A Risk-Based Approach to Waste-Containment Assurance

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98202, "Formation Damage and Its Impact on Cuttings-Injection-Well Performance: A Risk-Based Approach on Waste- Containment Assurance," by Q. Guo, SPE, and T. Geehan, SPE, M-I Swaco, and K.W. Ullyott, SPE, EnCana Corp., prepared for the 2006 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 15–17 February. With ever-tightening environmental regulations, drilling-waste disposal by downhole hydraulic fracturing is often the preferred waste-management option to achieve zero discharge. However, formation damage caused by solids-laden-slurry injection can cause uncertainties in injection-well performance and waste-containment assurance. Complicating the problems are the many formation-damage mechanisms that are difficult to model. A probabilistic approach based on Monte Carlo simulations has been developed to model formation damage and its contribution to injection-well performance and waste-containment assurance. Introduction The principal waste product of the drilling process is oil-contaminated drill cuttings. One of the preferred waste-management options is downhole injection into a deep geological formation by use of hydraulic fracturing. This technology is known as cuttings reinjection (CRI). A drill-cuttings-injection operation involves the collection of solid waste from solids-control equipment (i.e., a shale shaker) on the rig and its transportation to a slurrification unit, where the cuttings are ground into small particles and combined with water to make a slurry. The slurry then is transferred to a slurry holding tank for final slurry-rheology conditioning. The conditioned slurry is pumped through a casing annulus or a tubular into subsurface fractures created by injecting the slurry under high pressure into the disposal formation. The waste slurry often is injected intermittently in batches into the disposal horizon. Each batch injection may last from a few hours to several days or even longer, depending on the batch volume and injection rate. The waste-storage mechanism is similar to hydraulic fracturing in reservoir stimulation and circulation loss in drilling. However, because the created fractures from cuttings-slurry injection can be very large, because of the large volume of slurry injected, the extent of the fractures must be understood to ensure that the injected material will not migrate into areas where it could create problems, such as contamination of underground potable-water zones. Containment assurance often is provided by numerical simulations. The extent of the fractures created by cuttings-slurry injections must be predicted with confidence. However, the simulation results are only as good as the quality of the data entered into the simulators. Uncertainties are present in any projects involving subsurface geology. Prediction of fracture length and overall profile, which will contain the injected waste, and also the disposal capacity, depends strongly on how many fractures are generated from intermittent slurry injections. When and where secondary fractures are generated may depend on factors such as initial in-situ stress magnitude and orientations, local stress change from cuttings-injection operations, and many other geological conditions. While uncertainties will exist, these may be evaluated with increased confidence by use of information obtained during the actual CRI operations. Because it is never possible to have total certainty about all the geological and operational information, a probabilistic approach to waste-containment assurance and risk assessment of CRI operations is appropriate.