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Gas Huff-n-Puff PVT Experiment

In this module, the gas Huff-n-Puff (HnP) PVT experiment is simulated. The module quantifies a set of key performance indicators, such as cumulative EOR efficiency and recovery factor, from the following input data:

  • Reservoir fluid composition (generated with any of the available methods)
  • Reservoir temperature
  • Injection gas composition
  • Initial reservoir pressure
  • Number of Huff-n-Puff Cycles
  • Maximum and minimum cycling pressure
  • Surface process

The gas Huff-n-Puff (HnP) PVT experiment is a hybrid of the constant volume injection (CVI) experiment (=isovolume swell test) and the constant volume depletion (CVD) experiment repeated in several times (cycles). Read more about the PVT experiments here.

Recovery Factor (\(RF_o\))

The recovery factor of surface oil, \(RF_o\), is defined as the cumulative produced oil (\(N_p\)) divided by the original oil in place (\(N\)), i.e. \(RF_o\) = \(N_p\) / \(N\). The produced surface oil is calculated by processing the removed amounts through the wells associated surface process. As the number of cycles increase and the amount if will converge to 100%

Cumulative EOR Efficiency

EOR efficiency (E) is defined as the "incremental STB / Sm3" produced per "MMscf / Sm3E6" gas injected, i.e. E = \(N_{p}\) / \(Q_{inj}\). This is a key output from running the gas HnP PVT experiment, as it quantifies the "uplift per cost".

Saturation Pressure

The saturation pressure, and how it changes with cycle, communicates something fundamental about the gas Huff-n-Puff process. When producing at pressure above the saturation pressure, the mix of reservoir fluid and injection gas is single-phase (a pure dillution process = salt-water/freshwater mixing), which is associated with the highest efficiency (=mixing). When procuding at pressure below the saturation pressure, the mix of reservoir fluid and injection gas is two-phase, which is associated with the lowest efficiency (=vaporization). Hence, understanding how the saturation pressure is changing with cycle, and time, is fundamental to understand the efficiency of a specific gas HnP scheme.

Relative mixing uplift

Production above the saturation pressure is mixing dominated, and incremental production is due to volume increase ("swelling"). Production below the saturation pressure is vaporization driven. During the production (puff) period of a HnP cycle, the depletion from the maximum pressure to saturation pressure will have "max" recovery efficiency (pure-mixing salt-fresh water analogy). The subsequent steps below the saturation pressure are less efficient and calculated by differencing total cycle recovery minus “mixing” recovery down to the saturation pressure. The relative mixing uplift each cycle is calculated as \(N_{p,mixing}\)/\(N_{p,total}\).