Reconstruction of the shale digital core and numerical test of hydraulic fracturing

Shale oil reservoirs are typical unconventional oil and gas reservoirs with complex mineral composition. The types and distribution of mineral particles have a considerable effect on hydraulic fracture propagation in shale oil reservoirs. The research object is a continental shale oil reservoir in the Lucaogou Formation of Jimsar Sag. To realize an intensive study of the hydraulic fracture propagation law of deep shale oil reservoirs under hydraulic fracturing, digital rock cores were constructed based on the scanning images of 20 groups of shale samples within the depth range of 3684.62–3705.70 m. In addition, hydraulic fracturing numerical experiments of the digital rock cores were performed. Mineral composition and distribution were characterized using scanning electron microscopy and energy spectrum analysis. Realistic failure process analysis based on the finite element method was employed to construct the digital rock core and model the hydraulic fracturing process. The breakout pressure and hydraulic fracture geometry of each model were analyzed. Results reveal that mineral composition and porosity have a considerable effect on the breakdown pressure, hydraulic fracture initiation and extension, and complexity of hydraulic fractures in the shale oil reservoir. The breakdown pressure increases with increasing brittle mineral content (quartz, calcite, and dolomite) and exhibits a more evident linear relation with quartz content due to the high strength of quartz. Furthermore, the breakdown pressure decreases with increasing porosity. Pores weaken the strength of shale and provide seepage shortcuts for fracturing fluid. The initiation and extension of hydraulic fractures are primarily affected by in situ stress and pore distribution. Hydraulic fractures start from the pore near perforation, connect independent pore regions, and propagate along the connected pore regions. In areas far from the pore regions, hydraulic fractures extend along the directions perpendicular to the minimum principal stress. The complexity of hydraulic fractures in shale increases with quartz content and porosity and is also affected by mineral distribution. When hydraulic fractures come into contact with quartz minerals, they extend through or bypass them. When bypassing quartz minerals, hydraulic fractures form branched fractures, and the complexity of fracture geometry increases. When the quartz content or porosity is high and displays a large area of connected distribution, the expansion of hydraulic fractures is hindered. Large particles of quartz mineral dramatically increase the brittleness and breakdown pressure of the rock, inhibiting the formation of a complex fracture network. The connected pores form high-density lamellation, which results in a large amount of filtration of fracturing fluid, thereby leading to low stimulated efficiency and permeability enhancement.