Perforation prepacking refers to the methods that can be used to pack the perforation tunnels with gravel prior to filling the screen/casing annulus. The main objective of prepacking is to maximise the quantity of high permeability gravel placed in the perforation tunnels (or in cavities behind the casing) which is one of the key factors controlling the productivity of an IGP completion. Field experience indicates that in many cases standard gravel packing procedures do not allow proper packing of the perforations.

1 Guidelines for prepacking operations

1.1 Introduction

Prepacking can be carried out before or after the GP liner assembly is run in place. When carried out after perforating operations, prepacking results in more efficient and less damaging well kill operations, especially when large losses are incurred after perforating without prepacking.

1.2 Applicability

The perforation tunnels are packed with gravel immediately after backsurging or backflowing operations and then sealed with a graded salt LCM system. Hence the risk of impairment is strongly reduced by keeping the perforations clean. The subsequent LCM removal operations are also made easier as the LCM is placed as near as possible to the wellbore and the surface area that the LCM is required to cover is reduced. As an additional benefit, it is easier to size the LCM to bridge on gravel, which has a more uniform size distribution, than on formation sand.

There are basically no restrictions to the technique outlined below. However the following factors should be considered when selecting candidate intervals:

·Losses after perforating: Prepacking is especially applicable to intervals which require the use of fluid loss control material. There is however no reason why this technique could not be used in situations where serious losses do not occur. The use of graded salt particles to seal the perforation tunnels will then be superfluous.

·Interval length: Prepacking is considered especially beneficial for longer intervals as field experience shows that the perforation packing efficiency of standard gravel packing operations decreases very quickly with increasing interval length (>10 feet).

1.3 Operations outline

The following procedure is based on early prepacking trials carried out by BSP. Operators may need to adjust it to their specific needs. Although this procedure is based on TCP operations, prepacking can in principle be considered after perforation washing or backsurging operations.

1.4 Early BSP prepack procedures

Early BSP prepack

1.RIH Otis sump packer with expendable knock-out plug.

2.RIH TCP assembly.

3.RIH MWPT.

4.Detonate gun and flow well till liquid at surface.

5.Record buildup.

6.Retrieve MWPT.

7.Close PCT and open MIRV.

8.Reverse out influx (pump citric acid pill to clean tubing and casing.

9.Close MIRV.

10.Open PCT (in hold open option).

11.Unset packer.

12.Measure losse.

13.Tag fill.

14.Pull gun above perforations set packer.

15.Perform injectivity test.

16.Acidise if injection less than 30L/m perforations.

17.Pump prepack slurry through packer bypass:

-Slurry: 750 kg/m3 Gravel (20/40mesh)

-12.8 kg/m3 HEC

or

-53 L/m3 Shellflo-S

-30 L/m334% HCL

-015 L/m3HAI -65

-In 12% KCL inhibited brine

-Slurry volume: 40 L/m3 net perfs

18.Close packer bypass and squeeze slurry into perforations.

19.Close PCT open MIRV.

20.Reverse out excess gravel (measure returns).

21.Close MIRV, open PCT unset packer.

22.Measure loss rate.

23.Tag and measure fill.

24.If loss rate excessive and pack factor above 20 L/m then - spot graded salt pill.

25.POH.

26.RIH WWS with modified washing tool.

27.Washout fill till sump packer and measure return - use jet option if graded salt is used and wash perforations till pre-LCM loss rate is achieved.

28.Stab into sump packer.

29.Drop dart and set packer.

30.Slurry pack well.

1.5 Potential problems

• ·Tool sticking: This is a potential problem when the gravel slurry and LCM pills are pumped downhole. TCP guns may be dropped prior to pumping the gravel slurry, but this will result in difficulties in establishing prepacked gravel volumes. An alternative scheme is to pump the gravel through a ported sub located above the TCP gun. The guns must be pulled above the top perforations prior to pumping the gravel to avoid sticking problems.
• ·LCM removal: Salt LCM is removed by circulating undersaturated brine along the interval. Diversion of brine is a potential problem for intervals with a high permeability contrast.

2 Gravel packing procedures

Case where no lower telltale is used

The slurry is circulated down to the gravel pack ports at which stage back-pressure is applied up to a specified figure. The amount of back-pressure is calculated to limit the pressure on the formation to 200 psi below the fracture propagation pressure.

The objective of applying back-pressure is to squeeze part of the spacer and slurry into the formation. Losses during slurry packing are to be expected and are desirable. Injectivity is essential to ensure that a tight "pack" is obtained in the tunnels.

The following procedure is to be followed:

1.Set the liner hanger packer. Establish the squeeze and circulating positions of the GP tool.

2.With the GP tool in the reverse position, establish circulation pressures at 2, 4, 6 and 8 bbls/min. Check for losses.

