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High-tech mapping helps pinpoint opportunities

Technology plays a major role in the exploration and production of natural gas. While new drilling technology has made it possible for shale to become a profitable source of natural gas, 3-D seismic imaging technology is helping companies like Southwestern Energy determine exactly where drilling should occur. Through the use of seismic imaging, locating natural gas now goes beyond the geologists' valued studies of the earth’s surface and into computer-aided exploration of what lies beneath. Seismic data has become an invaluable tool in our industry as it paints a realistic picture of the complexity and depth of the geology below. The development of seismic imaging in three dimensions has greatly changed the nature of natural gas exploration, and there are many benefits to this process. Exploration teams can now more easily identify natural gas prospects, place wells more effectively, reduce the number of dry holes drilled, reduce drilling costs and cut exploration time. This leads to both economic and environmental benefits that can be enjoyed by all—individuals, communities and companies. "This technology-based process aids the industry by providing information that helps us know where to drill. Sound waves and powerful computers can create three-dimensional maps of the many layers of rock formations," said Mike Rhodes, geophysical operations manager, Southwestern Energy. "Technology is constantly changing," Rhodes said. "We are now able to record more than 1,000 channels of data at a time. More data means we can image subtle targets with a more detailed subsurface picture. There is no doubt that seismic imaging adds value to our business."

What is seismology?

Seismology refers to the study of how energy, in the form of seismic waves, moves through the earth's crust and interacts differently with various types of underground formations. A device dating back to the 1850s which detects and measures the earth's vibrations - the “seismograph” - is probably the most commonly-known tool of seismology due to its use in analyzing earthquakes. Since the 1920s, seismic technology has also been used to help locate underground hydrocarbon reserves.

The seismic process

The process begins with planning and permitting. After geologists identify areas for potential drilling, various permits are obtained. Southwestern Energy utilizes contractors that specialize in seismology to perform the work. Seismic permit agents research surface and mineral ownership, contact the appropriate individuals and businesses that own the rights and secure required permission to conduct seismic activities on the land. Once necessary surface permission has been obtained and notice has been given, seismic survey teams enter the property and use global positioning system data to determine precisely where sound sources and detectors should be placed. Specialized rigs are used to drill holes approximately 20- to 40-feet deep. A small source, which generates seismic waves, is then placed in each hole. The type of equipment used varies, in part, by the terrain. In open fields, for example, faster drilling units can be used. In more forested areas, hand-held drilling equipment is required. Once all holes are drilled with sound sources in place, small listening devices called geophones are placed throughout the area to record the sound waves that will be reflected back up from the earth. Cables about the size of an outdoor extension cord connect the geophones. The cables and geophones are moved most efficiently by helicopter, which helps avoid additional vehicle traffic in the seismic process. The sources are triggered by a radio signal, and a mobile truck-mounted central recording unit collects the sound readings from all the geophones.

Processing

The collected data is stored on computer discs and processed using specialized software to generate images that are later interpreted by geoscientists. The processors reconstruct the geometry of the acquisition by entering the placement of each recording geophone and the location of each source on the surface of the earth. They must also know the precise elevation of each geophone in order to understand how far the signal has traveled and whether or not it travels through a hill or valley as the distance varies. This process must be performed for every seismic recording. A typical 3-D will document thousands of these recordings. Next, the processors calculate the distance the geophones are from the source. The phones farther away receive the signal at a later time. The seismic data is required to get multiple recordings over a point in the subsurface. These recordings are processed to move the data back to their correct space under the surface and added together to create a clear picture of what lies beneath the earth’s surface. The final product is a 3-D volume or data cube, which may be processed further for rock properties or converted into a more useable product for the interpreter. When the final data volumes have been processed, they are shipped off to the interpreting geoscientist to begin the interpretation process

Interpretation

Geophysicists utilize the processed images to create subsurface maps of the rock formations deep below the surface. The computer-assisted interpretation allows the geoscientist to visualize the images from multiple directions -- similar to standing on a hillside or looking across a valley and seeing the changes on the surface. The seismic is "tied" to wells already drilled through the use of synthetics created from an electric log that is recorded in an existing well. After a well has been drilled, and typically before it is completed to produce or abandoned, a geophysical tool is lowered into the wellbore on a cable connected to a recording truck at the surface. This tool records various geophysical rock responses and sends a signal back to the recording truck for processing. Once the recordings, or well logs, have been processed, they are used to determine the depth of the rock formation and whether or not it is productive. After existing wells are "tied" into the seismic, the geoscientist is able to map the surface of interest and determine which direction to place an additional well. This subsurface map enables the geoscientists to predict the depth of the target zone that will be encountered and if it will be high or low to the existing wells. In some areas, geoscientists can also use the seismic data to determine the quality of the rock at the drilling site. A rock with higher quality will produce at a higher rate and possibly longer, which equates to a better return on the investment. Not only can the presence of porosity be predicted in select cases, but in certain areas the presence of oil or gas can be directly detected. This technology helps the geoscientists increase the possibility of finding and producing oil or gas at the selected drilling location. Interpreting seismic data is a complex process and can take months to translate. The process requires the input of the entire technical team to ensure data from all different disciplines has been analyzed. But once complete, the team can determine how big the prospect is, where the leases should be acquired and where to drill the well.
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