Study on the Influence of Injection Rate on Hydraulic Fractures Crossing Natural Fractures

Abstract

ABSTRACT The interaction between hydraulic fractures and natural fractures will directly affect the complexity of fractures, which is an important content that needs to be paid attention to in the fracturing process. To study the interaction between hydraulic fractures and natural fractures at different injection rates, a large-scale true triaxial hydraulic fracturing simulation system was used to conduct fracturing experiments on artificial specimens with prefabricated natural fractures. Results show that increasing the injection rate can generate more complex hydraulic fractures in formations with natural fractures. Hydraulic fractures can cross natural fractures and maintain the original direction of expansion easier by increasing the injection rate. When the injection rate increases from 4ml/min to 6ml/min, the time for the blunt hydraulic fracture to reach the opposite natural fracture is reduced by 29%, and the pressure rise in the fracture increases by 0.6MPa. The cementation at the end of natural fractures is weaker. When the natural fractures are fully activated, generating new fractures at the ends of natural fractures is easier by increasing the injection rate. INTRODUCTION Hydraulic fracturing is an important production stimulation method in reservoir exploitation. The generated hydraulic fractures provide migration channels for oil and gas, and the complexity of hydraulic fractures also directly affects the effect of fracturing. Early studies generally believed that the hydraulic fracture is a double-wing symmetrical shape. Faults, veins, and joints in rocks are collectively called natural fractures. (Gale, J. F., Laubach, S. E., Olson, J. E., Eichhubl, P., & Fall, A. 2014) Due to the presence of natural fractures, the actual fracture shape is more complicated. The impact of natural fractures is multifaceted: increased fracturing fluid loss, premature screen-out, fracture stop propagation, formation of multiple fractures, fracture offset, and high net pressure. There have been many experimental studies on the interaction between hydraulic and natural fractures. Lamont and Jessen (1963) found that when the approach angle between hydraulic and natural fractures ranges from 30° to 60°, hydraulic fractures can cross natural fractures and propagate along the direction orthogonal to natural fractures. Blanton (1982) and Warpinski (1987) showed that hydraulic fractures penetrate existing fractures at large stress differentials and large approach angles. At lower approach angles and stress differentials, natural fractures open, diverting fracturing fluid and preventing passage of induced fractures, at least temporarily. Daneshy (1974) studied the effect of natural fracture size and found that the crystal and matrix boundaries of granite blocks, small fractures (0.5 inches), have little or no influence on the direction of propagation of hydraulic fractures. In contrast, in a few cases, hydraulic fractures cannot penetrate large natural fractures.