Analysis of hydraulic fracture initiation and vertical propagation behavior in laminated shale formation

The extent of hydraulic fracture vertical propagation extent is significant in evaluating simulated reservoir volume for laminated shale reservoirs. Given that it is affected by the discontinuities (beddings, natural fractures, and other factors), fracture geometry is complex in the vertical plane and is different from a simple fracture in a homogeneous sandstone reservoir. However, the propagation mechanism of hydraulic fracture in the vertical plane has not been well understood. To clarify this mechanism, several groups of large-scale tri-axial tests were deployed in this study to investigate the fracture initiation and vertical propagation behavior in laminated shale formation. The influences of multiple factors on fracture vertical propagation were studied. The results showed that hydraulic fracture initiation and propagation displayed five basic patterns in the vertical plane of laminated shale formation. The ultimate fracture geometries could be classified into four categories: simple fracture, fishbone-like fracture, fishbone-like fracture with fissure opening, and multilateral fishbone-like fracture network. Furthermore, the favorable geo-stress conditions for forming the complex fracture network were as follows: vertical stress difference close to 6 MPa and vertical stress difference coefficient from 0.2 to 0.5. In addition, when q·μ-value (the product of injection rate and fracturing fluid viscosity) was roughly 3 × 10⁻⁹, a complex fracture geometry of fishbone-like fracture with bedding opening was formed; however, extremely small or extremely large values were both harmful. Variable injection rate fracturing with low viscosity fracturing fluid of 3 mPa·s was proved to be an effective treatment to improve the connectivity of induced hydraulic fracture with the discontinuities. Moreover, because of the influence of cementing strength on fracture communication effects between hydraulic fracture and the beddings, the overall propagation region generally displayed an ellipse in shape with beddings opening asymmetrically along two wings of the main hydraulic fracture.