Factors Contributing to Field Gas-Oil Ratio Being Lower Than Solution Gas-Oil Ratio in Petroleum Reservoirs

Possible Causes of Field GOR Below Solution GOR

The gas oil ratio (GOR) is a critical parameter in the oil and gas industry as it provides valuable information about the composition and behavior of reservoir fluids. In some cases, the field GOR (the ratio of gas produced to oil produced) can be observed to be lower than the solution GOR (the ratio of dissolved gas to oil at reservoir conditions). This phenomenon can have significant implications for production and reservoir management, and understanding the potential causes is essential for optimizing field operations.

Depletion and pressure drop

One of the main reasons why a field GOR is lower than the solution GOR is depletion and pressure decline within the reservoir. As the reservoir is produced, the pressure decreases, which can lead to the release of dissolved gas from the oil. This gas is then produced with the oil, resulting in a higher GOR. However, if the pressure drop is rapid or excessive, the gas may not be able to fully dissolve from the oil, resulting in a field GOR that is lower than the solution GOR.
This scenario is particularly common in reservoirs with high gas-oil ratios, where the rapid pressure drop can exceed the rate of gas dissolution. In such cases, the field GOR may start high and then decline over time as the reservoir is depleted, eventually falling below the solution GOR.

Fluid Separation and Gravity Drainage

Another potential reason for a field GOR to be lower than the solution GOR is fluid segregation and gravity drainage within the reservoir. In some reservoirs, gas and oil can segregate due to differences in density, with the gas migrating to the top of the reservoir and the oil accumulating at the bottom.

This segregation can lead to a scenario where the gas-rich portion of the reservoir is produced preferentially, resulting in a higher GOR in the early stages of production. However, as the reservoir is depleted and the gas-rich zone is exhausted, production can shift to the oil-rich zones, resulting in a lower field GOR compared to the solution GOR.

Compositional Gradients and Heterogeneity

Compositional gradients and reservoir heterogeneity can also contribute to a field GOR that is lower than the solution GOR. In some reservoirs, fluid composition can vary significantly within the reservoir, with higher gas-to-oil ratios in certain zones and lower gas-to-oil ratios in others.

This heterogeneity can be due to factors such as differential migration, lateral variations in the source rock, or the presence of compartmentalized zones within the reservoir. If production is biased toward the lower GOR zones, the field GOR may be observed to be lower than the solution GOR, which represents the average composition of the reservoir fluids.

Fluid phase behavior and changes in thermodynamic conditions

Finally, changes in the thermodynamic conditions of the reservoir can also cause the field GOR to be lower than the solution GOR. As the reservoir is produced, the temperature and pressure conditions can change, which can affect the phase behavior of the fluids.
For example, if the reservoir temperature decreases due to the influx of colder fluids or reservoir depletion, the gas solubility in the oil may increase, causing the gas to remain more dissolved in the oil phase. This can result in a lower field GOR compared to the solution GOR determined at initial reservoir conditions.

In addition, the formation of hydrates or the precipitation of heavy components can also alter the fluid phase behavior and result in a field GOR that is lower than the solution GOR.

In summary, the phenomenon of a field GOR that is lower than the solution GOR can have several potential causes, including depletion and pressure drop, fluid segregation and gravity drainage, compositional gradients and reservoir heterogeneity, and changes in the thermodynamic conditions of the reservoir. Understanding these underlying mechanisms is critical to effectively managing and optimizing oil and gas production operations.

FAQs

Here are 5-7 questions and answers about possible reasons for field GOR being below solution GOR:

Possible reasons for field GOR being below Solution GOR(Rs)

There are a few possible reasons why the field GOR (Gas-Oil Ratio) may be lower than the solution GOR (dissolved gas-oil ratio):

1. Depletion of the reservoir – As oil and gas are produced from the reservoir, the pressure decreases, causing some of the gas that was initially dissolved in the oil to come out of solution and leave the reservoir, lowering the field GOR.

2. Gas cap expansion – If the reservoir has a gas cap above the oil zone, as the pressure drops the gas cap can expand and push gas into the oil zone, diluting the oil and lowering the field GOR.

3. Preferential production of oil – If the well or field is being produced in a way that prioritizes oil production over gas production, such as through choke management, this can lead to a lower field GOR than the solution GOR.

4. Flashing of dissolved gas – If the pressure drops too quickly as the oil is produced, some of the dissolved gas can “flash” out of solution, again lowering the field GOR relative to the solution GOR.

5. Influx of water – The intrusion of water into the oil zone can displace some of the gas, leading to a lower field GOR.

How does reservoir depletion affect field GOR?

As a reservoir is depleted through production, the reservoir pressure decreases. This causes some of the gas that was initially dissolved in the oil to come out of solution and leave the reservoir, resulting in a lower field GOR (Gas-Oil Ratio) compared to the original solution GOR. The more the reservoir is depleted, the greater the drop in pressure and the more the field GOR will decline relative to the solution GOR.

What is the impact of gas cap expansion on field GOR?

If a reservoir has a gas cap above the oil zone, as the reservoir pressure drops during production, the gas cap can expand and push some of the gas down into the oil zone. This dilutes the oil, lowering the field GOR (Gas-Oil Ratio) compared to the original solution GOR (dissolved gas-oil ratio). The greater the gas cap expansion, the more the field GOR will decline relative to the solution GOR.

How can preferential oil production lead to a lower field GOR?

If a well or field is being produced in a way that prioritizes oil production over gas production, such as through the use of production chokes, this can lead to a lower field GOR (Gas-Oil Ratio) than the solution GOR (dissolved gas-oil ratio). The preferential production of oil leaves behind more of the dissolved gas in the reservoir, reducing the amount of gas produced relative to the oil and resulting in a lower field GOR.

What is the impact of gas flashing on field GOR?

If the pressure drops too quickly as oil is produced from the reservoir, some of the dissolved gas can “flash” out of solution. This gas flashing can lead to a lower field GOR (Gas-Oil Ratio) compared to the original solution GOR (dissolved gas-oil ratio). The more rapid the pressure decline, the more gas flashing can occur, further reducing the field GOR relative to the solution GOR.