The velocity of gas is high near the surface due to expansion factor of gas so it is suggested to use the tapered string. 3-1/2” X 2-7/8” tubing from surface to bottom of the well.
We must look at the metallurgy of the tubing considering 30% CO2, 60000 ppm saline formation water.
The coupling of the pipes should be compatible with our downhole condition. The whole idea is to complete the well keeping in mind the long life of the well productivity.
We need to reduce the time we take to drill the reservoir section. This will prevent the formation damage due to prolonged exposure to mud system
Oil base mud is not suitable for gas reservoir as it introduces third substance (oil) near the wellbore apart from water and gas. This will further damage the near wellbore permeability. Water based mud system is more suitable for our reservoir.
HEC (Hydroxy Ethyl Cellulose) is used in depleted reservoir to cover the loss of fluid, however normal HEC is suitable for only 120 Deg C, so their derivative is available for high temperature.
CaCo3 creates good filter cake where there is good permeability and which results in differential sticking. However CaCo3, is good for workover brine as this can be dissolved with HCl Acid.
Potassium Formate Brine is good for workover fluid as it has pH>9.5 so there is no corrosion issues, also we can get 1.02-1.04 SG brine
We’re introducing lot of Iron scales to the formation, as the mud tanks & water tanks are corroded. Need suitable corrosion fluid in the workover brine to control the corrosion.
We get Ferrus oxide (Siderite) in the flow back (reddish water) which is causing damage to the formation, as it is choking the rock permeability.
There is water condensation inside the formation which mixes with CO2 in the formation, secondly the produced water with high salinity responsible for permeability damage.
EDTA (suitable for 200 Deg C) can be used for cleaning the Ferrus oxide, however EDTA when comes in contact with oxygen its sustainibilty reduces, hence it should be mixed with suitable oxygen scavenger.