Quantitative Prediction of Deep Coalbed Methane Content in Daning-Jixian Block, Ordos Basin, China

Abstract

The potential of deep coalbed methane resources is substantial. Gas content is an important parameter for resource assessment. At present, the gas content test method commonly used for shallow coal reservoirs is not suitable for determining deep coalbed methane gas content. Therefore, it is urgent to establish a prediction method for deep coalbed methane gas content. This study aims to quantitatively predict the gas content of coalbed methane in deep coal reservoirs and uncover its influencing factors. For this purpose, we selected the Daning-Jixian area, a region in China with relatively advanced development of deep coalbed methane, as a case study. It established a prediction model for adsorbed gas and free gas content in deep coal reservoirs through a series of experimental tests, encompassing gas content, coal quality, isothermal adsorption, and nuclear magnetic resonance. The model sheds light on the impact of coal-rock characteristics, coal quality attributes, and pore characteristics on adsorbed gas and free gas content. The results show that adsorbed gas dominates in deep coal reservoirs with a high metamorphic degree and that the average proportion of adsorbed gas under reservoir burial depth is 80.15%. At a depth of approximately 1800~2000 m, a turning point is observed where adsorbed gas content begins to decline. Beyond this depth range, the detrimental effect of temperature on coalbed methane content surpasses the beneficial impact of pressure. Coal quality characteristics play a major role in controlling adsorbed gas content, and an increase in water content and ash yield significantly reduces the adsorption performance of coal reservoirs. The content of free gas increases with the increase in burial depth, with its controlling factors primarily being confining pressure and porosity. The increase in the proportion of micropores in the pores of deep coal reservoirs has an adverse effect on the content of free gas. The proportion of adsorbed gas in deep coal reservoirs gradually decreases with the increase in burial depth, while the proportion of free gas gradually rises with the increase in burial depth. The development potential of free gas cannot be overlooked in the exploration and development of deep coalbed methane.

Pore Size Distribution and Fractal Characteristics of Deep Coal in the Daning–Jixian Block on the Eastern Margin of the Ordos Basin

“…D 3 of the coal samples was less related to PV and SSA, indicating limited impact of mesopores and macropores on gas adsorption. Furthermore, the large proportion of micropores in the coal samples indicated that the adsorption properties of coal rock were primarily determined by micropores, further explaining the increase in free gas content in some deep CBM reservoirs …”

Dynamic Production Characteristics of Deep Coalbed Methane: A Case Study of Daning-Jixian Block

“…The reservoir and production characteristics of the deep CBM differ significantly from those of the middle and shallow layers. In conditions of high temperature and pressure, a portion of CBM undergoes a transition from an adsorbed state to a free state. , The deep reservoir exhibits the coexistence of adsorbed and free gas, along with conditions conducive to free gas preservation, resulting in the abundance of free gas in the deep coal reservoir . Additionally, the DJ block’s deep coal reservoir displays favorable conditions for reservoir formation, including high thermal evolution, excellent hydrocarbon generation capacity, well-developed reservoir micropores, and optimal preservation conditions .…”

Study of the Methane Adsorption Characteristics in a Deep Coal Reservoir Using Adsorption Potential Theory

“…In China, a large-scale deep coalbed methane block was built on the southern margin of Yanchuan [10]. Since 2021, several coalbed gas wells with a depth of more than 2000 m have produced tens of thousands of cubic meters of gas per day in the Junggar Basin and the eastern margin of the Ordos Basin [11][12][13]. Therefore, the study of methane adsorption characteristics in deep coal reservoirs is a hot topic, which can help predict the total exploitable methane content in deep coal reservoirs [14,15].…”

“…Coal with a high content of aromatic carbon has a strong adsorption capacity, whereas coal with aliphatic structures and oxygencontaining functional groups exhibits a weaker capability for methane adsorption [25,26]. Pressure has a positive effect, whereas temperature has a negative effect on the adsorption of methane [13]. There are various models used to describe the adsorption of methane on coal, including the Langmuir model, BET model, D-A model, and D-R model [1,3,4,27,28].…”