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Risultati finali
Magnetotellurics was, until about ten years ago, an underdeveloped geophysical method, due to the lack of adequate processing and interpretation algorithms. This lead to under-use of the method, but in the last few years there has been an increase in its application to various geological problems. For this reason there is a need for developing and implementing different approaches for solving processing and interpretation problems, including the exploration of hydrocarbon targets.
The main objectives of the project is to extend the exist technology of the application of the magnetotelluric (MT) sounding method into areas where the structure are very complex, such as a hydrocarbon target. Also to apply this geophysical method in areas where acquisition, processing and modelling of magnetotelluric data present significant problems.
The Magnetotelluric (MT) method is a maturing geophysical method. It is mostly used in projects for deep structure (up to 8Km for oil exploration and 100Km in academic research). Its advantage compared to other geophysical methods is that it is very cost effective. However, as a newly developed method (just fifty years of its first use) it is necessary to evolve the processing techniques especially since the method is sensitive to man-made noise sources. Also, on interpretation it is imperative to improve the sub-surface imaging techniques applicable to near-vertical structures with a strong degree of three-dimensionality.
An area in NW Greece has been selected as a test area and field measurements of MT and Time Domain Electro-Magnetic (TDEM) has been acquired. The area consist of an oil target in difficult terrane where other geophysical data exist and which will be used to verify the results of the electromagnetic (EM) methods applied in the area.
During this project a processing method based on the remote reference technique has been developed and implemented into software that has been tested and optimised through the first year of the project. The data acquired during the project were used in order to finalise the processing code implemented.
Analysis and implementation of an inversion code to be used for the interpretation of the MT data has been established. The code, able to produce 3-D imaging of the earth, has been tested into synthetic data and has also been used for modelling the MT data acquired. This included incorporating constraints mostly from wells and less from other available geophysical data. Based on the contrast of the physical properties of the strata, mapping of the reservoir was possible. This results in implementing the state of the art inversion code in two and three-dimensional modelling of magnetotelluric data.
The main project results could be summarised as follows:
- Processing code of magnetotelluric data;
- Data set of magnetotelluric time series;
- Two and three dimension modelling code of magnetotelluric data;
- Geophysical/Geological models of the test area;
- Environmental/Economic Impact of the application of the magnetotelluric method in hydrocarbon exploration.
The above-mentioned results are useful since we managed to:
- Reduce the time necessary to evaluate the dimensions of the hydrocarbon reservoir;
- Validate the proposed geophysical method in exploiting difficult geological structures;
- Transfer of knowledge to an end-user in the petroleum industry
Even before the end of the project and ever after the results of the project are and will be fully used from all the partners. In more details, the geophysical company involved (Geosystem Srl) is making use of the processing and modelling codes implemented. That will enhance its ability to manage in less time big magnetotelluric projects. The hydrocarbon company involved (Enterprise Oil Ltd), has already make use of the geophysical/geological models of the test area for further research or planning of use of the area potential. Also the environmental/economic impact of using the magnetotelluric method it is of great importance to the oil company since in this way a more environment friendly way for exploration is introduced and also the expenses for exploration could be extremely decreased in such a difficult environment. The University involved (Aristotele University of Thessaloniki) had the opportunity to be involved in the acquisition of a big geophysical survey and work together with field geophysicists and a production exploration team. Also the know how transferred from both private companied to the research team of the University is of great importance since it is a starting point for the further probe in the field of exploration geophysics.
Exploration teams in hydrocarbon exploration have been using the seismic geophysical method for more than seventy years. Explosives are mainly used for this method as an active source. In this case there is a negative environmental impact caused both by the explosion and by the drilling of the shot-hole. In addition, access of drill rigs often requires the use of bulldozed tracks that lead to increased erosion and to unsightly scars on the landscape. On the other hand the use of the magnetotelluric (MT) method is completely friendly to the environment. There is no active source since the natural electromagnetic field of the earth is the main source. The sensors are recording MT signals that are due to induction currents produced because of the different geo-electrical properties of the geological formations. Equipment and personnel arrive at the measurement points on foot, so no damage to the landscape occurs. In this ways there is no environmental impact and the area of study left as it was before the measurements.
Also, the seismic method is of very high cost, especially in areas of difficult topography as in the area of study. The typical cost of acquisition in this environment is 27000 euro per kilometre. Using this cost in order to cover the area of study an amount of 3.375 million euro would be necessary. On the other hand for the fieldwork applying the MT method only 0.110 million euros was necessary.
Of course one can say that the resolution of the two methods in imaging the earth might not be the same. In this specific case, we have shown that the resolution of the MT method has been increased especially with the processing and modelling techniques developed and implemented through this project, whilst the seismic surveys in the same area produced essentially no usable data.
Also, the time to the first three dimensional image of the earth structure is decreased using the MT method to a few days after the acquisition of the data, from a few months for the seismic method. The points above therefore show that there is a significant cost/benefit improvement by using the MT method in hydrocarbon exploration in difficult environments.
The magnetotelluric (MT) data set consists of two types of data; a) the time series of the recorded electric and magnetic field in 99 sites that follow a grid of 9 rows by 11 columns, where its grid point is at a distance of approximately 1.3 km from the adjacent one, and b) the transfer functions connecting the electric and magnetic fields at each site as a result of the processing code implemented through this project (result 1). Both data sets are on the deliverables of the project and belong to all the partners of the project. It is the first time in Europe that such a big amount of MT soundings exist in one area following a grid of approximately 100 square kilometres, that allow two and three-dimensional modelling.
Dissemination of these data sets has already been established through the project since the data has been used for: a) testing the processing code implemented b) testing the two and three dimensional modelling code implemented, and c) present a three dimensional geophysical (geo-electrical) model of the area of study.
After the end of the project this data set could be used further from the partners inside the consortium and their scientific groups for any other scientific result. Such possible scientific outcome could be the geotectonic information of the area revealed from the data set that could also be used for any hazard assessment.
However, as this has been done in the past in the International Magnetotelluric Community the data after two years from the end of the project will be available to any other scientific group for testing other processing/modelling codes. Except from the Oil Company that is one of the partners and already is the end-user of the project, there might be other public services or industrial companies that might have some profit of this result. Possible end-users are the general secretary of the district of Epirus, the prefecture of Ioannina, the ministry of environment, any private road construction company, any private geothermal company, etc.
By the further use of the data set it will be easy to develop more software codes by way of decreasing the time for the first model in order for real time calculation of the geoelectrical models to be accomplished so that the hydrocarbon (and geothermal) industry will have a direct benefit.
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