This paper presents the results of an integrated experimental and modelling studies to identify (and quantify) the various flow and phase behaviour phenomena that occur in the near wellbore region, during pressure depletion of a gas condensate reservoir. Laboratory studies were conducted to evaluate the characteristic effects of immobile and mobile condensate saturations on the mobility of gas in the near-wellbore region. The gas flowrates studied span both the Darcy and non-Darcy flow regimes. The experimental data were used in analytical pseudopressures and material balance models to estimate the well productivity changes due to the presence of condensate near the wellbore.
Analysis of experimental data and simulation results show that the net effect of the various phenomena occuring in the near wellbore region is a balance between those effects that reduce well productivity (inertial effects, permeability reduction), and those effects that increase well productivity (capillary desaturation, mist flow and viscous stripping). Experiments show that capillary desaturation and viscous stripping of condensate may occur in the near wellbore region and the productivity in gas condensate reservoirs may not be greatly impaired as predicted by theoretical considerations alone. The paper discusses and provides data/models for the following parameters or phenomena:
- the onset of velocity stripping,
- critical condensate saturations,
- gas permeability reduction due to condensate dropout,
- modifications to relative permeabilities at high velocities and/or low ifts, and
- non-Darcy flow parameters for gas condensate flow.
Three things are essential in the development of gas condensate fields:
- That in the original well testing of the field, accurate values of the gas/condensate ratio (GOR) are determined. This is important in the determination of initial in place reserves and fluid composition.
- That the GOR behaviour of the production wells is understood so that history matching to early data can be accurate.
- That the general long term behaviour of the reservoir and the liquid recovery factors expected in any planned gas injection process are realistic.
An understanding of the dynamic processes involved in the buildup of condensate around the wells is essential for the modelling of gas condensate flow near the wellbore The producing GOR is often used as indication of the efficiency of a producing gas condensate well. Many times the separator GOR's will be too high depending upon whether the well is being produced at too high or too low a flowrate (Fig. 1). At too low a flowrate holdup of the retrograde condensate in the formation results in excessive producing GOR's. At very high flowrate, as the pressure drops below the dewpoint the liquid will drop out and begin to collect in the near wellbore region. In so doing, produced hydrocarbons will contain less liquid than they should and therefore the GOR will be erroneously high.
Near Wellbore. The production of gas condensate below the dewpoint will result in the accumulation of liquid phase condensate in the near wellbore area, which in turn will ultimately impair the gas phase permeability. This near well accumulation of liquid is the result of the gas phase dropping out its associated condensate at a more rapid rate in its approach to the wellbore, in response to the pressure sink created by the producing well. This problem is made worse due to the fact that the throughput of the gas phase per unit rock volume progressively increases with decreasing distance from a wellbore with radial drainage. A dynamic equilibrium must be established where the rate of condensation is equal to the rate of liquid phase condensate flow.
