What Is Seismic Acquisition About?
Today many oilfields in the country are experiencing a decline in output. In such a situation, the exploration of greenfields and a detailed appraisal of brownfields become a must, although such surveys are quite expensive and pay back only after several years.
What Does Seismic Exploration Looks For?
According to the oilmen, seismic exploration helps significantly cut exploration drilling costs, as it is one of the most informative geophysical methods for studying the Earth’s crust. The method implies the generation of elastic waves that propagate into the Earth stratum, reflect from the interfaces of different rock, before refracting and partially returning to the surface. Studying the wave propagation time, the amplitude and then interpreting the collected data makes it possible to draw a conclusion about the depth and shape of geological boundaries, about the properties of a rock, and the formation of fluids.
Thus, seismic exploration allows to peek deeply into the Earth’s crust, looking for productive formations at a depth of thousands of meters. Exploration drilling is the only conditional alternative. It is conditional firstly because drilling provides information about a rock only near the wellbore area. Secondly, it is conditional because the cost of one well is comparable to the amount required to thoroughly study the target area using seismic methods.
However, it is necessary to understand that seismic surveying does not guarantee 100% accuracy in data collection and is not a substitute of subsequent exploration drilling. Yet, it helps to ensure maximum productivity in drilling, minimizing the number of dry wells.
Relying on their experience, oil companies are confident that seismic exploration costs will return with a vengeance, because the acquired information will help to plan deep drilling with higher quality and efficiency, allowing significant savings at this stage. This is the reason why no major company will ever start drilling until it has seismic data.
Irina Didenko, chief specialist at the Tyumen Petroleum Research Center, says that today there is a trend of companies starting exploration drilling only after they have conducted a thorough study of the seismic data. It is more profitable to invest in the improvement of the quality of seismic data.
A source of elastic waves is needed for seismic exploration. Elastic waves can be produced by blasting dynamite or TNT inside wells. The alternative option is a no explosion method (for example, the application of special vibration machines). The latter offers several advantages over explosive methods, as it requires no long and expensive preparation and remediation work, has a weaker environmental impact, and is safer for personnel. In addition, it can be repeatedly used at the same point.
On the other hand, the vibration source produces a smaller power than an explosion source. It is impossible to use poweful vibrators in remote areas such as forests, swamps, etc.
Surface pulse sources are also used in addition to vibration sources.
In offshore exploration, air guns producing short intensive acoustic signals are used to generate elastic waves. Air guns are towed by a special vessel.
One of the methods used to increase efficiency in seismic exploration is by increasing the number of signal sources in an area (surely, in this case non-explosive sources are implied). For example, sometimes several grouped vibrators are used. The stronger composite impact increases the power of propagating oscillations, and the process becomes continuous as, while one group of machines vibrates, the other group moves to another location.
However, the number of sources can be increased only in an open area – in steppe, desert or in the tundra. A larger number of source points has a stronger environmental impact, raising concerns from environmentalists, especially in case of offshore seismic exploration.
Taking Along a Seismic Receiver
The second indispensable element required for seismic exploration is a receiver of reflected or refracted waves. A seismic receiver converts the mechanical oscillation of elastic waves into an alternating current. The signals collected by seismic receivers (there can be hundreds of them) are transmitted to seismic stations. Oscillation charts are integrated to produce primary field data.
Today induction seismic receivers are the most commonly used. Under the impact of elastic waves, the body of the device moves inducing EMF in the induction coil. Piezoelectric geophones are used in offshore exploration. These geophones convert mechanical oscillation energy into electric energy. Multicomponent accelerometers have been recently used as seismic receivers.
Increasing the number of recorders is another way of making seismic exploration more informative. Its advantage is that it is applicable in any conditions, including remote areas. It is especially important in areas where it is impossible to increase the number of vibration sources. In any case, it is necessary to increase the density of seismic data to suppress noise and meet the demanding requirements on the quality of seismic information.
Data Processing and Interpretation
Acquisition of field data is just the first stage of the exploration. Its final result to a large degree will depend on the quality of the processing and interpretation of data. Seismic records that reflect the received oscillations on a time scale should be transformed into an image showing the depth of different rocks. Models of the areas being studied are produced based on generalized primary field data. Then, these models are correlated with real geological boundaries.
Irina Didenko explains: "If we are confident in the accuracy of the produced model we will be able to predict reservoir properties. A 3D model gives an idea about the approximate distribution of these properties in the reservoir."
