Early Evaluation Method for Recoverable Reserves in Deep Coalbed Methane Wells

This paper proposes an early evaluation method for recoverable reserves in deep coal-measure gas wells, overcoming issues such as significant parameter interpretation errors caused by controlled-pressure production, early transient flow stages, and gas-water two-phase interference in traditional Arps decline analysis and commonly used Rate Transient Analysis (RTA). Field applications confirm the feasibility of this method. By utilizing early-stage flowback, production data (water production, pressure), and the Compound Linear Flow (CLF) model, this method solves key parameters including reservoir permeability, fracture half-length, and dynamic drainage boundaries. These parameters are then used to establish a numerical model for gas-water two-phase flow to predict Estimated Ultimate Recovery (EUR). The CLF model, tailored for hydraulically fractured horizontal wells, identifies different flow regimes: intra-fracture transient flow, transitional flow, and inter-fracture transient flow. After the gas well reaches the inter-fracture transient flow stage, unique solutions for reservoir permeability and fracture half-length can be derived. These parameters enable the prediction of EUR through a two-phase numerical model, bypassing the limitations of traditional methods that rely on long-term stable flow data. Applications in multiple deep coal-measure horizontal wells in the Daji Block demonstrate that EUR can be predicted with minimal error within six months of production. This method provides a rapid EUR evaluation tool for deep coal-measure reservoirs (> 1500 m burial depth) characterized by “high pressure and high free gas content.”