Abstract:
Slurry injection is commonly used to dispose of oilfield wastes during the drilling and production phases of a well. Waste types include drill cuttings, drilling fluids, produced sands, and other types of wastes produced during production. Although slurry injection is most effective when hydraulic fractures are created, safe operations demand that the fractures remain contained below one or more confining layers that are situated above the permitted injection zone. This paper outlines how numerical simulations of 3D fracture propagation can be used to accurately forecast and monitor fracture containment in support of ongoing injection operations. In particular, simulation results are used to determine the accumulation of stress caused by ongoing deposition of solids within the fractures and near the wellbore. Five case studies highlight both the numerical methods and best practices for safely operating slurry injection wells. Field observations of pressure-fall off data and extrapolated near-well stress and fracture lengths are found to match closely with numerical results. Recent advances using cloud-based diagnostics of well performance also enable using real-time slurry rheology, injection rate and pressure data to drive numerical fracture simulations to predict how operational decisions impact fracture geometry and subsurface reservoir properties.
Introduction
Two common methods for disposing of exploration and production (E&P) wastes include landfills and deep injection wells. Each method has its own risks and potential drawbacks: landfills create environmental hazards, including potential ground-water contamination and impacts on nearby population centers, while deep injection of slurrified waste requires a thorough understanding of subsurface fracture propagation and long-term fluid containment. US shale production has led to a significant increase in the volume of E&P waste produced in regions that did not previously have the infrastructure to support safe disposal. Environmental sensitivities to E&P landfills, in addition to the lengthy regulatory process for siting new landfills, has made use of slurry injection wells for onshore E&P activity an important option.
Subsurface geomechanics plays an essential role in safe operations of slurry injection wells. The fluid rheology and pumping schedules drive much of the subsurface behavior: slurries are often composed of ground-up drill cuttings combined with a variety of drilling and production wastes that the injection facility receives, including: oil- and water-based drilling mud, produced water, rain water, and hydraulic fracturing fluids (flow-back, gels, acids, etc.). The facility is designed and operated with the sole purpose of supporting a sequence of day-long batch injections that may take place every few days. Batch injections are performed at very high flow rates to ensure that the bottom-hole pressure is large enough to stimulate one or more propagating fractures; these fractures may emanate from perforations made in the well's casing within the permitted injection interval or be created as off-shoots from an existing fracture produced during a prior injection. In both cases, the fractures may travel both laterally (away from the well) and vertically during continued injection.
