An Elastic-Electrical Joint Method for Fracture Evaluation in
The formation evaluation of fractured shale has been a challenging topic
in unconventional exploration engineering due to the complicated
formation conditions with strong anisotropy. The inversion result for
the fracture and the water/oil saturation could be less reliable if only
the single-physics model of either elastic or electrical is considered.
In this work, an elastic-electrical joint method involving both elastic
modulus and resistivity tensor is proposed to characterize the fractures
as well as predict the water saturation for the anisotropic formation.
In this method, we employ the three-order tensor invariants to
characterize the orthogonal resistivity tensor matrix of anisotropic
model. Both the elastic modulus and the electrical tensors of each
fractured model are computed with a FEM based multi-physics simulator.
By varying the fracture shape, alignment and the porosity (or water
saturation), the cross relation between elastic modulus and electrical
tensor invariants in terms of fracture shape is established. Therefore,
the water saturation can be predicted using a tensor-revised Archie’s
law which was proposed in our previous work. We validate the proposed
joint method using numerical experiments conducted on a set of idealized
fracture geometries (e.g. needle-like fractures and disk-like
fractures). The joint curve of elastic and electrical tensor invariants
is able to predict the fractures shape accurately with a known porosity.
In addition, the tensorial electrical parameters of formation can be
inversed from the joint curve as the elastic tensor and the electrical
scalar is determined by measurement. It provides an alternate mean to
obtain the electric tensor (e.g. resistivity tensor) when the
anisotropic measurement is not available in some practical engineering
scenarios. This work indicate that the proposed elastic-electrical joint
method can provide significant theoretical and engineering value for
well logging interpretation as well as formation evaluation.