CCE Test for Gas Condensate

PVT Analysis of Reservoir Fluids › 7. Constant Composition (Mass) Expansion — Gas Condensate

Reviewed by Coretest reservoir-fluids engineers · Updated 21 June 2026

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.

Purpose↑ Contents

The CCE on a gas condensate answers the foundational reservoir and production questions that gate every later study:

• At what pressure does the first drop of liquid form as the reservoir is depleted, i.e. the retrograde dew point pressure, \(p_d\), at reservoir temperature? This sets the boundary between single-phase gas production and two-phase (condensate-banking) behavior.
• How much liquid drops out of the gas as pressure falls below \(p_d\), and how does that liquid volume change with further depletion? This is the retrograde condensation, or condensate-dropout, curve, the single most important screening result for deciding whether gas cycling or pressure maintenance is justified.
• What is the volumetric behavior of the single-phase gas above the dew point, expressed through the total relative volume and the gas deviation (\(Z\)) factor, which feed gas-in-place and material-balance calculations?

A critical scope note carried over from the source: the CCE/CMD is a constant-composition process. No fluid is removed from the cell at any pressure step, so the overall composition never changes. The liquid that condenses remains in contact with the gas and is not produced. Because real reservoir depletion of a condensate reservoir removes gas while immobile condensate is left behind (a changing-composition, non-conservative process), the CCE dropout curve is not the correct input for compositional reservoir simulation. That role belongs to the Constant Volume Depletion (CVD) experiment. The CCE liquid-dropout curve characterizes the fluid and locates \(p_d\), it does not represent the depletion path of the reservoir. The reason both are quoted as percent liquid is that they are referenced to different base volumes and follow different mass-balance rules.

Physical principle↑ Contents

A gas condensate at reservoir conditions is a single-phase gas whose pressure lies above the cricondentherm-side dew-point branch of the phase envelope, with reservoir temperature \(T\) between the critical temperature \(T_c\) and the cricondentherm. As pressure is lowered isothermally at fixed overall composition, the system path is a vertical line on the p-T diagram. When this line crosses the dew-point curve at \(p = p_d\), the first infinitesimal liquid drops out. This is retrograde behavior: liquid appears on a pressure decrease (the opposite of the intuitive behavior of a black oil, where liquid appears on a pressure increase). Continued depletion increases the liquid volume to a maximum (the point of maximum retrograde liquid), after which further pressure reduction revaporizes the condensate and the liquid volume declines toward, in principle, the lower dew-point branch.

Governing relations used to reduce the data:

Relative total volume:

$$V_{rel} = \frac{V_t(p)}{V_d}$$

where \(V_t(p)\) is the total hydrocarbon volume (gas plus any condensed liquid) in the cell at pressure \(p\) and \(T\), and \(V_d\) is the cell volume at the dew-point pressure \(p_d\). \(V_{rel}\) is dimensionless and equals 1.0 at \(p_d\) by definition. (For oils the reference is the bubble point; for condensates the dew point is the natural reference.)

Liquid dropout (retrograde liquid volume), the condensate-dropout curve:

$$\text{LDO}(p)\ (\%) = 100 \times \frac{V_{liq}(p)}{V_{ref}}$$

where \(V_{liq}(p)\) is the volume of condensed liquid in the cell at pressure \(p\) and \(T\). The reference volume \(V_{ref}\) must always be stated, because two conventions are in use: - \(V_{ref} = V_d\), the dew-point volume: gives liquid as a percent of the saturated single-phase volume. This is the common CCE convention and is also called "% of dew-point volume." - \(V_{ref} = V_t(p)\), the current total cell volume: gives the instantaneous liquid saturation fraction. Mixing these conventions is the single most common reporting error for condensates.

