Characteristics of the pore and seepage system of deep coalbed methane in the Baijiahai uplift, Junggar Basin, Xinjiang

Objective Xinjiang boasts abundant deep coalbed methane (CBM) resources, with Jurassic coal seams in the Baijiahai uplift of the Junggar Basin identified as a favorable target area for deep CBM exploration and exploitation. The pore and seepage system of deep CBM directly determines its recoverability. Methods Based on appraisal well Caimei 2-004H drilled the most recently in the Xinjiang oilfield in 2023, this study investigated in detail the pore and seepage system of coals in the Jurassic Xishanyao Formation in the Baijiahai uplift using scanning electron microscopy (SEM) observations, mercury injection capillary pressure (MICP), low-temperature liquid nitrogen adsorption, and carbon dioxide adsorption, low-field nuclear magnetic resonance (NMR), CT scanning, and porosity and permeability experiments under varying temperature and pressure conditions, as well as mathematical modeling. Results and Conclusions Key findings are as follows: (1) Pores in deep coal reservoirs in the study area are dominated by plant tissue pores and other pores, along with fissures, which are mostly tabular and slit-shaped. These pores and fissures feature high connectivity and are open pores mostly. (2) The characterization results of full pore size distribution show that ultramicropores are the most developed in the deep coal reservoirs in the study area, followed by mesopores and transitional pores. Various types of pores are relatively uniformly distributed, with a larger total specific surface area of pores and a high total pore volume. Therefore, the deep coal reservoirs are similar to lean coals and anthracites, suggesting high adsorption and reservoir capacities. This occurs primarily because the coal seams comprise coals with ultra-low ash content (average: around 5%), macerals in the coals are dominated by fusinite and homotelinite, and faults are found in coal reservoirs in structural high parts. (3) Given the impacts of temperature and effective stress on porosity and permeability, this study determined the relationships between both porosity and permeability of the study area and the burial depth. Compared to deep coals in the Daning-Jixian block in the Ordos Basin, the deep coal reservoirs in the Baijiahai uplift exhibit high porosity and permeability, as revealed by whether conventional porosity and permeability results or the simulation results of the pore and seepage system under in-situ reservoir conditions. Specifically, the deep coal reservoirs exhibit porosity ranging from 8.60% to 10.21% and permeability from 0.04×10⁻³ μm² to 0.21×10⁻³ μm² under in-situ reservoir conditions. Therefore, the pore and seepage system in deep coal reservoirs in the study area boasts both a high CBM storage capacity and excellent conductivity.