Some reservoirs have a gas cap, which provides energy from gas expansion to help production from a wellbore; therefore, this is called “gas cap drive”. When oil is being produced, the gas cap expands and pushes oil downwards to a producing well (Figure 1).
Figure 1 – Gas Cap Drive
For this type of drive mechanism, it is imperative to keep gas within a reservoir as long as possible since it is an excellent energy source of the reservoir. Wells which are drilled into a high structure area where the gas cap is located must be closely monitored because this well will have more of a chance to produce gas.
When reservoir pressure declines, free gas will come out of a solution. If a well has good vertical permeability and the production rate is quite low, free gas will migrate and accumulate with an existing gas cap. This is an additional energy source to help production and ultimate recovery will improve. However, if a well produces at a high rate, free gas will be produced with oil and gas oil ratio (GOR) will go up; therefore, a well loses expansion energy from gas and the ultimate recovery will not be as high as it should be. Furthermore, with a high flow rate, gas will flow quickly into the oil because it has much lower viscosity than oil. So, this will create a situation called “gas fingering.”
For the gas cap drive, declining in production rate and reservoir pressure are slower than solution gas drive and ultimate oil recovery from this drive is around 20% – 40%.
Figure 2 – Gas Cap Drive Profile