Natural Gas: Oil Byproduct, Valuable Resource

Natural-gas use is growing across all economic sectors. Natural gas burns cleaner than oil or coal, and this environmental benefit has encouraged its use. While decades ago natural gas was seen as an unwanted byproduct of oil and may have been wasted, its value has been recognized today. Most natural gas is distributed by pipelines, which is a limiting factor for remote resources that are not near the major consuming markets. But there is considerable development of technology to convert natural gas to liquids to enable more widespread transportation.For more information on shale gas and horizontal drilling, see Modern Sha le Gas: A Primer from the U.S. Department of Energy.

Largest Natural Gas Reserves by Country – 1 January 2009

New Exploration Methods for Oil and Gas

In the unrelenting search for more oil and gas, innovation plays an unquestionable role. As large oil and gas fields become increasingly difficult to find, geologists, geophysicists and engineers employ new technologies, such as seismic, to uncover resources that just 10 years ago were unimaginable. Seismic is a technology that bounces sound waves off rock formations deep below the surface of the Earth to provide explorers with a picture of the subsurface, often revealing locations where oil and gas may be trapped. The technology of finding oil has even incorporated 3D visualization tools from Microsoft’s Xbox game console! The system will help geoscientists examine and interact with 3D models of the Earth.

In order to process the massive amounts of information collected from seismic surveys, mathematicians, physicists and other scientists are constantly developing new computer algorithms to find complex patterns that enhance our understanding of the land beneath us. If we are to continue finding new fields hidden deep inside the Earth, breakthroughs in computer processing power and data management are necessary.

How Do We Get to the Oil?

The oil and natur al gas we use today have been trapped deep inside the Earth for millions of years. Although it is tempting to think of oil and gas reservoirs as large pools and wells with giant straws that suck the fluid to the surface, oil and gas is actually locked inside the rocks like water in a sponge. Just like the small holes in a sponge that collect and hold water, there are tiny spaces or pores in rocks that fill with oil and gas. For the past 100 years, oil and gas was extracted from rocks with small pores that were still big enough that the fluids flowed easily. If you were a tiny molecule of oil, flowing through these rocks would be like driving on a highway in the express lane. During this time period, geologists and engineers knew about other large quantities of hydrocarbons trapped in rocks with even smaller and more complex pores, but were unable to harness the resource—the oil and gas flowed too slowly or not at all from these rocks. Instead of driving on a large and fast highway, flowing through these rocks would be like driving on a small two-lane road with many stoplights and intersections. Conventional gas wells drilled into these formations were considered uneconomic since the gas locked in the rock would flow out of the tiny pores in the rock at such low rates. This picture changed, and changed in a big way, with the advent of stimulated horizontal wells.

Drilling Location

Before the technology advances of the past few decades, the best place to put a well was directly above the anticipated location of the oil or gas reservoir. The well would then be drilled vertically to the targeted oil or gas formation. Technology now allows the industry to drill directionally from a site up to 5 miles (8 km) away from the target area. Engineers can even target an area the size of a small room more than a mile underground! This directional drilling technology means that the industry can avoid placing wells in environmentally sensitive areas or other inaccessible locations yet still access the oil or gas that lies under those areas.

Drilling Process

In simplified terms, the drilling process uses a motor, either at the surface or downhole, to turn a string of pipe with a drill bit connected to the end. The drill bit has special “teeth” to help it crush or break up the rock it encounters to make a hole in the ground. While the well is being drilled, a fluid, called drilling mud, circulates down the inside of the drill pipe, passes through holes in the drill bit and travels back up the wellbore to the surface. The drilling mud has two purposes:

• To carry the small bits of rock, or cuttings, from the drilling process to the surface so they can be removed.
• To fill the wellbore with fluid to equalize pressure and prevent water or other fluids in underground formations from flowing into the wellbore during drilling.

Water-based drilling mud is composed primarily of clay, water and small amounts of chemical additives to address particular subsurface conditions that may be encountered. In deep wells, oil-based drilling mud is used because water-based mud cannot stand up to the higher temperatures and conditions encountered. The petroleum industry has developed technologies to minimize the environmental effects of the drilling fluids it uses, recycling as much as possible. The development of environmentally friendly fluids and additives is an important area of research of the oil and gas industry.

Even with the best technology, drilling a well does not always mean that oil or gas will be found. If oil or gas is not found in commercial quantities, the well is called a dry hole. Sometimes, the well encounters oil or gas, but the reservoir is determined to be unlikely to produce in commercial quantities.

Technology has increased the success rate of finding commercial oil or gas deposits with less waste and a smaller impact on the surface. While conventional oil and gas wells are typically vertical, contacting only a limited amount of the target reservoir rock, horizontal wells look like a large “L.” The long horizontal wellbore, sometimes more than 4,000 feet long, contacts a large portion of the productive reservoir. The surrounding rock formation is then hydraulically fractured to release the oil or gas trapped inside. In hydraulic fracturing, massive trucks pump thousands of gallons of fluid into the rock at very high pressures in order to force the rock to crack. These cracks are then propped open with sand to allow a highly conductive passage through which the oil or gas can flow.

In shale fields, as many as 15 major fractures are placed along the horizontal wellbore, serving to connect all those small two-lane roads to wide boulevards and even larger, faster highways. Currently, the limits of this technology are being pushed back every day in order to unleash giant gas resources. In the future, this technology will have to go even farther to allow more fractures and longer horizontal wells. Advances in this area will undoubtedly transform our energy landscape.

For more information on shale gas and horizontal drilling, see  Modern Sha le Gas: A Primer  from the U.S. Department of Energy.

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