Бурение грунтовых зондов, установка энергетических колодцев

In the previous section, it has been theoretically verified that the casing tempera­ture can be lowered appreciably by applying coatings (mechanical barriers) to the tubing string. Experimental investigation by Leutwyler (1966) also shows that the overall heat transfer coefficient can be reduced to about 11% of the bare pipe level by the application of a coating of 1-in. calcium silicate insulation within an aluminium jacket (see Fig. 4.41). This has led to the development of well completions that utilize insulated tubing and a packed-off annulus.

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1 i i i i ]—i—i—i—i—|—

BARE PIPE

RUSTY AND SCALED

BARE PIPE

CLEAN AND PAINTED BLACK 3/1 6" ASBESTOS WRAP

PIPE COATED WITH ALUMUNIUM PAINT

1/2" CALCIUM SIUCATE INSULATION WITHOUT ALUMINIUM JACKET

1/2" CALCIUM SIUCATE INSULATION WITH ALUMINIUM JACKET

1" CALCIUM SIUCATE INSULATION WITH ALUMINIUM JACKET

TOC o "1-5" h z 1—- 1- 1__ I_ I__ I_ I_ I I I

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Fig. 4.41: Overall heat transfer coefficient for tubing with different types of insulation. (After Leutwyler, 1966: courtesy J. Petrol. Technol.)

Figure 4.42 shows the basic elements of a steam injector. The annulus is packed off with a high-temperature packer. Thermal elongation of the tubing is integrated into the packer assembly. To avoid steam breakthrough at the packer, nitrogen gas is injected into the annulus under wellhead pressure.

Figure 4.43 shows the various design elements of a typical insulated tubing. The inner tubing is welded to the outer tubing in a pre-stressed state to compensate for the differential thermal elongations. The annulus between the inner and outer

tubing is insulated with calcium silicate shells. To date, over a million feet of insulated tubulars of this kind have been manufactured and installed successfully. Besides using high-temperature insulation, the insulated tubing has gone through a number of additional improvements in design. As an example. Fig. 4.44 shows the Kawasaki integral tubing and tubing coupling. The insulation quality is improved by means of a multilayer fiber glass insulation material with radiation barriers and an evacuated annulus between the two concentric strings.

The thermal packers have always been one of the critical elements in completing injection wells. The typical leak-resistance time of thermal packers at temper­atures exceeding 570 °F has varied from several days to several months. An investigation conducted by Goetzen (1990) showed that no thermal packer was available to effectively seal the annulus for the normal injection period of ‘2 to 4 years.

To minimize the differential pressure at the packer, the annulus between tubing and casing is filled with nitrogen at wellhead pressure immediately after packer installation and annulus steamout. Loss of nitrogen can be overcome by main­taining a nitrogen bottle battery at the wellhead.

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