Post-flowback production data suggest oil drainage from a limited stimulated reservoir volume: An Eagle Ford shale-oil case

We analyze flowback and post-flowback production data from six multi-fractured horizontal wells completed in Eagle Ford shale formation. Petrophysical analysis of samples from an offset well shows that the target shale is in oil window with type II kerogen. We analyze two groups of wells with different well-completion and operational parameters. Wells fractured with slickwater, with higher total injected volume (TIV) and larger choke size show higher initial oil rate, but faster pressure depletion compared with the wells fractured with cross-linked gel, with lower TIV and smaller choke size. Both groups show supercharged fractures when flowback starts and reservoir pressure remains above bubble point during post-flowback period. Interestingly, rate-normalized pressure plots of oil and water from both groups during post-flowback period show pronounced unit slope indicating boundary dominated flow (BDF), while this trend is not observed during flowback period. Analysis of rate-normalized pressure and rate-decline data suggests that both oil and water are produced from a closed tank under BDF conditions. We propose a multiphase flowing material balance (FMB) model to describe water and oil production from an effective stimulated reservoir volume (ESRV) during post-flowback period. The proposed FMB model assumes negligible oil and water influx into ESRV. The driving forces considered are closure of fractures and expansion of oil and water in ESRV. Applying the proposed model on post-flowback data confirms that after fracture depletion, oil is produced from highly stimulated matrix around hydraulic fractures while water is mainly produced from fractures.