Constant Volume Depletion (CVD) Test

PVT Analysis of Reservoir Fluids › 8. Constant Volume Depletion (CVD)

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

Fluid type: gas condensate (retrograde gas)

Purpose↑ Contents

The Constant Volume Depletion experiment reproduces, in a single PVT cell, the pressure-depletion path that a retrograde gas-condensate reservoir follows under primary depletion when the produced gas is withdrawn and the liquid condensate that drops out is largely immobile and left behind in the pore space. It answers the central reservoir and production questions for a condensate field:

• How much liquid condensate drops out in the reservoir as pressure falls below the dew point, and at what pressure the maximum dropout occurs (the worst case for near-wellbore liquid loading and condensate banking).
• How the gas-phase compressibility factor (\(Z\)), the two-phase \(Z\)-factor, and the gas formation volume factor evolve with depletion, which sets material-balance gas-in-place and recovery calculations.
• How the composition of the produced wellstream changes with depletion, in particular the progressive leaning of the gas and the loss of intermediate-to-heavy components (\(C_{5+}\), \(C_{7+}\)) to the liquid phase, which drives plant liquid-yield forecasts.
• The cumulative recovery of each component and of total moles at abandonment pressure.

CVD is, together with the constant-composition expansion (CCE) that fixes the dew point, the primary laboratory data set used to tune an equation-of-state (EOS) model for compositional reservoir simulation (Eclipse E300, tNavigator, CMG GEM) and surface-process modeling.

Physical principle↑ Contents

A retrograde gas exists at reservoir temperature \(T\) above its cricondentherm region such that isothermal pressure reduction first reaches the upper dew point \(p_d\). Below \(p_d\) the system enters the two-phase region and liquid forms; this is retrograde condensation, the counter-intuitive appearance of liquid on pressure decrease. The amount of liquid grows to a maximum and, for many fluids, partially re-vaporizes at lower pressures (normal condensation behavior on the lower limb).

CVD imposes the constraint that the total cell (system) volume is returned to a fixed reference volume after each pressure step, namely the volume occupied by the fluid at the dew point, \(V_{cell} = V(p_d, T) = \text{constant}\). Gas is withdrawn at each step to enforce this. This mimics a reservoir of fixed pore volume from which gas is produced while the dropped-out liquid remains trapped.

Governing relations and symbols:

• Real-gas law: $$p V = Z n R T$$ where \(p\) = pressure, \(V\) = gas volume, \(Z\) = gas deviation (compressibility) factor (dimensionless), \(n\) = moles of gas, \(R\) = universal gas constant (\(8.314\ \text{J/mol/K}\)), \(T\) = absolute temperature (\(\text{K}\)). The single-phase gas \(Z\) at and above \(p_d\) is $$Z = \frac{p V_g}{n R T}$$
• Two-phase \(Z\)-factor below the dew point, used in \(p/Z\) material balance for condensate reservoirs: $$Z_{2phase} = \frac{p V_{cell}}{n_{total} R T}$$ where \(n_{total}\) is the total remaining moles of fluid (gas plus retrograde liquid) in the cell. Because \(n_{total}\) decreases only by the gas withdrawn while \(V_{cell}\) is held constant, \(Z_{2phase}\) captures both compressibility and the liquid present.
• Gas formation volume factor: $$B_g = \frac{V_{res}}{V_{sc}} = \frac{Z T p_{sc}}{Z_{sc} T_{sc} p}$$ in \(\text{rm}^3/\text{sm}^3\) or \(\text{rb/scf}\). With \(p_{sc}\), \(T_{sc}\), \(Z_{sc}\) approximately 1 at standard conditions, \(B_g = 0.350958\,Z T / p\) (\(\text{rb/Mscf}\), \(T\) in \(\text{degR}\), \(p\) in \(\text{psia}\)) or in SI \(B_g = Z T p_{sc} / (T_{sc} p)\).
• Conversion of step-produced cell gas to standard volume: $$V_{sc} = V_{cell\,gas}\,\frac{p_{cell}}{p_{sc}}\,\frac{T_{sc}}{T_{cell}}\,\frac{Z_{sc}}{Z_{cell}}$$ The source's \(V_{std} = V_{cell}\,(P_{cell}/P_{std})(T_{std}/T_{cell})\) is the ideal-gas form; the rigorous form carries the \(Z\) ratio.
• Retrograde liquid dropout (the primary CVD curve), volumetric definition: $$L(p) = \frac{V_{liq}(p)}{V_{cell}} \times 100\ \text{percent}$$ where \(V_{liq}\) is the retrograde liquid volume at step pressure \(p\) and \(V_{cell}\) is the reference (dew-point) cell volume. A mass-based form is \(\text{Dropout} = (m_{condensate} / m_{initial\,hydrocarbon}) \times 100\).
• Equilibrium \(K\)-values per component used in EOS tuning against the produced-gas composition: $$K_i = \frac{y_i}{x_i}$$ where \(y_i\) is the mole fraction of component \(i\) in the equilibrium vapor and \(x_i\) in the equilibrium liquid.
• Cumulative produced fraction of original moles: \(n_{p,cum} / n_{initial}\), and per-component recovery from compositional analysis of each produced gas increment.

