The Evolution of Fracture Systems in Rocks with Veins: Insights from 3D Discrete Element Models

Abstract:

Observations from natural vein systems suggest that preexisting veins can strongly influence orientation, continuity and connectivity of fractures in a rock even in cases where the orientation of the veins is incompatible with the orientation of the stress field.

We present a numerical method to model cycles of fracturing and sealing in a rotating stress field to simulate such systems, for different strength ratios of host rock and vein.

We study a layered model under vertical stress and uniaxial horizontal extension. This represents common conditions in sedimentary basins with layers of varying composition. The model with fractures that form during the first deformation phase is sealed and deformed again in a different direction to model the effect of a changing horizontal stress field. We find different types of fracture interaction with veins, depending on the strength contrast between veins and host rock and amount of rotation. The crack-seal and crack-jump mechanisms ensue naturally from the models as a result of the strength of the vein material relative to the host rock. Weak veins localize fracturing and reactivate, even in high misorientation to the extension direction. Connecting fractures between reactivated veins form at a higher angle to the veins than expected. In these systems, the connectivity of the fracture network is dramatically increased.

Veins stronger than the host rock have less influence on the new fractures. Most fractures crosscut the veins by the step-over mechanism. Deflection occurs for favorable vein orientations but the deflection length is very short.

The results are in good agreement with natural crack seal vein networks found in carbonate rocks of the Oman Mountains.

We find that preexisting veins can change the fracture behavior of a rock in a way that new fractures do not necessarily align with the principle extension direction and form a highly connected network with reactivated veins that dramatically enhances lateral hydraulic conductivity of the rock.

The results allow formulating constitutive rules for the fracture behavior of rocks with veins that can be used to make predictions on the orientation and connectivity of fracture networks on a reservoir scale to use in THMC models.

Furthermore they have implications for paleostress analysis based on crack seal vein networks.