Numerical Simulation of Sleeve Fracturing for In Situ Stress Measurement Using Cohesive Elements and Its Implications

Numerical analysis of growth of fractures from a wellbore under sleeve fracturing conditions demonstrates the impact of in situ stresses on initiation and propagation behavior of primary and secondary cracks at both field and lab scale. Crack initiation and propagation behavior are simulated in a Finite Element framework employing plane-strain cohesive element along the candidate crack paths. The model has been validated by benchmarking to Kirsch’s solution as well as by comparing predictions to results of a laboratory block test. Indeed, the field scale simulation results show that the first fracture always orients with the opening in direction of maximum in situ stress while the opening direction of the second fracture can vary with different in situ stresses. Furthermore, the deviation of wellbore displacement from the Kirsch solution due to the fracture initiation can be captured by the simulation, which can be further applied to interpret the initiation pressure and location of cracks based on field test data. In the context of past contributions on sleeve fracturing, these numerical results demonstrate that a relatively large region near the crack tip undergoes inelastic deformation, which contrasts with the assumptions required to treat this problem using linear elastic fracture mechanics (LEFM). Notably, the finite tip plasticity appears to eliminate unstable jumping of the crack length(s) predicted by LEFM solution. The finite tip plasticity also leads to the prediction that the secondary fracture often occurs in orientations other than 90° to the primary fracture. In contrast, the LEFM solution predicts none of this growth because the stress intensity factor (SIF) never reaches the rock fracture toughness in spite of the fact that these alternate orientations have the largest SIFs over limited periods of time. Finally, the finite tip plasticity shows the strength of materials approach (based on stress analysis around the hole) to give a lower bound estimate of the pressure required for crack extension.