Gas reinjection

Gas reinjection is the reinjection of natural gas into an underground reservoir, typically one already containing both natural gas and crude oil, in order to increase the pressure within the reservoir and thus induce the flow of crude oil or else sequester gas that cannot be exported. This is not to be confused with gas lift, where gas is injected into the annulus of the well rather than the reservoir. After the crude has been pumped out, the natural gas is once again recovered. Since many of the wells found around the world contain heavy crude, this process increases their production. The basic difference between light crude and heavy crude is its viscosity and pumpability—the lighter the crude the easier it is to pump. Recovery of hydrocarbons in a well is generally limited to 50% (heavy crudes) and 75–80% (light crudes). Recycling of natural gas or other inert gases causes the pressure to rise in the well, thus causing more gas molecules to dissolve in the oil lowering its viscosity and thereby increasing the well's output. Air is not suitable for repressuring wells because it tends to cause deterioration of the oil, thus carbon dioxide or natural gas is used to repressure the well. The term 'gas-reinjection' is also sometimes referred to as repressuring—the term being used only to imply that the pressure inside the well is being increased to aid recovery.

Injection or reinjection of carbon dioxide also takes place in order to reduce the emission of CO₂ into the atmosphere, a form of carbon sequestration. This has been proposed as a method to combat climate change, allowing mass storage of CO₂ over a geological timescale. Reinjection of carbon dioxide in the Norwegian Sleipner gas field saves the operators 1 million Norwegian Kroners per day in national carbon taxes.

Advantages

• It is an environmentally friendly method for the disposal of separated geothermal brine and steam condensate, especially when compared with surface discharge of waste geothermal fluid, which can results in thermal and chemical pollution of shallow ground water and water ways.

• Reinjection may help with the recharge of the reservoir and may provide pressure support, thus reducing reservoir pressure drawdown and preventing cold water infiltration.

• Reinjection also helps in reducing and managing subsidence that can arise from large scale fluid withdrawal.

• Reinjection provides the reservoir with low-gas working fluid compared to the higher gas content in the natural deep fluid. This can result in improved plant efficiency with less gas in the geothermal steam going through the turbines.

Disadvantages

• Difficulty and expense of siting suitable reinjection wells.

• Cooling of the production zones and quenching of steam wells.

• Dealing with large reinjection pressures.

• Groundwater contamination and leakage of reinjected fluid to the surface.

• Ground inflation.

• Change of chemistry in production wells. For example a change in chloride concentration and pH will change the solubility of solids and may trigger corrosion or scaling.

• Induced seismic activity.

External links

• GAS RE-INJECTION. Total experience, Elisabeth PROUST, 2006
• How Does Gas Injection Work? // RIGZONE