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Seismic Exploration in Challenging Conditions - Specific Features and Opportunities

by Alena Lipyavko | Oct 03, 2013

Oil and Gas Vertical, No.5 2012

At present, a significant portion of old and well-explored fields are in their final stage of development. In such conditions, major field operators trying to keep their levels of oil and gas production are focussing their efforts on a detailed follow-up exploration of old fields and exploration of new areas. Such activities can be conducted both in completely new regions, such as Eastern Siberia, and in poorly explored parts of already known fields. Here, companies often face certain challenges, because many of these areas are located in regions that are characterized by rough terrain, lack of infrastructure, swamps, and shallow waters. Therefore, operations in such areas have a specific character that depends on location. GEOTECH Holding has extensive experience of exploration in various conditions – in transit zones, in mountains, and in remote areas.

Transit zones  

Transit zones even in well explored areas quite often are left unstudied. This is explained by the high complexity of the operations required. Transit zones between the shore and deep waters always require a special approach to seismic exploration. Conventional equipment in this case is useless, because neither vessels nor onshore vehicles can approach the exploration site in shallow waters. Therefore, special equipment is required. But not all companies possess such equipment. The difficulties are compounded by the fact that drilling and explosive technology are sometimes not applicable due to environmental restrictions, for example, in protected areas. The efficiency of conventional pneumatic sources is also limited in super-shallow waters. The application of the pulse seismic sources Enisey can be an option in such cases. These sources are specially designed for application in shallow water areas (up to 5 m deep) and allow to obtain high-quality data. Operational conditions play a significant role. Unlike onshore areas that are characterized by two-three types of conditions (deserts, non-complicated areas and infrastructure complicated areas), transit zones can pose multiplie challenges. Some areas have a developed infrastructure, significant number of residential and industrial buildings, racks, power transmission lines crossing water areas, and natural and artificial barriers. In other areas like river deltas, plants that cannot be chopped pose almost an impassable obstacle. Thirdly, there are tidal effects that can affect a up to 1- to 3km wide coast strip. It is difficult to enumerate all factors that can complicate operations. Moreover, such areas are challenging not only from the performance point of view, but from the safety point of view as well, because there is the high risk of damaging expensive equipment. All these factors, such as relatively low performance, special equipment requirements, and tough terrain result in increased operational costs. In comparison with conventional field seismic exploration, the cost of a survey in a transit zone can be 1.5-4 times higher. Nevertheless, despite the complexity and high cost of operations, seismic exploration in transit zones is quite justifiable. Firstly, when structures are located at the boundary of the shore and sea, the dome of the field, i.e. the most productive part of the field, can be located exactly in the shallow water area. Secondly, in case of exploration of additional deposits in old and well-known fields, it is reasonable to conduct exploration in the poorly studies areas. Quiet transit zones are such areas. Thirdly, exploration of such zones allows to increase accuracy in forecasting and to improve the accuracy of the geological models that describe the structure of the subsea part of the area under exploration. Offshore drilling is much more expensive than onshore drilling, and for this reason accuracy in forecasting in this case is critical. Despite the pressing need for operation in transit zones, there are very few companies that conduct seismic exploration in such areas. The reason is that such operations require a sufficient amount of specialized equipment that is not used on a permanent basis. Either a large company providing a full range of services or a company that specializes only on transit zone seismic exploration all over the world can afford to conduct seismic exploration in transit zones. Currently, Russia has one-two companies, including GEOTECH Holding, which can conduct onshore exploration and survey transit zones. In all other cases, an alliance of onshore and offshore companies is always the case. Several types of sources and receivers are used for transit zone exploration to ensure high quality processing and a reliable image of the wavefield. Specialists need to have the required experience of combining different types of materials and reduction of data to the same spectrum, energy, etc. Surely, there are special  procedures embedded in all systems. Moreover, field survey procedures imply a degree of overlap of receivers and sources of different types with control points, which allows to define the required adjustment filtration and amplitude characteristics. Nevertheless, having experienced employees who can perform data processing is key. Surveys in transit zones are conducted much more seldom. Both contractors and customers have few specialists with the required experience and knowledge. For this reason, customers sometimes do not have a full understanding of the specific