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The natural energy of a reservoir can be used to move oil and gas toward the wellbore. Used in such a fashion, these sources of energy are called ''drive mechanisms''. Early determination and characterization of the drive mechanism(s) present within a reservoir may allow a greater ultimate recovery of hydrocarbons. Drive mechanisms are determined by the analysis of historical production data, primarily reservoir pressure data and fluid production ratios.

The natural energy of a reservoir can be used to move oil and gas toward the wellbore. Used in such a fashion, these sources of energy are called ''drive mechanisms''. Early determination and characterization of the drive mechanism(s) present within a reservoir may allow a greater ultimate recovery of hydrocarbons. Drive mechanisms are determined by the analysis of historical production data, primarily reservoir pressure data and fluid production ratios.

The three primary oil reservoir drive mechanisms are [[Drive mechanisms and recovery#Solution gas drive|solution gas drive]], [[Drive mechanisms and recovery#Gas cap drive|gas cap drive]], and [[Drive mechanisms and recovery#Water drive|water drive]]<ref name = pt10r5>Clark, N. J., 1969, Elements of petroleum reservoirs: Dallas, TX, Society of Petroleum Engineers, AIME, p. 66–84.</ref>. Reservoir pressure trends and producing gas-oil ratio trends of these three drive mechanisms are shown in Figures 1 and 2, respectively. A ''combination'' or ''mixed drive'' occurs when two or more of the primary drive mechanisms are present in the same reservoir. A combination drive may also occur when one or more of the primary drive mechanisms are assisted by ''gravity drainage''. Table 1 shows the energy sources and ultimate recovery ranges of the major drive mechanisms.

The three primary oil reservoir drive mechanisms are [[Drive mechanisms and recovery#Solution gas drive|solution gas drive]], [[Drive mechanisms and recovery#Gas cap drive|gas cap drive]], and [[Drive mechanisms and recovery#Water drive|water drive]].<ref name = pt10r5>Clark, N. J., 1969, Elements of petroleum reservoirs: Dallas, TX, Society of Petroleum Engineers, AIME, p. 66–84.</ref> Reservoir pressure trends and producing gas-oil ratio trends of these three drive mechanisms are shown in [[:File:Drive-mechanisms-and-recovery_fig1.png|Figures 1]] and [[:File:Drive-mechanisms-and-recovery_fig2.png|2]], respectively. A ''combination'' or ''mixed drive'' occurs when two or more of the primary drive mechanisms are present in the same reservoir. A combination drive may also occur when one or more of the primary drive mechanisms are assisted by ''gravity drainage''. Table 1 shows the energy sources and ultimate recovery ranges of the major drive mechanisms.

 

 

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==Solution gas drive==

==Solution gas drive==

In a solution (or dissolved) gas drive reservoir, the oil-bearing rock is completely surrounded by impermeable barriers. As the reservoir pressure drops during production, expansion of the oil and its dissolved gas provides most of the reservoir's drive energy (Figure 3). Additional energy is obtained from the expansion of the rock and its associated water.

 

 

In a solution (or dissolved) gas drive reservoir, the oil-bearing rock is completely surrounded by impermeable barriers. As the reservoir pressure drops during production, expansion of the oil and its dissolved gas provides most of the reservoir's drive energy ([[:File:Drive-mechanisms-and-recovery_fig3.png|Figure 3]]). Additional energy is obtained from the expansion of the rock and its associated water.

[[file:drive-mechanisms-and-recovery_fig3.png|thumb|{{figure_number|3}}Solution gas drive reservoir.]]

[[file:drive-mechanisms-and-recovery_fig3.png|left|thumb|{{figure_number|3}}Solution gas drive reservoir.]]

Depending on its discovery pressure, a solution gas drive reservoir can be initially either ''undersaturated'' or ''saturated''<ref name = pt10r25>Odeh, A. S., 1986, Reservoir fluid flow and natural drive mechanisms, in IHRDC Video Library for Exploration and Production Specialists, Manual for Module PE502: Boston, MA, IHRDC, p. 69–120.</ref>. In an undersaturated reservoir, the reservoir pressure is greater than the bubblepoint of the oil. No free gas exists in the reservoir while the pressure remains above the bubblepoint. The reservoir drive energy is provided only by the limited expansion of the oil, rock, and water. In a saturated reservoir, the reservoir pressure is at the bubblepoint. As soon as oil is produced, the pressure drops and bubbles of solution gas form in the reservoir. This solution gas liberation causes the oil to shrink, but the oil shrinkage is more than offset by solution gas expansion, the primary source of reservoir drive energy below the bubblepoint.

Depending on its discovery pressure, a solution gas drive reservoir can be initially either [[undersaturated]] or [[saturated]].<ref name = pt10r25>Odeh, A. S., 1986, Reservoir fluid flow and natural drive mechanisms, in IHRDC Video Library for Exploration and Production Specialists, Manual for Module PE502: Boston, MA, IHRDC, p. 69–120.</ref> In an undersaturated reservoir, the reservoir pressure is greater than the bubblepoint of the oil. No free gas exists in the reservoir while the pressure remains above the bubblepoint. The reservoir drive energy is provided only by the limited expansion of the oil, rock, and water. In a saturated reservoir, the reservoir pressure is at the bubblepoint. As soon as oil is produced, the pressure drops and bubbles of solution gas form in the reservoir. This solution gas liberation causes the oil to shrink, but the oil shrinkage is more than offset by solution gas expansion, the primary source of reservoir drive energy below the bubblepoint.

