Would Self-Supported Fracture Contribute to the Hydrocarbon Production in Shale Reservoir Besides Proppant-Supported Fracture

Abstract During hydraulic fracturing in deep shale gas reservoirs, it is difficult to pump proppants long distances, and self-supported fractures are formed at the far and upper ends of the fractures. The self-supported fracture could hold the fracture space by its surface structure again fracture closure. However, it faces various aspects of impairment during shale gas reservoir development, which affect its conductivity. Among the impairment, long-term production results in a decline of bottom hole pressure, which will compact the fracture space, and self-supported fractures without proppant are more sensitive than those with proppant. In this paper, we conducted a series of experiments with samples from a deep shale gas reservoir in the Sichuan basin of China. The self-supported fracture conductivity behavior at high temperatures and high-pressure conditions was studied from the aspects of various fracture dislocation distances, pressure reduction methods, and well-open and shut-in times. Then, the mechanism of self-supported fracture impairment was revealed from the change in fracture width, fracture surface roughness, and fracture surface microscopic morphology. With the ideal numerical model, the impact of self-supported fracture ratio, and pressure drop methods on the gas production were analyzed. The results indicate that the dislocation of self-supported fractures can improve its conductivity greatly. The self-supported fracture conductivity with 1 mm dislocation was increased by 18.5 times to 76.1 times compared with those without dislocation. With the production continuing, when the net pressure on the self-supported fractures is above 35 MPa, its conductivity was impaired significantly, reaching 51.2-92.0%. Small pressure reduction step/more frequent nozzle replacement and reducing the well open and shut-in times would help protect the self-supported fracture conductivity. When the net pressure on self-supported fractures increased from 10 MPa to 30 MPa, the equivalent width of fractures decreased by 53.3%. The average height of the fracture surface decreased by 30.7% after pressure compaction. The numerical modeling study indicates that the larger the volume of self-supported fractures, the higher the productivity of the gas well. In a 3-year production time, when the ratio of proppant-supported fractures to self-supported fractures increased from 1:0.25 to 1:5, the cumulative gas production increased by 220.8% (90.7×106). The small-pressure drop production has higher cumulative productivity than the high-pressure drop method. The cumulative gas production is 115.2×106 when a 10 MPa pressure drop is used, while it is 80.9 ×106 with a 50 MPa pressure drop. This study could provide a theoretical basis for hydraulic fracturing and production system optimization of deep shale gas reservoirs.