Low-permeability reservoirs are characterized by low oil and gas abundance, poor permeability, natural productivity lower than the lower limit of industrial oil flow, and rapid decline of formation energy. Hydraulic fracturing technology is widely used for the development of low-porosity and low-permeability reservoirs. The hydraulic fractures formed by using the hydraulic fracturing technology are the "underground highway" for oil and gas flowing from the deep reservoir to the wellbore. The combination between the long well section of the horizontal well in the reservoir and the wellbore can greatly improve the utilization of oil and gas resources in the reservoir. Therefore, hydraulic fracturing is the key technology to improve the production of low-permeability reservoirs. This paper establishes a volume fracturing propagation model by using the finite element method and discrete element method, and conducts the fracture propagation simulation of multi cluster fracturing in horizontal wells of low-permeability Nanyishan reservoirs in Qinghai Oilfield. The results show that when the stress difference (Difference between the maximum horizontal principal stress and the minimum horizontal principal stress) is less than 4 MPa, volume cracks tend to be generated. When the stress difference is higher than 5 MPa, plane cracks are mainly generated. The natural fracture density affects the volume fracturing effect only under the condition of low stress difference. When the stress difference is 3 MPa, the number of perforating clusters is increased, the cluster spacing is reduced, the improvement area is not significantly increased, the cracks between clusters are seriously interfered, and local cracks are merged. When the stress difference is 5 MPa, there is relatively weak interference between clusters. The increase of perforation clusters promotes the communication between natural fractures, volume fracture development, and expansion of reconstruction area. The research results of this paper provide strong technical support for optimizing the fracturing scheme, improving the single well productivity of low-permeability reservoirs, and developing low-permeability reservoirs in a cost-effective way.

The accumulation of multiple source components leads to the superposition of multiple pore throat systems and similar pore throat distribution may correspond to different seepage capacity in the mixed rock reservoir. Taking the mixed rock reservoir of III+IV oil formation in Nanyishan Oilfield, Chaidamu Basin, as an example, the development characteristics of pore throat system and its controlling factors in the mixed rock were summarized by means of thin slice, scanning electron microscopy, X-ray diffraction and high pressure mercury injection, and the reservoir physical property differences and their genetic controlling mechanism were analyzed. The results show that the reservoir space of the mixed reservoir mainly consists of intergranular pore fracture, micropore fracture, and micropore-dissolution pore. The diverse and complex pore types make the reservoir space combination type, capillary pressure curve shape and reservoir physical properties are not completely corresponding. The higher the content of brittle minerals, the higher the reservoir permeability. However, the development of iron dolomite mainly destroys the reservoir. The permeability of reservoir is mainly controlled by the development degree of macropores and the pore size which is the main contribution to permeability, and the proportion of macropores increases with the increase of permeability.

Low-permeability reservoirs are characterized by low oil and gas abundance, poor permeability, natural productivity lower than the lower limit of industrial oil flow, and rapid decline of formation energy. Hydraulic fracturing technology is widely used for the development of low-porosity and low-permeability reservoirs. The hydraulic fractures formed by using the hydraulic fracturing technology are the "underground highway" for oil and gas flowing from the deep reservoir to the wellbore. The combination between the long well section of the horizontal well in the reservoir and the wellbore can greatly improve the utilization of oil and gas resources in the reservoir. Therefore, hydraulic fracturing is the key technology to improve the production of low-permeability reservoirs. This paper establishes a volume fracturing propagation model by using the finite element method and discrete element method, and conducts the fracture propagation simulation of multi cluster fracturing in horizontal wells of low-permeability Nanyishan reservoirs in Qinghai Oilfield. The results show that when the stress difference（Difference between the maximum horizontal principal stress and the minimum horizontal principal stress） is less than 4 MPa, volume cracks tend to be generated. When the stress difference is higher than 5 MPa, plane cracks are mainly generated. The natural fracture density affects the volume fracturing effect only under the condition of low stress difference. When the stress difference is 3 MPa, the number of perforating clusters is increased, the cluster spacing is reduced, the improvement area is not significantly increased, the cracks between clusters are seriously interfered, and local cracks are merged. When the stress difference is 5 MPa, there is relatively weak interference between clusters. The increase of perforation clusters promotes the communication between natural fractures, volume fracture development, and expansion of reconstruction area. The research results of this paper provide strong technical support for optimizing the fracturing scheme, improving the single well productivity of low-permeability reservoirs, and developing low-permeability reservoirs in a cost-effective way.