Fluid Characterization and EoS Modelling of PVT Experiments

Abstract Equations of state (EoS) are used to simulate laboratory pressure, volume and temperature (PVT) experiments. Each PVT experiment mimics a form of phase behavior progression for the reservoir fluid from the reservoir and ultimately to the surface facilities. The use of a characterized fluid sample in compositional reservoir simulation study is precluded by a comparison and regression of EoS simulations to PVT laboratory data. In this study, EoS phase behavior software was applied to match laboratory PVT data of several reservoir fluid samples from offshore Niger Delta. The paper presents the fluid characterization routines, simulations, and regression capabilities of the software which was used for the determination of EoS parameters needed to match experimental data. Four EoS models are available through the generalized Martin's equation in the software used. This enabled the use of Redlich-Kwong (RK), Soave-Redlich-Kwong (SRK), Peng-Robinson (PR) and Zudkevitch-Joffe (ZJ) EoS. Two 3-parameter extensions of the PR EoS are also available, one based on a Peneloux et al. volume shift, the other being an implementation of the Schmidt-Wenzel (SW) two-parameter PR EoS. The SRK EoS similarly has a three-parameter extension. This gives a total of seven different EoS models. The predictive capability, strength and weaknesses of these models were subsequently compared and analyzed. Effect of the degree of input fluid characterization and pseudodization on EoS PVT experimental modeling was investigated i.e. characterization to C7+, C12+, C20+, C30+, on the simulation output for each of the EoS models available in the software. The results showed varying degree of simulated experimental data congruence depending on the PVT experiment being modeled. With regression, both the PR and ZJ EoS gave comparable and generally good agreement with laboratory data. Without regression or significant tuning of EoS parameters, neither EoS adequately predicts observed reservoir fluid PVT behavior. This work further elucidates the succinct effect of fluid models used in compositional reservoir simulations which forms the basis for different production scenario and realizations.