Identification of the coal structure and prediction of the fracturability in the No. 8 coal reservoir, Gujiao block, China

Abstract

Five boreholes were selected in the Gujiao block, Xishan, Taiyuan, China, as calibration wells to identify the coal structure. The geophysical-log responses of the coal structure in the No. 8 coal reservoir, Gujiao block, were investigated using coal-core logging combined with actual observations in the borehole profile of the coal mine. Natural gamma, density, apparent resistivity, and sonic travel time logs were extracted at 0.15 m intervals from 41 undeformed coal, 29 cataclastic coal, and 48 granulated coal regions. Box plots and Fisher’s maximum separation criterion were used to screen and identify sensitive log responses of the coal structure. The coal structure was identified in 31 boreholes using the available logs in the block, and the layered distribution patterns of the coal structure were discussed. The fracturability of the coal structure was divided into types using cluster analysis based on the thickness ratio of the coal structure and the relationship between the coal structure and its permeability. The results show that sensitive log responses for identifying undeformed coal and cataclastic coal are natural gamma, followed by density and apparent resistivity; log responses for identifying cataclastic coal and granulated coal-mylonitized coal are sonic travel time, apparent resistivity and natural gamma. The sensitive log response data were integrated and coal structure identification models were constructed based on the principle of amplifying the log responses to identify the coal structure in the No. 8 coal reservoir. The reservoir generally contains two or three dirt bands, and the coal structure is divided into several independent layers, with the cataclastic coal and granulated coal-mylonitized coal distributed in the middle of the reservoir. The coal structure was classified into four types and four subtypes through cluster analysis of the boreholes. Under the control of the Malan syncline, type I and type II are developed in the No. 8 coal reservoir in the southern part of the study area and in the east and north wings of the Malan syncline; they have good fracturability. Type III and type IV are mainly present in the No. 8 coal reservoir at the synclinal axis; they have poor fracturability. For type IV dominated by granulated coal, it is difficult to improve the reservoir permeability by fracturing; therefore, other strengthened permeability-improving measures should be considered.