It has been established that the preparation of HEC based gravel-pack slurries is critical to the performance of the gravel pack. It has also been shown that many techniques presently in wide use lead to numerous quality problems not all of which are readily apparent.
The cleanliness of the well bore and of all the fluids that reach the formation is paramount to keep impairment to a minimum and thereby realise the maximum well productivity. A procedure for preparing gravel pack slurries containing HEC as the viscosifier is described below.
1 Safety
With respect to health, safety and environment, adequate precautions must be taken throughout the entire process of mixing and pumping. This demands for a thorough awareness of the risks in handling the necessary materials and chemicals. For this, the local OPCO HSE-guidelines, safety data sheets and sole handling procedures must be consulted prior to any handling of chemicals.
2 Preparation of the gravel-pack slurry
The recommended mixing procedure is aimed at preparing a gravel-pack slurry containing 55 to 80 lbs/1000 gal HEC and 5 to 15 ppg gravel. Prior to addition of the gravel, the HEC gel is hydraulically sheared and filtered through a 10 micron absolute filter. More stringent filtration specifications may be required depending on formation characteristics. As the success of the gravel-pack job is highly dependent on the quality of the slurry pumped into the well, it is emphasised that great care must be taken to ensure the proper execution of this procedure.
The gel should be prepared well in advance, say 12 hours prior to executing the job, to provide sufficient time for the gel to fully hydrate and to enable slight modifications to the HEC concentration to be made. Excessive waiting time should be avoided, because the viscosity may start to deteriorate after 24 hours. The mixing recipe below should be verified in the field laboratory before being applied.
2.1 Clean equipment
A dominant parameter involved in performing a successful gravel pack is the cleanliness with which operations are done. In most cases, the formation damage caused by dirty fluids can never be completely removed. Therefore, make sure that each component of the surface equipment to be used for the slurry preparation and transport to the well head isTHOROUGHLY CLEAN. All tanks and tubulars have to be free from rust, fines, dust or pipe dope and loose remnants of paint or scale. Also the workstring should be thoroughly cleaned before running into hole.
The application of stainless steel gravel pack completion equipment should be seriously considered. A useful alternative in many cases (e.g. tanks) may be internal epoxy coating.
2.2 Make-up water
It is recommended that fresh water (potable water) be used as make-up water. Using water from other sources could result in the formation of precipitates (from e.g. Ca2+ , Mg2+, Fe3+ and even K+ ions) during the various stages of the procedure or during a possible post gravel-pack stimulation exercise.
2.3 Addition of defoamer
Apart from adding a defoamer agent, care should be taken to minimise air entrainment during mixing (e.g. limit agitation by mixing/circulating, hose outlets below the fluid surface) because the oxygen in the air may cause: a) corrosion and b) premature polymer degradation through oxidation. In addition, gas bubbles hamper the smooth dissolution of HEC (particles are not totally wetted) resulting in the formation of lumps/fish-eyes.
2.4 The growth of bacteria
Growth of bacteria may show up in the HEC fluid which is sometimes initiated due to exposure to sunlight or oxygen. This is indicated by the presence of flocs in the fluid. Addition of formaldehyde in the form of formalin, a well-known biocide, should inhibit this process. Other commercially available biocides may also be considered.
2.5 Selection of brine
A typical fluid containing 3 %w NH4Cl or 2 %w KCl is recommended as the basis for the HEC gel. Another candidate salt may be 4 %w NaCl. They give essentially identical viscosity properties to a HEC gel when they are added in the amounts indicated. 3 %w NH4Cl and 2 %w KCl are preferred since they are known to be, in general, more compatible with respect to rock/fluid interactions than 4 %w NaCl. Both ammonium chloride and potassium chloride provide excellent clay stabilisation.
When choosing between the use of NH4Cl and KCl the following should be borne in mind:
1.Ammonium chloride has fewer incompatibility problems with common stimulation fluids compared with potassium (or sodium chloride). These latter salts may create precipitates with HF-acid based stimulation fluids, which may cause formation impairment.
2.2. Where a brine of greater density than that given by 3 %w NH4Cl is requested, KCl (or NaCl) brines are recommended. The pH buffering capacity of NH4Cl makes it difficult to use at higher concentrations as adjustment of pH would then require excessive amounts of acid or caustic.
If the bottomhole pressure demands a slurry of a higher density, more or heavier salts, such as calcium chloride, calcium bromide and zinc bromide have to be used. If these salts are used then on raising pH to 8 - 9, precipitates are formed. Thus, do not adjust pH when using these salts.
HCl breaker should NEVER be used together with Br-salts. If HCl acid breaker is added to CaBr2/ZnBr2, bromine gas will be formed indicated by a red-brown colour developed in the solution. This is a highly toxic and corrosive gas. Bromine could also be released when using persulfate breakers together with Br-salts.
2.6 pH Adjustment to 5-6
To adjust the solution pH to 5 - 6, add small quantities of concentrated HCl or NaOH solution (70 g NaOH dissolved in 5 l of water). Allow sufficient time after each addition for the pH to stabilise.
When handling these materials, operators should wear adequate protective equipment as defined by the safe handling procedures.
The reason for adjusting the pH to 5 - 6 is to slow down the dissolution rate of HEC in water. This enables each individual HEC particle to become fully wetted before hydration starts, rather than allowing hydration to commence on clustered HEC particles, resulting in what is termed 'fish eyes'. Fish eyes hamper smooth gellation of HEC and are detrimental to formation permeability.
Adjustment of pH should not be made for the heavier salts mentioned in Section *4.5. Generally, hydration rates of HEC decrease significantly with brine concentration. So pH-adjustment would then be unnecessary.
