Estimating Effective Fracture Pore Volume and the Efficiency of Imbibition-Driven Oil Recovery Using Two-Phase Flowback Data

By providing sufficient time for oil to migrate from the matrix into the fractures through imbibition, the extended shut-in period contributes to immediate oil production in shale oil reservoirs. Previous studies have demonstrated that flowback data can be used for fracture characterization. However, the developed models mainly analyze water production and do not address the quantification of imbibition oil recovery. The objective of this paper is to propose a two-phase oil/water flowback analysis method to estimate the effective fracture pore volume (<i>V</i> _<i>efi</i> ) and the efficiency of imbibition-driven oil recovery, providing an early opportunity to understand the effects of fracturing operations. The method incorporates rate decline analysis, an extended flowing material balance (FMB) model, and producing oil/water ratio analysis to form a workflow for predicting fracture properties. The signature of fracture depletion is described through a set of diagnostic plots, which represent a key period for assuming the fracture system as a closed-tank system. Using water-phase flowback data, the semilog plot shows a linear trend of harmonic decline, indicating the water volume within the effective fracture system. By using oil-phase flowback data, the developed FMB model identifies the fracture depletion period and estimates imbibition-driven oil volume through a diagnostic plot. Moreover, the model incorporates two-phase oil/water flow in both propped and unpropped fractures. The imbibition recovery in different fracture domains is further determined by combining the production oil/water ratio analysis during flowback. The accuracy and applicability of the entire workflow are tested against numerical simulations. Under a series of variable set parameters, the inversion results show good accuracy and stability. Furthermore, we apply the new method to estimate <i>V</i> _<i>efi</i> and the efficiency of imbibition-driven oil recovery in different fracture domains using field flowback data. The results show a significant decrease in fracturing fluid efficiency after long-term well shut-in and demonstrate vastly different imbibition efficiencies in propped and unpropped fractures.