Permeability enhancement of low rank coal through acidization using H2S solution: An experimental study in the Kuqa-Bay Coalfield, Xinjiang, China

Abstract

Coalbed methane (CBM) wells with abnormally high H2S contents present higher CBM productivity in the low rank coal development areas of Xinjiang, China. The influence of abnormally high H2S contents on the coal reservoir properties is an important research topic. Therefore, acidization experiments using a H2S solution on low rank coal of the Kuqa-Bay coalfield were conducted. The pore structure characteristics and permeability at the horizontal and vertical stratifications were compared before and after the acidization experiments using various methods including X-ray diffraction (XRD), permeability test, high pressure mercury injection porosimetry (HPMIP), and field emission scanning electron microscope (FESEM). The results show that the H2S solution can significantly enhance the permeability of low rank coal. The higher the primary permeability, the greater the permeability amplification would be. The total pore volume (TPV) of the coal increased after the acidization experiment, and the pore distribution and connectivity among micropores, transition pores, and mesopores were also improved. Additionally, the pore volume ratio of the semi-closed pores increased, while that for the macropores showed an opposite trend. The permeability enhancement mechanisms after the H2S solution acidizing experiments indicated that the H2S solution can dissolve the carbonate minerals in the coal, thus, increasing the permeability, and this was considered as a positive permeability enhancement effect. Meanwhile, the clay minerals in the coal expanded with the H2S solution, and this decreased the permeability and was considered a negative permeability enhancement effect. The positive effect precedes the negative effect, leading to an increase in the coal permeability. Additionally, the acidization experiment indicated that the closed pores transform to semi-closed pores, improving the connectivity of the pores and indicating that the permeability of the coal can be further increased.