Three-Phase Flowback Rate Transient Analysis of Shale Gas Reservoirs

Abstract We present a three-phase flowback rate transient analysis (RTA) of the multi-fractured horizontal wells (MFHWs) in shale gas reservoirs by considering condensate dropout once the hydraulic fracture (HF) pressure drops below the dewpoint. We have developed a semi-analytical three-phase flowback model that considers 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, including two-phase water and gas infinite-acting linear flow (IALF), two-phase boundary-dominated flow (BDF), and three-phase water, gas, and oil BDF that appears once the HF pressure falls below the dewpoint. Meanwhile, IALF is considered in the matrix domain. Pressure diffusivity equations for the water, gas, and oil-phase flow are derived, and the material balance approach is used to obtain the average HF pressure with the modification of the matrix distance of investigation (DOI). We define new pseudo-variables for pressure and time to linearize the governing equations and construct the diagnostic plot for flow regime identification and the specialty plot for HF properties characterization. We obtain a diagnostic plot for each phase using the proposed semi-analytical solution. A half-slope straight line is observed in the early flowback period for the water and gas-phase diagnostic plots, indicating the IALF regime. This is followed by a unit slope line, implying the BDF regime. Furthermore, a unit slope line representing BDF is examined for the oil phase diagnostic after the HF pressure reaches the dewpoint. The numerical simulations are conducted to validate the developed flowback RTA model. Estimated HF permeability and half-length from the three-phase flow model closely match the set values in the numerical model with relative errors < 10%, confirming the proposed model's accuracy. The proposed three-phase flowback model is useful in understanding the early stages of HF dynamics and its attributes.