Seismic and its contribution to the energy transition
The use of seismic recording, processing and interpretation dates back to the first decades of the 20th Century. From its beginnings, reflection and refraction of waves were used, produced by an artificially controlled source. Since then, seismic has been an important tool in the search and development of oil and gas fields. Entering the 21st Century, the exploratory working phases are diminishing; however, seismic is positioned on several fronts to continue being applied in a reasonable and useful manner to search for alternative energy sources, as well as to help mitigate the effects of greenhouse gases. In this article, we will outline uses that are currently given to the seismic workflow.
Seismic enables geophysicists to view the subsurface at depths of 8,000 meters (m) and more. Sources and receivers are used in the field. The natural vertical axis is double time (which is how long it takes for a wave to go from the source to a reflector and to be picked up again by a receiver or geophone). The energy that arrives is defined by its phase and amplitude, which allows estimating, together with the arrival time, the characteristics of the rocks and, many times, the fluid they contain.
The energy transition (ET) is intended to replace fossil fuels with low-carbon energy sources. This change tends to avoid emission of greenhouse gases (GHG), which is the main driver of the climate changes. There is no human activity that does not have an associated impact on the environment, and energy is no exception.
The study of the ET, and the role that innovation and technology play in the development of solutions, has gained greater relevance in scientific communities, as well as the evolutionary systems on the sustainability of energy systems. The ET is the path to decarbonizing the global energy system, transforming it into a net zero-carbon industry. The accelerated deployment of renewable energies, and the implementation of ET, will positively impact the development of countries if the appropriate technological solutions generate greater welfare for society.
Cooperative effort required. The energy sector has a vital role to play in addressing the major global challenge of climate change, as it is responsible for a substantial amount of GHG emissions. However, the industrial, transport, agriculture and livestock sectors also generate significant quantities of GHG. Therefore, if sustainability objectives are to be achieved, in order to contribute to the fight against climate change, a major transformation process must be undertaken to decarbonize the economy, i.e. seek new forms of energy that generate low or zero emissions of CO2, the main greenhouse gas.
It is clear that cutting-edge technologies will be the foundation for the successful implementation of ET, together with positive political decisions. The age of fossil fuels should not end when supply dwindles, but when clean and renewable energies can provide the necessary support for the world's energy demand. The study of energy ET and the role that innovation and technology play in the development of solutions for energy sufficiency and energy systems development has become increasingly relevant in development studies, innovation systems, sustainability transitions and energy systems communities. The energy sector must play a vital role in addressing the global challenge of climate change, as it accounts for the largest volume of GHG emissions.
GEOPHYSICS AND ET
Geophysics, with its recent technological advances, uses powerful information transmission and processing capabilities. These include genetic and segmentation processing algorithms, artificial neural networks, and segmentation, as well as ocean bottom node technology. These technologies, combined with remotely operated vehicles for offshore seismic ,and the detection of reservoirs with petrophysical conditions suitable for subsurface storage, will help achieve net-zero.
Geophysicists discovered there are many places to apply various geophysical disciplines to help aid in the energy transition. But we will limit our discussion to the use of seismic. Historically, seismic was used to find oil and gas. But the push for net-zero has dictated that seismic be used to incrementally help other industries better understand the subsurface. Seismic’s ability to accurately predict vertical and horizontal subsurface formations makes it an excellent tool to achieve superior results. We will also explore seismic uses for carbon capture, utilization and sequestration (CCUS), lithium and geothermal.
Seismic and CCUS. Humanity faces a crucial problem caused by anthropogenic CO2 emissions from energy production, transportation, large-scale agriculture and industrial production (mainly steel and cement). Carbon dioxide capture and sequestration (CCS) is one of the main tools to address this problem. Geosequestration is defined as the capture, transport, injection and long-term storage of industrial CO2 in deep geological reservoirs. Subsurface storage can be depleted hydrocarbon reservoirs and saline aquifers. Geophysicists must define the challenges in the subsurface to achieve large-scale sequestration/storage and understand which geophysical methods will provide the most efficient solutions to the issues.
Geophysicists should study the best possible techniques and their practical applications to identify geological storage sites and monitor them over the long term without creating new environmental issues and risks. The application of seismic in the characterization of the subsurface for use in the storage of CO2 is similar to methods used in traditional oil and gas fields. The primary use of seismic data has historically been in the delineation of the subsurface and establishing the nature, shape and size of the trapping structure. The methods and processes used in these workflows are well established and can be directly employed in the subsurface characterization of CO2 storage locations. The acquisition and interpretation of seismic data is likely to have a key role in the monitoring of the injected CO2, Fig. 1. The objective is to characterize the containment system of a basin.
To achieve this objective, the following must be considered: 1) Site selection; 2) reservoir characterization; 3) seal characterization; 4) structural characterization; 5) formation mineralogy; 6) fault characterization; 7) hydrodynamics; and 8) geophysics and geosequestration.
The objective of geophysical analysis is to evaluate Advanced Seismic to determine the location and movement of CO2 in the subsurface, Fig. 2.
