Oil and Gas Vertical, No.15-16, 2012
Vladislav Votsalevskiy, Chief Geophysicist, GEOTECH Holding
The increasing complexity of the task of optimization of greenfields and extension of a brownfield’s life cycle faced by Russian oil companies underpins the necessity to apply cutting-edge technologies and solutions for the evaluation of the properties and characteristics of productive horizons. Only the collection of complete information about potentially productive targets allows to plan optimum exploration and a development drilling grid. Therefore, optimization of exploration and development plans minimizes costs in the most expensive operations, such as drilling and development of field infrastructure.
It is widely known that seismic exploration has been the most efficient method of exploration in the oil industry for many years. Currently, it is impossible to imagine a successful oil or gas field exploration and development project without 3D seismic. There are various types of 3D seismic acquisitions, which can be customized for specific geological tasks or a specific budget allocated for exploration or fulfillment of license obligations.
Some major oil companies in Russia determine priorities for each specific area and define seismic acquisition methods in tender ToR. However, unfortunately, the dominant trend is towards delivery of technical specifications based on previous experience.
Wide-azimuth seismic acquisition with a high data recording density has been a standard in the industry outside of Russia for many years. Russia significantly lags behind global trends in the application of wide-azimuth and rich-azimuth seismic technologies, replacing narrow-azimuth techniques that used to be accepted as the standard due to the limited resources of seismic recorders.
Modern trends in the global seismic exploration are focused on the most challenging geological tasks. The increasing complexity of recording techniques and higher density of seismic data significantly increase the amount of information used further for the development of geological field models. Therefore, an increased amount of input data allows to use more a powerful mathematical apparatus for the analysis and ensures maximum reliability of the geological results.
It is necessary to note that the efficiency of the most commonly used dynamic interpretation technologies rather strongly depends on the quality of the input seismic data. If there are no data with expanded spectrum both in the high- and low-frequency domains, even application of the best software for seismic inversion cannot guarantee an absolutely accurate result.
Foreign oil producers that have long operational experience and older fields faced the necessity of improving seismic survey efficiency long ago. Back in the times when there were no vertically integrated oil companies in Russia, western vertically integrated companies were addressing the tasks of improving the economic efficiency of their businesses at all stages, from exploration to the final field development stages. For this reason there are well technologies that allow to address the most complicated tasks at lower costs per unit of recorded data.
Currently, different combinations of methods and equipment are used to address specific tasks in especially challenging conditions: from a standard cable recording system with different types of sensors and different types of sources to cable-less recording systems with continuous recording and data sorting based on time tags with GPS/GLONASS synchronization. There are recoding systems using radio channels for data transmission that combine the advantages of cable and those of cable-less systems.
All seismic survey technologies and methods based on technical equipment have their strengths and limitations. Equipment that is a good fit in a particular seismic and geological condition can turn out to be completely unacceptable in other conditions.
All major seismic equipment manufacturers have been offering multicomponent recording systems for many years. These systems allow to collect information about the full field-reflected wave-field vector. Application of such data in many cases allows to make direct forecasts with differentiation based on the fluid saturation type. Unfortunately, this technology is rarely applied in Russia. The objective obstacle to its application is the surface conditions in the main production regions: namely, a very high watercut in the upper part of the section. The subjective limitation is the relatively high cost of multi-component seismic surveys. Another significant problem is high-quality processing and interpretation of multi-component data. There is a shortage of highly skilled specialists in this specific sphere.
The development of conventional seismic exploration technologies for the improvement of efficiency in gathering geological results followed two parallel paths: Increase in the number of th source points concurrently with an increase in the number of recording channels.
The largest global oilfield services companies have patented different methods for improvement of performance in seismic surveys using vibration sources. Manufacturers of recording equipment have developed systems with a higher number of active channels.
The first path, which implies an increase in the number of signal initiation points, can be efficiently applied only in areas with easy access for heavy equipment, such as deserts, steppes, and tundra.
The second option, which implies an increase in the number of active recording channels, is suitable for any conditions. Neither the availability of a developed infrastructure, nor forests, nor challenging terrain, presents an obstacle for increasing the density of signal receipt points.
If in certain regions we cannot use one of the seismic survey efficiency improvement options, it is necessary to focus on the other option that has no physical limitations.
As was said above, our northern tundra and partially steppe and semi-deserts in the south of Russia are suitable for the application of technologies with an increased number of signal sources. It is necessary to note that an increase in the number of blasting holes will result in an explosion of the cost of the survey. For this reason, globally seismic projects with high and super-high densities of signal initiation points are implemented only using surface vibration sources. In our country, in addition to foreign vibration sources, there are a sufficient number of Russian-made surface electromagnetic pulse sources, which are also efficient in super-high density surveys.
Considering current level of development of seismic exploration technology, it is possible to state that quantity has converted into quality.
For example, suppression of interference waves by technical equipment, which used to be achieved by changing the number of seismic receivers and the distances between them, is now replaced by suppression of interference waves using software and mathematic methods. The increased density of seismic data allows to reduce signal/noise ratio requirements for each specific recording trace or seismic record by initiating the signal but ensures significant improvement of the quality of the stacked data. Modern software processing packages allow to filter the useful signal from the noise and random noise and to collect high quality information even in areas with bad seismic and geological conditions.
