Numerical modeling of the step rate test using fully coupled hydraulic fracturing capabilities

This paper presents a fully coupled finite element model to simulate hydraulic fracturing (HF) in rocks. The step rate test, a common diagnostic test for designing HF treatments, is simulated to demonstrate the model's capabilities. In the model, the rock deformation is fully coupled with the pore-fluid Darcy flow and the fracture is modeled using specialized cohesive zone elements that simulate Darcy flow in undamaged state which smoothly transitions to a tangential Poiseuille flow when fully damaged. Fluid leak-off into the formation and near wellbore damage effects are also accounted for. Specialized pipe elements are used to model the wellbore fluid flow including the frictional losses. Using this model, a field-scale step rate test is simulated, with thoroughly calibrated material properties and in-situ stresses. It is shown that the model can match the entire pressure history of the test, including the flow regime transition from pure matrix flow to fully developed fracture growth and the shut-in leak-off response. It can also predict the formation breakdown pressure, showing it to be higher than theoretical estimates, which is critical for injection system pump design. The model is further used to gain insights on the fracture behavior.