Dynamic mechanisms of lamina-induced fracture propagation in tight oil-sand formations

Abstract

Lamina-induced fractures (LFs) are abundant in lacustrine formations, playing an important role in tight oil resource exploration. There is a lack of research on the dynamic mechanisms of LF propagation in heterogeneous formations characterized by complex combinations of lamina and surrounding rock. For this reason, we conducted triaxial compressive tests of 28 long column samples and Brazil split tests of 37 short column samples using six core samples with different lithologies, including nine column samples tested using a CT scanner and an acoustic emission apparatus. We designed this experiment to study the LF propagation process and its dynamic mechanisms within different lithologies. Our results show that (1) we classified three mechanical LF types based on the relationship between stress state and lamina angle: tensile LF (T-LF), shear LF (S-LF), and hybrid LF (H-LF); (2) H-LF only propagated in the middle lamina angle range (approximately 11°–64°); T-LF was inclined to propagate in the low lamina angles; and S-LF was inclined to propagate in the high lamina angles; and (3) in the Ordos Basin, T-LF, TF, and SF propagated mutually in outcrops with low confining pressures. T-LF, H-LF, and SF propagated mutually in the Yanchang Formation, an outcrop that greatly contributes to tight oil sweet spot exploration. Furthermore, unlike the in situ accumulation theory, we find that it is possible for hydrocarbon migration to occur in tight reservoir formations, in which the LF acts as a hydrocarbon migration pathway during the tectonic phase.