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Geological Expertise in Oil and Gas Exploration

(Graphic from Oil & Gas Portal)

Geologists use a variety of tools to discover underground pockets of crude oil and natural gas. Without their expertise, the oil and gas industry would not exist.

Exploration

The first thing geologists must determine is the location of geological formations that can trap oil and gas underground. They do this by determining what kind of sedimentary rocks form the reservoir and what kind of chemical elements are present in the rocks. If sandstones and carbonates are present, this is a good indication that ancient organic matter once existed in that area which decayed and formed hydrocarbons. The hydrocarbons became trapped underground in the form of crude oil and natural gas (Busby, 1999, p. 15).

Geological methods include drawing maps of the surface and subsurface region and gathering rock samples.

Topographical maps in 2-D and 3-D visualize layers of rock and the horizontal and vertical placement of those layers. Rock formations are given a two-part name, the geographical location, which is usually the name of the nearby town, and the predominant type of rock (Busby, 1999, p. 19).

Subsurface maps include three elements: structural, which shows the elevation of rock layers; isopach, which indicates thickness; and lithofacies, which reveal variations in a single layer of rock (Busby, 1999, p. 20).

When geologists take rock samples, they extract samples from the core and gather “cuttings” (rock chips) for “assessing the formation’s lithology, hydrocarbon content, and ability to hold and produce gas” (Busby, 1999, p. 20). If they can figure out how the rock layers were formed, they can determine if the conditions were right for “the generation, accumulation, and trapping of hydrocarbons” (Busby, 1999, p. 21).

Geochemical methods use chemical and bacterial analyses of soil and water samples from the surface and the area around underground gas and oil deposits to determine the presence of hydrocarbons. “Micro-seeps” of petroleum can be detected in this way (Busby, 1999, p. 21).

Vitrinite reflectance uses a reflectance microscope to measure the percentage of light which is reflected from vitrinite (plant organic matter found in shale). The percentage can indicate the presence of gas and oil (Busby, 1999, p. 21).

Geophysical methods use sound waves (seismic vibration) to “determine the depth, thickness, and structure of subsurface rock layers and whether they are capable of trapping natural gas and crude oil” (Busby, 1999, p. 22). Computers are used to gather and analyze the data. Geologists can now use 2D, 3D, and 4D seismic imaging in their analysis (Natural Gas, 2013).

On land, explosives and vibrations are used to generate sound waves. The energy that bounces off the rock layers is detected as echoes by sensors called geophones (jugs) (Busby, 1999, p. 23).

Bright spots and flat spots can reveal where deposits of gas-oil and gas-water deposits might exist underground. Amplitude variation with offset (AVO) and geology related imaging programs (GRIP) can enhance the resolution and analysis of bright spots (Busby, 1999, p. 24).

“Cross-well” seismic technology uses seismic energy in one well and sensors in nearby wells to retrieve high-resolution images that have been used successfully in determining the presence of crude oil. It is now being used in natural gas exploration (Busby, 1999, p. 24).

Gravity meters are used to detect salt domes and other rock formations capable of trapping gas and oil. Magnetometers detect the thickness of basement rock and find faults. Computer models create hypothetical pictures of subsurface structures from mathematical computations (Busby, 1999, p. 25).

Drilling

Once geologists determine the geological and economic feasibility of drilling a well, a group of geologists, geophysicists, and engineers pinpoint the site for the well and its potential reservoir. They decide how deep the well should be. The average well is about 5,800 feet deep in the United States. The drilling company must then get permission to drill from the owners of the land and determine who owns the mineral rights.  They sign a lease to use the land for a certain length of time.  The drilling company then breaks ground (spudding) and keeps well logs (measurements) to determine the possibility of gas and oil formation and the porosity and permeability of the rock. The contractors who own the drilling rigs sign an agreement to drill to a certain depth and detail what equipment they will need. A pit is dug at the site and lined with plastic that holds unnecessary materials (Busby, 1999, p. 29-30).

Rotary drills, driven by a diesel engine, are the most common type of drill used because they can drill hundreds and even thousands of feet per day. The drill bit must be changed after 40 to 60 hours of drilling. Other drilling techniques include directional drilling, which allows drilling in multiple directions, horizontal drilling, which is used to enhance gas recovery and to inject fracturing fluids, and offshore drilling, which uses special equipment to drill in ocean water (Busby, 1999, p. 31-35).

