| Abstract: | It is proposed that the growth of fractures is the basic process for generating and maintaining permeability in solid rock
(bedrock). Many extension fractures grow as hydrofractures, whereas many shear (and extension) fractures grow through the
formation of transverse fractures that connect the adjacent tips of existing fractures. In a boundary-element analysis, the
hydrofractures are modeled as being driven open by a fluid overpressure that varies linearly from 10 MPa at the fracture centre
to 0 MPa at the fracture tip. The host rock has a uniform Young's modulus of 10 GPa, a Poisson's ratio of 0.25, and is dissected
by vertical joints and horizontal contacts, each of which is modeled as an internal spring of stiffness 6 MPa m−1. The number of joints and contacts, and their location with respect to the hydrofracture tip are varied in different model
runs. The results of the analyses indicate that the tensile stresses generated by overpressured hydrofractures open up joints
and contacts out to considerable distances from the fracture tip, so that they tend to link up to form a hydraulic pathway.
Using the same Young's modulus, Poisson's ratio, and internal spring constant for joints as in the hydrofracture models, boundary-element
models were made to study the interaction stresses that cause neighbouring joints to become interconnected through the growth
of linking transverse fractures that, ultimately, may evolve into shear fractures. The models were subjected to tensile stress
of 6 MPa acting normal to the joint planes as the only loading. The offset (horizontal distance) and underlap (vertical distance)
between the adjacent tips of the joints were varied between model runs. The results show a concentration of tensile and shear
stresses in the regions between the neighbouring tips of the joints, but these regions become smaller when the underlap of
the joints decreases and changes to overlap. These stress-concentration regions favour the development of transverse (mostly
shear) fractures that link up the nearby tips of the joints, so as to form a segmented shear or extension fracture. Analytical
results on aperture variation of a hydrofracture in a homogeneous, isotropic rock are compared with boundary-element results
for a hydrofracture dissecting layered rocks. The aperture is larger where the hydrofracture dissects soft (low Young's modulus)
layers than where it dissects stiff layers. Aperture variation may encourage subsequent groundwater-flow channeling along
a pathway generated by a hydrofracture in layered rocks.
Electronic Publication |
