Propagation and complex morphology of hydraulic fractures in lamellar shales based on finite-discrete element modeling

Abstract

Abstract We explore the controls of stress magnitude and orientation relative to bedding on the resulting morphology/topology of hydraulic fractures using a combined finite-discrete element method (FDEM). Behavior is shown conditioned by the ratio of principal stresses $$\lambda ={\sigma }_{3}/{\sigma }_{1}$$ λ = σ 3 / σ 1 and relative inclination of the bedding. When the lateral pressure coefficient ( $$\lambda$$ λ ) is less than 0.67, hydraulic fractures predominantly initiate as tensile fractures along the wellbore, aligning with the maximum principal stress direction. Conversely, for $$\lambda \ge 0.67$$ λ ≥ 0.67 , shear cracks are favored to initiate for the minor stress difference, leading to a less predictable initiation and extension direction. Simultaneously, diminished stress differences correspond to elevated reservoir breakdown pressures, displaying a linear correlation with lateral pressure coefficients and little influenced by equivalent bedding orientation. Bedding plane orientation significantly impacts the mode and morphology of hydraulic fracture propagation. Bedding parallel to the direction of the minimum principal stress ( $${\sigma }_{3}$$ σ 3 ) favors layer-penetrating and bifurcated fractures, whereas inclined bedding facilitates the emergence of numerous steering-type and capture-type fractures. Especially at steeper inclinations ( $$\beta =60^\circ$$ β = 60 ∘ ), hydraulic fractures readily extend along the bedding surface, inducing macroscopic shear slip failure. Under high-stress disparities, the breakdown pressure exhibits greater sensitivity to bedding inclination, and its influence pattern aligns with the variations in tensile strength, typically reaching maximum and minimum values at bedding inclination angles of 0° and 60°, respectively.

Keywords

• Finite-discrete element method (FDEM)
• Bedding plane
• Hydraulic fracturing
• Crack propagation
• Breakdown pressure