GOR of liquid-rich shale reservoirs and its application in decline curve analysis
Conference Paper
Overview
Overview
abstract
Copyright 2017, Society of Petroleum Engineers. Unconventional volatile-oil reservoirs produced gas-oil ratio (GOR) behavior is quite different from that in conventional reservoirs. GOR is constant even at pressures below the bubble point. Reasons for this observed anomalous behavior is explored in this paper by using reservoir simulation, with a few field examples from Eagle Ford shale plays. The study shows that how GOR history and rate transient analysis (RTA) diagnostic plots can be used with decline curve analysis (DCA) methods, which were originally developed for single-phase flow, to forecast multiphase well production. We have performed extensive reservoir numerical modeling to simulate the non-linearity caused by the impact of nano-pores on rock and fluid properties, complex pore networks and the multiple porous media flow systems created by multi-stage fracture stimulation. We used the simulation model to generate synthetic datasets based on realistic reservoir fluid and rock properties under various rate/flowing pressure production conditions. The simulation model was used to explain the fundamental difference between GOR during linear vs. radial flow. Then we analyzed the producing GOR and pressure and saturation changes within the reservoir by traditional rate transient analysis methods. Finally, we simulated well performances at varied pressure drawdowns and performed decline curve analysis on data from flow regimes identified with RTA plots and GOR behavior. We found that fracture conductivity, fracture spacing, critical gas saturation, matrix permeability and bubble-point suppression are major factors impacting near-constant GOR behavior in LRS reservoirs. Examination of GOR behavior can provide better interpretation of where a well is in its GOR history and flow regime. We observed that GOR behavior correlates with flow regime changes identified on RTA diagnostic plots. Produced gas oil ratio flattens at a level higher than original solution gas-oil ratio during transient linear flow, and during most of the transition period that follows. A unit slope line on a log-log plot, as an indicator of boundary dominant flow regime, forms only when the entire reservoir pressure drops below the bubble point, and gas saturation exceeds the critical threshold, thus causing a sharp increase in GOR, and it can decline later if compound linear flow develops. This study showed that using different Arps b-parameters to represent different flow regimes in DCA methods, combined with drawdown-adjusted production rates, allows us to predict tight volatile oil reservoir performance with improved accuracy.
