Fracture Propagation,Fluid Flow,and Geomechanics of Water-Based Hydraulic Fracturing in Shale Gas Systems and Electromagnetic Geophysical Monitoring of Fluid Migration

摘要

We investigate fracture propagation induced by hydraulic fracturing with water-injection,using numerical simulation.For full 3D rigorous modeling,we employ a numerical method that can model failure due to tensile and shear stresses,dynamic nonlinear permeability,the dual continuum approach,leak-off in all directions,and thermo-poro-mechanical effects.From the numerical results,fracture propagation is not the same as propagation of the water front,because fracturing is governed by geomechanics,whereas water saturation is determined by fluid flow.At early times,the water saturation front is almost identical to the fracture tip,showing that the fracture is mostly filled with injected water.However,at late times,advance of the water front is retarded,compared to the fracture propagation,yielding a significant gap between the water front and the fracture top,which is filled with reservoir gas.We also find considerable leak-off of water to the reservoir.The inconsistency between the fracture volume and the volume of injected water cannot properly estimate the fracture length,when it is estimated based on the simple assumption that the fracture is fully saturated with injected water.As an example of flow- geomechanical responses,we identify pressure fluctuation under constant water injection,because hydraulic fracturing is itself a set of many failure processes,in which pressure drops every time when failure occurs.The fluctuation decreases as the fracture length grows. We also study application of electromagnetic(EM)geophysical methods,because the EM geophysical methods are highly sensitive to changes in porosity and pore-fluid properties,such as water injection into gas reservoirs.We employ a 3D finite- element EM geophysical simulator and evaluate the sensitivity of the crosswell EM method for monitoring fluid movements in shaly reservoirs.For the sensitivity evaluation,reservoir models are generated through the coupled flow-geomechanical simulator and are transformed via a rock physics model into electrical conductivity models.It is shown that anomalous conductivity distribution in the resulting models is closely related with injected water saturation but little with newly-created unsaturated fractures.The numerical modeling experiments demonstrate that the crosswell EM method can be highly sensitive to conductivity changes that directly indicate the migration pathways of the injected fluid.Accordingly,the EM method can serve as an effective monitoring tool for distribution of injected water(i.e.migration pathways)during hydraulic fracturing operations.