Oil reappears from time to time in old deposits and long ago exhausted oil wells. This phenomenon attracts attention of multiple researchers. Specialists of the Institute of Oil and Gas Problems under the guidance of Academician Dmitrievsky offer their explanation.
The earth's crust is similar to a sandwich cake, consisting of hard layers and fractured-porous layers saturated by various fluids, including oil. In some places, the crust is penetrated by an extremely dense network of fissures and ruptures. Ruptures form cavities located almost horizontally and united into a network. All this complicated system is in constant motion due to tectonic forces’ action. The layers are moving, fissures are widening and acting as a rubber bulb: liquid starts coming into formed interstice from surrounding porous layers. In case of significant tectonic tensions, liquid moves at large distances.
According to the researchers’ opinion, this mechanism of liquid movement in the crust is the most intense and universal among all possible ones. It acts both in ruptures and in thin fractured layers, which stretch at significant distances. Vibrations in the crust drive fluids along all possible directions, including horizontal and even downward directions. Migration occurs along lengthy cavities and fractures systems, located at the depth of 10 to 15 kilometers.
Liquid movement caused by widening of internal cavities is of vibrating character. Oil sometimes rushes in or sometimes floods back. The mode and period of vibration depend on the size of perturbed area. In large porous layers, the vibration period makes about 10 thousand years. In the ruptures, the period is shorter and it varies from a thousand to hundreds and even dozens of years, if rupture zones are located at small depths.
The researchers have investigated the carbohydrates migration process from the petroliferous stratum into the upper layers in several regions. An example can be the Romashinskoye oilfield in Tatarstan. The volume of produced oil there has significantly exceeded the previously asserted reserves. According to the TATANEFT Joint Stock Company’s data, more than 65% of oil in Tatarstan is produced in old oilfields exhausted by 80%. However, supplementary exploration of the known deposits allowed to increment reserves of oil by one and a half times within the last 25 years. In the Romashinskoye oilfield, the researchers also discovered old exhausted drillings with regenerated inflow of oil and oil with water. The space of oil pools and their reserves increase with increasing rupture network density. It is interesting to note that the depth of sedimentary covering in the zone of the gigantic Romashinskoye oilfield does not exceed 2 kilometers on average, and this mantle does not possess significant oil potential. Most likely, oil cames to these locations from the direction of Pre-Ural downfold.
In the researchers’ opinion, to overhaul old oil deposits is currently much more profitable and efficient than expensive geological exploration works at new locations.