Gas Produced by Gas Expansion
Reservoir gas produced by gas expansion is the process in which natural gas is extracted from a gas reservoir due to the expansion of the gas. This typically occurs when the pressure in the reservoir is decreased, either by natural depletion or through production activities. The efficiency and sustainability of this process depend on the reservoir characteristics and the management strategies employed to optimize gas recovery.
Key Points about
Gas Reservoir - A subsurface pool of hydrocarbons contained in porous or fractured rock formations. The gas is typically trapped by overlying rock formations that are impermeable.
Gas Expansion - When the pressure on the gas is reduced (by opening a well to production), the gas expands according to the principles of gas laws (such as Boyle's Law and Charles' Law). This expansion causes the gas to move towards areas of lower pressure, such as the wellbore, from where it can be brought to the surface.
Production Mechanism - As the reservoir pressure drops due to production, the gas in the reservoir expands. This expansion helps drive the gas towards the production wells. The energy required for this movement is primarily derived from the natural pressure of the reservoir.
Pressure Decline - Over time, as gas is produced, the pressure in the reservoir continues to decline. The rate of pressure decline depends on several factors including the permeability of the rock, the viscosity of the gas, and the rate of production.
Recovery Efficiency - The efficiency of gas recovery from a reservoir depends on the reservoir's properties, such as porosity and permeability, as well as the production strategy employed. Enhanced recovery methods, such as gas injection, can sometimes be used to improve recovery efficiency.
Process
Initial Production - When a well is first opened, the natural pressure of the gas reservoir pushes the gas towards the wellbore. This is typically the phase of highest production rates.
Pressure Decline - As production continues, the pressure in the reservoir drops, reducing the driving force for gas production. Production rates begin to decline unless measures are taken to maintain reservoir pressure.
Enhanced Recovery (if applicable) - To maximize recovery, techniques such as gas re-injection or other methods to maintain reservoir pressure may be employed.
Consider a natural gas reservoir with an initial pressure of 3000 psi. When production begins, the pressure at the wellbore is lower than the reservoir pressure, causing the gas to flow towards the well. Over time, as gas is produced, the pressure in the reservoir drops to 1500 psi, reducing the production rate. To maintain production, additional techniques might be applied to sustain pressure and enhance gas recovery.
Gas Produced by Gas Expansion Formula |
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| \( G_p \;=\; 43560 \cdot A \cdot h \cdot n \cdot ( 1 - w_{is} ) \cdot \left( \left( \dfrac{ 1 }{ G_{fi} } \right) - (1 - G_f ) \right) \) | ||
| Symbol | English | Metric |
| \( G_p \) = Gas Produced | \(ft^3\) | - |
| \( A \) = Drainage Area | \(ft^2\) | - |
| \( h \) = Thickness | \(ft\) | - |
| \( n \) = Porosity | \(dimensionless\) | - |
| \( w_{is} \) = Initial Water Saturation | \(dimensionless\) | - |
| \( G_f \) = Gas Formation Volume Factor | \(ft^3 \;/\; bbl\) | - |
| \( G_{fi} \) = Initial Gas Formation Volume Factor | \(ft^3 \;/\; bbl\) | - |
