Seismic surveys are perhaps the most important of all surveys in oil prospecting and exploration. They can be used to obtain detailed information about underground structures and traps with an accuracy of up to ±5 meters.
Seismic surveys are based on the study of the propagation of elastic vibrations in the rock strata. The general scheme of research is as follows. At (or near) the surface, a sound wave is generated and propagated deep into the subsurface by an expanding sphere. At the boundaries of rocks occur various effects of refraction, reflection of elastic waves, which are registered on the surface of the ground with special devices. The data obtained are recorded, processed, and brought to a unified format. The result is an image of the geological structure in the study area.
Elastic waves are generated in one of the following ways:
- Impact with compressed air using an air cannon;
- Drop of a heavy load.
Seismic surveys can be conducted both onshore and offshore.
Seismic surveys have been attempted since the 1920s to find oil fields. Until the 1990s, only two-dimensional (2D) seismic was used. With the development of computers, it became possible to analyze a significant amount of data, which led to the development of three-dimensional (3D) seismic. Whereas 2D seismic imaging shows a cross section of the structure of the earth’s crust, 3D seismic imaging can reveal a three dimensional image. Three-dimensional seismic allows not only to find a deposit and estimate its size, but also helps determine the most expedient points for drilling wells.
3D seismic is now commonplace. It is used in almost all oil-producing countries. Many countries are now using 4D seismic to monitor and control reservoir development, which is essentially 3D seismic that is conducted on the same reservoir at regular intervals. This kind of research allows us to evaluate changes that occur in the reservoir during the development process.
Seismic surveys take a long time. It takes about 2 years from planning to obtaining the results of the study. The more complex the survey, the longer it takes and the more expensive it is. The feasibility of conducting seismic surveys is determined by comparing the cost of conducting them with the opportunity cost of drilling exploration and prospecting wells.
Seismics is an effective tool for finding oil and gas fields. At the same time, this tool has its limitations. Seismic surveys can provide quite accurate and detailed information on the geological structure, but cannot unambiguously answer the question about the presence of commercial oil (or gas) reserves in the identified structures. To assess the potential of a deposit, one must know parameters that seismic surveys cannot provide (e.g., reservoir pressure, formation permeability). Thus, even the most accurate seismic surveys do not obviate the need to drill wells to test reservoirs and confirm the presence of commercial oil reserves.
While neither 2D, 3D nor even 4D seismic completely eliminates geological risks, seismic surveys can increase success rates in subsequent well drilling, optimize well placement, provide higher productivity and noticeably longer run times. Although the cost of seismic surveys is high, the skilful use of surveys in oil prospecting and subsequent development yields good economic returns.