A prior geophysical survey is required at sites being considered for storage. Going forward, this continues to be a major demand for geophysical monitoring and verification of CO2 injection during the life of the projects. As operators inject CO2 into the subsurface, there will be a requirement to image and monitor the evolution of the CO2 plume to ensure that it is injected at the correct locations and depths. In addition, it must be verified that the reservoirs are being filled efficiently, as predicted, and that CO2 does not interfere with other sources of production from aquifers, hydrocarbons and/or geothermal. It must also be monitored to ensure that CO2 does not flow into high-risk areas, such as major faults with uncertain seismic and flow properties, and that it remains sealed in the reservoir over time.
Due to the complexity of CO2 and geological structures, achieving these objectives requires combining seismology time-lapse (4D) subsurface monitoring techniques with complementary geophysical methods, such as electromagnetics gravity, along with satellite imaging and interferometrics. The 4D time lapse seismic involves repeating seismic recordings with active sources at different times or continuous seismic recording of microseismic sources and ambient noise data. The idea is to image and estimate the changes of physical properties in the subsurface over time. The ability to record changes over time in the field data depends on the magnitude of the signal and the level of noise in the data.
The power of the seismic signal produced by changes in physical variables will depend on these cases of CO2 injection into reservoirs with varying pressure and temperature and the changes in properties of reservoir rocks saturated with the mixture of fluids with CO2 after injection.
Seismic and geothermal energy. As climate change is apparently occurring at an accelerated rate, geothermal energy is one of the best resources to help offset GHG emissions. The idea of harnessing the earth's natural heat to produce geothermal energy is a method that is being used more often. Generally, geothermal systems require heat, permeability and water to be viable as energy generators. However, utilizing new techniques, it is now possible to generate energy with only heat. Water can be injected into the subsurface at elevated pressures, and permeability can be enhanced by hydraulic fracturing.
It is then possible to apply typical seismic characterizations of sandstones, similar to oil and gas studies. Seismic interpretation, in this case, also plays a major role. In order to be an economically profitable project, a detailed reservoir characterization is usually required to calculate clay volumes, porosity and permeability of the reservoir.
This means that information from wells, pre-stack seismic and geology must be integrated to obtain reservoir facies and properties. In this case, it would be advisable to study candidates where there is good porosity and permeability, the presence or absence (depending on the case) of faults and fractures, enough high temperature and structural knowledge of the reservoir.
The study can be done with 2D or 3D seismic. If the system to be developed is shallow, and the seismic is the traditional one used in hydrocarbon prospecting, it can be designed and processed to improve the fold at greater depths. In addition, simultaneous seismic inversion can be performed to improve the results.
The type of work will be closer to reservoir production than exploration. The extractive system resembles some cases of those used in secondary recovery (injecting water of lower temperature in a horizontal or vertical well to extract it at higher temperature in another) and take it to a plant where this change is exploited, Fig. 3.
Lithium seismic and exploration. As a result of new energy generation requirements, the world market has increased focus on technology, climate awareness, Covid-19 and the variation in price of oil and natural gas. There are other energy markets, such as the exploration and production of lithium, rare earths and critical minerals.
In the Argentine Republic, there are more than 30 salt flats within the so-called Lithium Triangle in different stages of development. Some are exploited and currently produce lithium carbonate and chloride from brines from Salar del Hombre Muerto Oeste and Salar de Olaroz. Other projects are in feasibility and construction stages. Brine is a natural solution hypersaturated in calcium, potassium, sodium and lithium, among other chemical elements, from which lithium carbonate can be manufactured with a simple industrial process.
From the geophysical prospecting point of view, the problem of searching for lithium deposited in salt flats is the limit of the physical response, since prospectively it presents conditions of low electrical resistivity, low acoustic impedance and much variability in density. There is little availability of subsurface information, and the factors that enrich the value of field data and subsequent image reconstruction are extremely useful. The different geophysical methods have strengths and weaknesses in the problem of lithium exploration, and their response can be studied through case studies. A workflow is needed from acquisition planning to dynamic model construction.
The exploration for lithium with reflection seismic could be performed through a 2D or 3D reflection seismic acquisition. The objective is to improve the geological model, followed by the drilling of exploratory wells. One of the possibilities is that the sedimentary fill is composed of halite levels intercalated with clastic sediments, which constitute the reservoirs of the lithium mineralized brine, Fig. 4.
The goal of the prospect is to determine the depth of the basement of the salt flat. Also, an attempt could be to identify the internal geometry and the continuity of the layers. In the example shown for this purpose (Fig. 4), 49 km of 2D seismic lines were recorded in 11 lines with 2,478 shots, along the length and width of the salt flat, covering even the dejection cones at the edges. Parameters were used to allow both adequate spatial and temporal sampling, using a vibrator as an energy source and telemetric recording equipment.
Processing consisted of noise attenuation, static corrections, interactive velocity analysis and pre-stack migration (PSTM). The processed lines have a spectral content dominated by high frequencies (spectral content greater than 40 Hz) with good image quality. The seismic was able to determine the depth of the basement, the main faults that limit the edges of the salt flat and internal minor direct faults, Fig. 5.
In this review, we have listed several studies where reflection seismic will continue to be useful, not only in the oil and gas industry, but also in other fields of alternative energies (lithium searching and geothermal) and even in the determination of areas for carbon sequestration and subsequent monitoring. In addition, a reinvention of an already centenary technique is still relevant and continues to be one of the geophysical techniques that provides more structural and stratigraphic studies to solve current problems. Seismic also has potential applications in medicine, applications to smart cities, geohazards and nuclear sites waste.
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