Injection involves making a suitable slurry out of the waste generated during drilling operations. This solids-laden slurry is pumped into the formation at pressures exceeding the fracture. Since this is the only disposal method not involving the surface or the sea bottom, waste streams that would be undesirable to surface-dispose of could be safely disposed of by injection. However, in the United States, only exempt waste can be injected. Nonexempt waste must be taken to an appropriate and approved UIC (underground injection control) well.
The use of injection as a method of disposal in offshore drilling operations has a history of mixed results. The first obstacle is having a conduit into a receiving formation. The slurry to be injected can be pumped down the inside of tubing or casing (through tubing injection), or it can be pumped down an annulus between casing strings into an open formation. Whichever method is used, the receiving formation must be isolated from the surface and other zones, especially up the cemented annulus. In many drilling operations, a conduit does not exist. The previous annuli have been cemented to surface, another well bore is not available, or a potential conduit simply does not exist. At other times, a satisfactory receiving formation does not exist.
Weak or unconsolidated sands are preferred as injection target. The sand can fluidize and repack, accommodating very large volumes of waste. Dedicated injection wells are preferred to annuli because the wells can be worked over if plugged by cuttings. Careful geotechnical surveys should be done to choose an appropriate zone. The zone must have a good seal above it, and the pipe must have a premium cement job.
If injection is to be used, then slurry must be made from the fluid and cuttings. The fluid and waste fluids are collected into slurry tanks to make the slurry. These slurry tanks are circulated with special slurry pumps designed to break up particles into natural grain sizes. If more fluid is needed because the slurry is too thick, then seawater can be added. The fluid may be too thin if excessive amounts of water are collected in the waste stream or if the mixing is insufficient. Frequently a shaker is used prior to the slurry tanks to prevent large particles or junk from entering the slurry tanks. In addition, there is a suction line screen to protect the pumps. There are usually two or three slurry tanks of 100-to 150-bbl capacity. This large volumetric capacity may be needed to handle large hole drilling.
As slurry is made in one tank, fluid and cuttings waste are diverted to another slurry tank. When the slurry is sufficiently mixed in the first tank and reaches an acceptable consistency, it is transferred to the slurry holding tank. This tank, or possibly series of tanks, is designed to hold the injection batch volume. The volume is based on the desired radius of injection. Care must be taken not to intersect nearby well bores, which may not be cemented to surface.
Weight (or space) limitations on the rig or platform frequently pose problems. Generally, batches of about 300 bbl of slurry are made (this is limited mainly by space and weight criteria). If 100 bbl of seawater are pumped before and after the slurry, then the total volume of each injection would be 500 bbl. Total slurry and equipment weight may be between 125 and 150 tons, which includes the slurry holding tanks in addition to the slurry skid. The footprint for the equipment may be as much as 40 feet wide by 90 to 120 feet long, depending on the equipment required. However, with a little innovation, the weight and space problems can be overcome.
The final problem is rate of waste generation. Offshore there is not a great deal of space in which to collect waste. For example, if 8Vi-inch hole is being drilled, then drilling 200 feet per tour would generate about 30 bbl of waste cuttings. An additional 50 bbl of oily wash water might also be generated. Total slurry volume would be about 100 bbl per tour. However, total slurry volume for 400 feet of 12%-inch hole in a tour would be about 300 bbl. Similarly, 500 feet of 17 VS inches may require handling as much as 1000 bbl of total slurry. For the large jobs, it is advantageous to have some storage (buffer) for generated waste. On the other hand, the cuttings from an 8 Yi-inch hole can be collected in boxes to be slurried and injected later while drilling the 6-inch hole section. This may save rental cost on the slurry equipment during the slower drilling intervals and more fully utilize the rental equipment.
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