Hydraulic Fracture Geometry and Pressure Responses During Growth Across Mudstone Barriers in Vertically Stacked Tight-Oil Reservoirs

Hydraulic fractures encounter successive sandstone–mudstone boundaries during the stimulation of vertically stacked tight-oil reservoirs. Eleven small-scale true-triaxial experiments were conducted on five-layer specimens assembled from Chang 6 outcrop sandstone and mudstone. Fracture reconstructions, post-test observations, and representative injection-pressure records were analyzed. Specimens 1–3 were reconstructed from X-ray computed tomography (CT); the remaining reconstructions combined fluid-tracer patterns with surface and internal observations. Specimens 1–3 showed direct penetration, partial penetration, and arrest within the barrier with 10, 20, and 30 mm barriers, respectively. The central sandstone thickness simultaneously decreased from 60 to 40 and 20 mm, making this a layer-configuration comparison. For the 20 mm barrier configuration, stress contrasts of 2 and 4 MPa corresponded to direct and partial penetration, respectively. Partial penetration persisted across tensile-strength contrasts of 1–3 MPa; the 3 MPa case exhibited a fracture-path deviation of 5–11 mm. The pressure histories showed rapid depressurization, recovery with fluctuations, or a prolonged high-pressure interval. Rapid depressurization in the direct-penetration record coincided approximately with pump shut-in and cannot be attributed to barrier crossing alone. These observations provide specimen-scale evidence for fracture growth across bonded sandstone–mudstone layers.