Abstract
As a reservoir reconstruction technology, hydraulic fracturing is a key method to improve the production of coalbed methane (CBM) wells. The CBM reservoir in eastern Yunnan, an important CBM exploration and development zone in China, is characterized by multiple thin coal seams. Compared to the fracturing of the single-layer coal seam, the combined seam fracturing technology is more difficult and complex. To study the fracture propagation characteristics and influencing factors of hydraulic fracturing in multiple coal seams, taking No. 9 and No. 13 coal seams as the research objects, the fracturing process was numerically simulated by using the finite element method and ANSYS software in this work. Based on the mathematical model of low permeable coal-rock mass, a two-dimensional hydraulic fracture model was established. In addition, the fracture geometries of combined seam fracturing were studied quantitatively. The results indicate that although No. 9 coal and No. 13 coal seams have similar rock mechanical properties, the propagation process and final geometry of a fracture are different. The reliability of the simulation results is verified by the comparison of experimental parameters and field investigation. The results prove the feasibility of combined seam fracturing in eastern Yunnan. The high Young’s modulus and thickness of the coal seam make the fracture geometry longer, but the fracture height is smaller. The low Young’s modulus, high Poisson’s ratio, and thickness of the No. 13 coal seam result in an increase in the length and height of the No. 9 coal seam. The increase in Young’s modulus of interlayer inhibits the propagation of fractures, while the high thickness and low Poisson’s ratio of interlayers facilitate the extension of the length and inhibit the extension of the height. This work provides a case reference for combined seam fracturing of coal reservoirs and has practical significance for the development of CBM characterized by multiple coal seams in eastern Yunnan.
Highlights
No 9 and No 13 coal reservoirs are taken as the research objects to carry out the numerical simulation analysis of combined seam fracturing
The influence of different parameters on hydraulic fracture geometry and the feasibility of combined seam fracturing can be studied by numerical simulation
An increase in the thickness of the No 13 coal seam resulted in a decrease in the fracture height and an increase in the fracture length, which further confirmed the conclusions proposed by Li et al (2014)
Summary
The coal matrix permeability is low in China; reservoir reconstruction is generally conducted to improve the performance of producing wells and increase recovery (Islam et al, 2009; Meng et al, 2011; Lekontsev and Sazhin, 2015; Li et al, 2020; Zhang et al, 2020). Jiang et al (2016) developed a hydraulic fracture numerical simulation model to quantitatively study the effects of different geological and operational factors on the fracture propagation in CBM reservoirs. Through the combination of triaxial compression test and numerical simulation of coal and sandstone, the effects of different confining pressure and cleat direction on rock brittleness and fracture characteristics were analyzed (Zhang and Bian, 2015; Xie et al, 2019; Eremin, 2020; Ren et al, 2021; Zhao et al, 2021). Investigation on the propagation of commingled crack in coal seams which are soft-hard interlaced strata is less studied and not well understood These relevant theories and techniques are mostly applied to the single coal seam or the thick coal seam fracturing treatment. Influencing factors such as fracturing pressure, Young’s modulus, Poisson’s ratio, and thickness, which affect the characteristics of fracturing propagation were investigated
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