Stimulated reservoir volume (SRV) has become the key means to the development of shale gas, and the natural fractures influence the complex of fracture network of SRV, significantly. However, currently, the researches on natural fractures are still focus on the geological description and the reservoir simulation, theoretically. Also, on the shale core experiment study, the influence between natural fractures, shale fracture characteristics and the change of the permeability is still unclear.
In this paper, the Wufeng (O3w)-Longmaxi(S1l) shale crop was taken to experimental study, which belongs to the Ordovician-Silurian formation in Fuling, Chongqing City, China. After the standard cylinder specimens were prepared, the natural fractured shale specimens were selected for the experiments by using the acoustic wave velocity test and manual evaluation. And then the CO2 steady-state method was used to measure the permeability of shale specimens under the triaxial compression condition until the specimen was damaged. At the same time, acoustic emission was monitored to measure the generation of fractures. After the experiments, the failure modes and the fragment distribution were studied, systematically.
The results reveal that: (1) the natural fractures reduce compression strength of the shale by 26.1% to 46.6%, and piercing cracks influence that significantly. (2) During the triaxial stress loading, different distributions of natural fractures affect the permeability of shale, obviously, and the effect trends are divided into the following three groups. In Group 1 (with a long piercing fracture), the initial permeability is high (0.7 mD), then decreases to 0.12 mD, due to the macro fracture is compressed; but when the specimen breaks, it increases to 2.0 mD. In Group 2 (with some small natural fractures), the permeability decreases and then increases, due to the compression and newly-generate of fractures; when the long piercing crack appears, the permeability increases to above 0.7 mD like that in Group 3. In Group 3 (without macro natural fractures), the permeability is too low to measure (nD) before the specimen damaged; but when the specimen damages and forms a long piercing crack, the permeability increases like that in Group 2. (3) There is a close relationship between natural fractures and fracture morphology: in Group 1, the specimen is broken into several larger pieces along the piercing natural fracture. in Group 2, the specimen is broken into many (more than 50) small and uniform pieces; in Group 3, the specimen isn’t broken completely and the scale of pieces is non-uniform.
Based on the triaxial compression experiment and permeability measurement, the change of mechanical characteristics and permeability are studied in the triaxial compression process of shale; that offer relative experimental basis and required parameters for the studies of fracture propagation mechanism, flow mechanism and the fracturing design program in the natural fractured shale gas reservoirs.