character of such surveys and set excessive requirements. The mountainous terrain of Central Asia is the ground zero in the ex-Soviet territory where surveys are conducted on cross-country terrain. This region has a high potential for large oil and gas deposits. Periodically, such surveys are conducted in the Caucasus. Though the region is quite well-explored, it also still has blank spots. The challenges of exploring for oil in mountainous regions are mainly related to the complicated movements and significant hazards attendant to such operations. Mountain slopes are not accessible to heavy equipment and machines, and hard rock cannot be drilled by conventional auger rigs. This is why, as in transit zones, special equipment such as light-weight drill rigs, rigs with compressor, and air purging is necessary. Conventional equipment does not allow to access survey sites. For this reason, either on-foot uncoiling is performed, which is very difficult and time-consuming, or helicopters are employed to deliver all equipment on the readied sites, then equipment is moved to the other site using the same method. In both cases, the result is a significant increase in the cost of operations. Costs can be reduced through the application of wireless technology, because it is much easier to deliver and install modules without cables. However, it is necessary to take into account the fact that to be able to collect high-quality geophysical data it is necessary to ensure redundancy of recording points to make sure the results don’t suffer in case of loss of part of the data. In this case, using wireless systems enhances the chances of success. Such systems allow to save time and money. The technology is used throughout the world. However, Russian customers are far from ready for such a technology. The reason maybe that it remains rather difficult to process data collected during surveys because of challenging geology and a high level of noise. This results in a number of problems related to the right application of statistical adjustments and development of a velocity model. This is one of the toughest cases that require the involvement of experienced staff with good knowledge of the entire wave process, the passage of a seismic signal and the factors that impact different distortions. Nevertheless, despite the complexity and high cost of data acquisition and processing, such areas cannot be avoided when they constitute a significant portion of an area with high potential. Absence of Civilization Currently Eastern Siberia is one of the growing markets in terms of oil and gas seismic exploration. Yet, it remains characterized by an almost complete absence of infrastructure, a scarce population, and almost no signs of civilization on a stretch of 500-1,000 km, which significantly complicates and increases the cost of operation. Operations in such areas are similar to operations in a mountainous area. But the problem here has to do with not only the complexity of movement, but also remoteness and, therefore, the mobilization-demobilization of manpower and equipment. Usually, preparation work starts in summer, while personnel and equipment are shipped in by helicopters in early winter. Air transport accounts for the bulk of the cost of operations in such areas, multiplying it by a factor of 3. Relations with customers play a significant role. All preparation is done in summer before the start of the season, and all agreements need to be signed in spring; otherwise, there is a serious risk of failure to complete the required work on time. The bulk of the operations are conducted in winter. Weather plays a significant role in the collection of high quality data. According to some experts, the quality of the material depends on the weather up to 60%. Bad weather, winds, and snowfalls create additional noise, which needs to be isolated at the processing stage. Today, crews are usually equipped with field-processing mini-stations that allow to keep out unnecessary interferences and collect high-quality data, which simplifies work and increases performance. The psychological factor also plays a prominent role in determining the quality of the work. In desolate areas with no signs of civilization over distances of hundreds of kilometers, crews after some time lose interest in what is happening around them, including their work. To avoid this, companies try to provide employees with 10- to 12-day vacations outside camp. Surely this approach results in additional costs, but here one has to choose between the quality of the collected data (an employee who is not interested in the result but simply wants to leave the site as soon as possible is unlikely to seriously perform his work) and low cost. The development of Eastern Siberia started relatively recently. It can be assumed that, as exploration continues, a local infrastructure will sprout up, making it easier to operate and reducing costs. 

 As a Conclusion  

Each of the abovementioned three zones has its specific challenges that impact data acquisition and processing, but they are all characterized by a high cost of operation, relatively low productivity, and significant manpower and time costs. Nevertheless, as traditional regions are now mostly explored, it is necessary to expand seismic exploration into other areas despite their more challenging conditions. Subsoil users should take into account this factor when planning seismic acquisitions costs, while exploration companies should be technically and technologically prepared and should ensure that their staff are reasy to work in especially challenging conditions.