Nitrogen (N2) injection-assisted flowback with coiled tubing (CT), CT flowback for short, is more and more widely used due to its convenience in implementation and flexibility in control. The unique feature of gas injection with CT is that the injection depth can be changed continuously (Zhou et al., 2011), which makes flowback become a more complex dynamic process. However, scarce research has been done on such a dynamic process, especially in the optimization of operation parameters such as CT running into hole (RIH) speed and N2 injection rate.
In this paper, numerical models are established using the transient multiple flow simulator to reproduce the dynamic process of CT flowback. And based on this, the characteristics of CT gas injection are explored by comparing fixed-point gas injection with variation-depth gas injection. The influence of formation flowback capacity, CT RIH speed and N2 injection rate on CT flowback process is analyzed. And finally the optimization suggestions for CT operating parameters are put forward.
The simulation results show that, compared with fixed-point gas injection, variation-depth gas injection (i.e. CT gas injection) can achieve a more controlled flowback, significantly reduce the gas injection pressure, and realize the gentle unloading of bottom hole pressure. Formation flowback capacity has an important effect on the CT flowback process, different formation flowback capacities result in different gas well unloading degrees. With the increase of formation flowback capacity, the unloading degree of gas well decreases. CT RIH speed can to a large extent control wellbore unloading rate, i.e. the flowback intensity. The increase in RIH speed is at the expense of the increase in gas injection pressure and will lead to a more aggressive bottom hole pressure unloading. So in determining the RIH speed, both surface gas injection facility limitations and downhole pressure control requirements need to be taken into account. For a given CT RIH speed, there is a minimum N2 injection rate, which is required to achieve the continuous drainage, i.e., the lower limit for N2 injection rate. On the basis of this lower limit, further increasing the N2 injection rate will accelerate the wellbore unloading, and reduce the peak of gas injection pressure, but there is also an upper limit for N2 injection rate beyond which the benefits are no longer apparent.