Study on Evaluation Methods of Sweet Spots for Shale Gas Infill Wells—A Case Study in the Changning Block

The evaluation of sweet spots for infill wells is critical to identifying premium reservoir zones, avoiding fracture hits, and achieving safe, efficient development with maximum production potential. Firstly, considering that geological and engineering factors—such as high fracability and good reservoir quality—are conducive to the formation of complex fracture networks and sufficient gas production after fracturing, quantitative evaluation indicators for fracability and geological properties have been established. Secondly, a classification method for different sweet spot tiers in infill wells was proposed. Lastly, taking the Changning infill pilot wells as an example, for sections not affected by fracture interference, higher sweet spot evaluation scores show a strong correlation with improved predictive performance of tracer-based gas production forecasts. Conversely, in fracture-interfered zones, a discrepancy was observed between the sweet spot evaluation results and actual gas production volumes. The horizontal wellbores were classified into a six-tier system (L1–L6), with tailored fracturing design recommendations provided accordingly. This study offers scientific guidance for the precise evaluation of sweet spots in infill wells and the design of customized staged fracturing, thereby significantly enhancing fracturing effectiveness.