2.7 Addition/dispersion of HEC
First of all, it must be ensured that the available HEC is, still, in usable condition and that it meets the specifications originally set by the manufacturer. Therefore, prior to field mixing, lab tests should be performed with the HEC to check for adequate gelling-up and gel viscosity. Acceptable commercial HEC brands should be established by operators.
HEC is best stored under cover in a dry atmosphere. Exposure to moister during storage could inhibit proper dispersion and/or hydration. When a bag is opened, the HEC should still be in powder form, dry, loose and free flowing. If this is not the case and the particles have formed lumps, the HEC should be discarded. The HEC should be added to the hopper through a strainer to remove partially hydrated clumps.
The addition of HEC to the mixing water should be done in a slow and continuous way but sufficiently fast in order to ensure that all polymer can be added before the fluid begins to gel. Addition of polymer too rapidly will result in lumps.
When the brine density is high, the viscosity of the HEC solution will increase accordingly.
When brine density is increased, the hydration rate of HEC will decrease. For the heavier brines this can result in significantly longer mixing times (e.g. 5-6 hours for a brine containing 1500 kg/m CaCl2-Br2). Heating of the solution, after adding and fully dispersing the polymer will accelerate the hydration process. It is emphasized that the solution should be allowed to cool before breaker is added.
2.8 pH adjustment to 8-9 for hydration
To adjust the blend pH to 8 - 9, add small quantities of the NaOH solution. Allow after each addition of NaOH sufficient time for the pH to stabilise.
A hydration time of 30 minutes is sufficient, provided no fish eyes are present. Hydration of fish eyes will take much longer or will not happen at all if the solution is not sheared.
2.9 Shearing and filtering
To minimise formation impairment, it is recommended to shear and filter viscous polymer solutions. Gels must be sheared and filtered to a minimum specification of 10 micron obsolete. A more stringent specification may be required.
Where a standard shearing device such as Baker's Polymer Shear Mixer or the Pall Well Technology 'Hydra Disc' system is not available, the actual conditions needed must be selected such that the polymer solution can be filtered to 10 micron without an excessive loss of viscosity occurring due to shear break down of the polymer. For filtration, a filter train consisting of a D.E. filter followed by a 10 micron absolute cartridge filter is generally used. It should be noted that shearing and filtering may reduce viscosity by 10 - 20%.
Suggested filterability test method
Suggested method to check whether the HEC gel can be filtered down to 10 microns absolute. (Method is being optimised by KSEPL).
1.Place 200 ml of the viscous fluid in the upper chamber of the Filterability Test Apparatus.
2.Place a wetted sheet of Whatman #41 filter paper between the chambers and assemble the chambers hand tight.
3.With valve to the top chamber closed, set the pressure regulator to 15 psig.
4.Open the valve to the top chamber and start timer.
5.Record the time necessary for the 200 ml test volume to pass through the Whatman #41 filter paper into the lower chamber.
a)If the time is less than 60 seconds, the fluid is considered to be filterable and should then be filtered.
b)If the filtration time exceeds 60 seconds, the fluid should be hydraulic sheared once more and the filterability test reperformed.
2.10 Monitoring of viscosity
Viscosity at high shear rates can be measured using the Fann-meter, the lower shear rates must be measured with a Brookfield LVT viscometer.
2.11 Gel viscosity break back and breaker selection
For the pumping of gravel, the gel break back time is not considered critical provided it is sufficiently longer than the duration of pumping of the slurry. Normally, a break back time of some two hours is sufficient, the actual choice depending on the job design. Break back times should be checked at high (100 sec-1) and low (0.07 sec-1) shear rates. The high shear rate occurs during pumping, while the low shear rate occurs as gravel settles out of suspension after screen-out.
HEC viscosity is easily broken by adding acids, oxidants or enzymes. The acid breaker is preferred because the control of the break-back time with acid is less sensitive to temperature and concentration than the persulphates and/or enzymes. In addition, powder agents such as persulphates and enzymes may suffer from a limited shelf life time resulting in reduced breaking activity.
In cases where acid breakers cannot be used (e.g. carbonate rich formations, resin coated gravel systems, low temperature formations) the production chemistry lab must advise on the use of an alternative (oxidative or enzyme) breaker. Enzymes can be used up to about 50 oC and have an optimum activity at 30 oC.
Simplified breaker selection criteria are as follows:
1.Always use an acid breaker rather than other alternative breakers (e.g., oxidative breakers, enzyme breakers or using no breaker) except when special restrictions dictate.
2.If acid breaker is acceptable, use HCl breaker or the next breaker decreasing in acid strength which has no restrictions. Alternative acid breakers to consider, in order of decreasing strength, are 1M formic acid + 0.1M HCl, 1 M formic acid, and 1M acetic acid. These breakers cover a broad range of breakback times. For guideline purposes, an acid breaker summary, after Scheuerman, is presented in Fig. 758.
It should be noted that, generally increasing temperature results in shorter viscosity breakback times and that increasing HEC-concentration results in increasing breakback times. Breakback time is dependent upon pH rather than acid breaker concentration.
2.12 Adding gravel
The gravel should always be checked against the required specifications. For this it is recommended to follow the API 58 recommendations. Only a final spot check should be carried out at the well site. It should be realised that in spite of a high gravel quality, some generation of fines is unavoidable. The gravel should always be added to the gel through a strainer to prevent the sack debris from entering the slurry. Broken sacks of gravel should be discarded because the gravel may have picked up fines.
Gravel should not be stored in cement silos as cement fines will cause similar damage. Gravel should be stored dry for it is difficult and time consuming to control the addition of wet gravel to the gel.