Apparatus and conditions↑ Contents

• A high-pressure, high-temperature variable-volume PVT cell (visual windowed cell or blind cell) rated above initial reservoir pressure; for deep condensates this means pressures to \(10{,}000\text{-}15{,}000\ \text{psi}\) and temperatures to \(150\text{-}200\ \text{degC}\). Volume is changed by a calibrated positive-displacement pump (mercury-based in older rigs, mercury-free piston/floating-piston cells in modern rigs).
• A precision back-pressure / metering valve for isobaric gas withdrawal at constant cell pressure.
• A gasometer or wet-test/dry gas meter plus a separator train to measure the volume of withdrawn gas at standard conditions, with gas chromatography (GC) on each withdrawn increment for compositional analysis. A liquid trap recovers any heavy ends carried out so the wellstream composition can be recombined.
• Cathetometer or capacitance/visual level measurement of retrograde liquid volume through the cell window.
• Thermostatic air bath or jacket holding \(T\) constant to typically plus or minus \(0.1\text{-}0.5\ \text{degC}\).

Held constant: temperature \(T\) (isothermal), and after each depletion step the total cell volume is restored to \(V_{cell} = V(p_d, T)\). Varied: pressure, decreased in stages from \(p_d\) down to abandonment.

Procedure↑ Contents

1. Recombine separator gas and separator liquid to the reported producing GOR to reconstitute the original reservoir wellstream, validate the recombination against the field GOR and the measured dew point, and charge a known mass/moles of this fluid to the PVT cell.
2. Bring the cell to reservoir temperature \(T\) and pressurize the single-phase fluid above the dew point; equilibrate.
3. Determine the dew-point pressure \(p_d\) at \(T\) (from a prior or in-line CCE) and record the cell volume at the dew point, \(V_{cell} = V(p_d, T)\). This becomes the fixed reference volume for the test.
4. Set the first depletion step: expand the cell (withdraw the displacing fluid / advance the piston outward) to lower the pressure to \(p_1\) below \(p_d\), allowing the total volume to increase. Retrograde liquid forms.
5. Allow full equilibration at \(p_1\) (gas and liquid in equilibrium, stable liquid level), then measure the retrograde liquid volume \(V_{liq}(p_1)\) and the total volume.
6. At constant pressure \(p_1\), withdraw gas through the metering valve while simultaneously reducing the cell volume, so that pressure stays at \(p_1\), until the total cell volume is returned exactly to the reference \(V_{cell}\). The liquid is not produced; only gas leaves.
7. Measure the volume of the gas removed in this step at cell conditions, and meter its volume at standard conditions; sample the increment and run GC to obtain the produced-gas composition \(y_i\) for the step.
8. Record the equilibrium gas \(Z\) (or two-phase \(Z\)) at \(p_1\) and the residual retrograde liquid volume now occupying the cell at \(V_{cell}\).
9. Repeat steps 4 through 8 at successively lower step pressures (a typical schedule of 6-10 stages down to a chosen abandonment pressure, e.g. \(700\text{-}1500\ \text{psi}\)), each time dropping to the new pressure, equilibrating, measuring liquid, withdrawing gas back to \(V_{cell}\), and analyzing the increment.
10. At the final (abandonment) pressure, measure the residual liquid volume and, optionally, recover and analyze the residual reservoir liquid for a full component mass balance.
11. Close the material balance: sum the moles of gas produced over all stages plus residual liquid moles and check against the moles originally charged.
12. Compute the per-stage \(Z\)-factors, two-phase \(Z\), \(B_g\), dropout curve, cumulative recovery, and stage compositions for reporting and EOS tuning.

Measured parameters↑ Contents

At each depletion stage the following are recorded directly:

• Stage pressure \(p\) and the constant temperature \(T\).
• Reference cell volume \(V_{cell}\) (constant) and the expanded total volume just before gas removal.
• Retrograde liquid volume \(V_{liq}(p)\) by the cell window/cathetometer.
• Volume of gas withdrawn at cell conditions and the same gas metered at standard conditions, \(V_{sc}\).
• Equilibrium gas composition of each withdrawn increment, \(y_i\), by GC (typically through \(C_{7+}\) or \(C_{12+}\) with measured fraction MW and density).
• Equilibrium gas \(Z\)-factor at the stage (from \(p\), \(T\) and the produced gas volume/moles).
• Cumulative gas (and per-component) produced to date.

