Best Practice of Accurate Phase Behavior Characterization and PVT Modeling for Supercritical Fluid Reservoir Coupled with Compositional Gradient–A Case Study of S Field, Southern Iraq

The supercritical fluid reservoir is especially unique compared to conventional oil and gas resources and usually exists at high pressure and high temperature (HPHT) conditions by exhibiting equivalent PVT properties and phase behavior for gas and liquid in the subsurface at initial state (Jiang, 2021; Luo et al., 1999; Praskovya et al., 2022; Peyman et al., 2009). A representative supercritical fluid reservoir is gas condensate that is extensively distributed in deep or ultra-deep formations with a typically reservoir pressure of 50MPa and temperature of 100°C (Shao, 2024). In the last decades, plenty of supercritical fluid reservoirs have been found around the world with coexistence of near-critical fluid reservoirs, like volatile oil (Gao et al., 2023; Panja et al., 2019;). Both supercritical and near-critical fluid reservoirs present similar phase behavior characteristics and play an important role in the global oil industry to meet the increasing demand of energy. In this study, our research will focus on not only supercritical (gas condensate), but also near-critical (volatile oil) fluid reservoir. Differing from traditional definition, the supercritical fluid reservoir discussed in this paper exclusively refers to both type of reservoirs. It is always difficult to exploit supercritical fluid reservoir with complex compositions and HPHT environment as it usually behaves extremely different variation of physical properties and production features in comparison with common black oil or dry gas reservoir once the reservoir pressure is below dew point for gas condensate or bubble point for volatile oil. In particular, a significant phenomenon of compositional gradient is frequently observed in these reservoirs (Lars et al., 2000; Mokhtari et al., 2013; Mehrdad et al., 2023), in which the mole fraction of components as well as associated PVT properties such as density, gas-oil ratio (GOR), saturation pressure, and formation volume factor (FVF) varies with depth. This phenomenon is particularly obvious in moderate-giant carbonate reservoirs and imposes huge impact on every aspect of oil-gas field development (Chen, 2022). Numerous studies (Cornelisse, 2014; Julius et al., 2014; Mohammad, 2020; Rafael et al., 1999) have shown that the sound understanding and accurate characterization of phase behavior and PVT modeling is the key for the successful development of such reservoirs. It is a fundamental step within the whole spectrum of oil and gas field exploitation including drilling, completion, production, surface facility, as well as pipeline gathering and transportation. The major challenges faced in the field implementation are (1) to acquire the originally representative fluid sample from target reservoir, (2) to obtain accurate fluid properties through PVT experiments, (3) to establish the correct PVT models, and (4) to achieve a good match of production history and predict future performance accurately by incorporating PVT models in the reservoir simulation.