Optimization of Proppant Placement Strategy Based on the Simulation of Overall Fracture Closure Shape and Productivity in Tight Sandstone Reservoir Volume Fracturing Under Non-Uniform Proppant Placement

Tight sandstone reservoirs are stimulated by large scale fracturing with slick water, but due to poor sand-carrying capacity, proppants settle to the bottom of the fracture rapidly, with no proppant at the top of fracture, causing non-uniform placement of proppant in fractures. Nevertheless, existing studies on post-fracturing productivity simulation often assume uniform proppant distribution and constant fracture conductivity, which contradicts actual conditions and leads to significant prediction errors and ineffective theoretical guidance. To address this, we explored the overall fracture closure shape under non-uniform proppant placement using a finite element model based on a differential approach. We then determined the conductivity of each fracture zone through conductivity tests and the cubic law. We established a productivity model for tight gas horizontal wells that considers the overall fracture closure shape to reveal the intra-fracture flow process after closure, as well as the influence of proppant placement morphology and conductivity on productivity. Results show that non-uniform proppant placement creates three distinct areas after fracture closure: an unpropped area, a propped area, and an arch area between them. Although the arch area only accounts for 0.85% of the overall fracture area, it contributes 37% of the gas flow rate due to its greatest conductivity and the lowest fluid pressure, which drives gas from the propped and unpropped areas to converge into the arch and then to the wellbore. The propped area contributes 52% of the gas flow rate. Thus, the arch and propped area are the main flow pathways. The analysis of proppant placement morphology on productivity indicated that, under a constant proppant dosage, longer proppant placement length leads to higher cumulative production, suggesting that extending the placement length should be a focus during fracturing. Furthermore, analysis of the influence of proppant type on productivity revealed that production using quartz sand was only 2.85% lower than that of ceramic proppant (although its conductivity decreased by 35%), due to the presence of the arch. Therefore, a strategy of replacing ceramic proppant with quartz sand is proposed. ZJ1 and ZJ2 are adjacent wells in the Shaximiao Formation of the Sichuan Basin. ZJ2 adopted quartz sand as the proppant; the volume (1680.5 m3) was 34% higher than that of ZJ1 which used ceramic (1250.7 m3), but the cost dropped by 60.7%. Furthermore, ZJ2's cumulative gas production (4997×104m3) is 6.6% higher than that of ZJ1 (4688×104m3). This field application proves that the research has high engineering application value, and the cost-effective strategy of replacing ceramic proppant with quartz sand can be extended to similar tight gas fields.