Surveying is one of the most important stages in the production of hydrocarbons from reserves. Some of the physical ways that are used to map underground formations include seismic, magnetic, electrical and electromagnetic methods. Seismic mapping is the revolutionary technology that has withstood the test of time to become the primary tool of exploratory geophysics for many companies.
When it comes to mapping the physical properties of both onshore and offshore sub-surface formations, the uncertainties and possibilities that can occur are endless. The variations in the structure of the different geographical formations in different parts of the world can best be unraveled by the use of this mapping technology.
Onshore Seismic Mapping
Seismic mapping as explained below is not rocket science. It works by sending out a perturbation that traverses the earth's layers in a planned trajectory. The intensity of this wave is dependent on various factors, the most significant ones being the amount of energy from the source and the acoustic impedance of the formation.
The principle behind the application of seismic waves is that when the wave hits an interface of two formations that have different impedances, some of the energy will be reflected back to the surface while some will refract into the formation.
The source of the seismic wave can be from a controlled explosive charge arranged a few feet from the surface or from a special truck called a vibroseis. The waves are reflected back to the surface by underground formations and recorded by strategically placed geophones.
The vibration that emanates from the vibroseis trucks is very strong but has no effect on nearby buildings or the environment. Seismic surveys have been carried out in towns like Paris without affecting the buildings.
Offshore Seismic Mapping
The equipment used offshore is a seismic streamer that consists of cables that are submerged in the ocean. The cable produces seismic waves in form of compressed air that is used to transmit acoustic energies. Precaution is taken so as to ensure no harm befalls marine life.
The waves are received by cables that contain very sensitive hydrophones and the data is relayed back to the ship. The travel time and the speed of the transmitted waves are raw data that is reconstructed using very powerful computers to produce an image of the underwater sub-surface. (Discover more uses for data in How Big Data is Transforming the Oil & Gas Industry.)
Processing Phase of Seismic Mapping
After the raw data is collected, the data goes through various computerized steps such as stacking and filtering to ensure the data can be interpreted. It is the duty of the geophysicist to interpret the image and compare the data with rock samples from the area and previous correlations or log analysis. This mapping and analysis eventually indicates anomalies in the formation that helps to identify where commercially viable reserves are located. The process can take up to seven months or more, which can translate into hundreds of thousands of dollars.
Benefits of Using Seismic Mapping for Oil and Natural Gas Exploration
The application of seismic surveys in exploration has revolutionized the oil and gas industry in many ways. It has reduced the risk of drilling dry wells by facilitating good planning that ensures the well is drilled strategically in the best location.
This technology helps to reduce the cost and preserve the surrounding environment. After analyzing the map, the nature of the reservoir is used to determine if the well should be drilled vertical, horizontal or even multilateral. This eliminates the need to drill extra wells in the same reservoir thereby reducing the costs further. (Related reading: Well Spacing - Challenges and Opportunities.)
The data that is accrued from the field is very valuable because such data, when licensed, can be used as leverage by companies and governments.
Understanding the Difference between 2D, 3D and 4D Seismic Mapping Technology
When seismic mapping was carried out along a line in the ground, the analysis of the data produced a 2-dimensional image that represented the subsurface formation. This is called 2D seismic mapping. This technology was quickly replaced by 3D mapping. By 1980, more than a hundred 3D surveys had been carried out over potential oil and gas zones. This process has gradually improved with the use of supercomputers.
Unlike 2D seismic mapping where there is only one source of energy and one receiver, in 3D mapping the energy sources and receiver phones are spread out over strategic positions covering the area to be mapped. The raw data is then relayed to computers and converted into a 3D image for later analysis. 3D technology has a very wide application and can cover large areas.
4D mapping technology factors in the changes that occur during production. These changes include temperature, connate water saturation, reservoir drive mechanisms and pressure. (Learn more about drive mechanisms in the article Understanding Reservoir Drive Mechanisms.) To account for these fluctuations, repeated 3D seismic mappings are carried out over a period of time, thus creating a trend or 4D seismic image.
Seismic mapping technology is an indispensable way to study subsurface oil and gas formations. The advancements and modifications to the technology keep growing, such as using drones to collect the reflected waves. Society and the oil industry benefits from this technology, which goes a long way to minimize dry wells and ensure more reserves are discovered every day.