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Section Milling during P&A

The Abandonator P&A unit was used to successfully remediate the well, set the cement plug, and further isolate the well.
NAM Case Study Photo

Background

A former gas producing well in the Netherlands, drilled in 1969, was due to be abandoned. The annulus pressure were monitored since the well stopped producing. The B annulus was showing a pressure build up and was being bled down when appropriate.

During pre-engineering of the well a cement logging run revealed bad cement over the interval of the caprock and samples of the bled down gas showed traces of reservoir gas. Because of this, it was decided to remediate the annulus. However, there was only 20m of good caprock available to set a cement plug with a 5m rat hole below.

The Abandonator P&A unit was used to successfully remediate the well, set the cement plug, and further isolate the well.

Objective

  • Retrieve 1760m of 5" tubing with fibreoptic cable to cutting depth
  • Mill a 21.5m window inside the 9-5/8" casing at 1735m with a 20° inclination
  • Under-ream the 20m window from 12-1/4" to 15" to create a good bond for the cement plug
  • Set a balanced cement plug

Execution

Milling Window

After successfully retrieving the 5" tubing with the control line, the section milling 144m BHA was ran in, fitted with the following:

  • 8-1/2" tapered mill
  • 4m extension
  • AMT section mill
  • Jetting sub
  • Ported float sub
  • Shock-sub
  • 7x 6-1/2" Drill Collars
  • Drilling Jar
  • 2x 6-1/2" Drill Collars
  • 5x 5" HWDP

After the BHA was ran into the hole and tubing stump tagged at 1762m , the process of locating the casing collars to determine the final cut-out depth was performed. At the cut-out depth the mud was conditioned to 1.58sg WBM mud with a viscosity between 40 - 50 Cp.

The Cut out was made with 2100 lpm, 135 Bar, 90 RPM, 6600 ft-lbs, and from there on the window was successfully milled from 1735m to 1756.5m.

Under Ream and Plugging

The next run planned was to under ream the milled section from 12-1/4" to 15" to guarantee a good cement bond against fresh formation.

The Under Ream BHA consisted of the following elements:

  • 8-1/2" tapered mill
  • 2.5m Extension
  • Perseus dress UR knives
  • Jetting Sub
  • Ported float sub
  • Shock-sub
  • 7x 6-1/2" Drill Collar
  • Drilling Jar
  • 2x 6-1/2" Drill Collar
  • 5x 5" HWDP

The same mud was used as on the previous run, 1.59 WBM with a viscosity between 40 - 50 Cp.

The next run was a 5" cement string consisting of 5" drill pipe combined with a closed end slotted mule shoe. A pancake type cement plug was successfully set and tagged.

Results

  • Successfully milled 21.5m section into 9-5/8" casing against poor cement/open hole, with the challenge of only a 7m rathole till top of tubing stump and 12m till to of production packer
  • Successfully opened up the milled section to 15" to create a proper bond for the cement plug
  • Successfully set 20m cement inside window + 130m inside 9-5/8" casing
  • Pressure build up on B-Annulus not observed after setting the cement plug

Challenges

  • Heatwave during operations which lead to outside temperature of up 35° C
  • High mud temperatures of 87° C BHT and 75° C at surface meant extra risk while cleaning magnets etc.