Stress sensitivity characteristics and control mechanism of different coal rank reservoirs

The stress sensitivity of coal reservoir reduces the permeability, which in turn affects the productivity of coalbed methane (CBM)wells. So how to reduce this effect from stress sensitivity deserves further study in the CBM wells drainage process. To clarify the stress sensitivity characteristic and differences of different rank coal reservoirs, coal samples collected respectively from Fanzhuang block (high rank coal), Baode block (medium rank coal) and Erlian Basin (low rank lignite) were employed in this study. The stress sensitivity experiments of different rank coal samples were systematically carried out during loading and unloading process. Meanwhile, stress sensitivity mechanism was analyzed. The results demonstrate that with the increase of coal rank, the stress sensitivity of coal samples gradually strengthens, and those samples containing obvious fractures showing stronger stress sensitivity. When loading effective stress is 10MPa, compared with the initial value, the permeability of Erlian low rank lignite coal decreases by 79.26% and the average irreversible permeability damage rate after unloading is 33.4%. For Baode medium rank coal, the permeability decreases by 79.4% and the average irreversible permeability damage rate is 51.4%. As for Fanzhuang high rank coal, the permeability drops by 92.33% after loading, and the permeability can only recover about 30% after unloading. For different rank coals, the stress sensitivity mechanism is mainly due to the different material composition, pore and fracture structure, and percolation form. The low rank lignite coals with a low degree metamorphism, large and medium pores are mainly developed, but cleats and fractures are not. The seepage is mainly through matrix pores and throats, and permeability is controlled by connected throats. When the stress is loaded, the large and medium pores are mainly compressed and deformed seriously, while the small-scale throats are less compressed and deformed, so the stress sensitivity of low rank lignite coals is relatively weak. However, high rank coal reservoir is dominated by micro and small pores. Thanks to high vitrinite content, cleats and fractures are developed, which control coal’s permeability. When the stress is loading, the micro and small pores are difficult to be compressed. But to the fractures, owing to their weak resistance to deformation, they are easy to fail due to ductile deformation or closure, and difficult to recover even unloading. Therefore, the high rank coal reservoir shows strong stress sensitivity. Considering the deeper burial depth and higher stress, stress sensitivity of high rank coal reservoir will cause greater damage to CBM well’s deliverability, so it is advisable to carry out with lower strength at the initial drainage stage to reduce irreversible permeability damage and expand formation pressure drop range. However, for the low rank lignite reservoir characterized by low stress and higher original permeability, stress sensitivity causes relatively small impact on productivity, therefore, early drainage speed can be appropriately accelerated to improve efficiency.