As the drill bit grinds downward through the rock layers, it generates large amounts of ground-up rock known as drilling waste. The conventional process of drilling oil and gas wells uses a rotary drill bit that is lubricated by drilling fluids or mud. This section discusses several alternative drilling practices that result in a lower volume of drilling waste generated.
Usually wells are not drilled from top to bottom at the same diameter, but rather in a series of progressively smaller-diameter intervals. Oil and gas wells are constructed with multiple layers of pipe known as casing. The top interval is drilled starting at the surface and has the largest diameter hole with large quantity of drilling waste. Drill bits are available in many sizes to drill different diameter holes. The hole diameter can be 20" or larger for the uppermost sections of the well, followed by different combinations of progressively smaller diameters.
Once a suitable depth has been reached, the hole is lined with casing that is slightly smaller than the diameter of the hole, and cement is pumped into the space between the wall of the drilled hole and the outside of the casing. This surface casing is cemented from the surface to a depth below the lowermost drinking water zone. Then, a smaller diameter hole is drilled to a lower depth, and another casing string is installed to that depth and cemented. This process may be repeated several times. The final number of casing strings depends on the regulatory requirements in place at that location and reflects the total depth of the well and the strength and sensitivity of the formations through which the well passes.
Traditional saying, wells were drilled to be relatively vertical and were completed at a depth to intersect a single formation. Thus, one full well was required for each completion. Modern technology allows modifications to several aspects of this procedure, thereby allowing more oil and gas production with less drilling waste generated.
It states how drilling can be done to intersect multiple targets from the same main well bore, how wells can be drilled using smaller diameter piping in the wells, how drilling can be done using techniques that minimize the amount of drilling fluid, and drilling fluid systems that generate less waste.
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