The subsurface injection of drilling waste has become an increasingly popular and well-accepted technology over the last several decades. The popularity of this technology is primary spurred by its economic advantages in meeting more stringent drilling waste management requirements, especially in remote and environmentally sensitive areas. Furthermore, its use has become more attractive with the dramatic development and improvement in the processes associated with surface and sub-surface engineering, fracture modeling, risks identification and mitigation options, injection monitoring and in-depth pressure analysis. Together, these advancements have improved considerably the assurance and efficiency of waste injection operations worldwide.
Nevertheless, despite the tremendous advancements in the fracture modeling attained from subsurface feasibility studies, a major uncertainty exists with the propagation of multiple-fractures that apparently accompanies the intermittent batch injection process, essential to the drilling waste injection operation. The propagation of multiple-fractures, along with their orientation and complexities, strongly influence the fracture design, ultimate disposal capacity and injection pressure behavior. Consequently, this uncertainty is a critical issue, both in drilling waste injection and re-fracturing in conventional stimulation treatments.
This paper describes the evolution of understanding of multiple-fracture mechanics in drilling waste injection, starting from the conventional "wagon-wheel" uniform disposal-domain concept to the branching multiple-fractures approach that becomes practical through mathematical computations of near-wellbore changes in the stresses resulting from prior fracture creation and solids accumulation. Moreover, the authors present four potential scenarios of subsequent fracture initiation and propagation during intermittent injections, and provide revised re-assessment of data from the joint industry Mounds Drill Cuttings Injection Field Experiment.