Experimental Investigation on Hydraulic Fracture Geometry of Perforated Well in Longmaxi Shale Gas Reservoir
Author:Jie Jing Yu
Supervisor:jiang guo sheng
Shale gas is a kind of unconventional natural gas with huge reserves,which brings a worldwide upsurge of exploration and development since the turn of the century.Since "the Nineteenth National Congress",it has become one of the development strategies of our country to promote the green low-carbon energy industry and ensure the healthy and sustainable development of economy.Shale gas,as a kind of clean energy,is of vital importance to China’s energy structure adjustment and energy security.Shale gas reservoir is typically characterized by low porosity and low permeability,poor connectivity,well bedded and anisotropy.In order to form complex fracture networks,"volume fracturing" has become one of the core technologies for shale gas reservoir development all around the world in recent years.As the bridge between the well and the reservoir,hydraulic fractures(HFs)are an important indicator for evaluating reservoir modification.The geometry characteristics and expansion path of HF directly determine the development effect of fractured wells.However,with the exploration and development work gradually proceeding to deep shale reservoirs,the problem of shale gas recovery reduction caused by poor reservoir stimulation has become increasingly prominent.Consequently,it is very significant for shale gas reservoir reconstruction to master the pattern of initiation and extension of HFs and reveal the formation mechanism of complex fractures.Based on Longmaxi shale formation in Sichuan Basin,experimental and numerical simulation is carried out to explore the geometry characteristics of HFs in this paper.The formation mechanism of complex fracture network in shale reservoirs is revealed from geological and engineering factors.This paper is divided into six chapters.The main contents are as follows:Chapter 1: Firstly,the geological characteristics of shale gas reservoirs and the exploration and development process are summarized,and then the purpose and significance of this study are introduced.Subsequently,the research status and shortcomings of relevant development at home and abroad are discussed.Finally,this paper describes the main research content,technical route and innovation points.Chapter 2: The anisotropic characteristics of Longmaxi shale are mainly discussed.In order to investigate the anisotropy of our shale specimen,X-ray Diffraction(XRD),field emission scanning electron microscope(FE-SEM)and atomic force microscope(AFM)are used in our investigation.Velocity measurement and uniaxial compression experiments have been carried out for standard cylindrical specimens with different coring directions as well.The anisotropy of Longmaxi Formation shale in mineral composition,micropore structure,acoustic properties,uniaxial compressive strength,Young’s modulus,Poisson’s ratio and other physical and mechanical properties are discussed.Finally,the energy dissipation and transformation form of shale specimen in uniaxial compression failure are analyzed.The failure mode of shale is further discussed based on the energy dissipation principle.Chapter 3: This chapter mainly discusses the process and results of physical simulation experiments of perforated horizontal wells hydraulic fracturing in Longmaxi shale formation.The shale specimens used in these fracturing experiments were cut to the size of 300 mm×300 mm×300 mm.Based on the distribution of tracer on HF surface,pump pressure-time curve and acoustic emission location results,the HF geometry characteristics of shale specimens with different perforation types are studied.Chapter 4: Emphasis is laid on the effects of perforation mode and fracturing fluid injection displacement on HF geometry.In the first place,the "SRA" method for evaluating indoor hydraulic fracturing experiments is improved,and a coefficient is proposed to characterize the complexity of HFs on experimental scale.In the second place,the fracture mechanism of oriented perforation,helical perforation,planar perforation and combinatorial perforation is discussed based on the physical simulation experimental results of hydraulic fracturing.In the third place,in terms of injection displacement,the propagation modes of HF under constant and step-displacement are discussed respectively.What’s more,based on the principle of energy dissipation,the mechanism of step displacement fracturing to enlarge fracture size is discussed.Chapter 5: Based on the results of physical simulation and extended finite element method numerical simulation,the interaction mode between natural structural plane and HF under different conditions,and the relationship between the geological factors(the anisotropy of micro-pore structure,in-situ stress difference coefficient,brittleness index)and the propagation path of HF are investigated.Chapter 6: The conclusions of the paper are summarized.The shortcomings and future research directions are described.Based on the research above,the following conclusions are drawn in this paper:(1)The physical and mechanical properties of shale show obvious differences because of the angles between the matrix and the bedding plane.For the standard shale cylindrical specimen with different coring angles α,with the increase of α,the uniaxial compressive strength and poisson’s ratio show a U-shaped anisotropic model,which first decreases and then increases.The elastic modulus and S-P wave velocity show a decreasing trend,and the development degree of micro-pore structure increases.Under uniaxial compression,the main energy dissipation forms of shale are fracture surface energy and plastic potential energy produced by shear slip.(2)Perforation mode is one of the main determinants of HF geometry in shale formations.The HF geometries induced from different perforations show great differences,in which the combinatorial perforation fracturing has a certain promoting effect on the formation of complex fractures.The fracture geometries are always planar under the effect of the oriented perforations,whose complexity is relatively low.Helical perforation and planar perforation fracturing have the potential to form complex fracture networks,but failure perforations will significantly lower the fracture scale.(3)Larger fracturing fluid injection displacement is a guarantee for the formation of complex fractures in shale reservoirs.Compared to the constant displacement fracturing,step-displacement fracturing can often further expand the scale of fractures.When the injection displacement increases,the fracturing fluid will also work towards the fracture surface.When the elastic strain energy is accumulated to a certain value near the fracture surface,new fractures might initiate.In the process of increasing displacement,within the allowable range of pumping equipment conditions,the increase amplitude of displacement should be large.(4)Results show that the HF geometry displays six basic patterns when intersecting with structural planes,namely,penetration/non-dilation,penetration/dilation,branching,derivation,capture and deflection,induction and deflection.These patterns are the basis for the formation of complex fractures.Fracturing fluid will enter the micro-cracks and micro-pore near the fracture surface and increase their width during the process of HF propagation.Larger in-situ stress difference coefficient will promote HF s to penetrate natural structural planes,thus further expanding the scale of fractures.Shale specimens with high brittleness index have stronger fracture-producing ability,larger fracture scale and smaller fracture tortuosity.Preliminary prediction of HF geometries can be made by pump pressure-time curve.