Coiled-Tubing (CT) continues to grow as an enabling technology that provides cost economy and time efficiency for the oil and gas industry. It has gained wider acceptance than ever due to efforts of the manufacturers, CT application engineers, and field operators. However, most CT applications (with the exception of velocity strings) are services to existing wells, with very few permanent production applications. A new CT application, using CT as a permanent production tubing string for artificial lift in shallow, low-production wells, has been developed. In this new application as described in this paper, CT is used to reciprocate a subsurface pump plunger. This CT pumping system is illustrated and engineering principles behind the system are presented. Engineering feasibility and reliability of the system, including fatigue problems, are investigated. Sensitivity of pump system fatigue to pump rate, CT and casing dimensions, CT material grade, pump length, vertical well depth, and fluid density is analyzed. Finally, a successful on-going field example is presented. More tests with the pump system are planned for deeper and higher production wells in the near future. It is believed that the new CT pumping system has the potential to significantly impact the economics of artificial lift systems, especially in shallow and low-production wells.
Coiled tubing has been used in the oil industry for a number of years, primarily for well service work, although some long-term downhole installations such as velocity strings have also been applied.
In 1995, an idea was developed that, for low volumes and shallow depths, pressure losses through small internal-diameter tubing would not prevent the CT from serving as a production string. A subsurface pump was designed for this application, and the design was patented.
This design allows for smaller, less costly holes to be drilled. Larger hole sizes are often drilled to accommodate sucker-rod and jointed tubing production strings.
Another concern in pumping wells is the need for an annular space that collects fluids commingled from different zones and that acts as a chamber for segregating associated gas.
Slim holes as a solution for producing wells was considered by YPF S.A. in early 1997 and a pilot test was commissioned.
At the beginning of the program, we knew that a slim hole cased with a small diameter such as 3.5 or even 2.875-in. string could be serviced with CT. Also, hollow sucker rods could pump and convey fluids to the surface.
However, lack of available hollow sucker rods in the right size required another solution. This led to the idea of using coiled tubing instead.
Tubing elongation caused by the tubing and fluid weight as well as pumping speed was a concern that was carefully considered. Simple calculations showed that elongation was not a problem at shallow depths, but a question remained regarding the effect of reciprocation speed on fatigue life. An actual test was the only way to know whether coiled tubing would sustain constant reciprocation, such as experienced by sucker rods.
Availability of a hydraulic pumping unit encouraged a test at slow speeds, aimed at reducing the risk of failures. But a more aggressive approach was taken by testing the coiled tubing production string with a standard beam-pumping unit.