Introduction
Exploration is the process of trying to find accumulations of oil and natural gas trapped under the Earth’s surface. Production is the process of recovering those hidden resources for processing, marketing and use.
To understand the challenges the oil and natural gas industry faces in exploration and production, it helps to understand how oil and gas accumulations – often called “reservoirs” – develop in the first place:
Oil and natural gas are formed when decaying plants and micro-organisms are trapped in layers of sediment and – over the course of millions of years – become buried deep within the earth, where underground heat and pressure turn them into useful hydrocarbons, such as oil and natural gas.
The layers of rock in which hydrocarbons are formed are called source rocks. High pressures underground tend to squeeze hydrocarbons out of source rocks into what are called reservoir rocks. These are rocks, such as sandstone, which feature pores large enough to permit fluids like oil, natural gas, and water to pass through them. Since oil and natural gas are less dense than water, they will float upward toward the surface. If nothing stops this migration, the oil and natural gas may reach daylight through what is called a surface seep.
More often, however, hydrocarbons’ path upward is blocked by a layer of impermeable rock, such as shale, or by some other geologic formation. These trap the oil and natural gas, either in an underground pocket or in a layer of reservoir rock, so that it may be recovered only by drilling a well.
Exploration
There isn't any way to be absolutely sure where new oil and natural gas reserves are located, so petroleum engineers need to collect clues as to what lies deep beneath the earth's surface. Advanced technology has revolutionized the exploration process for oil and natural gas, and helps them pinpoint potential reserves with greatly improved accuracy. This results in fewer wells, and lowered exploration costs.
Engineers can gather above-ground clues using airplanes and satellites to map the surface, to identify promising geological formations, and to look for oil and natural gas seeps. Ships can do the same for the ocean floor.
But engineers often get much more useful information by looking at geological structures and rock properties below the surface. They use a number of strategies including:
Seismic Surveys
Seismic surveys are done by sending high-energy sound waves into the ground and measuring how long they take to reflect back to the surface. Since sound travels at different speeds as it passes through different materials, computers can use seismic data to create a 3-D map of what lies below the surface.
Geologists and geophysicists – known as "explorationists" – use these 3-D seismic images to look for accumulations of oil and natural gas. Engineers then use the data to plan the safest, most cost-effective well path to the reservoir.
Once a reservoir has been located and put into production, a series of 3-D seismic surveys can be taken over time to see if all of the oil and natural gas reserves are being efficiently drained. If not, additional wells can be drilled to produce these bypassed pockets of reserves.
While seismic data are extremely useful to geologists, these surveys are also very expensive.
Exploration Wells
When the data indicate a likely site for oil and natural gas reserves, an exploration well is often drilled. Rock samples from the well are brought to the surface and analyzed. Well logs measure the electrical, magnetic and radioactive properties of the rocks.
By examining this information, a geologist can learn a great deal about the sub-surface structures and whether or not the site is likely to produce oil and natural gas in economic or "paying" quantities.
Gravity and Geomagnetic Surveys
These relatively inexpensive techniques can identify potential oil and natural gas bearing sedimentary basins and structures. High-resolution aero-magnetic surveys done by special aircraft can also show fault traces and differentiate between different rock types near the surface.
Creating a Drill Site
Drilling for oil and natural gas is a complex process, but advanced technology has made the job more efficient and productive while providing less impact on the environment. Want to see how the oil and gas industry does it? Click each numbered component in the drawing to learn more.
1. Some people believe that oil and natural gas companies can explore for oil wherever they want. This is not true. Companies must secure permission from the owner of the mineral rights, whether the owner is a private citizen or the government. Many mineral owners and the government allow oil and natural gas companies to compete to drill on their land. The companies assume all the costs and risks of drilling and, in return, pay the mineral owners a portion of what they find and a signing bonus to secure the drilling rights. The share of the production paid by the company to the mineral owner is called a royalty payment.
- The drilling derrick is used to position and support the drill string. Modern drilling equipment comes in a wide range of sizes. Many wells can be drilled with equipment that requires far less space than in the past.
- Drill rigs now run on electricity to supply the power to turn the bit and raise and lower the drill pipe and casing. Since most drilling occurs in remote areas, the electricity is supplied by electric power generators that run on diesel fuel. These generators make drilling rigs much quieter than in the past.
