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Coal Geology & Exploration

Abstract

Horizontal well perforation and fracturing in complex coal-bearing strata are key to increasing coal seam permeability and enhancing gas extraction efficiency, with the formation structure and perforation location determining fracture propagation. Considering the characteristics of formation structure and perforation locations, a fracturing geological model is established. Based on the finite element method, a numerical model is constructed to study the effects of perforation location, formation conditions, and the difference between vertical stress and horizontal stress on fracture perforation. Engineering verification is conducted, and construction suggestions are proposed. Key findings are as follows: There are three scenarios for the perforation locations: all within the coal seam, all within the roof strata, partially in the roof strata and the other partially in the coal seam. (1) When the perforation is within the coal seam, the fracture is “blocked” by the rock strata, which is beneficial to the fracturing of the coal seam. (2) When the perforation is within the roof and the bedding of the roof is developed, fractures can penetrate the coal seam through the bedding and interface if the vertical stress and the difference between the minimum horizontal principal stress exceed 2 MPa. However, when the roof is intact, fractures with a stress difference greater than −2 MPa can penetrate the coal seam under the influence of perforation induction. The presence of bedding and interface has a “blocking” effect on the vertical extension of the fractures, while complete formation facilitates the vertical extension of the fracture. (3) If partial perforation enters the coal seam, it will have a significant induction effect on the fracture formation and extension. Horizontal fractures extending along the interface and vertical fractures entering the coal seam can form regardless of whether the roof is intact or not. When the perforation is far from the coal seam, weak interface develop between the perforation and the coal seam, horizontal stress exceeds vertical stress, or the fracturing construction scale is insufficient, it is recommended to use engineering measures such as deep penetrating perforation, branch holes and other engineering measures to ensure the fracturing effectiveness. The research results have been applied in segmented fracturing construction in an underground coal mine in Hancheng of Shaanxi, and the gas extraction effect of the test perforation is good. Indicating its potential for proposing fracturing construction schemes under similar engineering and geological conditions.

Keywords

coal measure, horizontal well, directional perforation, hydraulic fracturing, fracture propagation

DOI

10.12363/issn.1001-1986.23.08.0496

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