The Basics of Gas Exploration, Production, and Distribution

by Dawn Pisturino on August 29, 2021

The Basics of Gas Exploration, Production, and Distribution

Gas and oil traps are formed by geological events such as tectonic plate shifting, glacier movement, and extreme temperature changes. As long as the gas cannot escape from the area, it will be trapped in place (Blewett, 2010).

When reservoir rock is subjected to high pressure and other conditions, it can become fractured or deformed, creating a space that can fill up with oil or natural gas. Anticlines are structural traps which occur when layers of rock are pushed upward, causing an arch. Synclines occur when the rock is pushed downward. Domes are similar to anticlines but have a more rounded appearance (Busby, 1999).

Faults occur when rocks crack due to outside forces and sections, or plates, slide out of alignment. Sections of rock can slide upward (dip-slip) or sideways (strike-slip). Thrust faults appear on the earth’s surface as mountain ranges. Fractures can divide traps into smaller compartments and increase the permeability of sedimentary rocks. Shales and chalks are normally porous and impermeable. When fracturing occurs, it can make these rocks more permeable, making it possible for gas to get trapped inside the rocks (Busby, 1999).

Stratigraphic traps are harder to access than structural traps because the gas and oil have been trapped within the layers of rock. These traps form as the result of changes in the porosity and permeability of the rock due to the way in which sediment has been deposited. Gas cannot escape the rock. Large fields of gas and oil can be trapped in this way (Busby, 1999).

Combination traps have the characteristics of both structural and stratigraphic traps. A salt dome occurs when a large quantity of salt gets trapped in sedimentary layers and breaks through the earth’s surface, forming “a plug-like structure” (Busby, 1999).

Carbonate rock reservoirs formed when ancient caves collapsed, causing fractures in the rocks. A new cave system was created, forming a reservoir for gas and oil to be trapped inside (Busby, 1999).

In order for any oil or gas field to be productive, there must exist the right combination of “reservoir rock, trap, and cap rock or other seal” (Busby, 1999). There must be “source rock that has generated gas or oil, reservoir rock to hold the gas, a trap to seal it off, and the right timing” (Busby, 1999). Without a trap in place, the gas will disperse out of the area (Busby, 1999).

The largest producing gas fields in the United States are as follows:

Marcellus Shale is an unconventional shale formation which stretches beneath two-thirds of Pennsylvania and parts of New York, Ohio, West Virginia, Maryland, Kentucky, and Virginia. This area is estimated to hold 6 trillion cubic feet of natural gas. The most productive wells lie 5,000 to 8,500 feet below the earth’s surface (Pennsylvania Department of Environmental Protection, 2020).

This natural gas can only be accessed through vertical and horizontal drilling and the use of hydraulic fracturing (fracking). The Pennsylvania Department of Environmental Protection inspects and monitors these wells “from construction to reclamation to ensure that the site has proper erosion controls in place, and that any waste generated in drilling and completing the well was properly handled and disposed. Also, unconventional well operators are required to submit a variety of reports regarding well drilling, completion, production, waste disposal, and well plugging” (Pennsylvania Department of Environmental Protection, 2020).

The Newark East gas field in Texas is composed of Barnett Shale. Currently, 5,600 wells and 150 rigs are in operation. The field is estimated to hold 1,951 billion cubic feet of natural gas (Geo ExPro, 2007; Oil Price, 2015).

The B-43 Area in Arkansas is estimated to hold 1,025 billion cubic feet of natural gas, but not much other information was available (Oil Price, 2015).

The San Juan Basin is found in Colorado and New Mexico. It is estimated that the field holds 1,024 billion cubic feet of natural gas. Not much other information was available (Oil Price, 2015).

The Haynesville/Bossier Shale formation is located in eastern Texas and western Louisiana. The natural gas is found at depths greater than 10,000 feet below the earth’s surface. The area is producing 2,680 million cubic feet per day of natural gas and 420 barrels per day of condensate (Railroad Commission, 2020).

The Pinedale gas field in Wyoming is the sixth largest gas field in the United States. It covers 70 square miles. Its layers of sandstone are 6,000 feet thick and form a 30-mile anticline. Operators use horizontal drilling to access the natural gas. In 2015, it produced 4 million barrels of gas condensate and 436 billion cubic feet of natural gas. Its gas reserves hold 40 trillion cubic feet of natural gas—enough to provide energy to the entire country for 22 months (American GeoSciences, 2018).

The Carthage natural gas field near Carthage, Texas produced 13,912,377 million cubic feet of natural gas in June 2020. Not much other information was available (Texas Drilling, 2020).

The Jonah field is located south of Pinedale, Wyoming. It covers 21,000 acres and is estimated to hold 10.5 trillion cubic feet of natural gas. Chevron is one of the energy companies involved in both Jonah and Pinedale (Wyoming History, 2014).

The Wattenberg field covers 180,000 acres in Colorado. Horizontal wells are drilled to access the natural gas. It has a complicated geological structure due to “crustal basement rock weakness [ caused by super-heated] organic Niobrara source rocks” (PDC Energy, 2020).

Prudhoe Bay in Alaska has been producing oil and gas for 40 years. It covers 213,543 acres and holds 46 trillion cubic feet of natural gas (NS Energy, 2020). Pump station 1, at the beginning of the Trans-Alaska Pipeline, is situated within the Prudhoe Bay field. The natural gas is held in place by “an overlying gas cap and in solution with the oil” (Department of Environmental Conservation, 2020). The Alaska LNG project will be using natural gas from the Prudhoe Bay field to produce liquefied natural gas (NS Energy, 2020).

