Patent Number: 061370282
Section: summary

FIELD OF THE INVENTION The present invention relates generally to the containment of hazardous waste within a manmade, subterranean cavity. BACKGROUND OF THE INVENTION Water, pipe scale, and sludge contaminated with naturally occurring radioactive materials (NORM) are wastes that often accompany the recovery of oil and gas from subterranean reservoirs. Most of the NORM-contaminated water is currently disposed of through injection back into the reservoirs from which it came. Unfortunately, solid NORM wastes, like scale and sludge, require extensive processing before disposal through underground injection into permeable rocks or land spreading can be accomplished. Because of the need to reduce the high costs involved in the conventional disposal of solid NORM wastes, it has been proposed that such be injected into salt caverns. Salt caverns are typically created by injecting fresh water into subterranean salt formations and withdrawing the resulting brine. This process is referred to as solution mining. Over time, numerous salt caverns have been solution mined by the petroleum industry for use in storing hydrocarbons and for disposing nonhazardous oilfield wastes (NOW). To date, salt caverns have not been used to dispose of NORM wastes due to concerns that they may leak radioactive materials into surrounding rocks and, perhaps, into fresh water aquifers. It has been noted that the release of NORM wastes from salt caverns could result from one of five scenarios: (1) inadvertent intrusion; (2) failure of the cavern seal; (3) leakage through cracks; (4) leakage through interbeds of permeable material; and, (5) a partial cavern roof fall. Risk estimates indicate that there is a very low probability of any of these scenarios occurring provided that the salt cavern is properly designed and operated. A properly designed salt cavern can be a leak-free repository for NORM waste. Nevertheless, the present inability to control the flow of water within a subterranean salt formation during solution mining has made it difficult for the petroleum industry to convert salt cavern design concepts into reality in the field. A need, therefore, exists for a new method for forming salt caverns in a controlled manner and then depositing NORM wastes therein. SUMMARY OF THE INVENTION In light of the problems associated with the known methods of disposing of oil field wastes contaminated with NORM, it is a principal object of the invention to provide a method for the disposal of NORM wastes in a salt cavern which is safe, cost effective, and can be performed with conventional, oil field equipment. It is another object of the invention to provide a method of the type described wherein a cavern of substantial strength and known dimensions can be formed in a salt formation being as little as a few feet thick. It is a further object of the invention to provide a method of the type described wherein the deposition of NORM wastes within a salt cavern accompanies the formation of the cavern itself thereby reducing the time required to dispose of NORM wastes. Briefly, the disposal method in accordance with this invention achieves the intended objects by including the steps of: drilling a pair of wells which intersect within a salt formation, providing a slurry containing NORM wastes and a carrier liquid, injecting the slurry through one of the wells into the salt formation wherein the NORM wastes settle, and removing the carrier liquid from the other one of the wells. One carrier liquid, fresh water, dissolves the salt formation to help form and enlarge a cavern for receiving the NORM wastes. The quantities of carrier liquid removed from the salt formation are disposed of by injection into a permeable formation remote from the salt formation. Prior to injecting the slurry, it is preferable to inject fresh water alone into one of the wells and simultaneously withdraw the resulting brine from the other one of the wells so as to dissolve a cavern in the salt formation. Such a cavern permits larger settling times for slurry positioned therein. Nonetheless, by employing fresh water as a carrier liquid, this step may be omitted in cases where the portions of the two wells open to the salt are relatively long, for example. Thus, the injection of slurry may be begun immediately after drilling the wells. The creation of a subterranean salt cavern with a well having a horizontal section as described below allows the geometry of the cavern to be controlled so that the strength of the cavern roof and the life of the cavern can be maximized. Horizontal well operations can be employed in any salt strata of sufficient thickness to permit horizontal drilling. The foregoing and other steps, objects and advantages of the present invention will become readily apparent upon further review of the following detailed description of the preferred method as illustrated in the accompanying drawing.