Mass Balance-Based Quality Control of PVT Results of Reservoir Oil DL Studies

Abstract

Fluid properties live at the heart of hydrocarbon reservoir engineering as it is related to the behavior of fluids under reservoir pressure depletion. To obtain their values, PVT experimental work needs to be performed. Although the experimental workflow is strictly defined, the accuracy of the measurements often suffers from significant errors which in turn affect severely all the following engineering calculations as well as the following equation of state (EoS) tuning step. In this work, a systematic methodology is developed to apply quality control (QC) on the PVT values reported in a typical lab report. Firstly, the equations expressing mass balance are developed to calculate the missing closing data, such as the residual oil density and the composition of oil collected at each stage of the depletion study. Subsequently, computational tools are developed to evaluate the physical soundness of the received results and provide insight as to whether the quality of the experimental data is sufficient. To demonstrate the proposed workflow, it is applied to a selection of representative reservoir fluids of varying volatility. We demonstrate that depending on the reservoir fluid properties, calculations may yield highly unrealistic results, which engineers should avoid using unless corrected. Specifically, differential liberation (DL) test properties reported for low-volatility oils are generally reliable. However, for high-volatility oils, particularly during the final depletion stages, the test outcomes often become unrealistic. Finally, instructions to fluid and reservoir engineers on how to handle those issues and protect the reliability of their calculations are provided.

Keywords

• reservoir oils
• differential liberation
• PVT
• quality control
• mass balance
• EoS model tuning