Multiwell Deployment of a New Abandonment System
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A new downhole-tool-based abandonment system was developed and deployed successfully on four wells for a major operator on a field in the North Sea. The operations were executed with each well taking less than 18.5 hours to secure. The successful operation saved the major operator considerable time and expense by eliminating the need for cutting and pulling the 10¾-in. casing to remove the oil-based mud (OBM) from the annulus before removing the wellheads.
Service companies were challenged by a major operator to create a solution to set a barrier against the overburden and to circulate OBM out of the annulus between the 10¾- and 13⅜-in. casings before pulling the wellhead.
The first stage of the operation was to run a perforation gun loaded for 1 ft with 18 shots/ft (spf) of a proprietary abandonment charge (single-casing perforation gun) to immediately below the wellhead at 475 ft. Then, the 10¾‑in. casing was perforated with 0.8‑in.-diameter holes without damaging the 13⅜-in. casing to create a circulation path.
The second stage was to run a retrievable bridge plug (RBP) with another 1-ft-long perforation gun below. The RBP was set and perforated immediately above the 13⅜-in. shoe at 2,300 ft; then, circulation was established up to the shallow perforations above and the OBM in the 10¾- by 13⅜-in. annulus was circulated out. After the circulation parameters were established, a wash pill was pumped around the annulus to clean out the OBM.
The third step was to set the actual overburden barrier in the A and B annuli. This was achieved by displacing cement through the ball valve of the RBP into the perforations below the RBP, placing the cement plug below and into the 10¾- by 13⅜-in. annulus. The ball valve was closed, and a cement plug was pumped on top of the RBP, completing the barrier.
The North Sea’s Leadon Field lies in 370 ft of water and is located in Blocks 9/14a and 9/14b of the UK Continental Shelf approximately 220 miles northeast of Aberdeen. Field development was enabled by the addition of two satellite fields, Birse and Glassel. The three fields were developed with subsea horizontal wells tied back to a floating production, storage, and offloading facility. The Lark and Horda formations produce in two well clusters, A and B. Cluster A has seven production wells and two water injectors; Cluster B consists of three production wells, two water injectors, and two aquifer wells. Both clusters have space for additional wells.
After a commercially successful period, production eventually declined, leading to a Cessation of Production Application being filed by the operator in 2004. A decommissioning program for the field was approved in March 2016.
Compliance and Risk
Two methods present themselves for OBM removal. In the traditional method, the 9⅝-in. or 10¾-in. casing is cut and is pulled in sections or section milled until the entire column of OBM is exposed and circulated to surface. A deleterious effect of this process is contamination of the milling fluid with OBM. The cutting and milling process is also the most time-consuming of all the alternatives. The second method uses perforating to create a circulation path and is the focus of the complete paper.
The most-attractive proposition is to create circulation ports above and below the annular column of OBM, then inject fluid into the lower ports and circulate the OBM up the annulus and back into the casing through the upper ports. The OBM can then be collected and disposed of with less need for separation. This method also eliminates the need to remove the casing.
With the possibility of an alternative circulation path established, the use of that path for placement of a cement barrier can be investigated. For cement to be placed effectively in the annulus, the 13⅜-in. shoe needs to be secured hydraulically; this can be verified beforehand with logs. With a secure shoe and an annular circulation path, a method of isolating the casing between the two sets of ports or perforations is required in order to circulate the annulus from the lower to the upper ports.
This abandonment process was performed on a four-well program in the Leadon Field starting with Well 9/14b-S2X. While all the wells were essentially similar, with near-identical programs, the first well contained an additional contingency step to isolate a second interval; that operation is discussed in the complete paper. The well operation for Well 9/14-S2X, which is representative of the other wells, is described in the following section. The planned scope of operation was to perforate the 9⅝‑in. casing to circulate out OBM in the annulus from 500 to 2,000 ft and plug permanently.
Run Number One. Run in hole (RIH) with one 21-ft perforating gun loaded for 1 ft with 18 spf of charges to produce 0.8-in.-diameter holes in the 9⅝-in. casing. It is important here to run the gun without centralization so that it lands on the low side where this charge will provide 120° of coverage. This charge was originally for 9⅝-in. 47- to 53-lbm/ft casing and was tested for 10¾-in. 55-lbm/ft casing.
