Coiled Tubing Times - oil & gas service, fracturing, coil tubing, well intervention, workover

Journal

Issue 39, March 2012

THE USE OF COILED TUBING AS A VELOCITY STRING

Productivity of a number of gas wells is decreasing as a result of water accumulation at the bottomhole leading to slowing-down the gas flow inside the well. This is extremely urgent for mature oil fields. If the liquid level in well is not attenuated, liquid is accumulated and aggravates the conditions of gas production, upon that well productivity could be significantly reduced up to complete output stop and well killing. It happens because of gas undercapacity to transport liquid accumulated inside the wellbore. Liquid accumulation leads to backpressure which precludes influx from the oil zone. In order to avoid liquid accumulation at the bottomhole water and condensate must be lifted to surface together with the gas.

The most efficient method for well recovery is installing smaller-diameter tubes in production string. Reducing the flow area of a gas well increases gas capacity to transport liquid up to wellhole. Liquid does not accumulate at the bottomhole and productivity restores. Upon that gas velocity should not exceed critical velocity needed for prevention of liquid accumulation at the bottomhole. Installing smaller-diameter strings can be done without killing a well. Used for this purpose string of smaller-diameter tubes are referred to as velocity string.

Velocity string is a string of smaller diameter tubes running inside macaroni tubing in gas well aimed at solving problems on lifting liquid out of the well.

Installing velocity strings lets to reduce liquid flow section and to increase its velocity that allows transporting liquid out of the well. As a rule coiled tubing is used as a velocity string.

Safe operations without killing the well and quick mobilization allow velocity strings to ensure economically efficient solution of liquid transportation out of the well. Safe operations without killing a well and quick mobilization let velocity strings of coiled tubing to ensure economically efficient liquid lifting out of the well.

Liquid accumulation at the bottomhole can be detected by routine analysis of well productivity. The most precise method for identifying liquid accumulation can be done by analyzing pressure variations in flow velocity and in bottomhole.

To get liquid lifted out of the well it is needed to create conditions when liquid mixes with gas and is raised up. Taking into account relatively low gas flow velocity gases are in water as separate bubbles defused in fluid medium. Gas bubbles at average moves faster than liquid. As far as the flow velocity grows the bubbles start form bigger bubbles. They are separating from liquid phase which could contain separate small gas bubbles. Further flow velocity grow leads to gas-liquid mixture - intermediate flow. As far as gas flow velocity grow turbulence and gas-droplet mixture are enhancing. It is typical for fast moving gas transporting droplets upward.

Liquid is removed from the well bore by the combination of interfacial shear and drag on the entrained droplets. That means that gas velocity must be sufficient to carry the entrained liquid droplets upward and is obtained determining a force balance between the gravity and drag forces on a droplet:

where C_d - drag coefficient;

d_k - drop diameter, m;

ρ_l - liquid density, kg/m³;

ρ_g - gas density, kg/m³;

v_g - gas velocity, m/s;

g - acceleration due to gravity, m/s².

Use this formula (1) to determine the minimum allowable gas velocity v_g (2):

With the use of the velocity string it is needed to determine the most effective coiled tubing size to install, optimum setting depth of coiled tubing and well production rate after the velocity string has been placed in operation:

where q - production rate, m³/day;

p_r - average reservoir pressure, Pa;

p_wf - well flowing pressure, Pa;

С - performance coefficient from well data;

n - exponent obtained from well tests.

The size and length of coiled tubing is dictated by previous pipe installation.

The following design parameters of coiled tubing string should be considered:

maximum load, interior and external pressure allowed.
running load.
load based on geometric well configuration.
Coiled tubing metal chemistry ensuring its performance while in service.

The use of coiled tubing as a velocity string is widely spread abroad. The experience of our colleagues abroad shows that the use of velocity strings is efficient way for rising debit of gas wells having liquid accumulation down the hole.

Currently the equipment for deployment of coiled tubing used as velocity string is produced abroad.

While performing velocity string it is needed to carry out coiled tubing intervention operation down a well one time instead of repeated running-retrieving operations while using coiled tubing for workover. Thus, rigid requirements for bend tolerance to this coiled tubing are not specified. The velocity string must be specified as sufficient mechanical data for one-time running and further performance.

Currently many steel types are produced in the country, they can be recommended for producing velocity strings that allows production of various-characteristic coiled tubing.

Using of domestic steel types will allow eliminating dependence on imported steel. Besides that, supply of domestic strip steel-rolled stock is done much faster that ensures short-term coiled tubing production.

REFERENCES

Lea, J. F., Nickens, H. V. Solving Gas-Well Liquid-Loading Problems / J. F. Lea, H. V. Nickens. - SPE 72092 // Journal of Petroleum Technology, April, 2004.
Wesson, H. R., Jr. Coiled tubing velocity/siphon string design and installation / H. R. Wesson // Coiled Tubing Operations & Slimhole Drilling Practices, Houston, 1993.
Brown, K. E. Production optimization of oil and gas wells by nodal systems analysis / K. E. Brown. - PennWell Publishing Company, Tulsa, 1983.
Adams, L.S. Design and Installation of Deep Coiled Tubing Velocity Strings / L.S. Adams // Coiled Tubing Operations & Slimhole Drilling Practices, Houston, 1993.
Matanović, D., Gaurina-Međimurec, N., Simon, K. The Use of Coiled Tubing as a Velocity String / D. Matanović, N. Gaurina-Međimurec, K. Simon // Faculty of mining, geology and petroleum engineering, Zagreb, 2004.
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