Fracture Characteristics and Change of Permeability Under the Influence of Natural Fractures: Experimental Study of Wufeng–Longmaxi Shale

Abstract

Summary Currently, substantial shale gas reserves have been recognized in many countries, including China and the United States. An improved comprehension of the structure of these shale plays can lead to improved shale gas producibility. Gas is produced through natural fractures (NFs) and through hydraulically induced fractures, together creating the so-called stimulated reservoir volume (SRV). Although NFs are advocated as significantly influencing the size and shape of an SRV, the effects of NFs on fracturing and permeability changes are incomplete. This paper describes experimental studies performed on calcite-filled fractures from outcrops of Wufeng (O3w)-Longmaxi (S1l) shale. These samples are representative of Ordovician-Silurian productive shales in the Fuling Basin, near Chongqing City, China. Basic parameters, such as acoustic wave velocity, porosity, and permeability, were first measured on representative samples. Subsequently, permeability and acoustic emissions were measured during triaxial compression experiments through the post-peak domain. After these evaluations, the fracture modes and the fragment distribution were evaluated. In the measurements made, the natural calcite-filled macroscopic fractures reduced the confined compressive strength of these shale samples from 26 to 47%. Permeability is also affected. During the triaxial compression experiments, different NF orientations affected the overall permeability differently. We divided the collected samples into three groups. Group 1 samples had axial fractures (AFs). The gas permeability reduced from 0.70 to 0.12 md. When the sample was broken, permeability increased to 2.0 md, whereas the axial stress dropped to zero. Samples in Group 2 had transverse natural fractures (TFs). When new fractures were generated, permeability increased to greater than 0.70 md. Because samples in Group 3 had no obvious, natural macroscopic fractures (NFs), permeability was too low to measure before catastrophic failure during triaxial compression. When these samples did fail in compression, significant fractures formed, and permeability increased, resembling Group 2. There was a close relationship between the initial pattern of NFs and the subsequently generated fracture morphologies. For Group 1, the samples broke into several larger pieces along the original AFs. For Group 2, the samples broke into numerous small and relatively uniform pieces. For Group 3, the samples did not break completely, and the constituent fragments were nonuniform. Our results are helpful for understanding fracture-propagation mechanisms and the evolution of permeability under in-situ loading processes (long-term geologic loading, stress alteration during well construction and stimulation, and reservoir modification during depletion).