Next, RIH to 12 ft below the wellhead datum at 474.36 ft. Pick up the Kelly cock, the side-entry Kelly cock, and a 5-ft pup-joint assembly. Space out, close the upper preventer, and ensure that the topdrive is disconnected. Open the Kelly cock, drop a firing ball, close the upper Kelly cock, and displace the 1-in.-diameter ball down to the firing head. Pressure up the string to 2,500 psi, and fire the guns. Continue then to pressure up to 3,500 psi to shear out the burst-disk sub if it has not already been burst by a shock wave from the guns. Note that there is a strong potential for gas in the annulus, so the annular must remain closed.
Bleed off any gas, after which the well should be static. Open the annular and perform a flow check. Circulate bottoms up if needed by circulating through the burst-disk sub, then pull out of the hole (POOH). Once the guns are at surface, inspect the scallops in the gun body to verify effective charge detonation.
Run Number Two. Pick up a 9⅝-in. locking-type RBP with a ball valve to use as a fundament plug, and run below it a 311-ft gun assembly with 1 ft of 12 spf for ½-in. holes RIH to 30 ft above the 13⅜-in. shoe at 2,375 ft and close the annular preventer. Set the RBP and set down the available string weight of approximately 50,000 lbm; set down 44,000 lbf on the plug.
Drop a 1-in. ball and pressure up to 2,500 psi to fire the guns; then continue to pressure up to 3,500 psi to burst the burst disks. Stage up the pumps and circulate the annulus through the 1.75-in.-diameter burst-disk sub port. Limit pressure to 560 psi surface to avoid breaking down the 13⅜-in. shoe.
If no circulation is possible, attempt to reverse circulate through the annulus and, again, limit the pressure to 560 psi. If reverse circulation is not possible, pressure up the drillpipe to 560 psi and hold for 2 hours. If returns are observed, very slowly increase the flow rate until the blockage clears and full return is observed. If circulation cannot be achieved, POOH with the plug and the release assembly and reperforate and circulate at a shallower depth. Pump 30 bbl of wash pill around the annulus and displace with seawater; then, with seawater inside the pipe and in the annulus, flow check the well for 15 minutes.
First Intermediate Isolation Plug. Mix and pump 30 bbl of cement to place a 500-ft cement plug below the RBP. Once cement is displaced to the fundament RBP, shut down the pumps and hold pressure. Close the ball valve. With left-hand torque in the string, set down 20,000 lbf to collapse the collet and pick up to release from the J-slot in the RBP. Watch for the pipe rotating free as an indication that the running tool is released. Perform a 5-minute flow check. Circulate bottoms up to clear the string of any green cement.
Intermediate Abandonment Plug. Mix and pump the abandonment plug to the cementing-company procedures for a 500-ft-high, 16-lbm/gal cement plug on top of the RBP fundament plug. Break off the cement line and check for vacuum or backflow. POOH through the cement plug slowly at 18–20 ft/min.
Continue to POOH above the cement plug at 25 ft/min to place the bottom of the string approximately 500 ft above the theoretical top of cement. Install a foam ball and make up the string to the topdrive. Stage up the pumps and circulate the well clean with seawater. POOH and rack back the drillpipe while checking the string for any internal or external cement sheath. Wait on the cement, then pressure test the well to 1,100 psi.
- When difficulty was experienced in achieving circulation, reverse circulation was found to be successful.
- When attempting alternative-path cementing, be sure to displace all lines with spacer.
- When using pump-and-pull techniques on tools in balanced cement plugs, set the pressure limit on the cement unit lower to prevent early shear out.
There are substantial efficiency gains to be had when pressure testing A and B annuli together for the regulatory authorities. From 21 hours on a conventional cut-and-pull operation, a two-trip plug-and-perforate operation reduced the time to 18¾–15½ hours. Moving to a single-trip operation further reduced the time to less than 10 hours.
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