1.1 General Introduction
Reservoirs act differently due to varying range of both rock and fluid properties and thus must be treated uniquely. During production, reservoirs are allowed to naturally produce their hydrocarbons until when production rates are mostly not economical viable then other support systems are used. Primary recovery is the natural stage of the reservoir to be able to produce without support thus depending on reservoir’s internal energy. There are different drive mechanisms known as a results of different energy sources. The drive mechanism of a reservoir is not known in the earlier life of the production but can be seen from production data with time. The knowledge about the reservoir’s drive mechanism can help improve reserves recovery and supervision during its middle and later life. The important drive mechanisms include: Rock and liquid expansion drive, solution gas/ depletion drive, Gas cap drive, Water drive, Combination drive and Gravity drainage drive.
Rock and liquid expansion drive has its oil existing at a higher pressure than the bubble point pressure and with only oil, connate water and the rocks. The rock and fluids expand as a result of their different compressibility as the reservoir pressure deplete. Formation compaction and expansion of different rock grains are some factors that affect reservoir rock compressibility. These factors are due to decrease of fluid pressure within the pore spaces which in turn reduce pore volume through porosity reduction. While the pore volume is reducing, the crude oil and water will be forced out of the pore space to the wellbore. Due to the compressibility (slightly) of both liquids and rocks, the reservoir will experience a rapid pressure decline. A constant gas-oil ratio equal to gas solubility at bubble point pressure is typical of this drive mechanism. A small percentage of total oil in place is recovered due to the less efficiency of this drive.
Other recovery methods like Secondary and tertiary (Enhanced) recovery methods are employed to help improve the recovery of the remaining hydrocarbons by providing additional or sustaining the energy. The efficiency of an enhanced recovery method is a measure of its ability to provide greater hydrocarbon recovery than by natural depletion at economically attractive production rate (Marcel et al. 1980). It depends on reservoir characteristics and nature of displacing and displaced fluids. Enhanced recovery methods seeks to improve the sweep and displacement efficiency. It has been basically grouped into three types; namely chemical processes, miscible displacement processes and thermal processes.