This is the stage that offers the best opportunities for improvement of the seismic exploration method. Developments in information technologies allow to create modern equipment and software that make it possible to collect and process huge amounts of data in highly productive fashion. Sometimes re-interpretation of previously collected data allows oil companies to add significant reserves.
Artificial earthquakes were first employed for scientific purposes in the middle of the 19th century. Then, the idea of using them in the exploration of mineral deposits occurred to Emil Wiechert, the first chairman of the German Seismological Society. In the 20s of the 20th century, seismic exploration was for the first time used for oil deposit prospecting. The methods of refracted and reflected waves were tested in the late 20s--early 30s.
Seismic refraction implies the recording of waves refracting in the Earth’s crust layers at certain angles at the interface between two layers. The method is employed today rather rarely, because it has poor accuracy and precision in comparison with other methods.
Seismic reflection is more common. It is based on recording the waves that reflect once from the layer interfaces. This method allows for a detailed (up to 0.5% of depth) exploration of geological structures and identification of those with oil and gas potential (usually, porous rocks - reservoirs). However, this method cannot provide a definite clue as to whether a reservoir contains hydrocarbons or not.
Usually, seismic reflection is complemented with the common depth point method (CDP). The large-scale application of this method began in our country after the 1960s. It implies multiple seismic reflections from each element of a geological boundary and their stacking. CDP significantly increases the informative value of exploration; however, it poorly suits complex tasks such as composition prediction.
The search for methods and techniques that would ensure high accuracy in seismic exploration while allowing to minimize costs still continues. The technology progresses in different directions. So let's look at the most important ones.
Only 2D seismic was being used before the early 1990s. Wave sources and receivers were arranged along a straight line and allowed to build a single section. Replacement of paper technologies by computers allowed to introduce 3D seismic.
In 3D seismic, the sources and receivers are arranged over the entire surveyed area and the outcome is a 3D model or a seismic cube. Both techniques employ the same data acquisition method – only first-section models are built, which are then stacked. This means that 3D seismic features no absolutely new capabilities unlike, for example, deeper surveys, etc. Yet, experts believe that a proper approach to interpretation can improve the reliability and informational value of the 3D method by 20-30%. Therefore, despite the higher cost (which is gradually falling), 3D seismic is applied quite widely nowadays.
But progress has not stopped on three dimensions. There is 4D seismic, also known as "time-lapsed seismic surveying.” In this case, after some time upon acquisition of the first cube, the survey is repeated with exactly the same arrangement of wave sources and receivers. A second data cube completely identical to the first one but reflecting the condition of the reservoir in different time periods is produced. The differences in the parameters of seismic data when two cubes are compared indicate changes in the surveyed reservoir.
At bottom, 4D seismic in an “ideal” method for seismic monitoring. One must understand that in most cases it is impossible to conduct completely identical seismic surveys and data processing. Nevertheless, even a far-from-ideal monitoring allows to study changes in the conditions of a field and make its development more efficient and consistent.
From 3D to 3C
In traditional seismic exploration only P-waves are used. They are characterized by a high velocity and propagate in almost all types of rocks. But the source actually generates waves of several types in the rock. Multiwave (multicomponent) seismic (3C) allows to use not only P-waves but also S-waves and C-waves.
The recording of all types of waves provides additional information about the structure of boundaries, composition and properties of a geological section, and it allows to localize fracturing zones. 3C seismic provides a detailed description of the internal characteristics of a reservoir, its structure, and offers clues as to whether or not its further exploitation is justified. It is evident that this method requires special equipment and implies significant costs. Therefore, it is relevant only in situations when detailed information is critical.
Quite often, a detailed description of the structure of developed deposits is required for additional appraisal and the development of discovered fields. In such cases, the high-resolution seismic method is employed. High-resolution seismic uses both low frequency (90-200 Hz) and high frequency (6-10 Hz) data of a desired signal.
High-resolution 3D seismic using reflected waves allows for a very detailed exploration of geological structures with diffused contours and a complex configuration of boundaries. Such vision of the environment opens opportunities for significant expansion of the range of economically profitably hydrocarbon traps.
In the process of its evolution, seismology has gone from the study of natural oscillations to the investigation of artificial oscillations. Modern technologies allow to return back to the analysis of the Earth crust’s natural oscillations, but with a new twist: I mean the study of the microseismic background for hydrocarbons exploration.
Infrasound microseismic technology is developed in Russia. Its inventors believe that oil and gas deposits generate infrasound waves (microseisms) which amplify external noise. Seismic-acoustic background noise is employed as a broadband signal, and the reservoir introduces changes in its spectrum.
