Vertical Propagation Behavior of Hydraulic Fractures and Fracability Evaluation in Shale Reservoirs: A Case Study of the Yongchuan Block, Sichuan Basin

Abstract

Vertical hydraulic-fracture propagation controls the effective stimulated thickness of deep layered shale, yet conventional fracability indices omit this response. Coupled hydromechanical cohesive-zone models were built for nine sublayers in the northern and southern Yongchuan sub-blocks, Sichuan Basin, using rock-mechanics tests, well logs, in situ stress evaluation, and measured layer thicknesses. Increasing bedding dip from 0° to 10° raised fracture height by 19.8% in northern Yongchuan and 41.7% in southern Yongchuan, whereas reducing tensile strength from 8.0 to 6.4 MPa raised it by 31.3% and 33.2%, respectively. These results indicate that, within the investigated parameter ranges, fracture height responded more strongly to tensile-strength variation than to bedding-dip variation, whereas the southern sub-block was more sensitive to bedding orientation. For northern Yongchuan, a 30-case full-factorial dataset was used to train a Gaussian process regression model of fracture height (leave-one-out R2 = 0.932, MAE = 0.77 m, RMSE = 0.97 m), and 12 additional off-grid simulations yielded an R2 of 0.864, an MAE of 0.93 m, and an RMSE of 1.06 m. The normalized vertical fracture-propagation response was then combined with a baseline fracability index through a weighted geometric mean. At the selected wells, the integrated index showed better agreement with average monthly gas production than the baseline index. The resulting evaluation framework provides a basis for fracturing feasibility screening in the Yongchuan area.