Intervention with scale milling on subsea wells have traditionally been performed with semi submersible rigs and heavy intervention equipment as coiled tubing. Riserless Light Well Intervention (RLWI) with wireline has so far been a practical and cost efficient alternative in the subsea intervention marked. The on-going development of wireline tractor technology including milling bits has increased the scope of work for RLWI to include mechanical scale removal.
This paper describes a case history of first time use of wireline tractor scale milling technology in a well on the high temperature Åsgard field deployed from a RLWI vessel. The well had a history of calcium carbonate scale build in the production liner first seen in 2003. The well had been intervened using coiled tubing in 2003 without being able to remove all the scale bridges completely.
The objective of the intervention was to remove a 23 m calcium carbonate scale bridge to provide access for installation of a high pressure high temperature (HPHT) bridge plug and permit subsequent additional perforation work. An electric wireline conveyed tractor with a rotational assembly was chosen to perform the job.
A qualification test programme was performed to confirm that the selected wireline tractor and milling assembly could operate reliably in an environment up to 166 °C for extended periods without tool failures. The maximum well temperature was 152 °C. Following successful trials in the test facility the equipment was mobilized to the RLWI vessel.
The scale bridge of 23 m was milled in 3 runs with a 5.72" custom designed milling bit. Subsequent operations were successful with installation of a HPHT bridge plug and completion of four additional perforation runs.
The production results after the intervention was an oil increase from 468 Sm3/d to 1204 Sm3/d. The Gas Oil Ratio was decreased from 3500 Sm3/Sm3 to 2750 Sm3/Sm3. The successful operation by scale milling on the Smørbukk field has demonstrated that wireline tractor scale milling technology can be used on high temperature wells using the RLWI concept.
The Åsgard development, located in the Haltenbanken area off Mid-Norway, consists of three independent discoveries (Smørbukk, Smørbukk Sør and Midgard) connected to two floating production installations and a condensate storage vessel (Figure 1). Åsgard is developed with subsea wells only, and started production in 1999 .
The operation described in this paper was performed at Smørbukk, a high temperature gas condensate field, producing from five independent reservoir layers. The wells are completed with 7" production tubing and 7" cemented and perforated liner. Most of the wells produce commingled from the different reservoir zones. The field is produced mainly by depletion with some pressure support from gas injection.