When we are discussing the hydrocarbon deposit mining, we should know that, the energy required could be supplied by nature itself, due to the thermodynamic conditions. Therefore, we could put to use even gravity, in order to have an efficient extraction while minimizing costs and of course, all that been environmentally friendly also.
We must bear in mind the three basic categories of deposits: Solution Gas Drive Reservoirs, Gas Cap Drive Reservoirs, and Water Drive Reservoirs.
In the first category, the gas is within the oil and at the time of extraction, due to the pressure reduction, the gas is released and replaces oil within the deposit.
Consequently, the effectiveness of this method depends on the amount of gas within the composition of the minerals, the properties of the fluids and the geological structure of the deposits.The replacement is however minimal, because the gas moves more than oil. For this reason it is a good case for the usage of water.
In the second category, the gas, due to density, is accumulated on the top level of the deposit. And when the oil extraction begins, the gas pushes upon its surface, and helps the whole process.
Of course, the volume changes of the gas, depends on the pressure and the level of the production that is desired.
In the third category, the gas or the oil have a direct contact with the water.
Indeed, this is what occurs in most cases. When the extraction begins and the oil is dwindling, then the water from below it, moves upwards to replace it, and in these cases we achieve rates, from 70% to 80%, while in the first category we obtain only 10 to 15%.
The extraction through gravity, can be the primary method for thick deposits which have a good vertical communication.
This process is slow, because the gas is ought to be found at the top level.
When this can’t be done, then the usage of water at high pressure occurs, in order to replace the oil.
In any case, the extraction with this method, minimizes the total cost and allows a better control of the entire process, which is important for the relationship between the State and the contractor.
In this context, we must function here in Greece, when we find ourselves in the mining phase in the Greek EEZ.