Abstract
Coal structure directly correlates to permeability and hydraulic fracturing effects. Underground coal mining indicates that a single coal section generally contains multiple coal structures in superposition, making how to recognise the coal structure combination and predict its influence on coal permeability a challenging problem. Based on well-drilling sampled cores, the geological strength index (GSI), and well-logging data, the DEN, GR, CALX, and CALY were selected to establish a model to predict GSI by multiple regression to identify coal structure from 100 coalbed methane wells. Based on fitting GSI and corresponding permeability test values, injection fall-off (IFO) testing, and hydraulic fracturing results, permeability prediction models for pre- and post-fracturing behaviour were established, respectively. The fracturing effect was evaluated by the difference in permeability. The results show that a reservoir can be classified into one of nine types by different coal structure thickness proportion (and combinations thereof) and the fracturing curves can be classified into four categories (and eight sub-categories) by the pressure curve. Up-down type I and type II reservoirs (proportion of hard coal >60%) and intervening interval type I reservoir (proportion of hard coal >70%) are prone to form stable and descending fracturing curves and the fracturing effects are optimal. Intervening interval type II (hard coal:soft coal:hard coal or soft coal:hard coal:soft coal ≈1:1:1) and up-down type III (hard coal:soft coal =1:1) form descending type II, rising type I and fluctuating type I fracturing curves and fracturing effect ranks second; up-down type IV and V (proportion of hard coal <40%), interval type III (proportion of hard coal <30%), and multi-layer superposition-type reservoirs readily form fluctuating and rising fracturing curves and fracturing effects therein are poor. The research results provide guidance for the targeted stimulation measured under different coal structure combinations.
Highlights
Coalbed methane (CBM) is a low-carbon, environmentally-friendly, unconventional energy resource and should be developed effectively
Based on the above issues, this paper mainly introduces the geological strength index (GSI) to conduct continuous and quantitative characterisation of the deformation of coal
The fracturing curves are mainly of the descending or stable types when the hard coal ratio is above 70%, and the fracturing effect is good
Summary
Coalbed methane (CBM) is a low-carbon, environmentally-friendly, unconventional energy resource and should be developed effectively. The coal reservoir in Qinshui Basin has experienced multistage tectonics, which form different coal structures of strong heterogeneity [1,2,3,4,5]. The complex coal structure leads to the difference in porosity and permeability, which is an important factor in determining CBM production [6,7,8,9,10]. Coal structure determines the choice, and efficacy of Energies 2020, 13, 4559; doi:10.3390/en13174559 www.mdpi.com/journal/energies. The accurate identification and distribution of coal structure is of great importance for CBM exploration, development, and gas leakage prevention [19,20,21]
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