Objective  In recent years, significant breakthroughs have been achieved in the exploration and exploitation of deep coalbed methane (CBM), which has emerged as a major area for the reserve growth and production addition of CBM in China. Variations in the temperature and pressure conditions of deep formations inevitably cause substantial differences in the commingled-production compatibility of deep and shallow superimposed CBM systems. Therefore, revealing the depth effects of the commingled-production compatibility is critical to enhancing the efficiency of the multi-layer commingled production of deep CBM and even coal-measure gas.

Methods  Using a systematic literature survey of the exploration and exploitation of deep CBM and the commingled-production compatibility of superimposed CBM systems, this study organizes the research advances in both fields and seek common regularity and synergistic breakthroughs. Furthermore, it explores the potential controlling factors, discriminant indicators, and prediction methods for the commingled-production compatibility of deep superimposed CBM systems and proposes future research trends and key scientific issues.

Results and Conclusions  Comparison of the geological conditions and production mechanisms of deep and shallow CBM reveals the presence of four key factors governing the depth effects of the commingled-production compatibility: fluid pressure regime, gas saturation (i.e., gas occurrence state), reservoir porosity and permeability, and fracability. The co-evolutionary patterns of these factors under the coupling effects of temperature and in situ stress provide a geological foundation for understanding the gas accumulation mechanisms and the commingled-production compatibility of deep superimposed CBM systems. The fracability of deep coal reservoirs can adjust other geological properties of the reservoirs, playing a key role in the geology-engineering integrated evaluation for the commingled-production compatibility and the R&D of interference reduction technologies. The discriminant parameters for the compatibility and their thresholds should be determined by fully considering reservoir characteristics including overpressure, rich free gas, high gas energy density, elevated desorption and production capacities, low water saturation, and intact coal structures, as well as the similarities and differences between the energy driving mechanisms underlying CBM production under depressurization via gas and water drainage. This is the only way to establish the discriminant indicators for the compatibility and their threshold system that are suitable for the geological characteristics and production behavior of deep CBM. The methods for predicting the commingled-production compatibility include theoretical analysis, dynamic productivity analysis, physical simulation, numerical simulation, and the geochemistry of produced fluids. These methods have found widespread applications in the geological research and practices of the multi-layer commingled production of moderately deep to shallow CBM, providing effective guidance for the identification of gas and water sources, the selection of the optimal pay zone combination, and productivity potential evaluation. However, these methods neglect the special geological properties and gas occurrence states of deep reservoirs under high-temperature and high-pressure conditions. Furthermore, they mostly focus on the static evaluation results of the compatibility while omitting its dynamic transformation process. These limitations lead to the inadequate elucidation of the interference behavior and dynamic compatibility effects of the multi-layer commingled production of deep CBM. Therefore, it is necessary to highlight four types of integration: the integration of macroscale and microscale factors, the integration of static and dynamic factors, geology-engineering integration, and theory-practice integration. It is recommended to address three key scientific issues as a priority using this methodology: the co-evolutionary patterns of critical geological properties of superimposed CBM systems (through the integration of macroscale and microscale factors), the depth response mechanisms behind the commingled-production compatibility (through the integration of static and dynamic factors), and prediction models and evaluation methods for the commingled-production compatibility (through geology-engineering integration). These efforts will help establish a comprehensive theoretical and methodological framework for the depth effects of the commingled-production compatibility of superimposed CBM systems (through theory-practice integration) and develop both geological theories on deep CBM production and evaluation technologies for the commingled-production compatibility of superimposed CBM systems. In the future, further attention should be paid to the geological issues in frontier fields such as the co-occurrence, joint exploration, and co-production of multiple types of natural gases in deep coal measures, along with underground coal gasification-enhanced CBM/coal-measure gas recovery (UCG-ECBM). The purpose is to advance the integrated, efficient exploitation and utilization of clean coal measure energy.