Gas Huff-n-Puff PVT Experiment

The gas Huff-n-Puff (HnP) PVT experiment is a hybrid of the Constant Volume Injection (CVI) experiment, also known as an isovolume swell test, and the Constant Volume Depletion (CVD) experiment. The process is repeated over multiple injection and production cycles.

More detail can be found in PVT Experiments.

1. Generating a Huff-n-Puff Calculation

In this module, the gas Huff-n-Puff (HnP) PVT experiment is simulated using the selected reservoir fluid composition, surface process, injection gas composition, and cycling pressure schedule.

The module calculates key performance indicators, including cumulative EOR efficiency, oil recovery factor, saturation pressure changes, and relative mixing uplift.

The required inputs are:

1.1. Reservoir Fluid Composition

Select the reservoir fluid composition to be used in the HnP calculation. The composition can be generated using any of the available fluid definition methods described in the Fluid Definition section.

The following reservoir fluid inputs are required:

• Initial reservoir fluid composition.
• Initial reservoir pressure.
• Reservoir temperature.

1.2. Surface Process

Select or edit the surface process associated with the well. The surface process is used to calculate surface oil volumes and generate the black-oil table used in the experiment.

1.3. Injection Gas Composition

Define the injection gas composition used during the injection period of each HnP cycle.

Input the injection gas mole fraction or molar percentage for each relevant component, such as $N_2$, $CO_2$, $H_2S$, $C_1$, $C_2$, $C_3$, $iC_4$, $nC_4$, $iC_5$, $nC_5$, and heavier hydrocarbon fractions as applicable.

1.4. Huff-n-Puff Cycling Schedule

Define the pressure schedule and number of cycles used in the HnP experiment.

The following inputs are required:

• Number of Huff-n-Puff cycles.
• Maximum cycling pressure.
• Minimum cycling pressure.

The maximum cycling pressure represents the pressure after gas injection, while the minimum cycling pressure represents the pressure reached during the production, or puff, period.

2. Terminology

2.1. Recovery Factor

The surface oil recovery factor, $RF_o$, is defined as the cumulative produced surface oil, $N_p$, divided by the original oil in place, $N$:

$$RF_o = \frac{N_p}{N}$$

The produced surface oil is calculated by processing the removed reservoir fluid through the surface process associated with the well.

As the number of HnP cycles increases, the cumulative recovery factor typically increases and eventually approaches an asymptotic value. This reflects the decreasing incremental recovery contribution from later cycles.

2.2. Cumulative EOR Efficiency

Cumulative EOR efficiency, $E$, is defined as the incremental surface oil produced per unit of gas injected:

$$E = \frac{N_p}{Q_{inj}}$$

where $N_p$ is the cumulative produced surface oil and $Q_{inj}$ is the cumulative injected gas volume.

This is a key output from the gas HnP PVT experiment because it quantifies the production uplift relative to the amount of gas injected. In practical terms, it represents the oil gain per gas volume injected.

2.3. Saturation Pressure

The saturation pressure provides important insight into the gas Huff-n-Puff process and how the fluid behavior changes from cycle to cycle.

When production occurs at pressures above the saturation pressure, the mixture of reservoir fluid and injection gas remains single phase. In this region, the process is primarily mixing dominated, similar to a dilution process (saltwater/freshwater mixing). This condition is generally associated with higher recovery efficiency because the injected gas remains mixed with the reservoir fluid.

When production occurs below the saturation pressure, the mixture becomes two phase. In this region, recovery is more strongly influenced by vaporization and phase separation, which is generally less efficient than single-phase mixing.

Therefore, tracking how saturation pressure changes with cycle number and pressure depletion is important for evaluating the efficiency of a specific gas HnP scheme.

2.4. Relative Mixing Uplift

Relative mixing uplift describes the portion of total produced oil that is attributed to mixing-dominated recovery.

Production above the saturation pressure is considered mixing dominated, where incremental recovery is mainly caused by fluid swelling and single-phase dilution. Production below the saturation pressure is considered more vaporization driven.

During the production, or puff, period of an HnP cycle, depletion from the maximum cycling pressure down to the saturation pressure represents the mixing-dominated portion of recovery (max recovery efficiency) such pure-mixing salt-fresh water analogy. The remaining production below the saturation pressure represents the lower-efficiency vaporization-dominated portion and calculated by differencing total cycle recovery minus mixing recovery down to the saturation pressure.

The relative mixing uplift for each cycle is calculated as:

$$\text{Relative Mixing Uplift} = \frac{N_{p,mixing}}{N_{p,total}}$$

where $N_{p,mixing}$ is the produced oil attributed to the mixing-dominated pressure interval, and $N_{p,total}$ is the total produced oil for the cycle.