In the heart of China, the Ordos Basin is emerging as a pivotal battleground for the exploration and development of deep coalbed methane (CBM), a resource that could significantly bolster the country’s energy portfolio. Recent findings published in Meitan kexue jishu (Coal Science and Technology) shed new light on the geological factors controlling the enrichment of deep CBM, offering a roadmap for more efficient and targeted extraction methods.
The study, led by Liyong Fan of the National Engineering Laboratory of Exploration and Development of Low-Permeability Oil and Gas Fields in Xi’an, delves into the intricate interplay of material composition, depth, and preservation effects that govern the differential enrichment of deep CBM. Fan and his team analyzed data from numerous deep evaluation CBM wells implemented by China Petroleum Changqing Oilfield Company, employing mathematical statistics, model calculations, and geological analysis to unravel the complexities of CBM enrichment.
One of the key findings is the identification of “three controlling effects” that influence the differential enrichment of deep CBM. “The material component is the primary factor controlling the enrichment of CBM,” Fan explains. “Low ash content and high metamorphic coal with high gas content are crucial parameters in this regard.” This insight underscores the importance of selecting the right coal seams for CBM extraction, a factor that could significantly enhance the efficiency and profitability of mining operations.
The depth effect, another critical factor, reveals that the porosity of coal seams decreases with increasing burial depth. This reduction in porosity slows down the movement of free gas and leads to a critical conversion depth for adsorbed gas, typically ranging between 2000 and 3000 meters. Within this depth range, the total CBM content is higher, making it an optimal zone for extraction. However, beyond 3000 meters, the depth effect intensifies, leading to a gradual decrease in total CBM content, often dropping below 20 cubic meters per ton.
The preservation effect, the third controlling factor, is particularly important for the enrichment of both adsorbed and free CBM. The type of cap rock, groundwater total dissolved solids, and pressure coefficients play pivotal roles in this effect. Fan notes, “The coal-rock combinations of coal-mud, coal-ash, and coal-ash mud in the study area have good capping properties, corresponding to high groundwater salinity. This results in higher gas content, often exceeding 20 cubic meters per ton within the buried depth of 2000 to 3000 meters.”
These findings have profound implications for the energy sector. By understanding the geological controls on CBM enrichment, energy companies can optimize their exploration and extraction strategies, potentially leading to higher yields and reduced operational costs. The proposed deep CBM enrichment model, which couples depth and preservation effects, provides a valuable tool for delineating high-potential zones within the Ordos Basin.
As the energy sector continues to diversify and seek sustainable solutions, the insights from this research could pave the way for more efficient and environmentally friendly CBM extraction methods. The work by Fan and his team, published in Meitan kexue jishu, represents a significant step forward in the quest to unlock the full potential of deep CBM resources in the Ordos Basin and beyond. The implications of this research extend far beyond the immediate region, offering a blueprint for similar studies in other parts of the world. As the global demand for clean energy sources continues to grow, the lessons learned from the Ordos Basin could play a crucial role in shaping the future of the energy landscape.