Gas deviation (compressibility) factor, valid above \(p_d\) where the system is single-phase:

$$Z = \frac{pV}{nRT}$$

where \(p\) is absolute pressure, \(V\) the gas volume at \(p\) and \(T\), \(n\) the number of moles of gas in the cell, \(R\) the universal gas constant (\(8.314\ \text{J mol}^{-1}\,\text{K}^{-1}\), or \(10.732\ \text{psia ft}^3\,\text{lb-mol}^{-1}\,\text{R}^{-1}\)), \(T\) absolute temperature. Below \(p_d\) the cell is two-phase, so a single bulk \(Z\) no longer describes the gas alone unless the liquid volume is subtracted.

Gas formation volume factor (single-phase region, \(p \ge p_d\)):

$$B_g = \frac{V_{res}}{V_{sc}} = \frac{ZT}{p} \times \frac{p_{sc}}{Z_{sc} T_{sc}}$$

with \(V_{res}\) the reservoir-condition gas volume and \(V_{sc}\) the volume of that same gas at standard conditions; \(p_{sc}\), \(T_{sc}\), \(Z_{sc}\) are the standard-condition pressure, temperature, and (essentially unity) \(Z\). In field units \(B_g = 0.02827\,\frac{ZT}{p}\) (reservoir ft\(^3\) per scf, \(T\) in R, \(p\) in psia), or \(B_g = 0.00504\,\frac{ZT}{p}\) (reservoir bbl per scf). The source's compact form \(B_g = \frac{ZRT}{p}\) captures the proportionality \(B_g \propto \frac{ZT}{p}\), the standard-condition normalization being the constant of proportionality.

Y-function linearization (used to refine \(p_d\) and smooth the single-phase data):

$$Y = \frac{p_d - p}{p\,(V_{rel} - 1)}$$

where \(p\) is the absolute pressure at a measured step, \(p_d\) the dew-point pressure, \(V_{rel}\) the relative volume at \(p\). For a well-behaved fluid \(Y\) is linear in \(p\); the straight-line fit is used to smooth \(V_{rel}\) above the saturation pressure and to check the consistency of the visually picked dew point.

Isothermal compressibility of the single-phase gas (\(p > p_d\)):

$$c_g = -\frac{1}{V}\left(\frac{\partial V}{\partial p}\right)_T = -\frac{1}{V_{rel}}\left(\frac{\partial V_{rel}}{\partial p}\right)_T$$

with units of 1/pressure.

Apparatus and conditions↑ Contents

• Cell: a high-pressure, high-temperature visual (windowed) PVT cell is strongly preferred for condensates, because the retrograde dew point is detected by the appearance of the first liquid (mist/fog) and liquid volumes are read against a calibrated scale or a cathetometer. Modern practice uses a mercury-free cell with a floating piston (the volume being set by a calibrated, motor-driven positive-displacement pump tracking the piston), although classical work used mercury-filled visual cells. A full-visual (full-bore window) cell or a cell with a movable window is favored so the small condensate volumes near \(p_d\) can actually be seen.
• Volume metering: a precision pump or piston position gives total cell volume \(V_t(p)\) to high resolution; the condensed-liquid volume \(V_{liq}\) is read optically (liquid level against graduations or imaged and integrated).
• Temperature: held constant at reservoir temperature \(T\) throughout (isothermal). The cell is held in an air bath or jacket controlled typically to within about \(\pm 0.1\) to \(0.5\ \text{C}\).
• Pressure: the controlled variable, stepped downward from an initial pressure well above \(p_d\) (often at or above reservoir pressure) to below the point of maximum liquid dropout.
• Held constant: overall composition (no fluid withdrawn), mass, and temperature. Varied: pressure and, in response, total volume and phase split.

