Study On Mechanism And Characterization Of Multi-scale Hydraulic Fracturing

The complex fracture network exhibits multi-scale characteristics.The time scale varies from microseconds(dynamic hydraulic fracture)to hours(quasi-static hydraulic fracture).The space scale varies from micrometers(micro fracture)to hundreds of meters(main fracture).It has been a main concern of researchers in petroleum-related rock mechanics that how to characterize this multi-scale problem and how to address this complex issue.From a time and space perspective,the hydraulic fracturing process is intrinsically a multi-scale combined fluid-mechanics-related and fracture-mechanics-related problem.Whereas it is still constrained by the difficulty in observing technics.The study here mainly focuses on the characterization of multi-time-scale and multi-space-scale hydraulic fracture.Experiments and theoretical analysis are conducted on mechanism of multi-time-scale hydraulic fracture,multi-time-scale hydraulic fracture blunting,characterization of Mode I shale fracture and the mechanism of cohesive zone in shale.The contents of this thesis are as follows:(1)Experimental and theoretical study of multi-time-scale hydraulic fractureFor the first time,the direct observations and theoretical analyses of the relationship between the crack tip and the fluid front in dynamic hydraulic fracture are presented.The quasi-static propagation of hydraulic fracture is characterized by optical microscope and digital image correlation.A laboratory-scale hydraulic fracturing device is built and verified by a combined method of digital image correlation and Williams’series.The momentum-balance equation of the fracturing fluid is established and numerically solved.The theoretical predictions conform well to the directly-observed relationship between the crack tip and the fluid front.The kinetic energy of the fluid occupies over half of the total input energy.Using dimensionless analyses,the existence of equilibrium state of the driving fluid in this dynamic system is theoretically established and experimentally verified.The dimensionless separation criterion of the crack tip and the fluid front in dynamic situation is established and conforms well to the experimental data.Different classic crack profiles are considered.The dynamic analyses show that the separation of crack tip and fluid front is dominated by the crack profile,equilibrium fluid velocity,injection pressure and fluid density.This study provides a better understanding of the dynamic hydraulic fracture.The dimensionless separation criterion can be used to distinguish dynamic hydraulic fracture from the quasi-static hydraulic fracture in numerical simulation.(2)Multi-time-scale blunting of hydraulic fractureThe propagation of hydraulic fracture is a process comprising multiple initiations and arrests.The initiation pressure to some extent depends on the crack blunting level.Hydraulic fracture blunts due to the development of the process zone.The study here focuses on the multi-time-scale crack blunting using PMMA specimen.Using a combined static and quasi-static method,the crack blunting index is measured.Results show that hydraulic fracture can be blunted by stress concentration from 1 millisecond to 6×10~5milliseconds.The morphology depends on the status of the stress concentration.It is also shown that the hydraulic fracture propagates in a step-wise manner.The fracture toughness is not a constant but depends on the arrest duration.The level of micro-crack connection has a positive relation with the arrest time of the crack.(3)Multi-space-scale characterization of shale Mode I fracture and mechanism of micro-scale cohesive zone in shale.The micro cracks near the main crack play an important role in the enhancement of oil and gas production.The study focuses on the multi-space-scale characterization of shale Mode I fracture and the mechanical properties of micro-scale cohesive zone in shale.A mini double cantilever beam device is built to study the crack length and crack width.Environmental scanning electron microscope is used to characterize the morphology of the fracture under stress concentration.Digital image correlation method is used to measure the crack width and the divider method is used to characterize the crack length.Results show that crack width is a multi-space scale parameter.The crack length is not a constant but depends on the divider length.The concept of cohesive zone unit that stands for the basic unit in cohesive zone is proposed.Several fittings of cohesive zone models are conducted.Particle flow code is used to simulate the mechanical properties of cohesive zone unit.The simulations conform well to the experimental results.The study provides direct observation of the development of micro cracks in shale.The behavior of the micro cracks is well characterized by the cohesive zone model.