Flow behavior analysis of two-phase (gas/water) flowback and early-time production from hydraulically-fractured shale gas wells using a hybrid numerical/analytical model

Abstract

In recent years, quantitative analysis of flowback and early-time production data for interpretation of hydraulic fracture properties in shale has received significant attention. With high-resolution phase rates and flowing pressures, analysis of flowback and early-time production data provides an early opportunity to determine key parameters for well performance forecasting. The main purpose of this paper is to quantify flow characteristics associated with two-phase flowback and early-time production of shale gas wells exhibiting both planar fractures and a complex fracture network using a rigorous and efficient flow model.To study flow characteristics of this complex system, a hybrid numerical/analytical model is developed in which matrix and hydraulic fracture flow is accounted for. The finite-difference method is used to numerically model unsteady-state two-phase fracture flow. The analytical matrix flow model, derived using the line-source function, is dynamically coupled with the fracture flow model by imposing continuity of pressure and flux on the fracture surface. The main advantage of the hybrid solution over more conventional numerical simulation is reduced model setup and run time, without loss of the important physics.Detailed flow regime analysis reveals that the gas/water flowback and early-production of a horizontal well with a planar fracture geometry may the following complex sequence of flow-regime: 1) radial flow of water in the fracture; 2) a first transient linear flow of water in the fracture; 3) a second transient linear flow of water in the fracture; 4) boundary-dominated flow of water in the fracture and gas linear flow in matrix. Water primarily depletes in the first three flow periods. Development of the water boundary-dominated flow and gas linear flow indicates the system indicate that the system is at the end of flowback period and proceeds early-time production. Specially, the second water linear flow period develops when both gas and water production are dominating the system, and is caused by the pressure maintenance associated with gas flux from matrix. A complex fracture network with the same total fracture length as a planar fracture system will exhibit the same sequence of flow regimes. Due to the accelerated water drainage caused by multiple contact points between fractures and the wellbore in complex fracture network, the system also exhibits a shortened second water transient linear flow period and earlier water boundary-dominated flow.Finally, the new model is also applied to match flowback data obtained from a multi-fractured horizontal well completed in the Marcellus Shale. The results demonstrate the practical application of the new model for deriving reservoir/fracture properties from flowback data and for forecasting.