Procedure↑ Contents

1. Validate and charge the sample: confirm the recombined gas condensate (or a properly conditioned single-phase bottomhole sample) is at or above its saturation pressure and is homogeneous. Charge a known mass/quantity into the evacuated PVT cell at a pressure clearly above the expected dew point.
2. Bring to test temperature: heat the cell to reservoir temperature \(T\) and let it equilibrate. Agitate (rock/rotate the cell or use a magnetic stirrer/circulation) to ensure single-phase homogeneity before any volume reading.
3. Record the starting datum: at the highest pressure, with the system confirmed single-phase, record \(p\) and total volume \(V_t\). This is the first point of the relative-volume curve.
4. Expand one step: increase the cell volume a controlled increment (withdraw piston / displace metering fluid), lowering the pressure to the next target level at constant \(T\) and constant mass.
5. Equilibrate: stir/rock, then allow the system to reach mechanical and thermal equilibrium at the new pressure. Wait until pressure and any visible liquid level are stable. Equilibration near \(p_d\) is slow because the amount of condensing liquid is tiny; allow generous time and do not rush the reading.
6. Read the single-phase points: above \(p_d\), record \(p\) and \(V_t\) at each step. No liquid is present; the cell remains all gas.
7. Detect the dew point: continue stepping down in fine pressure increments as \(p_d\) is approached. Identify \(p_d\) as the pressure at which the first liquid (fog/mist, then a thin film/meniscus) is first observed in the visual cell. Bracket it with small steps; confirm by the change in slope of the p-V (relative-volume) data and by the Y-function intercept.
8. Record the saturated volume: at \(p_d\), record the dew-point volume \(V_d\). Set \(V_{rel} = 1.0\) here.
9. Step into the two-phase region: lower the pressure in further increments below \(p_d\). At each step, equilibrate, then read both the total volume \(V_t(p)\) and the condensed-liquid volume \(V_{liq}(p)\) against the cell scale.
10. Map the dropout to its maximum and beyond: continue depletion through the point of maximum retrograde liquid and into the revaporization region, recording \(V_{liq}(p)\) at each step so the full retrograde-condensation curve is captured.
11. Optional consistency check: revaporize by recompressing in steps back above \(p_d\) to verify the dew point repeats and to confirm no leakage or composition loss occurred.
12. Reduce the data: compute \(V_{rel}\), \(\text{LDO}(p)\), \(Z\) (single-phase), \(B_g\), and the Y-function fit; cross-check \(p_d\) from the visual pick against the Y-function and p-V slope break.

Measured parameters↑ Contents

At each pressure step: - Cell (absolute) pressure \(p\). - Total cell volume \(V_t(p)\) (gas plus any liquid). - Temperature \(T\) (held constant, logged to confirm isothermal operation).

In the single-phase region additionally: - Confirmation that no liquid is present.

In the two-phase region additionally: - Condensed-liquid volume \(V_{liq}(p)\), read optically against the calibrated cell scale.

At the saturation boundary: - Dew-point pressure \(p_d\) (first liquid observed). - Dew-point volume \(V_d\).

Calculated / derived parameters↑ Contents

• Relative total volume: \(V_{rel} = \frac{V_t(p)}{V_d}\) (dimensionless, \(= 1.0\) at \(p_d\)).
• Liquid dropout curve: \(\text{LDO}(p)\ (\%) = 100 \times \frac{V_{liq}(p)}{V_{ref}}\), with \(V_{ref}\) stated as either \(V_d\) (dew-point-volume basis) or \(V_t(p)\) (instantaneous-saturation basis).
• Maximum retrograde liquid: max of \(\text{LDO}(p)\) and the pressure \(p\) at which it occurs.
• Gas deviation factor (\(p \ge p_d\)): \(Z = \frac{pV}{nRT}\).
• Gas formation volume factor (\(p \ge p_d\)): \(B_g = \frac{ZT}{p}\,\frac{p_{sc}}{Z_{sc} T_{sc}}\); field units \(B_g = 0.02827\,\frac{ZT}{p}\) (rcf/scf) or \(0.00504\,\frac{ZT}{p}\) (rb/scf).
• Y-function: \(Y = \frac{p_d - p}{p\,(V_{rel} - 1)}\); its linear fit smooths single-phase \(V_{rel}\) and validates \(p_d\).
• Isothermal gas compressibility (\(p > p_d\)): \(c_g = -\frac{1}{V_{rel}}\left(\frac{\partial V_{rel}}{\partial p}\right)_T\).

