A Method of Predicting Revaporization of Retrograde Condensate by Dry-Gas Injection
- James W. Givens (Continental Oil Co.)
- Document ID
- Society of Petroleum Engineers
- Society of Petroleum Engineers Journal
- Publication Date
- March 1969
- Document Type
- Journal Paper
- 21 - 27
- 1969. Society of Petroleum Engineers
- 5.7.2 Recovery Factors, 1.6.9 Coring, Fishing, 5.4.2 Gas Injection Methods, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 4.6 Natural Gas, 5.8.8 Gas-condensate reservoirs
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This paper presents a mathematical model for predicting the revaporization of retrograde predicting the revaporization of retrograde condensate liquid by dry-gas injection. The primary assumptions in the model are (1) that complete phase equilibrium exists between the gas and phase equilibrium exists between the gas and liquid in the model; (2) the liquid saturation is less than the mobile liquid saturation; and (3) that the dry gas does not bypass any rich gas as it sweeps through the model. The model calculates the phase compositions and saturations for all 10 cells, liquid phase recovery, and produced gas composition as a function of cumulative injection.
The predicted results for two synthetic systems were found to agree favorably with the results of laboratory displacement studies. The systems investigated were a methane-normal pentane system and a sour system containing mainly methane, hydrogen sulfide, normal pentane, and normal heptane.
The revaporization of retrograde liquid is a very important factor that must be considered in selecting the optimum operating procedure for a gas condensate reservoir. Some reservoirs may be cycled at pressures below their dew point without a significant loss of liquid recovery. In order to ascertain the revaporization characteristics for a reservoir, the reservoir fluids and available injection gases must be studied at reservoir temperature and the desired pressure levels. This type of laboratory study requires considerable time and expense; therefore, the mathematical model presented here was developed to reduce the time, presented here was developed to reduce the time, expense and laboratory work necessary to evaluate the revaporization process.
The model presented and this paper simulates the linear displacement and revaporization for a core initially filled with condensate gas and liquid. The model assumes that the revaporization process occurs (1) at constant temperature and pressure, (2) at liquid saturations below the mobile liquid saturation, (3) at complete equilibrium between the vapor and liquid phases, and (4) at 100-percent sweep efficiency for dry gas displacing the rich gas. It is also assumed that 10 cells are sufficient to simulate the process adequately.
The general steps of the calculational procedure are:
1. The liquid and vapor compositions and liquid saturation are calculated for all 10 cells, using the combined composition for the liquid and vapor phases (total composition), which results from a phases (total composition), which results from a differential liberation to the desired temperature and pressure.
2. The incremental injection volume is selected based on the gas saturation of the cell containing liquid that is nearest the injection end of the model. For the example shown in Fig. 1, the gas volume in Cell 2 would be the incremental injection volume. By choosing the injection volume equal to the gas volume of this cell, there will not be any over or under flow of this cell's boundaries and numerical dispersion will be minimized.
3. The injection increment is added to the first cell after moving an equivalent volume of vapor across the boundary between all cells. The vapor removed from the 10th cell is the produced gas. The GPM (gallons of liquid/Mscf) content of the produced gas is calculated from its composition produced gas is calculated from its composition and used to determine the terminal point for a run.
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