There is also the technology of seismic localization of emission sources. It implies sounding of the in-depth processes occurring in the field. Microwaves of the seismic emission and man-induced noise represent the input information. Seismic monitoring based on this technology allows to collect information about such reservoir continuously without time limitations, unlike 4D seismic.
Borehole Seismic Surveys
Seismic surveys can be conducted not only on a rock’s surface, but sometimes elastic waves are recorded at depth in wells. This method is called vertical seismic profiling (VSP). VSP is not, in effect, a new method. It was first proposed in our country half a century ago, but at the time it was a breakthrough of global scale. It remains relevant today and continues to evolve.
Verification by drilling shows that the accuracy of the structures plotted based on VSP data is 3-5 times higher in comparison with surface seismic survey data. This has to do with the direct measurements of the velocity and the small size of the surveyed borehole area.
Quite often, VSP is employed for online drilling support. The locations of the first wells are selected using surface seismic data, and VSP conducted in these wells allows for optimum planning of the localization of the next phase of drilling, etc. Sometimes the trajectory of wells is corrected based on VSP data while these wells are drilled.
How to Do Without Cable?
One of the most interesting new technologies is cableless seismic surveying: without the cables used to transmit the control signals and seismic data and to supply the power needed to run the equipment. The absence of cables allows to resolve a number of problems.
The cables and the associated equipment required for a standard surface seismic survey weighsignicantly, which directly affects transportation costs. The unwinding of cable systems, as well as their maintenance and repair, requires significant manpower. Seismic exploration companies sometimes spent up to 50% of working time on cable troubleshooting and maintenance.
In addition, a complicated network architecture negatively impacts the performance of a recording system, the risk of increasing failure. This imposes limitations on surveys, sometimes making it hard to take into account the features of the terrain. Cables put a limit on the number of data links, which eventually affects the quality of the final image.
The new cableless technology allows for the use of wireless data links. Each receiver has an independent link to the central electronic systems. Receivers can be placed in the desired locations and not on a regular grid. Data is stored locally at the receiving point. There is no need for transmission of all data in real-time, which allows to reduce power use and bandwidth requirements. Each module has a GPS system, which reduces mapping costs.
Seismic Exploration in the 21st Century
New methods are being introduced to address the challenging and very specific tasks related to seismic exploration. These methods allow not only to study the structure of a section, but they also help to detect clues of the presence of hydrocarbons: for example, the absorption method and velocity dispersion. If an abnormal increase of absorption and dispersion of the phase velocity of seismic waves is recorded in a section, this indicates that the reservoir is promising.
The differential seismic method is employed to explore complex geological structures with high accuracy and to study the composition of a section. It implies local conversion of input seismic data on small bases at the interpretation stage.
Sometimes, to collect more information, wave sources and receivers are placed outside the area of interest. This method is employed by a seismic locator of the later survey. It is based on the analysis of scattered waves. This method helps to study fractured zones where conventional seismic-reflected waves attenuate and scattered waves are produced.
Can It Be Trusted?
Although seismic exploration is far from being a new method, discussions around its efficiency arise now and then. What is at issue is that subsequent drilling sometimes does not confirm seismic data. In any case, predictions always have been of a probabilistic character. This is the reason why the method quite often is subjected to harsh criticism. This is also the reason why the efforts to make it more accurate and reliable have never stopped.
In any case, experts agree that despite all its limitations, seismic exploration has no alternative today. It is obvious why the biggest oil companies investment heavily in the seismic exploration of their greenfield and brownfield assets. The oilmen expect significant returns from such investments, because exploration does not only help to save money on inefficient drilling, but it also adds reserves.
Yuriy Romashkov, head of exploration, Geophysics Department, chief geophysicist, OJSC Permneftegeofizika, is confident: “Methodologically, seismic exploration can help address tasks in the most challenging orohydrographic conditions - mountains, swampy areas, transit zones, open water areas, etc. I think it has not alternative. Nowadays, seismic acquisition methods are the most reliable methods for exploration of the Earth’s crust. If anyone is disturbed by the flatness of this statement, just bring me any example when owners of a license have spudded at just a single well based on the recommendation of a magnetic or gravimetric survey."
Igor Dibtsev, IVP Upstream, TNK-BP, believes: "Seismic exploration is an indispensable condition from the point of view of exploration, development of the field and bringing it on stream. It is impossible to start anything without such work. This is an extremely important and necessary step, the starting point in approaching any field. This is why currently we put a sharp focus on seismic exploration."