Coil Tubing Plug Retrieval/Fishing Operations On Deep, High Pressure Sour Wells
Authors: Stadlwieser J. , Howes M. , Cawston R. 
Published: March 2010

Abstract
This paper discusses coil tubing operations performed on three sour gas wells in which coil tubing was used to retrieve plugs where conventional wireline methods proved ineffective.
All three wells are located in the Obed Field of Alberta, which produces high pressure sour gas. The field was suspended in March 1992 with wireline plugs set to isolate the down-hole formation. All of the wells had severe corrosion problems prior to suspension, making plug setting and retrieval difficult.
Coil tubing offered the versatility of jetting and circulating, allowing access to the down-hole plugs in a sloughing scale environment.
On the first two reactivations, coil tubing was used for descaling tubing and washing onto plugs. During the third reactivation, the coil tubing was parted, leaving 3,000 m of 31.8 mm coil tubing (CT) down-hole. The 31.8 mm CT was successfully retrieved using 50.8 mm CT in conjunction with wireline chemical cutters.

Introduction
The Obed Field is located near Hinton, Alberta. The field was purchased and reactivated by Chevron Canada Resources in August 1992. The field is comprised of five producing wells, three of which are completed in the Nisku formation and two in the Leduc formation. The wells are approximately 4,000 m deep and produce gas with 26–33% H2S and 6–9% CO2. Bottom-hole static pressures for the wells range from 35 to 38 MPa. The tubing in all of the wells is 114.3 mm, L-80 VAM ACE with 88.9 mm VAM ACE corrosion resistant alloy (lncoloy/Hasteloy) tailpipe. Figures 1, 2, and 3 are schematics of the wells.
All of the wells have a history of severe scale and corrosion problems. Prior to being suspended in March 1992, the wells had been on a Diaryl Disulfide (DADS) batch treatment program for sulphur deposition prevention.
In August 1992, attempts were made to retrieve the down-hole wireline set plugs from three wells. All three plugs were leaking and could not be pulled by conventional wireline methods due to scale buildup. Some sulphur buildup was encountered down to 1,000 m, with a mix of carbonate and iron sulphide scale existing from 2,000 m to bottom.

Options Available to Retrieve Plugs
When Chevron reactivated the Obed Field, it was assumed that the plugs could be retrieved using conventional wire line methods. Normally, plug retrieval using wireline is quite effective in vertical wells wiLh up to 60 ° deviation.
The following options were available for scale control, plug retrieval and fishing operations:Wireline prong/plug retrieval.Wireline bailing for scale removal.Wireline fishing.Coil tubing jetting/acid washing for descaling tubing.Coil tubing used for washing over fish.Coil tubing used to clean out sloughed scale and pull prongs/plugs.
Initially, pulling the prongs and plugs with wireline techniques was selected, since it was the most economical and, historically, had been successful in the Obed field. Initial runs were unsuccessful due to sloughed scale which made it impossible to reach the prongs/plugs. Consequently, an attempt was made to bail out the fill, but this also proved to be ineffective.