Distribution Characteristics and Backflow Flow Pattern of a Slickwater Fracturing Fluid in Shale Based on Low-Field Nuclear Magnetic Resonance

The distribution and characteristics of the fracturing fluid in shales play a crucial role in determining the shale gas production system for the mining field. By use of low-field nuclear magnetic resonance (NMR) technology and an online displacement system, the distribution of the fracturing fluid in shale reservoirs during hydraulic fracturing was investigated. An online monitoring method was established to track the filtration loss, well soaking, and backflow 3 processes of fracturing fluids in shale reservoirs. The full-scale pore-throat size of the shale in the CN block predominantly distributes within the range of 0.4-8.0 nm. Low-field NMR technology accurately captured the variations in the water-phase sorption and flow in different pores within the shale. During the filtration loss stage, the fracturing fluid entered the core in a stepwise manner, indicating the development of microfractures during fluid invasion. During the well soaking stage, the fracturing fluid preferentially entered the small pores under capillary forces with a stable sorption time of approximately 8 h at the standard core scale. The peak value of the T-2 spectrum gradually decreased with increasing backflow time. Within 135 min, the liquid in the large pores and microfractures in the shale was predominantly expelled. After 135 min, the fracturing fluid in the small pores became immobile and formed bound water. After 200 min, the backflow of the fracturing fluid reached a stable state. During the backflow stage, the fracturing fluid in the large pores was primarily expelled, making the greatest contribution to the backflow volume. Weighted analysis of 3 major production parameters showed that the order of importance for influencing the backflow rate in shale was backflow pressure difference > soaking time > outlet pressure. To fully flow back the fracturing fluid after fracturing, it is recommended that the pressure difference be gradually increased, the soaking time shortened, and the outlet pressure optimized.