Definition
Non-Condensable Gas Co-Injection (NCG) belongs to the in-situ exploration and production of oil sands.
Non-Condensable Gas Co-Injection refers to the process which is to inject NCG together with the steam in the oil sands reservoir.
Comments
Canada is the country with the biggest unconventional oil resources called bitumen.
This bitumen lies in oil sands reservoirs.
Most of these oil sands reservoirs (90%) are too deep in the ground to be developed with mining techniques.
Instead, the oil and gas companies invented an in-situ process to extract this bitumen from the oil sands.
This in-situ process is combining vertical wells for Cyclic steam injection and horizontal wells pairs for steam injection, called Steam Assisted Gravity Drainage (SAGD).
This SAGD method has proven to be efficient in causing the bitumen to melt under the effect of the steam injected by the upper well and collected by the lower well back to the surface together with the melted bitumen.
Anyway this SAGD process meets several limits:
– It requires a large quantity of steam, thus water even though a part of it is recovered
– Producing steam has a significant costs based on the local energy
– Its performances may vary highly from one oil field to another depending on the nature of the surrounding ground of the oil sands.
To improve the performances of the SAGD process, the operating companies inject solvents with the steam in order to increase the fluidity of the melted bitumen.
These solvents are partly collected back to the surface together with the bitumen and the water, it is treated to be re-injected.
But in some cases, solvents are not enough, especially when the oil sands reservoir is embedded in permeable zone.
These reservoirs are called unconfined reservoirs and are characterized by low pressure due to the:
– Limited caprocks
– Gas and water mixture at the top of the resevoir
– Important quantity of water at the bottom of the reservoir.
That is where the non-condensable gas co-injection is used.
These non-condensable gas are in fact natural gas liquids (NGL) such as methane which are injected together with the steam and the solvents.
When injected in the reservoir, the non-condensable gas behaves a seal to quench the oil sands containing the bitumen from the gas and water mixture at the top and the water at the bottom.
Therefore the non-condensable gas reduces the losses of steam in the permeable zones and the quantity of water collected together with the bitumen.
The seal effect of the non-condensable gas has additional vertues:
– It applies also to insulate the heat of the steam inside the reservoir, so that the thermal impact can last much longer on the bitumen.
– It increases the pressure in the reservoir.
If the impact of the non-condensable gas may vary from one reservoir to another, worst is the overburden of the oil sands field, more efficient is the non-condensable gas co-injection together with the steam and solvents.
In some cases, like Christina Lake in Alberta, it can reduce the steam consumption by 25%.