Use a low bottom hole pressure ball in the crossover tool if losses are expected.

3.With the GP tool in squeeze position, establish the squeeze pressures at rates of 1/2, 1 and 2 bbls/min not exceeding the surface pressure calculated (BP-max).

The type of pack (slurry pack or conventional pack) installed is determined by the injectivity test. Where the stabilised injection rate is more than 1 bpm, then the slurry pack will be performed. Where the injection rate is less than 1 bpm, an acid squeeze (15% HCL, 20 gal/ft of perforation) will be required and the injectivity test repeated. If the repeated injection rate is more than 0.5 bpm, then the gravel will be slurry packed, otherwise it should be conventionally placed.

4.With GP tool in gravel pack position:

• -pump 8 bbls of carrier fluid spacer;
• -pump gravel slurry from mixer/blender;
• -pump 2 bbls of carrier fluid spacer;
• -displace front spacer to the GP ports at 5-6 bbls/min, applying a backpressure sufficient to avoid U-tubing due to the higher gradient of the slurry;
• -when the front spacer is 2 bbls from the GP port s, increase the backpressure to the value calculated above (BP), while reducing the pumprate slowly to 2 bbls/min;
• -continue pumping at 2 bbls/min to screen out pressure of 1500 psi above the applied backpressure;
• -open choke completely and reconfirm screen out with 1500 psi at less than 1/2bbls/min.

5.Set GP tool in reverse circ. position and reverse drillpipe clean. Use settling tank to determine the quantity of gravel returns.

6.Reconfirm screen out with 1500 psi without backpressure. If no screen out is observed, perform conventional top up.

7.Reconfirm screen out after 1 hr, with 1500 psi at less than l/2 bbls/min pumprate. If satisfactory, POH. Otherwise inform base.

Case where a lower telltale is used

1.Set the liner hanger packer. Establish the squeeze, upper and lower gravel pack position of the GP tool.

2.With the GP tool in the lower gravel pack position establish circulation pressure at 2, 4, 6 and 8 bbls/min. Check for losses.

3.Move into squeeze position and check injection rate for at least two minutes. Do not exceed 3 bpm or FPP whichever come first.

4.With the GP tool in lower gravel pack position:

• -pump 8 bbls of carrier fluid spacer;
• -pump gravel slurry from mixer/blender;
• -pump 2 bbl of carrier fluid spacer;
• -circulate slurry in place at 5-6 bbls/min. Reduce rate when slurry reaches the X-over port to 3 bbl/min. Screen out with 500 psi above maximum established injection rate;
• -pull into upper gravel pack position and recheck screen out with 1500 psi pump pressure at less than 0.5 bpm pump rate.

5.Set GP tool in reverse circulating position and reverse drillpipe clean. Use settling tank to determine the quantity of gravel returns.

6.With the GP tool in the upper gravel pack position reconfirm screen out with 1500 psi pump pressure.

7.If no screen out is observed, perform conventional top up.

8.Reconfirm screen out in upper gravel pack position after one hour, with 1500 psi at less than 1/2bbls/min pumprate. If satisfactory, POH otherwise inform base.

3 External gravel packing procedures

1.With the liner set and the gravel pack tool in the circulating position establish circulation at 0.5-5 bbls/min.

2.Commence adding gravel at ±l/2 sack per min (i.e. + 118 sack/bbl).

3.Having reached 80-85% of theoretical fill (or earlier if pressures indicate) reduce the gravel concentration to 1/8-1/4sack/min. When getting close to screen out, it may be necessary to add 1-2 sacks and displace these completely before adding the next batch of gravel. Care should be taken that the string is not full of gravel at screen out.

4.Continue packing to screen out at reduced rate with 500 psi above initial pressure at 0.5 bpm. Do not exceed the maximum surface circulation pressure specified in the programme.

5.If there is no screen out or pressure build up after 100% of theoretical annular fill, this could be due to washing out of the underreamed section or slumping of the pack due to unconsolidated formation or tool failure resulting in gravel falling on top of running tool or BOL. In this case, check tool is still free. If there is no indication of tool failure, continue packing at reduced rate to screen out or a pressure build up followed by losses. (It is a common observation in some wells that there is only a slight pressure build up usually in conjunction with losses).

6.If no screen out occurs after 150% theoretical annular fill, double check system for leaks, packer failure, fill inside liner, etc. Consideration can be given to run gravel pack log to determine quality of pack (Base to advise).

7.After initial screen out reverse out excess gravel in DP. Wait for one hour and recheck screen out at same pressure, in circulating position. Repack if necessary.

8.If screen out occurs before 90% theoretical annular fill, consideration should be given to running a gravel pack log, as it may be necessary to check for bridging to determine whether the bridge should be jetted or the liner pulled (Consult base).