Primary Drive MechanismsPrimary drive mechanisms essentially recover hydrocarbons from the reservoir using the natural energy of the reservoir, without the need for external energy. The primary driving mechanisms will directly or indirectly decrease the reservoir pressure or, in other words, cause pressure depletion. Solution Gas Entrainment Solution gas entrainment is classified as petroleum reservoirs that initially contain no free gas but rather develop free gas upon depletion of pressure and this entrainment requires the reservoir rock to be completely surrounded by impermeable barriers. This drive comes into action when the pressure falls below the bubble point and the expansion and exsolution of dissolved gases becomes the primary energy source. Other forms of energy can come from the expansion of rock and water. This type of reservoir is called undersaturated or saturated. If undersaturated, the reservoir pressure is higher than the pressure at the bubble point and vice versa for the saturated condition. If it is an undersaturated tank, there will be no free gas until the tank pressure falls below the bubble point pressure and energy is generated by massive expansion of reservoir rock and fluids. For a saturated reservoir, oil production comes from the expansion of gas bubbles caused by the decrease in pressure. The oil and water will decrease slightly, but the volume of gas expelled and its subsequent expansion compensates for this. From this reader, information such as pressure, GOR and fluid production history can be known. The pressure can drop significantly if the reservoir is initially undersaturated due to the fact that the compressibilities of rock water and oil are lower than those of gas. F...... middle of paper ...... of mechanisms. Compaction Drive A reservoir is considered compaction drive when the contraction of pore volume takes a significant place in the overall expansion while the reservoir is saturated. This drive is supplemented by dissolved gas drive and may or may not be driven by a water/gas plug. This reservoir acts like their non-compaction counterparts, except they feature improved recoveries. For example, oil recovery will be greater for dissolved gas entrainment, in which the compaction entrainment will act like a normal dissolved gas entrainment tank. This is due to the direct consequence of the additional rock expansion that compaction reservoirs actually exhibit. Due to the additional compaction, some production occurs. For example, permeability may decrease, fracture may occur and subsidence, but all these problems are manageable and the compaction result is very favorable..