“Siberian Oil” (Corporate journal ОJSC “Gazprom Neft”) / №99 (March 2013)
In December 2012 Gazprom became the first Russian company to conduct field seismic exploration using the innovational technology UniQ. Through the use of this new system, the company wants to boost reliability in its data acquisition and minimize development costs on new fields.
Text: Sofia Zorina
The method of depth surveys using seismic waves proposed in 1898 opened a new avenue in exploration for mineral resources and, at the same time, imposed continuous improvement in technology. Even the first, relatively primitive methods yielded impressive results. For example, 10 years of field surveys resulted in the discovery of 50 salt domes* associated with hydrocarbon deposits in the USA by 1929. "Geology and chance” had helped discover just a single dome over the same period.
Further developments in seismic exploration were aimed at improving Earth crust data acquisition, transmission, and processing methods. The effort was accelerated by digital technologies, which are used in all modern systems. As a result, exploration success rates in general and, in seismic surveys in particular, soared.
FORWARD AND DOWNWARD
The necessity for continuous improvement in seismic exploration technology is dictated by the imperative to reduce field development costs - maximum complete information about potentially productive targets allows to optimize the localization of exploration and production wells and to extend their lifecycle. Global, vertically integrated oil companies have been addressing the issue for many years now, while in Russia its relevance remains far from for many oil producers. Such an attitude to a large extent is rooted in history: in Soviet times, the focus was traditionally on achieving maximum results within the shortest timeframe, while long-term oil recovery was not a priority. Huge investments in the oil industry allowed to ramp up production by drilling a large number of exploration wells, while large deposits of easily accessible oil did little to encourage implementation of new exploration technologies, including seismic acquisition. The collapse of the USSR had, likewise, a negative impact as it marked the beginning of a steep decline in oil production that continued up to 1997. For obvious reasons, investments in exploration were reduced to their minimum level in those years.
However, growth in production amid tough competition both from Russian and international companies compelled Russian oil companies to take a new look at seismic exploration as one of the most efficient methods for studying the Earth’s crust.
The main stages in seismic exploration include excitation of elastic waves using an artificial source, recording of these waves by receivers knows as geophones, subsequent processing of the seismic records using mathematical models, and geological interpretation of the results. The reliability and accuracy of the information collected regarding the structure and composition of the environment being studied depends on the number of wave sources and receivers, the adequacy of the mathematical tools applied, and the experience of the interpreter.
Depending on the arrangement of the wave receivers on the surface, seismic methods are divided into narrow-azimuth (2D seismic) and wide-azimuth (3D seismic) methods: in the former case, the seismic wave receivers are arranged on a quasi-straight line, and in the latter case the receivers are arranged in a certain order on the plane. Therefore, 2D seismic provides only a vertical section of the Earth’s crust, while 3D seismic gives geologists a three-dimensional image of the area under study.
It is evident that a wide-azimuth survey provides a maximum of information with its high density of recorders. It is the defining progress achieved in seismic technologies, among which the UniQ system developed by Schlumberger is the most innovational.
quantity into quality
The efficiency of the UniQ technology is predicated on the application of a significant number of wave sources and receivers. The increase in the number of receivers is coupled with a high density of distribution, a prerequisite for the maximization of the output signal-to-noise ratio and extended range of frequencies of the recorded signals. This provides geoscientists with a better picture of the studied environment and allows for a more accurate correlation of seismic and well data.
The implementation of this seemingly simple idea of a higher number of source and receiving points per unit of surface to produce more detailed seismic datasets until recently was hindered by the objective obstacle of the limited traffic capacity of cable channels. The arrival of fiber-optic technologies provided a solution. Today the number of active data transmission channels used in UniQ is practically unlimited and can amount to hundreds of thousands in real applications, which is a by an order of magnitude higher than in conventional 3D survey systems. In reality, that means that a high-density grid of seismic wave sources and receivers can cover thousands of square kilometers, while all of the information collected flows simultaneously into a data processing center.
The scale of the traffic can be judged by the following numbers: a standard wide-azimuth survey covered around 3,000 sq.km on Gazpromneft's fields in 2012, producing around 40 terabytes of data; the survey using the UniQ system covering 350 sq.km of surface area produces around 50-55 terabytes of data.
With respect to the increased density of receivers, one crucial aspect is worth noticing: unlike standard techniques, UniQ records data using point receivers while the distance between the receiving points is significantly reduced. Conversely, in case of a conventional 2D or 3D seismic survey, linear groups of seismic receivers are used to collect signals: several receivers are grouped on a single base, and the composite signal is recorded on a single seismic trace. The grouping of receivers increases reliability in signal acquisition: however, part of the recorded noise is not suppressed, and the composite signal is smoothed, losing some useful information. The stepout between grouped receiving points is usually 25 or 50 meters, while the point receivers are installed at a distance of 12.5 meters from each other. Such a density of data acquisition allows to suppress noise using mathematical methods and to ensure a stable signal quality.
Increased density of seismic data allows to conduct a detailed analysis of the upper part of a section (which is almost not accessible using conventional techniques), to find hidden karst zones, to eliminate the impact of the relic permafrost, and to get images of the steep boundaries of formations.
Thus, UniQ is an irreplaceable tool in the development of greenfields with a complex geology and in the detailed appraisal of brownfields, where other seismic exploration methods are inefficient. This technology likewise has no climate or terrain limitations (except for water bodies) and can be customized to the meet geological requirements of a specific field.
UniQ helps to produce a much more detailed body of data in comparison with conventional 3D techniques.
UNIQ at GAZPOMNEFT
In recent years UniQ has been successfully used by the leading oil and gas giants. All major oil companies present in the Mideast use this system in field exploration. Gazpromneft pioneered the technology in Russia. The respective agreement was signed in 2012 with Geotech Holding, the exclusive UniQ provider in Russia.
Under the agreement wide-azimuth seismic surveys will target the areas of the Chonskiy project (Ignyalinskiy, Vakunaiskiy, Tympuchikanskiy) in Eastern Siberia.
The efficiency of 3D seismic at Gapzpromneft has already been proven: 3D surveys have been conducted on most of the assets of the company. The application of the UniQ technology should be the next step in improving the quality of the acquired seismic data.
* A salt dome is a portion of a rock salt layer that intrudes in the form of a dome in a upper sedimentary rock formation. Salt domes are frequently accompanied by combustible gas shows and oil deposits.