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

Bo Huang,Gang Wang,Wanghong Zhang,Yong Qin

doi:10.1177/0144598717723815

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.

Highlights

The Gujiao block in the Xishan coalfield, Shanxi Province, China, has rich coal-bed methane (CBM) resources (Mo et al, 2008) and is one of the blocks constructed in recent years for the large-scale development of CBM in China
Coal structure was classified into three types—undeformed coal (I), cataclastic coal (II), and granulated coal-mylonitized coal (III–IV)—based on the fracturability of the coal and the distinguishability of the coal structure using geophysical logging
Based on the sensitive log responses of the coal structure to the physico-chemical differentiation in the reservoir, undeformed coal and cataclastic coal can be identified by natural gamma, density, and resistivity, in order of preference

Summary

Introduction

The Gujiao block in the Xishan coalfield, Shanxi Province, China, has rich coal-bed methane (CBM) resources (Mo et al, 2008) and is one of the blocks constructed in recent years for the large-scale development of CBM in China. The high heterogeneity of the coal structure results in considerable variations in the permeability of the main coal reservoir (Li, 2001; Li et al, 2012; Meng et al, 2011; Pan et al, 2012; Wang et al, 2013). Scientific prediction of the coal structure in the main coal reservoir is the primary technical issue that guides CBM exploration and development in this region. Differences in the physical and chemical properties of coal result in a good correlation between coal structure and log responses (Fu et al, 2009a; Peng et al, 2008; Teng et al, 2013, 2015; Yao et al, 2011). Geophysical logging is the most cost-effective method to have high reliability (Chen et al, 2013; Li et al, 2016; Gan et al, 2016)

Methods

Conclusion