The most common technique for drilling wells is rotary drilling because “it can drill several hundreds or thousands of feet in a day” (Busby, 1999). A long piece of steel pipe with a drill bit on the end is suspended from a rig and driven into the ground by a diesel engine. The rotating bit drilling into the earth “creates the wellbore or borehole” (Busby, 1999). The bit must be changed after 40 to 60 hours of drilling.

Directional drilling is being used more commonly now to access oil and natural gas in unconventional traps (tight formations). Rotary rigs can now drill in many different directions to reach gas in multiple areas, drill offshore, or drill under populated areas (Busby, 1999).

Horizontal drilling can increase the recovery of natural gas “from a thin formation . . . a low-permeability reservoir . . . isolated productive zones . . . by connecting vertical fractures . . . prevent production of excessive gas or water from above or below the reservoir . . . to inject fracturing fluids” (Busby, 1999).

Offshore drilling is more expensive because the average rig drills down to around 10,400 feet. “An offshore exploratory rig must be able to move across the water to different drilling sites” (Busby, 1999).

Drilling barges are used in shallow waters. Jack-up rigs can be raised or lowered and drill down to a depth of 350 feet. A semisubmersible platform floats on pontoons and anchors at the drilling site. These platforms can drill down to 2,000 feet. Drill ships float over the drill site and can drill down to almost any depth (Busby, 1999).

Once a productive field has been discovered, a fixed or a tension-leg platform is permanently anchored at the site. The legs on fixed platforms can be anchored with piles driven into the ocean floor; whereas, tension-leg platforms float above the field and are anchored by “steel tubes connected to heavy weights on the sea floor” (Busby, 1999).

Drilling a dry hole can be one of the biggest expenses associated with drilling wells. More common issues include something breaking inside the well or objects falling into the hole. Drilling must then be stopped and the problem corrected (Busby, 1999).

Pressures become higher as the rig drills deeper. When this occurs, gas and water “can flow into the well, dilute the drilling mud, and reduce its pressure” (Busby, 1999). When the flow of fluids is uncontrolled, this is called a blowout.

“Natural gas is produced from most reservoirs by expansion, where the pressure of the expanding gas underground forces it into the well” (Busby, 1999). When the pressure drops in the well, gas production decreases. It can be stimulated with the use of a compressor (Busby, 1999).

Once the gas has been purified and processed, it is transported through pipelines from the gas field to distribution companies and industrial customers. Compressor stations along the line maintain the pressure needed to keep the gas flowing smoothly through the pipe. The gas flow is measured at the beginning and end of each pipe section, at each compressor station, and each intersection where the pipe branches off into two pipelines. Large industrial customers receive natural gas directly to their facilities, which “requires high-volume meters” (Busby, 1999).

Economically, it is essential to measure natural gas flow accurately at all points of the supply chain because “an error of 1% in measuring 300 million ft3 of gas per day can lead to a difference of about $2 million per year” (Emerson, 2016). After all, customers pay for the amount of energy delivered.

Differential pressure (DP) meters “measure volumetric flow through a calibrated orifice (generally a plate), are inexpensive, and simple in concept” (Emerson, 2016). Measurements must be corrected for density (mass), temperature, pressure, and gas composition. DP meters are not as acceptable as more advanced technologies (Emerson, 2016).

Ultrasonic meters measure volumetric flow rates by measuring “speed and sound in the gas” (Emerson, 2016). They have an accuracy of 0.35% to 0.5%. Some are available with an accuracy of 0.25% (Emerson, 2016).

Coriolis meters “measure mass flow and density” (Emerson, 2016) but temperature, pressure, and gas composition still need to be measured. These meters tend to be rather expensive (Emerson, 2016).

Flow computers “measure, monitor, and may provide control of gas flow for all types of meters” (Emerson, 2016). They record data from volumetric flow measurement, temperature, gas composition, and density in order to calculate flow rate. Every calculation is dated and timed (Emerson, 2016).

Shale gas is usually composed of less than 50% methane and roughly 50% of ethane, propane, butane, pentane and other gases. CO2, H2S, and sand can also be present. DP meters are excellent meters to use at the gas field site and when impurities are removed from the gas (Emerson, 2016).

Once the natural gas has been purified of water and CO2, the natural gas is processed through liquid separators and H2S separators. At this point, a Coriolis meter or ultrasonic meter is used (Emerson, 2016).

Ultrasonic meters are generally used on transmission pipelines, while Coriolis meters are used on distribution lines. To accurately calculate the Btus (British thermal units) per pound, a gas chromatography device is used (Emerson, 2016). One Btu equals “the energy released by burning a match” (U.S. Energy Administration, 2020).

Dawn Pisturino

Thomas Edison State University

October 30, 2020

References

American GeoSciences. (2018). The pinedale gas field, wyoming. Retrieved from

https://www.americangeosciences.org/geoscience-currents/pinedale-gas-field-wyoming.

Blewett, R.L. (Ed.) (1999). Shaping a Nation: A Geology of Australia. Canberra: Australia

National University.

Busby, R.L. (Ed.). (1999). Natural Gas in Nontechnical Language. Tulsa, OK: PennWell.

Department of Environmental Conservation. (2020). Prudhoe bay fact sheet. Retrieved from

https://www.dec.alaska.gov/

Emerson. (2016). Selecting flow meters for natural gas fiscal measurement. Retrieved from