Abstract
Understanding multi-phase flow in retrograde reservoirs is essential due to retrograde behavior of gas condensate. Maintaining reservoir pressure above dew point is essential to avoid condensation of heavier fractions for an efficient condensate recovery. Conventional evaluation of production logs cannot distinguish condensate phase from gaseous phase due to dynamic pressure transient conditions and tool response to associated phase behavior in complex mixture of fluids. The paper proposed an effective workflow to evaluate time- lapse production logs and pulsed neutron logs to delineate retrograde gas-condensate profile in gas producers.
Phase redistribution and tool sensitivity complicates identification of two-phase flow during production logging. Modified isochronal tests were conducted to find variations in condensate yields over time to understand drawdown conditions and choke sizes required for PLT flow passes. Condensate dropout and liquid saturation varies based on phase envelope under depletion conditions. As condensate does not flow until it reaches critical condensate saturation, pulsed neutron logging (PNL) was recorded simultaneously with PLT to evaluate hydrocarbon phase saturations. Time-lapse logging with varied drawdowns was implemented to identify retrograde phase behavior. Neutron and sigma parameters were derived for PNL data processing using PVT data of gas and condensate.
Time-Lapse PLT and PNL logs confirmed downhole condensate production under critical drawdown conditions. Phase saturations from Time-Lapse PNL revealed mobile condensate above critical condensate saturations. When the size of liquid droplet was below measurable range in baseline log, optical probe sensitivity of production logs was diminished to identify liquid phase from gaseous phase. Dynamic compositional variations were observed along drain hole length according to retrograde phase envelope of reservoir gas. Measured flow profile and pressures along the well were used to fine tune the vertical resolution of reservoir model along well trajectory to simulate condensate saturations over time for comparison. Simulation results corroborated increased condensate saturations along the well and confirmed that the subject reservoir gradually transitioning into two-phase on phase envelope. Differences were observed in grid scale saturations over log scale saturations. Time-Lapse PLT results concluded that the liquid saturation buildup is predominant in region 1 of drainage area. Distinct mobility regions were not observed in PBU analysis of horizontal gas producers; thus condensate buildup was dominant in near wellbore and the reservoir phase envelope was so far similar to initial sate at reservoir level.
The paper investigated flow behavior of gas-condensate reservoirs and effectively delineated retrograde gas-condensate flow profile according to phase envelope at reservoir conditions. Integrated PLT and PNL log evaluation identified condensate saturation build-up which can be used for prediction of future well performance. The work demonstrated a pragmatic method to distinguish multi-phase flow in retrograde gas reservoirs which is scarcely discussed in current literature.
