In conventional electrical exploration methods, such electrodynamic parameter as electrical resistivity is examined, that, as a rule, brings little information on hydrocarbon targets. Electrical exploration proposes studying induced polarization (IP) of rocks as an effective supplementary method. In this case, it is considered to be proved that polarization increases above hydrocarbon pools. Investigations of a medium by methods of electrical resistivity and IP are supplemented by the vertical electric current soundings (VECS) method, which determines one more parameter that is manifested over hydrocarbon pools and is very informative today. And what is more, there are hydrocarbon pools that can be characterized by the VECS method. We propose to use a system of works for electrical prospecting intended to studying electrical resistivity, IP, and a parameter being manifested in works by VECS. The integra-tive approach permits validity of reservoir delineation to be enhanced, especially if the VECS method is involved into the system. The VECS method has been widely tested for hydrocarbon targets. This permits us to assert that this method as very effective.
When works are carried out over oil fields, the areal signal owes mainly its existence to halo variations over oil pools. The VECS signal is highly associated with the medium material composition and its using can be considered as a direct method for hydrocarbon prospecting. Surveying by the VECS has proved or disproved the prospectivity for oil of many targets (uplifts) suggested by seismic exploration. The presence of a halo is indicative of oil evidence in uplift even though the accumulation itself was not achieved. It can be stated that now electrical exploration in the name of VECS significantly supplements seismic exploration giving information on the material composition of a medium and, in part, on the availability and distribution of even minor hydrocarbon traces.
It is conformed in practice that applying VECS allows one to enhance the efficiency of exploratory drilling. It is known fact that a third part, at least, of exploratory wells is drilled to no purpose. Therefore, involvement of the VECS into a system for prospecting and exploration works prior drilling is well substantiated. It should be understood that applying VECS makes it possible to locate sites of wells with greater confidence rather than it may be done using only the results of a seismic exploration. With an allowance for a high cost of seismic works, applying VECS may be performed at the first stage of prospecting works, for example in a conjunction with geochemistry.
Applying VECS for monitoring of the long-exploited deposits is of significant interest. During monitoring of the Krasno-Oktybrsky (Tatarstan) oil field in 2001, three supplementary anomalies, which were overlooked by a seismic exploration, were revealed. In the course of development of these targets, the additional amount of oil had been produced.
Using the system for electromagnetic prospecting works proposed by us, the VECS being a key method involved into the system gives the following advantages:
- VECS permits one to outline an oil accumulation, as well as to evaluate prospectivity for revealed new oil accumulations within the limits positive structures identified by seismic exploration. Works by the VECS-М method provide the outline of oil showings. We pay attention to the fact that VECS-M, as opposed to other electrical prospecting methods, mainly gives information on a medium beneath the measurement point. Increase in the density of VECS-M survey grid is worthwhile since a signal is nearly free from information on an average medium between the measurement point and circular electric dipole (CED).
- Areal VECS signal is free from the background of a host medium and it is caused only by local geoelectric heterogeneities, which are underground resources.
- Due to the vertically focused electric current, increase in the depth of investigations and spatial resolution are attained as compared with conventional sources of an electromagnetic field (loop and grounded electric line).
- Near surface methods oriented to study of a halo, geochemistry for example, greatly depend on near surface contaminations and these methods give only the image of a halo outcrop even under favorable conditions.In the VECS method, the halo is traced down to a deposit. The VECS shows the deposit outline at times corresponding to depths of oil strata.
- The Er component possesses the high sensitivity to layers with a high resistivity and to variation in medium polarizability. Both qualities are extremely important in studying hydrocarbon fields. Oil pools are usually associated with layers of increased resistivity. Rocks with the increased polarization para-meter also overlain oil beds.
- Involvement of conventional IP parameters increases the interpretation adequacy of data acquired during electrical prospecting for hydrocarbons. Therefore, we propose a method of sounding by counter lines (SCL) as addition to VECS. The Cole-Cole model of frequency dispersion of a medium resistivity is applied for interpretation of SCL data. We use the SCL method due to its higher sensitivity to polarization parameters as compared with conventional IP methods of electrical prospecting.
- For studying resistivity, we use data of conventional method of transient processes TDEM (time-domain electromagnetic sounding) acquired earlier, or involve TDEM into the current surveys.
- The very important point is that inferences on arrangement of hydrocarbon deposits are made based on two different electrical prospecting methods such as VECS and SCL. VECS and SCL study the distribution of different electrical parameters of a medium.
- The proposed system of methods permits one just as to carry out preliminary profiling surveys using the SCL method and to carry out surveys by the VECS in zones of increased polarizability in order to obtain a map of deposits, so conversely, to approve the results acquired by the VECS method with studying polarizability.
-Our integrated works (using VECS, SCL and TDEM) are intrinsically combined in the survey system. All the facilities needed for VECS, SCL and TDEM are parts and components of the VECS instrumentation.
Articles
V.S. Mogilatov, A.V. Zlobinskiy. The complex prospecting of a variety of electromagnetic parameters of the medium above seismic uplifting for the detection of oil deposits. // Geophysics, 2013, No. 2, p.51-57 1 Мб, pdf-file
About VECS
Electrical prospecting by the method of vertical electric current sounding (VECS) as applied to oil targets. 2 Мб, pdf-file