How to reduce heat loss on couplings of Vacuum Insulated Tubing and Casing of OCTG
VIT is one important kind of OCTG for steam injection in heavy oil field. It can reduce heat loss during steam injection, improve steam injection and oil recovery efficiency, and protect oil well casing and cementing sheath from thermal stress damage.
In the existing structure of ordinary VIT string, most of the tubing couplings have no vacuum insulation layer, and their thermal resistance is too small so that the radial heat loss is far greater than other parts loss. After calculation, the coupling heat loss is about 70% of the whole tubing, which has become one of the main reasons affecting the heat insulation effect
At the same time, the thermal stress in this part of the area is the largest, and it is easy to form an unsafe area after many times of steam huff and puff operation.
In order to solve this problem, some Chinese enterprises developed direct connected VIT to replace ordinary one. The thread structure has been improved so the connection strength and compression resistance have been improved. However, the thread connection part is still not covered with thermal insulation layer, only part of the heat loss is reduced with unsatisfactory effect.
Sanjack improved the design and invented one kind of directly connected VIT with full length insulation, which is especially suitable for heavy oil recovery.
The VIT invention comprises the inner tube, insulating layer, outer tube, internal thread and external thread.
One end of the inner tube is processed with internal thread, the other end is processed with external thread.
The inner tube at the internal thread end is directly connected with the inner tube at the external thread end of another VIT joint.
The end of the inner tube of the external thread is normally thicken, and the end of the inner tube of the internal thread also needs thicken, however, the thicken OD of is 10-16mm larger than that of the inner tube end of the external thread.
The outer tube end of the internal thread end needs flaring; the clearance between the ID of the flaring part and the OD of the thickened end of the internal pipe at the internal thread is δ.
The thickness of the insulation layer between the inner tube and the outer tube is θ, and δ≈ θ.
The insulating layer is also wound in the annular space at the inner tube upsetting part of the inner thread end and the outer tube flaring part of the internal thread end.
The insulation layer is wound at overall length in the annular space between the inner tube and the outer tube.
A large amount of heat loss of the connection is avoided and the heat preservation effect is greatly improved.
The insulation layer is wound at overall length in the annular space between the inner tube 1 and the outer tube 3, realizing the full-length heat preservation, greatly improving the heat preservation effect, improving the steam injection efficiency and saving the energy.