| Some research achievements have been made on the true triaxial mechanical properties of natural rock,but the study of the mechanical properties of fluid-bearing rocks has focused on uniaxial and conventional triaxial tests under different water conditions.In the actual exploitation of oil fields,most of the reservoirs are in a complex seepage environment with varying degrees of saturation.Therefore,the further research of the true triaxial mechanical properties of rock in the presence of fluid plays a significant role in the study of the mechanical properties of underground rock and the improvement of the fracturing technology of underground tight reservoirs.The gray-white block-shaped medium-slender rock sandstone in Panlong Town,Yan’an City,Shaanxi Province was selected,and the true triaxial test of natural sandstone,water-bearing sandstone and oil-bearing sandstone was carried out by using YSZS-2000 multi-function electro-hydraulic servo rock true triaxial testing machine.The variation law of the true triaxial mechanical parameters of rock under different fluid occurrence conditions is analyzed.The main research results are as follows:(1)With the increase of the intermediate principal stress,the peak stress and elastic modulus of the natural sandstone also increase linearly,and the Poisson’s ratio changes irregularly.Under the same intermediate principal stress condition,with the increase of confining pressure,the peak stress and elastic modulus increase,and the Poisson’s ratio changes irregularly.(2)The natural water absorption curve of P sandstone is divided into two types: Pa type and Pb type.With the increase of water saturation f,the peak stress of the rock decreases linearly,the elastic modulus value decreases,and the decreasing amplitude becomes smaller and smaller;the Pb sandstone increases with the water saturation f.The peak stress of the rock decreases linearly,and the value of the elastic modulus changes irregularly.There is only one natural oil absorption curve for P sandstone.With the increase of rock oil saturation f,the peak stress and elastic modulus of the rock decrease linearly.(3)When the fluid is immersed in the rock,it will weaken the mechanical properties of the rock.As the water(oil)saturation increases,the peak rock stress decreases.Under the same conditions,the natural sandstone samples have the highest peak stress,and then are oil-bearing sandstone,Pa-type sandstone and Pb-type sandstone.The reason is that the part of the pore throat existing in the Pb type sandstone is slightly larger than the Pa type sandstone,which causes the dissolution or reaction of the mineral in the part of the pore throat under the action of water,and the degree of action of water on each sample is different;When oil is immersed in the rock,it does not react with the rock.Therefore,when the saturation is the same,the oil has the least impact on the rock and is the most stable.(4)Under the conditions of confining pressure=9.5MPa and intermediate principal stress=10MPa,when the rock is saturated with water,the peak stress of Pa-type sandstone is about 11.48% lower than that of natural sandstone,and the Pb-type sandstone decreases by16.19%;when the oil saturation of the rock is 0.639,the peak rock stress decreases by approximately 5.28%.When the rock is saturated with water,the elastic modulus of the Pa-type sandstone decreases by about 16.80%;when the oil saturation of the rock is 0.639,the elastic modulus of the sandstone decreases by about 11.35%. |
