Examples of internal gravel packing procedures are given below. These programmes contain specific information which may only be relevant for a particular field, and hence may require to be adapted for use in other areas.
*1 Workstring cleaning procedure
Cleaning the wellbore is often difficult and because all wellbores seem to be unique in this respect, it is difficult to programme a set procedure. Decisions have to be made at the wellsite to obtain the desired degree of cleanliness in the shortest time.
1.M/U bit and Rotavert Scraper, P/U 31/2" DP and RIH to float collar. Use thread compound sparingly.
2.With bit at float collar circulate well to drillwater and observe well stable.
3.Scrape three times casing from float collar to 50 m above top packer setting depth while circulating viscous pills. Repeat scraping until returns are clean.
4.With bit at float collar pump the following cleaning pills:
- -20 bbl viscous pill, 20 bbl sand pill and 20 bbl viscous pill. Chase at maximum rate with drillwater using rig pumps.
- -20 bbl viscous pill, 25 bbl citric acid pill and 20 bbl viscous pill. Circulate slowly to give ten minutes contact time with the casing.
- -Circulate with clean brine at maximum rate until the turbidity of the returns is some 25 NTU. Plot turbidity as function of circulating time.
5.Displace well to completion brine with density to give 50 psi overbalance at top packer setting depth. Observe well stable. POOH.
*2 Guidelines for prepacking operations
*2.1 Introduction
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.
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.
Besides the technical limitations of the various prepacking methods described below, a desirable requirement for candidate well selection is the feasibility of rapid and conclusive evaluation through testing immediately after completion or after early hook-up.
*2.2 Applicability
The original techniques described here were developed and field tested as improved well killing procedures for wells which incurred large losses after perforating. 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). There is in principle no limit to the length of the interval that can be prepacked but for initial field trials we suggest a maximum length of some 100 feet.
*2.3 Design considerations
·Graded salt LCM pill system: The graded salt fluid loss control system marketed by TBC-Brinadd is basically composed of a viscosifier (Xantham gum), a fluid loss agent (starch) and graded salt particles in a salt saturated brine. The salt particles have a carefully selected size distribution to act as an efficient bridging agent. TBC-Brinadd will advise on the appropriate particle size and mix ratio for specific wellbore conditions such as gravel size, fluid properties and density requirements, magnitude of fluid losses, bottom hole temperature, etc.
·Perforation packing factors: This factor determines the amount of gravel to be pumped downhole. If it is underestimated then the perforations will not be adequately packed and LCM will be placed inside the perforations from where it is difficult to remove.
On the other hand too large a volume may result in excessive gravel fill inside the casing. Industry rules of thumb for standard gravel packs are in the range of 0.3 ft3/ft of perforations for new wells to 0.5 ft3/ft for wells which have produced significant quantities of sand. Prepacking operations larger pack factors, i.e. some 0.7 ft3/ft perforations, should be considered. Please note that in practice it is difficult to accurately measure the pack factor.
·Carrier fluid viscosity and gravel mix ratio: The use of Shellflo-S should be seriously considered in view of its potential advantages. Recommended HEC concentrations as a function of the chosen gravel mix ratio are given in the manual. As viscosity is an important controlling factor for fluid leak-off, optimal viscosifier concentrations (and hence gravel mix ratios) will have to be established locally for specific wellbore conditions.
·Breakers: The viscosity breakback schedule does not appear to be a critical factor for prepacking operations. It is suggested to aim for a breakback time somewhat in excess of the expected pumping time. With Shellflo-S, the use of of breakers can be avoided by designing the transition temperature below the reservoir temperature.
*2.4 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
*2.4.1 Early BSP Prepack Procedures
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 losses
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
18.In 12% KCL inhibited brine
19.Slurry volume: 40 L/m3 net perfs
20.Close packer bypass and squeeze slurry into perforations
21.Close PCT open MIRV
*Reverse out excess gravel (measure returns)
23.Close MIRV, open PCT unset packer
24.Measure loss rate
25.Tag and measure fill
26.If loss rate excessive and pack factor above 20L/m then - spot graded salt pill
27.POH
28.RIH WWS with modified washing tool
29.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
30.Stab into sump packer
31.Drop dart and set packer
32.Slurry pack well
Slurry:750 kg/m3Gravel (20/40mesh)
-12.8 kg/m3HEC
or-53 L/m3Shellflo-S
-30 L/m334% HCL
-0.15 L/m3HAI -65
*2.4.2 Outline of early BSP acid prepack procedures
*2.4.2.1 Perforate the well twice with TCP
The fill should be tagged after each time perforating prior to POH to ensure that the TCP assembly for the second perforation run or the wash tool does not stand up.
*2.4.2.2 Perforation washing
Wash the double perforated intervals with the wash tool (Selective Injection Packer).
