Flowback Behavior of Deep Coalbed Methane Horizontal Wells

Significant differences exist between deep and medium-shallow coalbed methane (CBM) reservoirs. The unclear understanding of flowback and production behavior severely constrains the development of deep CBM resources. To address this challenge, guided by the gas-liquid two-phase flow theory in ultra-low permeability reservoirs, and integrating theoretical analysis, numerical simulation, and insights from production practices, this study classifies the flowback and production stages of deep CBM well considering the Daning-Jixian Block, Eastern Ordos Basin as a representative case. We summarize the flowback characteristics for each stage and establish a standard flowback production type curve, aiming to guide field operations. The results indicate that: (a) The production process of deep CBM horizontal wells can be divided into five distinct stages: initial single-phase water dewatering stage, initial gas appearance to peak water production stage, gas breakthrough to peak gas production stage, stable production and decline stage, and low-rate production stage. (b) Based on reservoir energy, two standard type curves for horizontal well flowback production are established: the ‘Sufficient Reservoir Energy’ type and the ‘Insufficient Reservoir Energy’ type. The former achieves a higher initial gas rate (up to 12 × 10⁴ m³/d) but exhibits poorer stability, while the latter achieves a lower stable rate (up to 8 × 10⁴ m³/d) but demonstrates stronger stability. Numerical simulation confirms these behavioral patterns and reveals the underlying mechanisms related to the effectively drained area where pressure is significantly depleted. The findings from this study have guided the flowback production operations in 53 deep CBM wells with positive results, demonstrating high potential for broad application.