==Production trends==

==Production trends==

Once reservoir pressure reaches the bubblepoint pressure or if the reservoir was initially saturated, the reservoir pressure declines less quickly due to the large compressibility of the gas bubbles forming in the reservoir. The producing GOR rises quickly as the bubbles link up and begin to flow and can increase to as much as ten times the initial GOR. If reservoir pressure continues to fall, the producing GOR will eventually drop as the gas expands less and less as it flows up the wellbore.

Once reservoir pressure reaches the bubblepoint pressure or if the reservoir was initially saturated, the reservoir pressure declines less quickly due to the large compressibility of the gas bubbles forming in the reservoir. The producing GOR rises quickly as the bubbles link up and begin to flow and can increase to as much as ten times the initial GOR. If reservoir pressure continues to fall, the producing GOR will eventually drop as the gas expands less and less as it flows up the wellbore.

Oil production rates fall quickly once the producing GOR begins to rise. Wells must be placed on [[artificial lift]] early in their life (see “[[Artificial lift]]”). Initially, little or no water is produced. As reservoir pressure drops, a small amount of water may be produced as the interstitial water saturation expands and exceeds the critical value required for flow.

Oil production rates fall quickly once the producing GOR begins to rise. Wells must be placed on [[artificial lift]] early in their life. Initially, little or no water is produced. As reservoir pressure drops, a small amount of water may be produced as the interstitial water saturation expands and exceeds the critical value required for flow.

===Recovery===

===Recovery===

==Gas cap drive==

==Gas cap drive==

In a gas cap drive reservoir, the primary source of reservoir energy is an initial gas cap, which expands as the reservoir pressure drops (Figure 4). Additional energy is provided by the expansion of solution gas released from the oil. Less significant drive contributions are provided by the expansion of the rock and its associated water.

In a gas cap drive reservoir, the primary source of reservoir energy is an initial gas cap, which expands as the reservoir pressure drops ([[:file:drive-mechanisms-and-recovery_fig4.png|Figure 4]]). Additional energy is provided by the expansion of solution gas released from the oil. Less significant drive contributions are provided by the expansion of the rock and its associated water.

[[file:drive-mechanisms-and-recovery_fig4.png|thumb|{{figure_number|4}}Gas cap drive reservoir.]]

[[file:drive-mechanisms-and-recovery_fig4.png|thumb|{{figure_number|4}}Gas cap drive reservoir.]]

In a water drive reservoir, the oil zone is in communication with an aquifer that provides the bulk of the reservoir's drive energy. As oil is produced, the water in the aquifer expands and moves into the reservoir, displacing oil. Depending on the aquifer's strength, additional energy may be provided by solution gas expansion. Much less significant contributions are provided by the expansion of the reservoir rock and its associated water.

In a water drive reservoir, the oil zone is in communication with an aquifer that provides the bulk of the reservoir's drive energy. As oil is produced, the water in the aquifer expands and moves into the reservoir, displacing oil. Depending on the aquifer's strength, additional energy may be provided by solution gas expansion. Much less significant contributions are provided by the expansion of the reservoir rock and its associated water.

The geometry of the aquifer determines whether it is a ''bottom water'' or an ''edge water'' drive (Figure 5). In a bottom water drive, the aquifer is present below the entire reservoir and water influx moves vertically upward into the oil zone. In an edge water drive, the aquifer is located on the flanks of the reservoir and the water moves upward along the reservoir dip.

The geometry of the aquifer determines whether it is a ''bottom water'' or an ''edge water'' drive ([[:file:drive-mechanisms-and-recovery_fig5.png|Figure 5]]). In a bottom water drive, the aquifer is present below the entire reservoir and water influx moves vertically upward into the oil zone. In an edge water drive, the aquifer is located on the flanks of the reservoir and the water moves upward along the reservoir dip.

[[file:drive-mechanisms-and-recovery_fig5.png|thumb|{{figure_number|5}}Edge water versus bottom water drive reservoirs.]]

[[file:drive-mechanisms-and-recovery_fig5.png|thumb|{{figure_number|5}}Edge water versus bottom water drive reservoirs.]]

==Combination drive==

==Combination drive==

Most oil reservoirs produce under the influence of two or more reservoir drive mechanisms, referred to collectively as a combination drive. A common example is an oil reservoir with an initial gas cap and an active water drive (Figure 6).

Most oil reservoirs produce under the influence of two or more reservoir drive mechanisms, referred to collectively as a combination drive. A common example is an oil reservoir with an initial gas cap and an active water drive ([[:file:drive-mechanisms-and-recovery_fig6.png|Figure 6]]).

[[file:drive-mechanisms-and-recovery_fig6.png|thumb|{{figure_number|6}}Combination drive reservoir.]]

[[file:drive-mechanisms-and-recovery_fig6.png|left|thumb|{{figure_number|6}}Combination drive reservoir.]]

===Production trends===

===Production trends===