Chapter 4: Drilling Oil Wells

Chapter 4:
Drilling Oil Wells

While the equipment used to drill and complete an oil well is usually quite simple, the engineering required to drill one properly can be highly complex. Whether the drilling rig is offshore or onshore, they all have the same basic structure and use the same equipment.

Anatomy of an Oil Rig

The purpose of a rotary oil rig is to drill a hole to a predetermined depth. And, hopefully, in the drilling process, passing to or through one or more oil and gas bearing reservoir rock formations. Since rigs are expensive to manufacture, they need to be movable. In addition, since the rock chips created by the drilling bit must be removed, mud is pumped through the drill pipe to the bit and back up the annulus or space between the drill pipe and the outer casing that is added as drilling proceeds. The mud is mixed, usually with water but sometimes with chemicals, in a chemical tank, then sucked from the mud pit and pumped via a standpipe and rotary hose to a swivel that is attached to a Kelly, which is itself attached to the drill pipe. The returning mud and rock chips that reach the surface move by gravity down a return line to a shale shaker designed to separate the returning mud from the shale for reuse. The remaining rock chips travel down a shale slide to a reserve pit.

Chart courtesy of Earth Science World and Exxon-Mobil

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The kelly is a special section of pipe that has flattened sides that are either square or hexagonal in shape. The kelly fits inside an opening called a kelly-bushing. The kelly bushing, in turn, fits into a part of the rotary table called the master bushing. As the master bushing rotates, the kelly bushing rotates. The turning kelly rotates the drill stem and thus the bit. Since the kelly slides through the opening in the kelly bushing, the kelly can move down as the drilling progresses.

Rotary Drilling Table Power to rotate the drill stem comes from the rotary table. This is equipped with its master bushing and kelly bushing. When the kelly and kelly bushing are removed, the hole left in the master bushing accommodates slips that have teeth-like gripping elements called dies. These are placed around the drill pipe and keep it suspended in the hole when the kelly is disconnected and an additional section of drill pipe and/or casing is attached.

A recent innovation is the advent of the power swivel. This is a top drive system that eliminates the need for the kelly and rotary table. With a power swivel drill pipe, joints can be added three at a time versus one at a time, vastly speeding drilling time.

Both the mud transfer pumps and drilling pipe require power to operate. Usually both are handled by two or more 500 hp to 1,000 hp diesel engines. Additional power and engines are required to supply electricity to the rig since the rig typically drills 24 hours a day.

Well Bore Engineering

The engineering becomes more complex as the well is being drilled to its total depth. The deeper one drills, the more pressure that is exerted on the lower strata. Drilling engineers maintain a density (weight) of mud in the hole in order to counter the natural pressures of fluids and gases that might otherwise release into the well hole. But at certain depths and conditions, it becomes impossible to either keep the mud from penetrating a formation or for fluids to release into the well hole. A control panel on the surface is linked to various parts of the rig to keep an eye on down-hole pressure, mud volume, weight on the drill bit and other aspects of the drilling operation. At some point, however, it becomes necessary to pull the drill stem and bit from the hole, insert casing in the hole, and fill the annulus (space) between the casing and the wall of the hole with concrete.

Anytime the drill stem and bit are removed, whether to case the well or retrieve a broken tool, the process is call "tripping out." The cement used to cement wells is not very different from ordinary concrete. The cement is pumped into a special valve called a cementing head. As the cement arrives at the head, a plug called a bottom plug, is released from the cementing head and precedes the concrete slurry down the inside of the casing. This plug keeps the cement and the mud ahead of it from mixing. The plug travels downward until it reaches the float collar. At the collar, the plug stops, but continuing pump pressure breaks the seal in the top of the plug and allows the slurry to pass on. The slurry flows through the plug and starts up the annulus.

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When the estimated amount of cement required has been pumped into the casing, a top plug is inserted and water, usually salt water, is pumped in behind the top plug. The top plug keeps the cement from being contaminated by the following displacement water. When the top plug reaches the bottom, the pumps are stopped, and the cement is allowed to harden. Depending on well conditions, it may take from a few to 24 hours or more for the cement to harden. Then drilling may be resumed. To resume drilling, the drill stem and a new, smaller drill bit that fits inside the casing must be tripped back into the hole. This process is called "tripping in."

Well Completion

well completion cross-section Once a well has been drilled and logged, a decision must be made whether to complete the well or plug it. The vast majority of wells drilled in existing fields do produce petroleum. However, examination of pierced reservoir rock formation porosity and permeability may indicate that the potential flow of oil and gas from the well will be worth less than the cost to complete the well. In these cases, the well is plugged with concrete, sometimes in several places, and the well is abandoned.

