Three-Phase Flowback Rate Transient Analysis of Shale Gas Reservoirs

Summary We present a three-phase flowback rate transient analysis (RTA) of multi-fractured horizontal wells (MFHWs) in shale gas reservoirs, considering condensate dropout when the hydraulic fracture (HF) pressure drops below the dewpoint. A semi-analytical three-phase flowback model has been developed to account for the multiphase flow in the HF and two-phase water and gas flow in the matrix to analyze flowback data. Three HF flow regimes are considered—two-phase water and gas infinite-acting linear flow (IALF), two-phase water and gas boundary-dominated flow (BDF), and three-phase water, gas, and oil BDF occurring when HF pressure falls below the dewpoint. In addition, IALF is considered in the matrix domain. Pressure diffusivity equations for water, gas, and oil-phase flows are derived using the material balance approach to obtain the average HF pressure by modifying the matrix distance of investigation (DOI). New pseudo-variables for pressure and time are defined to linearize the governing equations, constructing diagnostic plots for flow regime identification and specialty plots for HF properties characterization. Diagnostic plots for each phase use the proposed semi-analytical solution. In the early flowback period, the water- and gas-phase diagnostic plots show a half-slope straight line, indicating the IALF regime, followed by a unit slope line for the BDF regime. A unit slope line represents BDF once the HF pressure reaches the dewpoint for the oil-phase diagnostic plot. Numerical simulations validate the developed flowback RTA model, showing that estimated HF permeability and half-length from the three-phase flow model closely match the set values in the numerical model with relative errors under 10%, confirming the model’s accuracy. The developed model’s applicability is examined by analyzing flowback data from an MFHW in Marcellus shale, demonstrating its utility in understanding the early stages of HF dynamics and attributes.