Tortuous hydraulic fractures (HFs) are likely to be created in heterogeneous formations such as conglomerates, which may
cause sand plugging, ultimately resulting in poor stimulation efficiency. This study aims to explore HF growth behavior in
conglomerate through laboratory fracturing experiments under true tri-axial stresses combined with computed tomography
scanning and acoustic emission (AE) monitoring. The effects of gravel size, horizontal differential stress, and AE focal
mechanisms were examined. Especially, the injection pressure and the AE response features during HF initiation and
propagation in conglomerate were analyzed. Simple HFs with narrow microfractures are created in conglomerate when the
gravels are considerably smaller than the specimen, whereas complex fractures are created when the gravels are similar in
size to the specimen, even under high horizontal differential stresses. Breakdown pressure and AE rates are high when a HF
is initiated from the high-strength gravel. A large pressure decline after the breakdown may indicate the creation of a planar
and wide HF. Analyzing the focal mechanism indicates that the shear mechanism generally dominates with an increase in
the HF complexity. Tensile events are likely to occur during HF initiation and are located around the wellbore. Shear
events occur mainly around the nonplanar and complex matrix/gravel interfaces.