A Novel Method to Measure the Phase Behavior of Black Oils: Saturation Pressure and Liquid-Vapor Fractions

Abstract

Abstract Building a robust PVT model critically relies on accurate phase behavior data which has been traditionally obtained using PVT cells. While the PVT cell can provide accurate data, it requires a large volume of downhole or recombined samples which are usually expensive to collect. A novel microfluidic chip design and method is presented in this work to rapidly measure bubble point and liquid-vapor volume fractions of black oil systems at multiple pressures and temperatures. The chip was initially charged with a representative single-phase live oil at a reservoir temperature. Afterward, the pressure was lowered to subsequent pressure steps to measure the saturation pressure, and liquid and vapor volumes. The waiting time at each pressure step was adjusted to ensure that the equilibrium condition was achieved. The aforementioned procedure was performed at multiple temperatures to measure corresponding saturation pressures and L-V fractions, ultimately generating a partial phase envelope of the test oil sample. The measurements were conducted for various oil samples with a wide range of API gravity. The high-resolution optical access along with an in-house developed automated image analysis algorithm were used to detect the saturation pressures and quantify the L-V fractions. The saturation pressures for each of the tested crude oils were compared with those obtained from conventional Constant Composition Expansion method, showing a tight agreement between the data (i.e., within less than 5% deviation). The measured microfluidic L-V fractions of each sample are also in strong agreement with those obtained by conventional methods, where available. Given the very small volume of oil sample, easier control on operating parameters, and faster run-time and analysis time required for this microfluidic approach, the phase envelope of a testing oil can be determined in a day. The microfluidic platform developed in this work can be an alternative approach to some of the conventional PVT tests with an order of magnitude higher lab throughput. This makes PVT data accessible by reducing cost, and sample size requirements, and potentially moves the energy industry to a data-on-demand model. With a much smaller physical size inherent to microfluidic devices, this platform can be deployed to operations sites, alleviating the sample handling and shipment challenges that industry currently struggles with.