Calculated and derived parameters↑ Contents

• Retrograde liquid dropout: $$L(p) = \frac{V_{liq}(p)}{V_{cell}} \times 100\ \text{percent}$$ The defining CVD curve.
• Gas deviation factor of the produced/equilibrium gas: $$Z = \frac{p V_g}{n_g R T}$$ with \(n_g\) from the metered standard volume, $$n_g = \frac{p_{sc} V_{sc}}{R T_{sc}}$$
• Two-phase \(Z\)-factor: $$Z_{2phase} = \frac{p V_{cell}}{n_{total} R T}$$ \(n_{total}\) = moles originally charged minus cumulative moles produced. Used directly in the \(p/Z_{2phase}\) versus cumulative-production material-balance plot for condensate gas-in-place.
• Gas formation volume factor: $$B_g = \frac{Z T p_{sc}}{Z_{sc} T_{sc} p}$$ from each stage \(Z\).
• Equilibrium \(K\)-values: $$K_i = \frac{y_i}{x_i}$$ where \(x_i\) is obtained from the residual-liquid composition (measured at abandonment and/or back-calculated from material balance), used to constrain the EOS.
• Cumulative recovery: fraction of initial moles produced, \(n_{p,cum} / n_i\), and cumulative recovery of each component (especially \(C_{5+}\) / \(C_{7+}\) liquid recovery) from sum of (\(y_i\) times stage moles).
• Produced wellstream and plant liquid yield (\(\text{stb/MMscf}\)) as a function of depletion pressure from the evolving \(y_i\).
• Equilibrium gas gravity and heating value per stage, derived from \(y_i\).

Reporting and typical curves↑ Contents

The standard CVD report includes:

• Liquid dropout curve: \(L(p)\) (percent of dew-point volume) versus pressure, falling from 0 at \(p_d\), rising to a maximum (the maximum retrograde liquid), then often declining at low pressure. The maximum and the pressure at which it occurs are the headline numbers; they indicate condensate-banking severity and whether revaporization helps at late depletion.
• \(Z\)-factor versus pressure (single-phase \(Z\) above \(p_d\), equilibrium gas \(Z\) and two-phase \(Z\) below \(p_d\)).
• \(p/Z_{2phase}\) versus cumulative produced gas, a near-linear material-balance plot used to back out condensate gas initially in place.
• Cumulative recovery (total and per component, e.g. \(C_{7+}\)) versus depletion pressure, read to forecast ultimate recovery and the steady leaning of the gas.
• Produced-gas composition table per stage (\(y_i\)) and the residual-liquid composition at abandonment.
• \(B_g\) versus pressure and produced gas gravity / yield versus pressure.

These curves are read together: the dropout curve sizes the in-situ liquid loss, the composition stages quantify how much of that loss is intermediates that never reach surface, and the \(Z\) and \(p/Z\) data anchor volumetrics. All of it is regressed simultaneously when tuning the EOS so the model reproduces dew point, dropout, \(Z\), and stage compositions before it is exported to the field simulator.

Practical notes and pitfalls↑ Contents

• Sampling and recombination dominate the data quality: a non-representative wellstream (wrong GOR, separator sampling during liquid carryover or below saturation) shifts the dew point and the whole dropout curve. Always validate the recombined dew point against CCE.
• Equilibration time below the dew point is long because retrograde liquid forms slowly and clings to surfaces; insufficient settling overstates the gas fraction and understates dropout. Verify a stable liquid level and pressure before withdrawal.
• The constant-volume constraint must be enforced precisely. Returning to \(V_{cell}\) at constant pressure during gas removal requires careful coordinated pump and valve control; if volume is not exactly restored, the imposed depletion path and the two-phase \(Z\) are biased.
• Low-dropout, near-critical, and very lean condensates challenge liquid-volume reading: the small retrograde liquid level near the window can be below visual resolution, so use high-resolution level detection and, where possible, mass-balance cross-checks.
• Prefer mercury-free cells; mercury rigs carry HSE and disposal issues and mercury can amalgamate or adsorb, and mercury vapor can bias trace measurements. If mercury is used, account for its compressibility and meniscus in volume calibration.
• Dead volume and tubing holdup in the withdrawal line cause loss of heavy ends and underreported \(C_{7+}\) in the produced increment; minimize and calibrate line volumes, heat-trace the lines to prevent condensation, and trap and analyze any line liquids.
• The fixed reference is the dew-point volume, not the original single-phase volume; using the wrong reference corrupts \(L(p)\) and the cumulative-production normalization.
• Do not produce liquid: only gas should be withdrawn. Any condensate co-produced through the valve violates the CVD assumption and must be trapped, measured, and accounted for.
• GC of every increment is essential; relying on a single average composition loses the leaning trend that drives plant-yield forecasts. Characterize the plus fraction (MW and density) consistently across stages so \(K\)-values and EOS regression are stable.