Surveying is one of the most important stages in the production of hydrocarbons from the reserves. Some of the physical ways that are used to carry out the mapping of 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 the 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.
Application of seismic mapping
Onshore:Seismic mapping as explained below is not rocket science. It is carried out by sending out a perturbation that traverses the earth 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 refract into the formation. The sources 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 amount of vibration that emanates from the trucks is so strong but has no effect on the buildings and environment around it. Seismic surveys have been carried out in towns like Paris without affecting the buildings.The sent waves are therefore reflected back to the surface by underground formations and recorded by strategically placed geophones.
Offshore: The equipment used offshore is a seismic streamer that consists of cables that are submerged into the ocean. The cable has sources of seismic waves in form of compressed air that is used to sending out acoustic energies. Precaution is taken so as to ensure no harm befalls marine life. The waves are received by cables that have very sensitive hydrophones attached to it and the data is sent back to the ship. The travel time and the speed of the sent waves are raw data that is reconstructed by using very powerful computers to come up with an image of the covered sub-surface.
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 finally indicate anomalies in the formation which then helps identify where commercially viable reserves are located. This process can take up to seven months or more which can translate into hundreds of thousands of dollars.
Benefits of using Seismic mapping in the exploration of oil and natural gas
Application of seismic surveys in exploration has revolutionized the oil and gas industry in many ways. It has reduced the risk associated with drilling dry wells because of good planning that ensures that the well is drilled strategically in the best location.
This technology helps in reducing the cost and conserving the surroundings. After analyzing the map, the nature of the reservoir is used to determine if the well to be drilled will be vertical, horizontal or even multilateral. This eliminates the need for drilling extra wells in the same reservoir thereby reducing the costs further. The data that is accrued from the field is very precious. This is because such data when licensed can be used as leverage by companies and governments.
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 formed a 2D image that represented the subsurface formation. This is called 2D seismic mapping. This technology was quickly replaced by 3D mapping of data. By the year 1980, more than a hundred 3D surveys had been carried out all over the potential oil and gas zones. This has gradually improved with the incorporation of super computers.
Unlike in 2D where there is only one source of energy and one receiver, in 3D the energy sources and receiver phones are spread in strategic positions covering the area to be mapped. The raw data is then relayed to computers to be converted into a 3D image for analysis. 3D technology has a very wide application and can cover large areas.
When it comes to 4D mapping, the technology factors in the changes that occur during production. These changes include temperature, connate water saturation, reservoir drive mechanisms and pressure. To account for these fluctuations, repeated 3D seismic mappings are carried out over a period of time hence creating a trend or 4D seismic image.
In conclusion, seismic mapping technology is an indispensable way of studying subsurface oil and gas formations. The advancements and modifications to the technology keep growing every day with even drones being used to collect the reflected waves. The oil industry can only be so grateful to such a technology that goes a long way in minimizing dry wells and ensuring more reserves are found every day.