| Unconventional oil and gas resources are attracting more and more attention as the world’s demand for oil and gas continues to grow beyond conventional resources.Unconventional oil and gas reserves are abundant,mostly occurring in tight reservoirs.Tight reservoir rocks are characterized by low porosity,low permeability and complex pore structure due to complex sedimentation and diagenesis.Therefore,it is of great significance to study the structural characteristics and rheological damage characteristics of tight reservoir rocks for tight reservoir reconstruction and improvement of unconventional oil and gas exploration and development level.In this paper,CT scanning and particle flow discrete element PFC software were used to characterize the pore structure characteristics of rocks in tight reservoirs.Through the laboratory triaxial compression test and uniaxial rheological test of tight sandstone,the mechanical parameters and rheological damage characteristics of the rock were determined.Using the discrete element model of pore structure of reservoir rock,uniaxial rheological simulation is carried out by using PFC software.The main conclusions are as follows:(1)The pore structure of the tight sandstone reservoir in the study area is complex and its spatial distribution is irregular.The pore and throat diameter distribution obeys normal distribution,and the normal distribution curve is shifted to the direction of smaller diameter.Most pore and throat diameters are distributed between 4～10μm,and most pore diameters are distributed between 20～100μm.(2)When the stress of tight sandstone is low,the creep deformation will gradually stabilize at a specific value,the creep deformation rate will gradually go to zero,and the rock creep will enter a stable creep state.When the tight sandstone is subjected to high stress,the creep will go through constant velocity creep for a certain period of time and then enter the accelerated creep stage until the specimen is damaged.The creep failure mode of tight sandstone is the same as that of triaxial compression,which is shear failure.There is a damage threshold for the rheological damage of tight sandstone.When the load of rock is less than this value,the rheological damage will eventually reach stability;when the load is greater than this value,the rheological damage of rock will continue to develop until the failure of rock occurs.(3)The parallel bond model can better simulate the mechanical properties of rock under uniaxial compression,and the effective modulus of particles increases linearly with the elastic modulus of the model.The relationship between particle stiffness ratio and Poisson’s ratio is a power function.The relationship between particle tensile strength and model compressive strength increases linearly.The bond strength and friction Angle increase linearly with the compressive strength of the model.The combination of parallel bond contact model and Burger’s contact model can give consideration to the strength characteristics and rheological properties of rock,and can better simulate the rheological properties of tight sandstone.(4)When the stress of tight sandstone is low,the rock creep tends to be stable gradually.When the tight sandstone is subjected to high stress,the creep will go through constant velocity creep for a certain period of time and then enter the accelerated creep stage until the specimen is damaged.The creep failure mode of tight sandstone is the same as the triaxial compression failure,both of which are shear failure.There is a damage threshold for the rheological damage of tight sandstone.When the load of rock is less than this value,the rheological damage will eventually reach stability;when the load is greater than this value,the rheological damage of rock will continue to develop until the failure of rock occurs. |
