Numerical Simulation Of Mechanical Behavior Of Hydraulic Fractures In Shale Reservoirs

Accelerating the exploitation and development of shale gas resources is one of the major strategic plans on an upgrading version of China's economy.Hydraulic fracturing technology is the critical technique of shale gas development;thus,there is a pressing need to establish an effective method to solve some fracture-related scientific problems which include how to predict the propagation behavior of hydraulic fractures created in the shale gas reservoirs and what is the mechanism of the complex fracture network formed during hydraulic fracturing process.A large number of experimental data have confirmed that the size and complexity of the fracture network created in shales were strongly affected by the natural fractures.The classic hydraulic fracturing theory is unable to predict and specify the mechanical behavior of hydraulic fractures propagating in the shale formation that is abundant with micro-cracks,fissures and macro-natural-fractures,and it could not explain the mechanism of complex fracture network created during stimulation operations.Most of the existing numerical simulation methods ignore the differences between micro-cracks and macro-fractures,thus these methods cannot help the researchers understand the way of impacts of micro-cracks and macro-fractures on the propagation of hydraulic fracture which limiting the application in shale formations.This dissertation targets on the mechanical behavior of hydraulic fractures,micro-cracks and macro-natural-fractures based on experimental phenomena and numerical simulation technique.It discussed a novel method for simulating the mechanical behavior of the complex fracture network created in shale gas reservoirs.It includes:(1)Based on the theory of classic fracture mechanics and elasticity mechanics,the distribution of stress fields around hydraulic and natural fractures were analyzed,and the mathematical model of mechanical stability of the natural fracture with approaching hydraulic fracture was established to calculate the points on which the shear failure occurred and to determine the failure conditions.Moreover,it elaborated the interaction behaviors between hydraulic fracture and the natural fracture;then some new criteria for testing the interaction behavior were proposed according to the Least Resistance Law.These new criteria showed the excellent agreement with the experimental results.(2)Aiming at the simulation of macro-natural-fractures in the shale gas reservoirs,and based on the study of characteristics and the factors influencing the distribution of natural fractures,the Monte-Carlo simulation technique was adopted to simulate the stochastic natural fractures with several reasonable probabilistic distribution functions of critical parameters including the location,geometry and direction of the natural fractures.Finally,the distance function method was used to mesh the target area,which is containing abundant natural fractures.(3)The damage mechanics theory was adopted to depict and predict the hydraulic fractures propagating in cracked and fissured shale formations,which is hard even impossible to be studied via classic fracture mechanics methods.In this dissertation,each of the elements was assigned with different strength values and hydraulic characteristics stochastically to simulate the micro-cracks that were randomly distributed over the entire target,then a kind of reasonable damage variable and damage evolution equation were introduced to study the deterioration of shale rock and its impacts on the mechanical behavior of the hydraulic fracture.Last,a coupling model among fluid flow,effective stresses and the rock damage was proposed to study the hydraulic fracture propagation mode and its mechanical behavior via finite element method.(4)The existing fracture network models cannot consider the impacts of both the micro-cracks and the macro-natural-fractures on the mechanical behaviors during hydraulic fracturing process.This dissertation,combining the interaction criteria between hydraulic fracture and the macro-natural-fracture and the theory of damage mechanics,while handling the micro-cracks and macro-natural-fracture with different simulation methods,introduced a novel but embryonic simulation method to study the propagation behavior of complex fracture network,and some preliminary simulation results were also obtained.It will lay the foundation for further research on simulation of hydraulic fracturing of unconventional reservoirs and fracturing optimization design and its implementation.