure falls below the dew point pressure at a given reservoir temperature. When the condensate oil begins to drop out from the gas, a two-phase fluid system develops and a bank of condensate oil builds up, inducing severe productivity losses. While the production rate is constant, different mobility zones are formed around the wellbore corresponding respectively to the original-gas-in-place (OGIP) away from the well, the condensate bank with only gas flow, and two-phase gas and oil flow near the wellbore. Thus, the behaviors of gas condensate systems are complex and difficult to interpret. In this thesis, a single well model is built to evaluate the dynamic performance of an infinite and homogeneous gas condensate reservoir. Firstly, apparent compressibility is defined by integrating PVT properties. The application of modified pseudo-pressure and pseudo-time linearizes the partial differential equations with the non-linearity caused by gas properties. Secondly, a three-region method accounts for the composition changes in the reservoir. Fluid flow towards the well during depletion can be divided into three concentric main flow regions, from the wellbore to the reservoir. An analytical model could have been built directly from the three-region method. Thirdly, on the basis of the three-region method, the semi-analytical model is developed by dividing the whole reservoir into multiple sub-radial regions. In the modeling process, the discretized subradial regions are hydraulically coupled with nearby sub-radial regions so that an ultimate linearized system is generated to obtain bottomhole pressure responses. Finally, a moving boundary is also taken into consideration to investigate the difference between a consistent boundary model and a moving boundary model. All models have been validated and can be successfully used to analyze pressure and production data of gas condensate production wells. This thesis has contributed to production from gas condensate reservoirs with detailed studies on the inherent PVT properties, condensate banks, and the interference of adjacent regions. The modeling results provide reliable perspectives of transient pressure analysis in gas condensate reservoirs and help characterize and estimate the drainage areas of the three regions mentioned above, which is critical in gas condensate reservoir development. Furthermore, this model builds a consolidated foundation for further investigation of reservoir heterogeneity in the development of unconventional reservoirs.