Some of the drilling problems that come up include drilling a dry hold; a breakage inside the well; things falling into the well; and high pressures underground causing gas or water to flow into the well, changing the balance of the pressure in the well. Drilling must be halted then and the problem corrected (Busby, 1999, p.33).

Geologists use various tests to measure the probability that the well will produce enough oil and gas. Drilling-time measurements measure the rate of the bit’s penetration into the rock; mud logs measure the chemistry of mud and rock cuttings, looking for traces of gas; wireline logs sense electrical, radioactive, and sonic properties of rocks and fluids; electrical logs test rock for resistivity; gamma ray logs measure radioactivity; neutron logs measure rock density; caliper logs test the type of rock; dip logs look for the placement of rock layers; sonic/acoustic velocity logs measure the speed at which sound travels through rock. Traditionally, these tests were conducted on bare, uncased wells. But new technology allows testing to be done with the casing in place (Busby, 1999, p. 36-37).

If the well comes up dry, the well is plugged up and abandoned. If the well holds promise of a productive well, the bare well is “cased, or lined with metal pipe to seal it from the rock” (Busby, 1999, p.29). A foundation of cement is created. Then the casing is drilled with holes so gas can flow into the well. The flow rate of the gas is measured, and if productive, valves and fittings are installed in order to control the flow. Oil and gas products are separated at the wellhead. A gathering system is built after several wells are completed. Flow lines gather gas from several wells and transport it to a centralized processing facility (Busby, 1999, p. 37-38).

Transmission

“The pipeline industry carries natural gas from producers in the field to distribution companies and to some large industrial customers” (Busby, 1999, p. 43) through large pipes with high pressures, from 500 to 1,000 psi or 3,400 to 6,900 psi). Compressor stations along the lines maintain the pressures in the pipes. “As of the 1990s, more than 300,000 miles of gas pipelines criss-cross the United States, serving nearly 60 million gas customers” (Busby, 1999, p. 43).

Pipes are laid in trenches and coated inside with chemicals to prevent corrosion, improve light reflection, reduce water retention, reduce absorption of gas odorants, and to improve gas flow (Busby, 1999, p. 46-47).

Gas demand depends on weather, the season, and its use in power generation. Pipeline operators try to spread the costs over the whole year. Gas meters are used “to reduce costs and increase the accuracy of gas flow measurement” (Busby, 1999, p. 51).

Pipeline inspection and maintenance have to be done on a regular basis to detect gas leaks, address corrosion, repair damage, and to keep the gas flowing smoothly (Busby, 1999, p.  51-53).

Economic Concerns

During exploration, there is no guarantee that all the money spent on research, testing, and drilling will be recouped. If a well is productive, royalties must be paid to the owner of the mineral rights after all production costs are paid. State and federal governments regulate how many wells can exist per 640 acres and how much gas and oil can be produced over a certain time period. Offshore drilling, which has become more common, is very expensive because these oil rigs use special equipment and can drill as deep as 10,400 feet. When wells are losing pressure and running dry, companies must spend money on well stimulation. In fact, companies give priority to this because it costs less than exploring for new wells. It’s been estimated that the oil and gas companies spend roughly $5 billion on treating natural gas before it is ever transmitted through a pipeline. Pipelines and compressor stations must be built, inspected, repaired, and maintained. Environmental regulations cost companies money on research and new technologies (Busby, 1999, p. 15-54).

If oil and gas supplies diminish or are suddenly cut off, access to energy is decreased, and costs sky-rocket. When pipelines break or oil rigs are damaged or destroyed, this causes a disruption in the oil and gas supply. If the disruption lasts long enough, it can raise costs to the consumer. Political conflicts affect oil and gas supplies, energy costs, and the ability of companies to find new sources (Busby, 1999, p.15-54).

Dawn Pisturino

Thomas Edison State University

October 22, 2020; March 18, 2022

Copyright 2020-2022 Dawn Pisturino. All Rights Reserved!

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

Natural Gas. (2013). Natural gas and the environment. Retrieved from

       http://www.naturalgas.org

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