Losses: In the case that the losses are unacceptable for the rig the following procedures for LCM pill pumping will be followed:
a. Pump a 5 m3 Shellflo-S pill
Composition:
·Shellflo-S in completion brine @ 80 l/m3
·Add HCl (30% concentration) @ 50 l/m3 + corrosion inhibitor prior to pumping
If the losses remain unacceptable
b. Pump a 5 m3 Shellflo-S pill
Composition:
·Shellflo-S in completion brine @ 160 l/m3.
·Add HCl (30% conceptration) @ 50 l/m3 + corrosion inhibitor (HAI-65 1 l/m3) prior to pumping.
If Shellflo-S is not available and losses are unacceptable a 5 m3 HEC pill should be spotted.
Composition:
·HEC-10 in completion brine @ 10 kg/m3.
·Add HCl (30%) @ 10 litres/m3.
*2.4.2.3 Run the gravel pack assembly
Run the tools as per standard instructions.
After the gravel pack packer is set continue as follows:
a. Find and mark the relevant tool positions:
·Squeeze position
·Neutral position
·Circulation position
·Reverse circulation position
b. Shear the ball seat with 21,000 kPa with the tool in the reverse circulation position
c. Perform a pickle of the running string
Pickle procedure (only for bottom and middle zone)
Have gravel pack tool in the reverse circulation position.
1.0.5 m3 Shellflo-S pill (50 l/m3 from diverter)
In the case that Shellflo-S is not available use 0.5 m3 HEC (4.8 kg/m3).
This is pumped from the 2000 gallon tank pumped via Clamblender to Halliburton Unit for downhole displacement.
2.0.7 m3 Super Pickle.
3.1.5 m3 15% Fe Acid + 5% Musol + 0.5% Losurf 251 + 0.2% HAI-85.
This is already pre-mixed for the acid prepack and will be fed by gravity to the Halliburton Unit for downhole pumping.
4.Displace the treatment till the Shellflo-S pill (or HEC pill) reaches the cross-over tool.
Reverse circulate out till clean brine returns.
Returns to be disposed of overboard.
At the moment that the dirty returns with inside tubing debris are coming back, a 2 litres sample has to be taken. Samples to be labelled and to be sent to the Production Chemistry Laboratory.
For the top zone Super pickle will not be used.
Replace 3b with 1.0 m3 DMA (or diesel) for the top zone.
d. Perform circulation rates/pressures with the tool in the circulation position at 900, 600, 450, 300, 150 and 75 l/min
With the tool in the squeeze position, obtain injection pressures at 75, 150, 300 l/min and at the FCP.
The FCP should not be exceeded. Establish the squeeze rate at the FCP.
*2.4.2.4 Perforating the acid prepacking operation
Prior to any acid mixing, the concentration of the raw acid should be accurately known.
a. Mix and spot the following treatment with the tool in the reverse position
·At the moment that 0.5 m3 acid is below the crossover tool go to the squeeze position.
·Pump at a rate as high as possible but stay below the FCP.
An attempt should be made to keep the injection rate as constant as possible.
1.4.0 m3 15% Fe acid + 5% Musol-A + 0.5% Losurf 251 + 0.2% HAI-85 M (0.60 m3/m)
This volume is fed by gravity through the Otis manifold to the cement pump.
2.6.7 m3 12/2HF Acid + 5% Musol-A + 0.5% Losurf 251 + 0.2% HAI-85 M (m3/m).
This volume is fed by gravity through the manifold to the cement pump.
3.8.0 m3 3% Clayfix + 0.5% Losurf 251 (1.2 m3/m).
Gravity fed through the manifold to the cement pump.
4.3.0 m3 Divertor pill of 106 kg/m3 gravel (7 sacks)
(PF = 30 l/m sand on net perforations) and Shellflo-S (concentration of 50 l/m3). Add HCl (30%) as breaker @ 50 l/m3 + corrosion inhibitor.
5.0.5 m3 3% Clayfix + 0.5% Losurf 251.
This is gravity fed through the manifold to the cement pump.
The above treatment is based on interval lengths of 6.7 m of net perforations and leads to a calculated pack factor of approximately 30 l/m.
The steps 1, 2, 3, 4, 5 have to be performed for a total of 3 times.
b. Last stage pumped and displaced with brine
Once the last stage has been pumped and displaced with brine into the perforations, move the tools to the circulation position and establish circulation with induced losses of 50%. Check the losses before the moment that these are induced
*2.5 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.
*3 Gravel packing procedures
*3.1 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.
note: 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.
Refer also to Section 17.2.5.4 for alternative procedures to measure 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 1/2bbls/min pumprate. If satisfactory, POH. Otherwise inform base.
*3.2 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.
Refer also to sSection 17.2.5.4 for alternative procedures to measure 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.