Re-injection of oil contaminated drill cuttings is attracting considerable attention as a cost effective means of complying with environmental legislation concerning discharges of oily wastes.
The concept of disposing of waste drill cuttings by means of re-injection down hole has already been applied successfully by several operators worldwide. The technique has proven to be viable in many different areas and formations around the world, with the most activity in the North Sea, Alaska, Gulf of Mexico and Venezuela. Re-injection of drill cuttings normally involves collection of the waste from solids control equipment on the rig, followed by transportation to a cuttings processing station. Cuttings are slurried in this unit by being milled and sheared in the presence of water, usually seawater. The resulting slurry is then disposed of by pumping it into a dedicated disposal well, or through the open annulus of a previous well into a fracture created at the casing shoe set in a suitable formation.
Operations are usually batch by nature and carried out at low pump rates (2.0 - 8.0 bpm). These kinds of operations have been carried out all over the world, with disposal into many different types of strata.
On logistical and cost grounds the means of disposing of waste cuttings from platform based operations can usually be narrowed down to one of two choices. These are either re-injection into a dedicated disposal well, which if newly drilled can be re-completed as a producer or water injector at a later date, or re-injection through the annulus of a well drilled prior to the current live well. Drilling and cuttings disposal into the same well is possible but to date, because of well control concerns, it is not a preferred option with operators.
Sequential annulus injection is invariably the preferred means of disposing of cuttings, particularly in offshore locations. This is because of its flexibility and that it avoids the cost of drilling a dedicated disposal well. For cost reasons, dedicated re-injection wells are usually only practical on land or in shallow water. They do have advantages, however, including ease of cleaning out with coiled tubing in the event of plugging, can be designed to accommodate high volumes of waste, the ability to inject larger sized solids and a reduced risk of tubing plugging. Even so, unless annular cuttings re-injection is not viable, for example because of lack of annular access to a suitable deposition horizon, drilling a dedicated disposal well is usually ruled out on cost grounds. Thus, annular re-injection of waste cuttings is invariably the method of choice. Typically the 13-3/8" by 9-5/8" annulus is selected as the disposal location.
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