Computational Analysis of Stress Interference Effect for Hydraulic Fracturing in Waste Injection Wells

摘要

Hydraulic fracturing is a key mechanism for injection into waste disposal wells. To successfully inject drill cuttings slurry or produced water under fracturing conditions and prevent unwanted migration, it is essential to predict the extent of hydraulic fractures accurately based on understanding the fundamental fracture mechanisms. Popular hydraulic fracture models for injection wells continue to rely on pseudo-3D fracture technology. However, when injection takes place in multiple, closely spaced wells, or under the influence of offset producers, stress interference effect on fracture geometry can hardly be predicted based on simple single-well methods. If the distance between the multiple injection wells is small such that pronounced stress perturbations exist, the stress interference could influence the fracture geometry and orientation. This paper presents a study on the stress interference effects on fracture orientation and spatial extent caused by multiple injection wells using a hybrid computational method incorporating finite element and boundary element fracture models. To investigate the stress interference effects, a pore pressure cohesive zone model was developed to predict hydraulic fracturing for injection wells through finite element analysis and sub-modeling technology. The model was applied to investigate the stress perturbation effect on fracture geometry caused by long-term, high-rate, multi-well waste injection into a heterogeneous geological environment. Boundary element analysis was performed to simulate the possible fracture reorientation and fracture merging caused by the changing of the stress field with the propagation of multiple hydraulic fractures. Injection wells with different well distances and stress boundary conditions were investigated. A 3D analytical tool has been developed to simulate the changes of stress due to the depletion in pore pressure from an offset location. The computational analysis on the stress field was verified with the analytical solution. Based on the finite element and boundary element analyses, revised stresses were incorporated into the tuned pseudo-3D models to update the hydraulic fracturing prediction to account for stress interference effects. Ultimately, the fracture predictions were quantified under the influence of stress interference.