Development of HP-HT Coiled Tubing Unit.
This paper presents results on the feasibility studies, development and first operations of a coiled tubing unit for use in deep high pressure and high temperature oil wells.
The coiled tubing unit was required to work primarily in the Villa Fortuna/Trecate oilfield in Northern Italy. This field had an initial reservoir pressure of about 14500 psi at 18400 ft TVD and static wellhead pressure of 8500 psi; reservoir temperature is 330 F. A coiled tubing unit was required to carry out operations such as acid/solvent treatments, washing, fishing etc.
This paper is focused in the following items:
laboratory and field tests, performed for the evaluation of stripper rubber life at 10.000 psi, and tubing failure through ovality.
The modifications of a coiled tubing unit standard components.
Pipe design optimisation with stress analysis software to ensure that the operations could be carried out with the tubing in its maximum allowable ovality state.
Quality control: new tubing ovality monitoring and data acquisition system for coiled tubing O.D. measurement.
Furthermore the first "live" well operation and the first operation in Villa Fortuna/Trecate field are reported in the paper.
The development of the Villafortuna Trecate field led to the necessity of extending the operations performed with standard coiled tubing unit in high pressure and high temperature wells.
On the basis of this industry need a research project started in 1990 for the design and construction of a coiled tubing unit able to perform the following operations in high pressure and high temperature wells:
asphaltene deposit removal with chemical solvent;
matrix stimulation by acid placement across the perforations;
displacement of high density brines in emergency well control situations,
At the time of the project the main operative limits for a standard coiled tubing unit were: maximum wellhead working pressure 5000 psi, maximum depth 17.000 ft, maximum pulling force 25.000 lbs. The industry standard injector head of that time had a nominal pulling capability of 40.000 lb, however, under field conditions slippage of the tubing within the gripper system often occurred when pulling forces exceeded 25.000 lb. It was clear that such behaviour is the result of differing structural compliance for the tubing and the injector chains. Furthermore the wear capability of the stripper rubbers was long enough only with a wellhead pressure less than 3000 psi.
In order to have access at Villafortuna Trecate wells the operating limits had to be increased: in particular the maximum wellhead working pressure had to be 10.000 psi, the maximum depth 22.000 ft, maximum pulling force 60.000 lbs. In order to perform the operations in a safe condition the stripper rubbers wear capabilities had to be satisfactory at 10.000 psi and 240 F.