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Abstract:

Abstract Summary

Hydraulic fractures in unconventional reservoirs are often assumed to be vertical, but growing field and laboratory evidence shows that fractures can also propagate horizontally along bedding planes. These horizontal fractures are influenced by both rock properties such as shale anisotropy and stress relaxation and operational factors like well spacing and fluid volumes. Understanding the interaction between geology and stimulation intensity is critical for predicting and managing horizontal fracture formation.

Key Points

• Both rock properties and operational choices control fracture geometry.

Challenging the vertical-only assumption

• Field data increasingly shows hydraulic fractures can propagate horizontally along bedding planes.

Field evidence of horizontal fractures

• Observed using tiltmeters, microseismic monitoring, fiber-optic data, casing deformation, and vertical core samples with propped horizontal fractures.

Shale rock properties matter

• Shales are finely layered and highly anisotropic in:
• Elastic properties
• Fracture toughness
• Tensile strength
• Tensile strength anisotropy makes it easier to initiate fractures along bedding planes than across them.

Horizontal fractures can form even under higher vertical stress

• Laboratory experiments confirm that sub-horizontal fractures can initiate despite vertical stress exceeding horizontal stress.

Stress relaxation increases fracture risk

• Shale viscoelasticity and creep cause stress relaxation, moving the stress state toward hydrostatic conditions.
• This significantly increases the likelihood of horizontal fracture initiation.

Operational factors amplify the effect

• Multi-well developments, higher fluid volumes, and tighter well spacing increase reservoir stress.
• Accumulated stress shadowing raises minimum horizontal stress until vertical fractures are no longer favored.

Rock has a stimulation limit

• Each formation has a threshold beyond which horizontal fractures are likely to form.
• Exceeding this limit is a function of both geology and operational intensity.

Implications for operations

• Stimulation designs must be tailored to formation-specific limits if horizontal fractures are to be avoided.