Tight oil production has increased dramatically and contributed to 61% of total US oil production in 2018.However, recovery factors for primary depletion with multistage fractured wells are low, typically less than10%. Gas huff-n-puff emerges as a promising technique to push the recovery factor beyond 10% in tightoil reservoirs, based on laboratory studies, simulation and field pilot tests. A CO2 huff-n-puff pilot wasimplemented in the Midland Basin, and data collected demonstrated significant incremental oil recovery,but with higher than expected water-cut rise. To understand the excessive water production, a compositional model was built. Eight pseudo-components were used to match the PVT lab results of a typical oil sample in the Wolfcamp shale. Alab scale model was established in our simulator to match the results of gas huff-n-puff experiments incores, where key parameters were identified and tuned. A half-stage model consisting of five fractureswas built, where stress-dependent permeability was represented by compaction tables. Then a sensitivityanalysis was conducted to understand the roles of different mechanisms behind the abnormal high water-cutphenomenon on this scale. Our simulation results have shown that initial water saturation, IFT-dependentrelative permeability, reactivation of water-bearing layers, and re-opening of unpropped hydraulic fracturesmay all affect water-cut after gas injection. Among them, re-opening of unpropped hydraulic fractures wasthe most critical one. Data from a pilot test imply substantial water production after gas injection, which may impede oilproduction, but the underlying mechanisms are poorly understood. A numerical model is developed to studypossible mechanisms for high water-cut pilot results. This study also intends to quantify the impact of highwater cut on cyclic gas injection.
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