Author: Team PetroPak

The knowledge of petroleum fluid phase behavior plays a crucial role throughout the Oil & Gas industry, from reserves estimation to reservoir production to surface processing to transportation and storage. Phase behavior modeling is vital, but often taken for granted. Whether an equation of state or a simple black-oil model is used, it needs to be specifically built and tuned to the experimental PVT data for each unique fluid. Only then can the model be used to make the necessary engineering predictions.

In physics and thermodynamics, an equation of state is a thermodynamic equation relating state variables which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature (PVT), or internal energy. Equations of state are useful in describing the properties of fluids, mixtures of fluids and solids. In a practical context, equations of state are instrumental for PVT calculations in reservoir engineering problems, such as petroleum gas/liquid equilibrium calculations. A reservoir fluid model based on a calibrated equation of state is the key to proper design, evaluation and prediction of fluid flow in hydrocarbon projects.

PVT measurements derive from a series of processes/experiments to establish phase behavior and physical properties/parameters of the reservoir fluid. The experiments through which these parameters are determined in the laboratory assume that a valid fluid sample is available for the tests, either through i) obtaining a single phase sample in the field, or ii) through careful recombination of separator gas and liquid in the laboratory. Validation of field samples and recombination are integral parts of PVT measurements. The majority of PVT measurements are derived from a relatively straightforward suite of experiments depending upon fluid type. All PVT measurements are carried out by measuring in a PVT cell the total fluid volume and compressibility over a wide range of pressures extending beyond initial reservoir pressure to pressures below the anticipated separator pressures. Depending on fluid type, the following are the common PVT experiment.

1. Constant Volume Expansion (CCE)
2. Differential Liberation
3. Constant Volume Depletion (CVD)
4. Separator tests
5. Measurement of physical properties

Measurement of phase behavior or pressure/volume/temperature (PVT) analysis is a key component of optimum utilization of hydrocarbon resources. Representative fluid samples are required to enable advanced physical and chemical analyses to be carried out in specialized fluid testing lab. Fluid samples are collected in the field either subsurface in the wellbore or at surface separator. Collected samples are then transferred from the field to the PVT lab under controlled conditions. Once in the lab, the following QA/QC tests are conducted to make sure the samples are representative:

1. Opening Pressure
2. Saturation Pressure at Separator Temperature
3. Air/Water contents
4. Compositional Analysis

Although thorough sample-checking procedures can identify some of the most obvious problems, there is never absolute certainty that the fluid under study is truly representative of the reservoir fluid. On occasion, laboratory measurements can show that a fluid is definitely not representative (e.g., saturation pressure is significantly higher than reservoir pressure

Special core analysis, often abbreviated SCAL are laboratory procedures for conducting flow experiments on core plugs taken from a hydrocarbon bearing formation. Special core analysis is distinguished from “routine core analysis” by adding more experiments, in particular including measurements of two phase flow properties, determining relative permeability and capillary pressure.

Special core analysis (SCAL) measurement capabilities:

• Capillary Pressure
• Relative Permeability
• Steady-state and Unsteady state
• Wettability Determination
• Reservoir Condition Core floods
• Petrophysical Correlation Measurements
• Archie Exponents – a, m, n
• Core GeoMechanics, Core Mechanical Properties