- The drill bit uses three conical shaped cutting surfaces to grind rock into rice-sized particles. The newest bits drill 150 percent to 200 percent faster than similar bits just a few years ago! The drill string consists of lengths of pipe fastened to each other and to the drill bit. The drill string transmits power from the top drive to the drill bit.
- As the drill cuts into the rock, drilling mud is added to the hole. This helps cool the drill bit, and the mud is circulated to bring cuttings to the surface. The weight of the drilling mud keeps the hole open. It also helps counteract the pressure of any gas or fluids encountered along the way, in this way preventing a well from loss of control or "blow out.”
- Protecting the aquifer from contamination is a major concern of the oil and natural gas industry. Casing made of steel or high-tech alloys is lowered into the hole and cemented into place to protect fresh water aquifers. The casing also keeps the hole open so that oil and natural gas can be brought to the surface.
- To reduce waste, the drilling mud is passed through a sieve where the ground rock particles or cuttings can be removed. Then the mud is recycled back into the hole.
- Dirt and rock cuttings are removed from the hole and temporarily stored nearby. Holding areas are carefully sited, lined and often times covered with nets to protect local wildlife.
- All aspects of the drilling operations are closely monitored to ensure efficient drilling and safety. Electronic sensors measure drilling rates, vibration, pressure, rock type, mud properties and many other drilling parameters. Computers monitor operations and collect data from inside the well. With advanced communications technology, drilling personnel can share and review this data with engineers and geologists located thousands of miles away. If a problem is detected, the rig can be safely and quickly shut down.
Drilling Rigs
There are many different types of drilling rigs. Which rig selected depends on the specific requirements of each drill site. Roll your mouse over each picture to see what kind of rig it is.
Land Based Drilling Rigs - The land-based drilling rig is the most common type used for exploration. This site is using a conventional, land-based drilling rig that is smaller and more efficient than those used in the past.
Slim Hole Drilling Rig - A conventional drill bore might be 18 inches in diameter; a slimhole bore can be as little as 6 inches. A slimhole well drilled to 14,760 feet may produce one-third the amount of rock cuttings generated by a standard well. The size of the drill site can be as much as 75 percent smaller, since slimhole equipment requires less space than conventional equipment. However, slimhole drilling is not technically feasible in all environments.
Coiled Tubing Drill Rig - Conventional wells are drilled using sections of rigid pipe to form the drill string. In some cases, coiled tubing technology can replace the typical drill string with a continuous length of pipe stored on a large spool. This approach has many benefits, including reduced drilling waste and minimized equipment footprints, so it is especially useful in environmentally sensitive areas. This technology is best suited to re-entering existing wells, and when multiple casing wells are unnecessary.
Jackup Drill Rigs – These rigs may be used in relatively shallow water -- less than 300 feet deep. A jackup rig is a floating barge containing the drilling structure that is outfitted with long support legs that can be raised or lowered independently of each other. The jackup, as it is known informally, is towed onto location with its legs up and the barge section floating on the water. Once at the drilling location, the legs are jacked down onto the seafloor, and then all three legs are jacked further down. Since the legs will not penetrate the seafloor, continued jacking down of the legs raises the jacking mechanism attached to the barge and drilling package, and slowly lifts the entire barge and drilling structure to a predetermined height above the water. These rigs are extremely strong, since they have to withstand ocean storms and high waves. These rigs are moved by simply by moving the legs up and down, which makes them cost-effective and easily shifted out of harm's way during storms.
Semi-Submersible Rigs – Drilling in water deeper than 300 feet demands some kind of floating platform to hold the rig. Semi-submersible rigs are floating vessels supported on large pontoon-like structures that are submerged below the sea surface. As with jackup rigs, the operating decks are elevated as much as 100 or more feet above the pontoons on large steel columns. This design has the advantage of submerging most of the area of components in contact with the sea and minimizing loading from waves and wind. Semisubmersibles can operate in a wide range of water depths, including deep water. Semi-submersibles can either be attached to the ocean bottom using strong chains and wire cables or may utilize dynamic positioning to remain stationary during drilling without anchors.
Drill Ship - For exploration targets farther offshore, specially designed rigs mounted on ships can drill a well in water depths up to 10,000 feet. These rigs float and can be attached to the ocean bottom using traditional mooring and anchoring systems, or utilize dynamic positioning to remain stationary during drilling without anchors.
