Research on Cross-Layer Propagation of Fractures in Coal Measures Strata

ABSTRACT: Hydraulic fracturing in coal measures strata encounters difficulties in the longitudinal penetration of fractures. To figure out which engineering parameters can hydraulic fractures penetrate through coal measure strata, a series of mechanical tests, in-situ stress measurement and true triaxial fracturing experiments were carried out. We found that the mechanical strength of mudstone is significantly higher than that of coal: in terms of the average values, the elastic modulus, uniaxial compressive strength, tensile strength, and fracture toughness of mudstone are 2.65 times, 1.47 times, 2.27 times, and 2.78 times those of coal, respectively. The higher of the fracturing fluid viscosity and injection rate, the easier for hydraulic fracture to penetrate. According to fracture penetration chart, a critical line was fitted. When injection rate > 6 m3/min & viscosity > 50 mPa·s or injection rate > 4 m3/min & viscosity > 150 mPa·s, hydraulic fracture can penetrate through coal measures strata. 1. INTRODUCTION As an unconventional nature gas resource, coalbed methane (CBM) has the characteristics of clean, efficient, and large resource reserves. It is one of the important energy reserves. According to the prediction of CBM, the resource volume of CBM in the land area with a burial depth of 300-2000 m in China can reach 3.146×1012 m3(Chenglin et al., 2009). The exploration and development of CBM has great significance. However, in western China, there exists a thin interbed reservoir with alternating mudstone and coal (Fig. 1). The production of CBM after fracturing is lower than expected due to inadequate communication in the vertical fracture direction. Previous fracturing experiments focus on hydraulic fractures propagate in coal, pay little attention to the research of hydraulic fracture penetration in coal measures strata. Consequently, in this study, cores of coal and mudstone were employed to carry out macroscopic mechanical tests. The mechanical parameters of mudstone and coal were compared. The in-situ stress was restored through the Kaiser stress test. On this basis, True triaxial fracturing experiments were designed and conducted to explore the effects of injection rate and fracturing fluid viscosity on the fracture's cross-layer penetration capability. Eventually, a chart of fracture cross-layer penetration conditions was developed to provide a basis for on-site fracturing design.