The first oil wells, created thousands of years ago, were holes dug by hand at spots where oil seeped to the surface. These first oil wildcatters collected small amounts for medicines and pigments. Once the gooey black liquid’s potential for fuel was discovered, enterprising locals had to seek deposits deeper than it was safe or economical to dig. Drilling was born.
The earliest drilling methods used materials close at hand. In China, bamboo poles were repeatedly hammered into a hole and pulled out, the mud and dirt from the bottom collected in the hollow at the end of the pole. Once the hole reached an oily layer in the soil, the liquid could flow to the surface. This drilling method was limited by the hardness, diameter, and length of the tool that was being pushed into the deepening well. A similar technique is still used, though with modern materials, for shallow water wells in soil.
With the invention of internal combustion engines, drillers needed to be able to drill deeper and faster, and drill into rock instead of soft soil. The answer was the cable tool drill rig: this machine repeatedly raises a heavy tool with a sharpened end and drops it to the bottom of the hole. Broken pieces of rock collect in a basket, to be emptied each time the tool is pulled up to the surface. Hundreds of thousands of cable-tool wells were drilled around the world, producing billions of barrels of oil. The economics of cable-tool drilling, however, are unfavorable for the wells needed to reach deep reservoirs: hauling the tool back back to the surface after each drop takes ever longer as the well deepens. Though still used for an occasional shallow oil well and for water wells a few hundred feet deep, cable-tool drilling has gone the way of the treadle sewing machine and the hand-cranked wall phone.
Enter Howard Hughes. Not the famous Hollywood aviator and director who designed the “Spruce Goose”; his father. Howard Hughes, Sr., invented and patented the basic tool used in modern drilling, the rotary drill bit. Baker-Hughes Corp. is still one of the world’s largest suppliers of drill bits to the oil industry. Rotary drilling uses the same principle as the cordless drill you gave Dad for Father’s Day: a drill bit’s sharpened tip grinds away at the bottom of a hole, and the shavings get moved to the top just like debris collects on the floor below a hole drilled in the wall. With wood bits, those shavings move along flutes on the side of the bit. It would be highly impractical, however, to make a single drill bit thousands of feet long with flutes on the side. Rotary drilling must use a different approach.
Rotary drilling operations can be separated into multiple systems; but the most important are the drill rig, the drill string, and the mud system. The rig is the giant steel structure you might see at the land or water surface. It includes a tower, or derrick, with a massive pulley system for lifting and lowering heavy weights; and a powerful diesel engine to turn the drill string. The rig is at the center of activity on the surface. It is where roughnecks hook together pieces of the drill string and where a “tool pusher” controls the rig’s operations. Drilling rigs run 24/7/365, stopping only to wait for service and other operations.
The drill string consists of the drill pipe and a bit. Located at the “business end” of the drill string, the bit is high-tensile steel covered with a forest of diamond-encrusted teeth and studded with holes called mud jets. The drill pipe consists of 30-foot sections of hollow steel pipe, somewhat smaller in diameter than the bit at the end, that screw together. As the hole deepens, more drill pipe is continually added to the top of the string. The ever-increasing weight of the pipe aids the bit as it grinds into the rock at the bottom of the well. The rig motor at the surface must turn the entire string of pipe to turn the bit at the bottom. That string of pipe can be very substantial: the current record for the longest string of drill pipe is just about 37,000 feet.
Drill pipe is hollow for a reason: the mud system. Drilling fluid, called “mud” in the industry, is as essential to the process as the Hughes drill bit. Throughout the entire drilling process, the hole remains filled with mud that is in constant circulation: it’s pumped into the top of the drill string, squirts out of the mud jets on the bit at the bottom of the hole, and rises back to the surface through the space around the drill pipe. In the process, the mud serves several purposes: it lubricates and cools the bit and and any places where the drill string contacts the wall of the hole, and the rock shavings or “cuttings” float back to the surface in the rising column of mud. An extremely important job of the mud is to keep the borehole from collapsing in on itself, and the weight of the column of mud filling the hole keeps water, oil, and gas under high pressure at the bottom of the hole from exploding out of the top of the well.
Once a well has reached its full depth, the walls of the hole are lined with heavy steel pipe called casing that is fixed in place with special high-temperature, high-pressure cement. Only then can the mud be pumped out of the well. It is believed that the BP Macondo well blowout that destroyed the Deepwater Horizon drill ship was caused when a cement job thousands of feet below the ocean bottom failed, allowing uncontrolled high-pressure fluids to enter the well.
Though the basic principles of rotary drilling are now more than a century old, this essential industry continues to advance with new techniques, materials and refinements of the technology.
For more Information:
Schlumberger Oilfield Glossary: Rotary Drilling; NaturalGas.org: Rotary Drilling
