Classification of Coal Structure Combinations and Their Influence on Hydraulic Fracturing: A Case Study from the Qinshui Basin, China

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.

Coal Structure Prediction Based on Type-2 Fuzzy Inference System for Multi-Attribute Fusion: A Case Study in South Hengling Block, Qinshui Basin, China

“…To some extent, the petrophysical quantity plates can provide insight into the statistical distribution law related to the coal seam. By referring to information from the neighboring areas [29], it was determined that the…”

Optimal Injection Parameters for Enhancing Coalbed Methane Recovery: A Simulation Study from the Shizhuang Block, Qinshui Basin, China

“…3 coal seam. Previous studies have shown that its burial depth is 500-1300 m, the coal thickness is 4-10 m, and the gas content is 8-28 m 3 /t [30,31]. Based on the above, basic parameters of the basic model are described as follows: the depth of the coal seam is 1000 m, the thickness of the coal seam is 6 m, and the gas content is 24 m 3 /t.…”

Characteristics and the Model of Thermal Evolution and Gas Generation of Late Paleozoic Coal in the Qinshui Basin, Based on Hydrous Pyrolysis

“…The Qinshui Basin is located in the southeast of the Shanxi Province, covering an area of about 26,000 km 2 (Figure ). It is rich in coal and coalbed methane resources, and it is the only large basin in China to develop coalbed methane on an industrial scale. − Structurally, it is located in the central part of the North China Craton and is generally a large-scale complex syncline with axial NNE–SSW. , After a series of tectonic influence, especially during the Yanshanian and the Himalayan period, the Qinshui basin formed a complex structural setting. − …”

“…32−34 Structurally, it is located in the central part of the North China Craton and is generally a large-scale complex syncline with axial NNE−SSW. 35,36 After a series of tectonic influence, especially during the Yanshanian and the Himalayan period, the Qinshui basin formed a complex structural setting. 37−41 After a series of tectonics and discontinuous deposition, the Qinshui Basin became a residual structural basin with a thickness of 200−3000 m of coal-bearing strata, 26 and under the influence of intermittent uplift and subsidence, multiple periods of transgression occurred in the early and late Carboniferous and early Permian, resulting in obvious regional cycles of coal seams, carbonate rocks, mudstones, and sandstone in the sedimentary strata from the Taiyuan Formation to the Shanxi Formation 43−47 (Figure 1).…”