Divisions of GEOTECH Holding have conducted 2D high-density pilot surveys with the seismic receiver pointed data recording at 5 m stepouts. The obtained results demonstrated a significant incremental informational content of the seismic exploration, a significant expansion of the signal spectrum, and improvement in the detail level of time sections. At the same time, it is necessary to note that the signal/noise ration exceeded all standard limits, and that there were no grouping effect that suppressed noise during recording between pointed groups. Application of standard methods of evaluation of the quality of field data would result in the rejection of thee collected data even before preliminary processing.
Thus, implementation of modern field survey methods requires simultaneous changes in the field data quality evaluation approaches. The criteria used in formalized approaches to the determination of dominant frequencies, signal/noise ratios in several analysis windows and other estimates were developed for specific field survey methods in specific seismic and geological conditions. The principles and the need in formalized estimates cannot be subjected to doubts especially in conditions of a shortage of highly skilled staff. However, flexible approaches are required for each specific method and seismic and geological conditions. Only some oil companies demonstrate a reasonable approach to this problem and lead in the application of the modern world-class technologies ensuring high-quality seismic survey results.
Unfortunately, it is a common practice when technical parameters of quality evaluation suitable for low- and average-resolution seismic surveys are mechanically applied to high and super-high data density surveys, which significantly limits the options for the implementation of modern, efficient surveys. Excessive quality criteria result in a reduction in the estimated performance and respective increase in price.
The results of seismic exploration performed using all modern equipment and methods at the field stage, processed and interpreted using cutting-edge software packages significantly exceeds the results of standard surveys in efficiency.
Currently, the latest hardware and software system featuring all the capabilities of seismic exploration is the UniQ system developed by WesternGeсo (Schlumberger). This technology allows to use from dozens to hundreds of thousands active channels recording the seismic signal with any required density. If necessary, it is possible to use high-performance seismic survey methods with vibration or pulse surface sources, central recording or managing system supporting dozens of groups of sources in any configuration.
Field data collected using the UniQ technology allow to address all challenging geological tasks within the physical limitations of the seismic survey method.
The main advantages of this technology are as follows:
1. Super-dense observation systems for reduction of the spatial data sampling stepout.
2. Application of new-generation field equipment that expands the frequency range at the hardware level. The unique design of pointed signal sensors ensures the recording of frequencies from 4 Hz instead of the standard 10 Hz.
3. The super-high number of active channels allows to create any random configurations of observation systems both with the linear and pattern apertures of the recording groups.
4. Processing of UniQ data using algorithms and approaches allowing to suppress interference during processing, collect data with a high signal/noise ratio and significantly expand the frequency range (both from the high- and low-frequency end).
Therefore, it is possible to ensure significant improvement in the obtainment of results in exploration in especially challenging areas with subsalt structures, thrusts and other complicated tectonic features, trap intrusions, finely laminated reservoirs, and small varieties of acoustic impedances of productive and non-productive formations.
As a result, the system addresses the tasks of increasing resolution and improving the geological results of seismic surveys, characterizing productive layers in order to improve efficiency in production drilling, and defining bypassed productive zones at the old fields.
Actually, when using the UniQ technology optimized for a specific survey area, the system concurrently addresses a wide range of other tasks requiring different types of surveys when using standard methods.
In addition to the abovementioned, it is necessary to note several aspects that complicate the application of modern technologies.
Confidentiality requirements limit experience exchange and other interaction between the geological and geophysical departments of different oil companies. Therefore, it significantly limits the possibility of rapid propagation of new seismic survey technologies that help improve efficiency in exploration even when it is successfully applied by at least one oil company.
Unlike in the times of the Ministry of Geology and Ministry of Oil Industry of the USSR that actually established the modern resource potential of all oil companies, the situation in the industry is far from homogenous today. Oil companies that set strong and efficient geological and geophysical divisions conduct seismic surveys that ensure maximum payback from exploration-stage expenses. Other oil companies focusing only on minimum prices use conventional technologies that theoretically allow to address geological tasks but are not capable of ensuring the required development of resources.
We can provide examples of a balanced approach to exploration based on recent experience.
3D-wide azimuth seismic surveys with 20 active data recording lines is actually a standard at TNK-BP. Information collected using this methodology significantly expands opportunities for the definition of the reservoir properties of productive layers and allows to optimize well placement. Lukoil has repeatedly commissioned seismic surveys that use the truly rich-azimuth method with 36 active recording lines in one of its challenging fields, collecting information about the structure of the salt roof and subsalt deposits. Given the complexity and high cost of drilling in subsalt horizons, some increase in seismic exploration costs will result in a multifold reduction in costs at the main stages of development of the field.
Application of the rich azimuth seismic survey technology is also planned for shallow waters offshore exploration in the northern part of the Caspian Sea. These surveys will allow to define the zones with better reservoir properties to optimize development costs.
Implementation of such projects requires simultaneous usage of up to 15-20,000 recording channels. Such operations can be conducted only by the largest seismic exploration companies that can mobilize the required equipment and experienced, highly skilled staff.