Abstract

The problem of nonhazardous oil field waste disposal is a critical cost consideration in the oil and gas industry. The problem becomes even more critical when that waste contains naturally occurring radioactive material. The primary concern is the disposal of the waste solids, predominantly from drilling operations or site remediation efforts. Many companies have investigated the injection of slurried solid waste for disposal into depleted production wells. They have all included the need for controlled fracturing, whether classic, dissolution, or both, to provide a conduit for waste passage and the storage volume in which to place the waste. This requires that the operator exceed the fracture pressure and cause the formation to part to allow fluid passage. Regulatory agencies are very concerned with such efforts due to the perceived potential for “uncontrolled” fractures intersecting underground sources of drinking water aquifers or other wellbores and allowing direct or indirect contamination of those aquifers.

This article describes a concept and process by which subpressured or underpressured zones are utilized to take slurried waste at pressures below the theoretical fracture pressure. As such, the required injection pressure is often provided by the head of the fluid in the wellbore. In oil field vernacular, the well takes the fluids “on vacuum.” Under this scenario, the likelihood of the formation being hydraulically fractured is very low due to the inability of pressure to build to the frac gradient. The chance of injected fluids migrating back to the surface is also extremely low because a subpressured zone exists due to isolation compartmentalization through structural seals (such as well-sealed fractures). Additionally, the hydraulic head of the normally pressured formations geologically above the disposal zone is greater than is the injection pressure and provides a secondary impediment to upward mobility.