Study On Mechanical Properties Of The Hard Sandstone Under Hydrochemical Corrosion

Rock pressure manifestation caused by hard roof has always been a problem puzzling the safe and efficient mining of coal mine.With the development of coal mining to the deep,this problem is becoming increasingly prominent.Based on this,the team proposed the technology of surface drilling hydraulic fracturing to weaken the high hard roof.With the aid of hydraulic fracturing,many cracks are formed in the hard roof to weaken and treat the hard roof.Fracturing fluids usually contain chemical substances.It is of great significance for the optimization of this technology and the treatment of hard roof to clarify the mechanical properties of fracture surface under chemical action and its influence on hard roof.In this paper,based on the "two acids and three minerals" acidification model,combined with the physical and chemical reaction mode between hydrochemical solution and rock,the rock fracture surface and its mechanical properties under corrosion are analyzed by using Shimadzu test machine and coal-rock triaxial rheological test machine,and compared with the mechanical properties of non-corroded rock,so as to optimize and finish.It provides theoretical basis and basic guidance to improve surface drilling hydraulic fracturing weakening and treatment of high hard roof technology.The research results are as follows:(1)The triaxial compressive strength and shear strength of sandstone decrease in varying degrees after hydrochemical solution corrosion,while the roughness and porosity of fracture surface increase gradually,and the damage of mechanical parameters of sandstone increases with the increase of immersion time.(2)The scanning electron microscopy(SEM)test was carried out after the corrosion of hydrochemical solution,and the change of the fracture surface microstructure was obtained.After corrosion by hydrochemical solution,the particles on the fracture surface of sandstone specimens appear to fall off obviously,the surface roughness increases,and with the increase of the concentration,the corrosion holes become larger gradually.The fillings such as clay minerals and feldspar particles in the corroded pore can be seen to form relatively obvious acid-etched holes on the fracture surface after corrosion with different concentration of hydrochemical solution,which improves the connectivity between the pores.(3)By introducing damage parameters,the variation law of crack surface roughness is quantitatively described.The results show that with the increase of soaking concentration and soaking time,the roughness of crack surface increases gradually,and the change of roughness is the greatest when the concentration is 5%.(4)The direct shear test of sandstone specimens under hydrochemical corrosion was carried out,and the deterioration rule of shear strength of fracture surface was revealed.Under different test conditions,the peak shear strength of sandstone specimens soaked in hydrochemical solution decreases with the increase of concentration of hydrochemical solution.With the continuous action of external loads,the specimens reach the peak shear strength and then break down instantaneously.Damage parameters are introduced to quantitatively describe the deterioration law of sandstone strength after corrosion.The relationship between damage parameters and porosity and the relationship between shear strength and damage parameters of acidized rock are obtained by regression analysis.(5)The deterioration of triaxial compressive strength of sandstone under hydrochemical corrosion is revealed by using GCTS rock triaxial test system.The modulus of elasticity of sandstone specimens decreases in different degrees after soaking in different concentration of hydrochemical solution.When the concentration is 1%,the modulus of elasticity of sandstone specimens keeps above 50 GPa after soaking in different time.When the soaking time is 30 days and 45 days,the modulus of elasticity decreases slightly.With the concentration of hydrochemical solution increasing,the modulus of elasticity decreases sharply,but with the concentration of hydrochemical solution increasing.With the increase of elastic modulus,the decrease of elastic modulus decreases gradually.