The capillary string methodology is part of the ‘gas well deliquification’ also referred to as "gas well dewatering", and is the general term for technologies used to remove water or condensates build-up from producing gas wells.
When natural gas flows to the surface in a producing gas well, the gas carries liquids to the surface if the velocity of the gas is high enough. A high gas velocity results in a mist flow pattern in which liquids are finely dispersed in the gas. Consequently, a low volume of liquid is present in the tubing or production conduit, resulting in a pressure drop caused by gravity acting on the flowing fluids.
As the gas velocity in the production tubing drops with time, the velocity of the liquids carried by the gas declines even faster. Flow patterns of liquids on the walls of the conduit cause liquid to accumulate in the bottom of the well, which can either slow or stop gas production altogether.
Possible solutions to this problem include the installation of a ‘velocity string’ and or of a ‘capillary string’.
The capillary string is a technology used in the marginal wells/field to lift by nitrogen or to inject foamer in order to help the production of the bottom hole accumulated liquids. The positioning of the capillary string does not require any workover rig, just a coiled tubing unit and at the end the string will result hanged to the well head.
This methodology usually consists of a string of coiled tubing, usually with a diameter of ½”, 3/8” or 1”, descending into the well and anchor to the wellhead, through which to inject the foamer and lighten the hydrostatic column.
The technology proposed by Excellence Logging allows the application of this methodology in wells that require the presence of a safety valve but were completed without any external control line. This technology can be performed in wells completed with tubing from 2 3/8" to 3 1/2" and higher, where the only requirement is the presence of a landing nipple for anchoring the safety injection valve.
A special safety injection valve was developed to ensure the safety of the well and at the same time allow the injection of emulsifier / foamer. The valve is fail-safe: when pressurized from above before it first opens the 'safety section' and then, further increasing the pressure, opens the 'injection section '. The return from the well (production) is via the 'safety section'. When decreasing the pressure of injection, it first closes the 'injection section' and then the 'safety section / production'.
The whole operation is performed with our "mini coiled tubing unit" and the installation of the system is executed in two phases, both with the well always under pressure and therefore with coiled tubing BOP-installed:
Step 1: Run the first section of the capillary string (lower section) in the well, with a check valve at the bottom and connected to the lower part of the safety injection valve that is set into the dedicated landing nipple by means of a self-lock mandrel. At this point the valve is fastened and secured in the closed position: the well is safe!
Step 2: Run a special snap-in, snap-out-stinger connected to the bottom of what will become the upper section of the capillary string to tie back the top of the safety valve and ensure the sealing and hydraulic continuity. Once the stinger is inserted in the valve, the upper section has to be cut and suspended in the wellhead through the hager/connector provided by the manufacturer of the wellhead. Once landed in the wellhead the upper capillary tube section is going to have full hydraulic continuity.
As an alternative and where possible a WH feed-through can be also used for the suspension of the capillary to the wellhead.
At this point pumping from the surface moves the safety valve in the open position and then, while increasing the pressure, open the injection valve and thus the injection of the emulsifier fluid in the well can start. Decreasing the injection pressure first stops the injection phase, and then lock closes the safety valve.