Classes of fracturing curves and their formation mechanism under different combinations of coal structures and geostress

Abstract

ABSTRACT Exploring the classes and causes of fracturing curves under different combinations of coal structures and stress (CMCSSs) in coal measures can provide direction guidance for parameter optimization during hydro-fracturing. The Shizhuang south block, south central Qinshui Basin (Shanxi Province, China) was investigated. By coupling various methods including core drilling for control, geological strength index (GSI), and multi-parameter fitting of logging, the coal structures along the longitudinal direction in coal measures of 58 coalbed methane (CBM) wells in the research area were partitioned. According to the theory of measuring geostress by using hydro-fracturing method, nine CMCSSs were attained. Fracturing curves were divided into five classes (ten sub-classes) based on the pressure fluctuation amplitude and curve shape. The causes and fracturing effect of different fracturing curves were analyzed by using the theoretical rock mechanics and gas-production conditions, to propose the direction and improvement measures for parameter optimization under different CMCSSs. The result showed that the zig-zag fracturing curve was likely to appear when the proportion of soft coal was less than 30% in the roof and floor of a coal seam and a tensile stress was applied, showing optimal fracturing effect. On condition that the proportion of soft coal was less than 30% under transition or compression stress, wave-like or steadily rising fracturing curve was likely to form, with a moderate fracturing effect. The steady and decreasing fracturing curves exhibited an insignificant relationship with fracturing effect. If the soft coal accounted for more than 70%, when under compression, conventional hydro-fracturing showed the worst fracturing effect. The research result provided a reference for the implementation of targeted improvement measures for increasing yield under different CMCSSs.