Fluid Characterization of Oil Reservoirs: A Comprehensive Evaluation Between Bottomhole and Surface Samplings
Authors: Hendraningrat L. , Kechut N. I. 
Published: October 2024

Reservoir fluid samples provide crucial information for HSE (Health Safety and Environment) compliance, field and facilities development, flow assurance, asset integrity, and contaminants analyses. Failing to collect these samples may result in missing critical data needed for effective development upon completion of operations. Representative reservoir fluid samples are essential for obtaining high-quality data. Therefore, accurate sampling and fluid characterization are of utmost importance. This study presents a comprehensive analysis of bottomhole and surface separator samples from several undersaturated oil reservoirs with indicated multiple flow assurance issues.

Any fluid sampling program, whether surface or bottomhole samplings, should be designed based on the modeling requirements. There are several factors influencing the sampling and analysis program, which need to be addressed before conducting the activities. Among them are analysis objectives, reservoir fluid types, reservoir conditions, reservoir state, well design (including construction and accessibility), flow assurance issues, schedule, cost, and safety condition. These considerations are essential for determining whether surface or bottomhole sampling is required as well as the volume of the sample. However, when assessing fluid characterization in undersaturated oil reservoirs with multiple flow assurance issues such as wax and produced sand as discuss in this paper, using a combination of bottomhole and surface samples can provide a more comprehensive understanding of the reservoir fluids phase behaviour. In this study, four samples from two main oil reservoirs were studied, with each zone has both surface and bottomhole samples. Standard experimental procedures were employed to measure PVT fluid properties, including expansion and depletion studies. The fluid characterization was performed using PVT equation of states (EOS) model which was validated with laboratory data. Flow assurance analysis was conducted to observe and predict issues that might be evident in both bottomhole and surface samples. Black oil tables were generated from matched EOS models to observe deviations that could affect the fluid flow dynamic model.

All samples were analysed across measured reservoir temperatures range from 227°F to 250°F. Based on fluid compositional analysis, both bottomhole and surface samples showed a consistent number of wellstream components and did not show any complex naphthenic or aromatic mixtures. In terms of mole percentage, both sample types were consistent with acceptable deviation. The PVT EOS fluid characterization closely matched all laboratory experiments, by fine-tuning the critical properties. The advanced thermodynamics model from bottomhole and surface samples consistently predicted wax precipitation would occur in one of the reservoirs. These predictions were validated against actual field history data. Integrating PVT laboratory data, employing EOS models, and evaluating the benefits of each sampling method are crucial for developing accurate black oil tables for dynamic reservoir simulation studies.

This paper assists engineers make informed decisions whether bottomhole sampling is required or surface sampling is good enough for a comprehensive fluid phase behaviour study. If flow assurance issues are identified, using both bottomhole and surface samples can provide a more comprehensive understanding of the reservoir fluids behaviour.