One of the most common applications of coiled tubing is the removal of sand or other debris from wells. The typical operation involves circulating fluids down the coiled tubing and up the well. The fluids are designed to carry the sand from the bottom of the well to surface, without killing or over pressurizing the well.
Because most sand producing wells are under pressurized, gas is typically required to keep the hydrostatic weight of the return fluid column under that which the reservoir can support. This gas is typically nitrogen. Difficulties arise in many locations around the world where the supply of nitrogen is problematic, or at least very expensive. A cleanout method that does not require nitrogen removes the need for a timely, reliable and cheap source of nitrogen as well as the need for cryogenic pumps and tanks.
This paper details the use of concentric coiled tubing and a down hole jet pump to conduct cleanouts in the balanced or underbalanced condition without the use of nitrogen. This technique has been the subject of previous papers, demonstrating its applicability in difficult, deviated well profiles. This paper describes the enhancements made to the system to make it suitable to simple wells, focusing on low-cost, lower-tech operations as opposed to difficult applications. The first case histories are included.
Much attention, of late, has been focused on how to clean difficult wells, several papers having been written on highly deviated wells and large bore wells1,2,4,6,8,10. Little attention is paid to the simpler vertical wells, near vertical wells, or wells with small internal diameters.
The reason for so little attention being focused on these wells is that technically, they are very easy. Often, no gels are required, simple water mixed with nitrogen, pumped through coiled tubing at relatively low rates will do the job nicely.
However, the most common or obvious method may not be the most commercially attractive method. For example, there are instances where logistically, pumping nitrogen can be very difficult and expensive. The technique described in this paper provides for an alternative method under these conditions.
This first part of the paper identifies why nitrogen is required to clean depleted vertical wells, and what factors determine just how fast nitrogen has to be pumped for how long. Figure 1 shows the simple case of a sand particle falling though a fluid, the fluid itself simultaneously rising in the well bore. The net velocity of the particle is the upward velocity of the fluid minus the falling rate of the particle in the fluid.
Figure 1 represents the simplest case where a liquid fills the well. Typical fall rates of particles in water are of the order of a few inches per second. Generally, an upward liquid velocity of 10in/sec will provide for a good cleanout.
In reality, it is usually not possible to have the wellbore full of a liquid steadily moving up. The full column of liquid would exert a pressure over that which the reservoir below can hold, so lost returns would result, leaving little of no fluid returning to surface. To combat this, nitrogen is added to the liquid to ensure that the hydrostatic pressure of the well contents (strictly plus their friction pressure) does not exceed the reservoir pressure.