Water flowback behaviors of unconventional gas reservoirs have been studied for gaining useful insights pertinent to fracture geometry characterization after fracturing stimulation. Field observations indicate that flowback profiles, such as fluid flow rates, flowback time, fracturing fluid recovery, may vary significantly among different wells. Stimulated fracture geometry has been widely postulated to be a dominant factor influencing flowback characteristics. This study attempts to identify the flowback signatures under various fracture geometries and configurations and to investigate effects of stimulated fracture geometry on flowback characteristics.
In this study, an in-house three-dimensional coupled fluid flow and geomechanics simulator is used to simulate the flowback behaviors in unconventional gas reservoirs. The developed model incorporates essentially all the relevant physical mechanisms controlling flowback characteristics: capillarity, fracture closure, stress-dependent reservoir properties, and gravity segregation. Various complex fracture geometries are efficiently modeled through the Embedded Discrete Fracture Model (EDFM). Geomechanical deformations of the hydraulic and natural fractures are quantified based on widely-adopted fracture constitutive models. The flowback signatures under various fracture geometries are characterized by plotting different diagnostic plots. Fluid phase and pressure distributions are presented to help explain the mechanisms controlling observed characteristics.
Two prominent regimes are observed for the whole flowback process: two-phase transient flow in fractures (early flowback period, EFP) and transient linear flow with considerable gas influx from matrix to fractures (late flowback period, LFP). Semi-log plots of water rate vs. cumulative water production (RVP) show near straight-line trends in EFP, and the slope of this straight-line increases as the total fracture volume increases. The curve moves left downward as the fracture interface increases under the same fracture volume. Complex fracture geometry leads to lower water recovery due to more water trapping in fractures. The existence of gas-filled natural fractures increases both the gas rate and ultimate water recovery, characterized by high gas water ratio (GWR) and a large slope in RVP plot during EFP.
This study examines the salient signatures in flowback profiles for a variety of fracture geometries. The outcomes have offered new insights for the effect of fracture geometry on water flowback, facilitating the reduction in uncertainty of the stimulated fracture characterization using flowback data.