Evaluation of Coal Body Structures and Their Distributions by Geophysical Logging Methods: Case Study in the Laochang Block, Eastern Yunnan, China

Abstract

The coal body structure is one of the key parameters in the exploration and exploitation of coalbed methane (CBM). Accurate identification of the structure of a coal seam is important for CBM exploration and development. In the present study, we collected coal seam logging data and drilling data from six CBM wells in the Laochang block in eastern Yunnan, China. Four logging curves, including gamma-ray logging (GR), density logging (DEN), acoustic travel time logging (DT) and compensated neutron logging (CNL), were selected. Principal component analysis was used to establish a geophysical identification approach for the coal body structures in the Laochang block. The geophysical identification results for the coal body structures were in good agreement with the actual coring data. We found that if the coal body structures score (F value) was less than 12, the coal seam belongs to Type I coal; otherwise, it is Type II coal. With increasing coal seam thickness, the proportion of Type I coal showed a decreasing power function trend. When coal seam thickness exceeded 1 m, it was dominated by Type II coal. The coal seams near well YW-04 had the highest degree of overall damage, followed by wells YW-02, YW-03 and YW-01. The coal seam in well YW-04 was affected by the superimposed regional and local tectonic tensile stress fields; the coal seam in well YW-02 was subject to the regional tensile stress field and the goaf, and the coal seams in wells YW-01 and YW-03 were subject to the joint effects of the regional tensile and local compressive tectonic stress fields. The distributions of the coal body structures in the Laochang block were the result of the combined effects of the coal thickness, goaf, tectonic type and tectonic stress field. These results may provide guidelines for the exploration and development of CBM in the Laochang block in eastern Yunnan. Furthermore, the scientific and quantitative evaluation method developed in this research can be applied to the identification of coal body structures in other CBM blocks worldwide.