Effects of Fracture Characteristics on Spontaneous Imbibition in a Tight Reservoir

Horizontal well fracturing technology is widely used in the development of shale gas resources. After the fracturing operations, natural and artificial fractures exist in the reservoir at the same time, which has significant effects on spontaneous imbibition to increase shale gas production. Based on nuclear magnetic resonance technology, this work studies the spontaneous imbibition property of fractured shale, including artificial and natural fractures, surface cemented minerals, boundary conditions, etc. The results show that the relaxation time spectra reflect the characteristics of water and gas migration in the rock pore space. The micropores and the small pores are the main storage space for water in shale. After water imbibition starts, the imbibition volume is roughly proportional to the square root of time. Through the comparative experiments on artificial fractures, natural fractures, and seamless cores, it is found that the existence of fractures can significantly promote water imbibition, and the imbibition speed of artificial fractured cores is faster than those of others. Because of the existence of artificial and natural fractures, the number of pores for gas discharge increases, and the imbibed volume is slightly more. When the cemented minerals such as quartzite and calcite are attached to the core surface, they have poor water sensitivity and block some of the pores, thus resulting in a slower imbibition speed. Different boundary conditions will not only affect the contact area between the core and water but also influence the types of imbibition, which changes the water flow path and imbibition speed. This study provides an important insight into spontaneous imbibition in gas shale after the fracturing operations.