If, however, the well readings indicate that the well will be commercially productive, the well is completed. If the well is to be completed, production casing is run down the hole and cemented. Once the casing is in place, a tool called a "perforating gun" is lowered into the well-bore to blast holes through the casing, cement and into the reservoir. These holes are made in order for the oil bearing reservoir to have access to the production casing. Tubing may then be lowered into the casing. A plug may then be set below the perforations and a packer set above the perforations as a barrier between the production casing and the tubing. This allows the earth's natural pressure to push hydrocarbons to the well-bore and to the surface through the tubing unless a pumpjack is necessary to raise the fluids to the surface.

Several steps are taken at this time to cut out excessive costs from the production process. A large drilling rig will be replaced by a smaller, moveable completion rig. Also, a completion team will use a swabbing method to force the reservoir to give up fluids naturally. This natural flow rate will be measured and compared to other wells in the area. If it is not up to par, then further measures will be taken to increase the volume of production. These measures include chemically or physically treating the reservoir to stimulate the flow of fluids. Acid treatment can be used in a reservoir containing limestone to open pore spaces by dissolving sections of limestone. Using a physical method, fluid containing small beads is pumped into the earth under great pressure to crack open the reservoir. Then the beads are used to keep the fractures open and allow the flow of fluids to increase.

When a satisfactory rate of production has been established, the well will be tested to calculate the maximum production for the well over a period of twenty-four hours. This is termed the "open flow potential." This and other completion information may be required by the state and will aid other geologists and analysts scouting for oil and/or natural gas in the same area.

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If a well contains more than one zone of interest, the operator will usually begin by producing the lowest zone in the well bore first and then work their way up the well bore as each zone becomes depleted. When a zone is completed and the well is near the production process, a multi-valve device will be connected to the surface called a "Christmas tree." This device is placed at the top of the production casing and will allow connections to flow the oil and gas. Equipment to process the recovered oil and gas is placed near the well to make sure that no contaminants remain in the oil or gas. This equipment is used to make the oil or gas ready for transportation.

Production

Production is the process of extracting petroleum from the underground reservoir and bringing it to the surface to be separated into gases and fluids that can be sold to refineries. Production begins with a high level of production and decreases through time until the well is ultimately plugged and abandoned. This decrease in production is a natural result of the inevitable decline in original pressure within the reservoir. The time period for commercial production can span from three to 50 years, and the production amount can vary between 30 to over 1,500 barrels a day.

Either gas expansion and/or water encroachment provides the principal natural energy for most petroleum reservoirs to produce. Both can operate as reserves are taken from the reservoir. The reduction in pressure around the well bore as hydrocarbons are extracted causes other hydrocarbons to move into their space. This process continues until the energy is depleted and/or the well makes too much water to be commercially productive.

Engineers take the past performance of a well and use it to project the future reserves of a well. One way of predicting future production is to measure the percentage of decline in production over a given period of time and use this rate of decline to estimate future reserves.

Reservoir Engineering

Reservoir engineering is the application of scientific principles to develop and maintain petroleum reservoirs to maximize economic benefit. For example, carefully spacing wells over a reservoir can make a huge difference in its overall productivity. In 1904, Anthony Lucas, who had discovered Spindletop, spoke about the decline in production. He claimed that "the field had been poked with too many holes and that the cow was milked too hard." Oil operators in that day gave little thought to reservoir depletion as they completed wells. They produced a well at the highest rate they could without regard for well spacing. As a result, in the 1920's the federal government questioned the wasteful treatment of reservoirs and decided to initiate studies. These studies consisted of applied mathematics, geology, chemistry, fluid dynamics, and physics to aid in the analysis of hydrocarbons within a reservoir. Reservoir engineering began as engineers implemented what the government learned.

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Enhanced Oil Recovery

Enhanced oil recovery once the natural flow of gas and oil ceases, the reservoir will have yielded only 10 to 25 percent of the total volume of the oil it contains. The rest is trapped in unconnected rock pockets or is thick enough to cling to the rock and refuses to migrate toward the well-bore.

secondary oil recovery in an existing oil field

Chart courtesy of Earth Science World and Exxon-Mobil

Petroleum engineers have developed a number of ways to coax this reluctant oil to migrate. The most common approach is to drill adjacent wells and use them to inject water into the reservoir to force the oil to move toward the production well. Another is to inject gas into adjacent wells to slow the rate of production decline or to enhance gravity drainage. Both approaches are referred to as secondary recovery processes. Even after secondary recovery steps have been taken, more than 50 percent of the oil in the reservoir will remain.

Enhanced oil recovery, also known as tertiary recovery method, is a technique used for increasing the amount of oil, which can be extracted from an oil field after secondary recovery efforts are no longer effective. Using this method, 10 to 20 percent more of the reservoir's original oil can be extracted versus the amount retrieved using primary or secondary methods. Gas injection, thermal recovery, or chemical injection can be used to encourage additional flows, although gas injection is the most commonly used. Thermal recovery involves using heat to improve the flow rates and chemical injection is used rarely to lower the surface tension in the reservoir. The cost of using these types of recovery is usually high; however, when the costs of oil are at historically high levels, the economics of the enhanced oil recovery methods rapidly improve.

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