Analysis of the Key Factors Affecting the Productivity of Coalbed Methane Wells: A Case Study of a High-Rank Coal Reservoir in the Central and Southern Qinshui Basin, China.

Abstract

In the present study, three CBM blocks in the central and southern Qinshui Basin, China, including Fanzhuang, Zhengzhuang, and Changzhi blocks, were selected. Combined with the data, such as the physical properties of coal reservoirs, logging, hydrofracture operation, injection/drawdown well testing, microseismic fracture monitoring technology, and over 2000 days gas production rate, the key factors affecting the gas production rate of CBM wells were analyzed comprehensively and systematically. Unimodal and bimodal models can be identified according to the long-term gas production rate data. The unimodal model corresponds to a declining pump pressure curve, meaning that caprock integrity is destroyed during hydrofracture operations, commonly causing poor gas production performance. The bimodal model is associated with fluctuating-rising and stable pump pressure curves, indicating good hydrofracture consequences. On the premise of the relatively high gas content, the gas saturation/critical-reservoir pressure ratio, permeability, and coal deformation are the major geological factors that affect the long-term gas production performance of CBM wells. Engineering factors, including pollution by the drilling fluid and cement paste, the type of the fracturing fluid, tonstein intercalation, coal deformation, and in situ stress, affect gas production performances via the following four mechanisms: the effect of hydrofracture operations on caprock integrity, the effect of fluids pumped on the pore-fracture system, the initiation and propagation of artificially induced fractures, and the performances of proppants pumped. This work can provide guidelines for the optimization and development of high-rank CBM blocks.