Effect of Rock Transverse Isotropy on Stress Distribution and Wellbore Fracture

Unconventional oil and gas, which is of major interest in petroleum industry, often occur in reservoirs with transversely isotropic rock properties such as shales. Overlooking transverse isotropy may result in deviation in stress distribution around wellbore and inaccurate estimation of fracture initiation pressure which may jeopardize safe drilling and efficient fracturing treatment.

In this work, to help understand the behavior of transversely isotropic reservoirs during drilling and fracturing, the principle of generalized plane-strain finite element formulation of anisotropic poroelastic problems is explained and a finite element model is developed from a plane-strain isotropic poroelastic model. Two numerical examples are simulated and the finite element results are compared with a closed form solution and another FE program. The validity of the developed finite element model is demonstrated. Using the validated finite element model, sensitivity analysis is carried out to evaluate the effects of transverse isotropy ratios, well azimuth, and rock bedding dip on pore pressure and stress distribution around a horizontal well.

The results show that their effect cannot be neglected. The short term pore pressure distribution is sensitive to Young? modulus ratio, while the long term pore pressure distribution is only sensitive to permeability ratio. The total stress distribution generally is not sensitive to transverse isotropy ratios. The effective stress and fracture initiation are very sensitive to Young? modulus ratio. As the well rotates from minimum horizontal in-situ stress to maximum horizontal in-situ stress, the pore pressure and stress distributions tend to be more unevenly distributed around the wellbore, making the wellbore easier to fracture. The pore pressure and stress distributions tend to "rotate" in correspondence with the rock bedding plane. The fracture initiation potential and position will alter when rock bedding orientation varies.