Advanced Drilling Techniques
Oil and natural gas wells have traditionally been drilled vertically, at depths ranging from a few thousand feet to as deep as five miles. Today, advances in drilling technology allow oil and natural gas companies to reach more reserves while reducing environmental impact by:
reducing the surface “footprint” of drilling operations,
drilling smaller holes and generating less waste
creating less noise,
avoiding sensitive ecosystems, and
completing operations more quickly.
Here are some technologies used:
Horizontal Drilling - Horizontal drilling starts with a vertical well that turns horizontal within the reservoir rock in order to expose more open hole to the oil. These horizontal “legs” can be over a mile long; the longer the exposure length, the more oil and natural gas is drained and the faster it can flow. More oil and natural gas can be produced with fewer wells and less surface disturbance. However, the technology only can be employed in certain locations.
Multilateral Drilling - Sometimes oil and natural gas reserves are located in separate layers underground. Multilateral drilling allows producers to branch out from the main well to tap reserves at different depths. This dramatically increases production from a single well and reduces the number of wells drilled on the surface
Extended Reach Drilling - Extended Reach Drilling - Extended reach drills allow producers to reach deposits that are great distances away from the drilling rig. This can help producers tap oil and natural gas deposits under surface areas where a vertical well cannot be drilled, such as under developed or environmentally sensitive areas. Wells can now reach out over 5 miles from the surface location. Offshore, the use of extended reach drilling allows producers to reach accumulations far from offshore platforms, minimizing the number of platforms needed to produce all the oil and gas. Onshore, dozens of wells can be drilled from a single location, reducing surface impacts.
Complex Path Drilling - Complex well paths can have multiple twists and turns to try to hit multiple accumulations from a single well location. Using this technology can be more cost effective and produce less waste and surface impacts than drilling multiple wells.
Well Evaluation
Rock and fluid properties will determine how much oil and natural gas can be recovered from a reservoir. After an exploratory well has been drilled, it is evaluated to determine if there is enough oil and natural gas in the reservoir to make it economically feasible to initiate recovery operations.
Drill Cuttings and Core Samples - As the drilling mud is brought to the surface, it is run through a sieve to removed the drill cuttings (pulverized rock) before the mud is recycled down into the well. Small pieces of rock are selected for microscopic analysis to determine the type of rock being drilled, how porous it is, and whether oil is present. The drilling mud also is analyzed with sensors to see if trace amounts of oil or natural gas are present — an indication of a possible accumulation at depth. In the past, rock cuttings were the principal source of well information.
Well Logging - A special bit can be used to cut a cylindrical piece of rock that can be brought to the surface for analysis. The core is sent to a laboratory where the exact porosity and permeability can be determined. This gives a good indication of how well oil or natural gas would flow through the rock. Fluid samples can be taken and analyzed to determine the amount and type of hydrocarbon present in the rock.
Wells are completed for production if the value of the recoverable oil and/or natural gas is greater than the cost of drilling and producing them and delivering them to market. If not, the well is plugged In accordance with industry standards and federal or state requirements (depending on the location) and the site is restored.
Completion
Preparing a well for production is a complex process. Press the Play button to see each step in the process.
Step 1 - A pipe, called the casing, is lowered down the drilled hole. Sections of casing fit together just like the drill pipe. Cement is then pumped through the bottom of the casing so that it fills the area between the casing and side of the well. The casing prevents oil, gas and deep brines (underground salt water) from entering and contaminating aquifers (underground fresh water).
Step 2 - Because the casing and the liner must remain in a well for a long time and their repair or replacement would be costly, another string of pipe is placed in the well through which oil or gas is usually produced. This string of pipe is called "tubing". This is like a double-hulled tanker in that it provides an extra layer of protection for groundwater supplies. Tubing is pulled out of the hole on occasion and inspected to see if it needs to be repaired. (Casing is cemented in and can be repaired, but not as easily as the tubing).
Step 3 - Operators do not want anything but oil and natural gas to enter the well. To allow underground fluids to enter the pipe and flow to the surface, the tubing and casing must be perforated. Explosive charges are lowered to the precise depth of the oil reservoir. Detonating the charges forces holes in the casing. Fluids can then flow into the casing and up the tubing toward the surface.
Step 4 - A "Christmas Tree" is a device that is placed on the well at the surface. It regulates the flow from the well into the pipelines that take the oil and natural gas to facilities for processing and sale. It consists of a series of valves that are opened and closed to regulate flow for optimum field production or to shut down a producing well if a problem is detected. Some Christmas Trees have computer systems that allow them to be monitored, opened and closed remotely.
source :http://www.adventuresinenergy.org
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