Study And Application Of Evaluation Method For Dominant Fracturing Section Of Mud Shale Reservoir

Shale oil and gas resources are abundant and have become a hot area of exploration and development in China.Because of the low porosity and permeability of shale reservoir,hydraulic fracturing must be used to achieve industrial exploitation.In order to improve the development efficiency,it is necessary to study the characteristics of shale reservoirs carefully to find favorable development section.At present,most of the researches on shale reservoir evaluation in China are based on the experience of shale development in the North America.The standard quantitative description is not accurate enough,which seriously affects the reliability of the evaluation results.And there is no unified evaluation method for the shale reservoir fracability,which seriously restricts the efficient development of shale oil and gas.Therefore,it is necessary to establish a reliable quantitative evaluation method to accurately find the dominant fracturing section to improve the efficiency of fracturing development.Taking the shales of the Es₃^l Formation in the Zhanhua Sag,Bohai Bay Basin as the research object,the organic geochemical characteristics of shale were tested.The porosity and permeability parameters of shale were tested by nuclear magnetic resonance,scanning electron microscope,low temperature gas adsorption and high pressure mercury injection.The three-dimensional pore and throat model of shale was established by focused ion beam scanning electron microscopy.Mineral composition tester was used to test the mineral composition of shale.The mechanical parameters of shale and the influencing factors were tested by MTS rock mechanical testing system.Combined with analytic hierarchy process,the key parameters were selected through analysis and comparison,and the evaluation model of dominant fracturing section of shale reservoir was established.The results show that the shales of the Es₃^l Formation have high total organic carbon content,the types of organic matter are I and II₁,which are in the stage of low maturity-maturity.The porosity is 5.67%,the permeability is 0.008×10^-3?m²,and the fluid mobility in the pore is poor.Shales mainly develop six types of pore,such as organic pore,intergranular pore,intragranular pore,intergranular pore,dissolution pore and micro-fracture.The three-dimensional pore throat model shows that organic pore has great influence on porosity and connectivity.Micropore provides the main adsorption space for shale oil and gas,mesopore and macropore provide the main storage and migration space.The content of carbonate mineral is the highest,brittle mineral is the second,and clay mineral is the lowest,it has a high brittleness.The compressive strength,Young's modulus and Poisson's ratio of shale decrease with the increase of maturity,increase with the increase of confining pressure,and the failure mode changes from split to shear after confining pressure is added.Confining pressure will restrain the rupture of shale and reduce the fracability.The higher maturity will enhance the brittleness and fracability of shale.The established mathematical model for dominant fracturing section evaluation of shale reservoir comprehensively considers reservoir resource factors and engineering transformation factors.The reservoir resource factors including total organic carbon content,porosity and permeability,the engineering transformation factors including rock mechanics brittleness index,mineral brittleness index,diagenesis and confining pressure.The model was used to evaluate the mud shales in the Es₃^l Formation and find that the reservoir at about 3052.4m is the most dominant fracturing section.