Reporting and typical curves↑ Contents

The standard CCE/CMD report for a gas condensate contains:

• A table of \(p\), \(V_t\), \(V_{rel}\), \(Z\), \(B_g\) for the single-phase region and \(p\), \(V_t\), \(V_{liq}\), \(\text{LDO}\) for the two-phase region, with \(T\) and the picked \(p_d\) called out.
• Relative volume vs pressure: \(V_{rel}\) plotted against \(p\). Above \(p_d\) the curve is the smooth single-phase gas expansion (read for \(Z\), \(B_g\), \(c_g\)); the slope changes at \(p_d\), and \(V_{rel}\) is normalized to 1.0 there. The change of slope is the volumetric signature of the dew point.
• Condensate (liquid) dropout curve: \(\text{LDO}\) (%) vs pressure. Read it as follows: the curve rises from zero at \(p_d\), reaches a peak at the pressure of maximum retrograde liquid, then declines as condensate revaporizes. The peak value (e.g. expressed as percent of dew-point volume) is the headline number for screening recovery and condensate-banking severity. Always read the axis label to know whether the basis is \(V_d\) or \(V_t(p)\), because the two curves differ in shape and magnitude.
• Z-factor vs pressure (single-phase region).
• Y-function vs pressure, shown as a straight-line fit to demonstrate data quality and to corroborate \(p_d\).

The dropout curve is the primary deliverable: a low maximum dropout suggests a lean condensate where depletion may suffice, while a high maximum dropout flags a rich condensate where significant liquid is lost to the reservoir and gas cycling or pressure maintenance should be evaluated (with the quantitative depletion behavior then taken from CVD, not CCE).

Practical notes and pitfalls↑ Contents

• Single-phase, representative sampling: the recombined sample must be at or above the true saturation pressure and recombined at the correct producing GOR. A sample that has dropped below \(p_d\) in the bottle, or that was recombined at the wrong ratio, shifts \(p_d\) and the entire dropout curve. Condition and homogenize before charging.
• Dew-point detection is the hardest measurement: the first liquid is a faint mist and the condensed volume near \(p_d\) is extremely small. Approach \(p_d\) in fine pressure steps, use the visual cell, and corroborate with the Y-function and the p-V slope break. Picking \(p_d\) from volume data alone tends to underestimate it; visual confirmation is preferred for condensates.
• Equilibration time: near and below the dew point the liquid forms slowly and can cling to walls and windows. Allow ample settling and stable-reading time at each step; rushing produces a falsely low \(p_d\) and a distorted dropout curve.
• Mercury-free operation: prefer a piston cell. With mercury cells, account for mercury vapor and the safety/contamination burden, and ensure clean separation between metering fluid and hydrocarbon. With piston cells, calibrate the piston-position-to-volume relation and watch for piston friction (stiction), which biases low-pressure volumes.
• Dead volume and calibration: characterize the cell dead volume (lines, ports, window recesses) and the volume calibration over the full pressure and temperature range. Small dead-volume errors are amplified in the tiny near-dew-point liquid readings and in \(V_{rel}\) close to 1.0 where \((V_{rel} - 1)\) appears in the denominator of the Y-function.
• Liquid-volume readout error: optical reading of thin condensate films on a curved window is error-prone; use a cathetometer or imaging with proper meniscus and parallax correction, and ensure the window is clean.
• Wall and window wetting / hysteresis: some condensate adheres and revaporizes sluggishly. Run the optional recompression check to confirm the dew point repeats; a non-repeating \(p_d\) signals leakage, composition loss, or insufficient equilibration.
• Do not use CCE dropout for simulation: the CCE conserves mass and composition, so its dropout curve overstates in-reservoir liquid relative to actual depletion. Use it to locate \(p_d\) and to characterize the fluid; take depletion-path liquid behavior and produced-gas compositions from CVD.
• Isothermal discipline: temperature excursions move the path on the p-T diagram and shift the apparent \(p_d\). Verify bath stability before each reading and log \(T\) at every step.