PVT Analysis of Reservoir Fluids: The 14 Core Tests Explained

The Constant Composition Expansion (CCE), historically also called Constant Mass Expansion (CME) or flash expansion, is the foundational depletion experiment in any black-oil, volatile-oil or gas-condensate PVT study. It is almost always the first test performed after recombination/validation of the sample, because it locates the saturation pressure that anchors every subsequent test (differential liberation, separator tests, CVD).

Differential Liberation, also called Differential Vaporization or Differential Gas Liberation (and historically the "differential gas-oil ratio" experiment), is the second cornerstone depletion study run on a reservoir oil sample. Together with Constant Composition Expansion it forms the backbone of the black-oil PVT report and supplies the reservoir-condition fluid properties that drive material-balance and reservoir-simulation calculations.

The multiple-contact miscibility test, also called the multiple-contact mixing or multcontact experiment, reproduces in the laboratory the repeated, stage-wise mass exchange that occurs when an injection gas (\(CO_2\), \(CH_4\), \(N_2\), lean or enriched hydrocarbon gas, or a flue-gas blend) advances through reservoir oil during a gas-drive enhanced-oil-recovery (EOR) process. Unlike the single-stage flash, the differential liberation, or the swelling test (each of which contacts the oil with gas only once), the MCM test forces the two phases through a sequence of equilibrations so that the compositional path of the displacement front, and ultimately the development of miscibility, can be observed directly.

For gas condensate fluids the Constant Composition Expansion is commonly run under the names Constant Mass Expansion (CME), Constant Mass Depletion (CMD), Constant Mass Study, or Pressure-Volume (P-V) relation. It is the first volumetric experiment performed on a recombined gas condensate sample and establishes the upper anchor of the entire condensate phase envelope: the retrograde dew point.

Fluid type: gas condensate (retrograde gas)

The dew point determination is the cornerstone constant-composition experiment for any gas condensate reservoir. It anchors the entire phase envelope on the gas side and is the single most consequential number extracted from a condensate PVT study, because retrograde behavior, condensate banking, and ultimate liquid recovery all key off where, in pressure space, the first hydrocarbon liquid appears.

The retrograde condensation (liquid dropout) test maps how much liquid phase forms inside a gas condensate as reservoir pressure declines below the dew point. It is one of the defining experiments for gas condensate reservoirs because, unlike black oils, these fluids exhibit the counter intuitive behavior in which lowering pressure first creates liquid, drives it to a maximum, and then partially re vaporizes it.

The Gas Depletion test reproduces, at the cell scale, the non-equilibrium material-balance history that a dry or wet gas reservoir undergoes as it is produced down its decline curve. Unlike a Constant Composition Expansion, where nothing leaves the cell, here gas is physically withdrawn at each pressure step, so the test couples a thermodynamic state determination with a production accounting that an equation-of-state (EOS) simulator cannot reproduce in a single equilibrium flash.

The Gas Flash test takes a sample of reservoir gas at reservoir pressure and temperature and subjects it to a single, abrupt pressure reduction to a defined flash pressure (separator or atmospheric). By measuring the gas volume before and after the expansion and reducing both to standard conditions, the test characterizes the real-gas compressibility behavior and the surface-to-reservoir volume relationship that govern gas accounting and facility design.

Gas recombination is not a "measurement" in the same sense as a CCE or CVD experiment: it is the sample-preparation and quality-control step that creates the live reservoir fluid on which every subsequent gas-condensate or wet-gas PVT study depends. For a gas reservoir sampled at separator conditions, recombination is the bridge between what was physically collected at surface and what actually exists in the reservoir at \(p_R\) and \(T_R\). Done wrong, it silently biases dew point, liquid dropout, \(Z\)-factor, and CVD recovery for the entire study. Done right, it delivers a single, well-characterized live fluid that all downstream tests can share.

PVT (pressure-volume-temperature) analysis reproduces the reservoir pressure and temperature path in the laboratory on a representative fluid sample to quantify how the fluid changes phase, volume, density and composition from reservoir to surface. Its results feed reserves estimation, reservoir simulation and equation-of-state modeling.

Black-oil and volatile-oil studies rely on constant composition expansion, differential liberation, separator and flash tests, plus swelling and miscibility tests for gas injection. Gas-condensate studies center on constant composition expansion to the dew point, constant volume depletion, the dew point test, the retrograde condensation curve and K-values. Dry and wet gas use gas depletion, gas flash and gas recombination.

Constant composition expansion keeps the total fluid mass and composition fixed while pressure is lowered, so it locates the saturation pressure and total-system volume behaviour. Differential liberation removes evolved gas in stages at reservoir temperature, modeling how solution gas leaves the oil during depletion to yield Bo, Rs and oil density on a residual-oil basis.

Modern PVT work uses a high-pressure, high-temperature variable-volume cell with a sapphire window for direct observation of the first bubble, first dew and retrograde dropout. The Coretest PVT 400/1000 is a 400 mL full-visibility cell rated to 1000 bar and 200 C with CO2 and H2S service.