Technical Bulletin

Kelco Oil Field Group 10920 W. Sam Houston Pkwy North, Ste 800 Houston, TX 77064 (713) 895-7575 www.kofg.com

Xanthan Formulated Systems General Guidelines Unique to xanthan biopolymers, such as KELZAN® and XANVIS®, is that they can be used with or without bentonite, i.e. as a secondary or primary viscosifier/suspending agent. Unlike other polymers (cellulosics, polyacrylamides, starches), xanthan develops significant viscosity at low shear rates (LSRV) to provide suspension of weight materials in high-density fluids. KELZAN XC and KELZAN XCD, or XANVIS can be used in a wide range of makeup waters, ranging from fresh water to saturated sodium chloride, in formate brines, and near-saturated calcium chloride. Following are some general guidelines to achieve optimum fluid efficiency in the field: 1. As with any polymer-based system, the use of

efficient solids control equipment is recom-mended, including high speed, fine screen shale shakers, hydroclones, and decanting centrifuges. Overall stability is based on maintaining low-density solids in the ≤5% by volume range, and using the biopolymers to increase LSRV.

2. Bingham Plastic parameters, the plastic viscosity

(PV), yield point (YP) will often be inverted, preferably varying between a PV:YP ratio of 1:2 to 1:3 with a PV in the 10-20 range for unweighted systems. Using KELZAN XC, XCD or XANVIS to improve carrying capacity will automatically maintain a low PV while increasing viscosity at low shear rates (≤5.1 sec-1), as measured with the Fann 35 and Brookfield viscometers.

3. Using the Power Law model, “n” values, as

calculated between 3 and 300 rpm, should be maintained between 0.25 and 0.35 for optimum shear thinning characteristics and hydraulic efficiency. “K” values >10 dynes.secn/cm2 are recommended to assure sufficient LSRV for suspension. Hole conditions should dictate the need for increasing “K”. Fill on trips, settling, barite sag, and trouble making connections are all symptomatic of hole cleaning problems brought about by inadequate LSRV, and indicate the need to add KELZAN or XANVIS biopolymers.

4. In saturated brines or relatively high concen-trations of salt, non-dispersible KELZAN XC or the XANVIS L slurry may be the preferred product since they will hydrate at a faster rate than KELZAN XCD under these conditions.

5. For optimum stability, pH of xanthan systems

should be maintained between 8.0 and 9.0. Avoid high pH above 10.0 with soluble calcium such as in drilling cement since precipitation of the polymer can occur. Pre-treat with SAPP or bicarbonate of soda to minimize the effect.

6. Under high temperature applications above

200ºF, use of an oxygen scavenger will minimize thermal degradation.

7. In low pH, fresh water and low salinity brines

(<10% salt) a biocide is highly recommended especially if the fluid is to be stored.

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Formulation and Maintenance

Fresh Water. Depending on the application, KELZAN® XC, KELZAN XCD, and XANVIS® can be used independently or in conjunction with bentonite.

Table 1 Fluid Formulation for Fresh-Water-Based Systems

Additive Concentration (lb/bbl)

Function

KELZAN XC, XCD or XANVIS 1.0 - 2.0 Viscosifier Bentonite 0 - 10 Viscosifier NaOH, KOH 0.2 - 0.5 pH Control Polyanionic Cellulose, LV 1.0 - 2.0 Filtrate Control Derivatized Starch 2.0 - 4.0 Filtrate Control Polyacrylamide 0.25 - 1.0 Shale Inhibition Deflocculant as needed Deflocculant, Dispersant Barite as needed Weight Material Glutaraldehyde 500 - 1000 ppm Biocide

NOTE: Fluid loss additives may be used independently or in conjunction with each other, dependent on the application.

Salt Water Systems This category of fluids includes systems formulated with seawater, potassium, sodium or calcium chlo-ride, formates, bromides, and low concentrations of potassium salts such as acetate. Fluids containing dissolved electrolytes are typically inhibitive systems used to minimize shale hydration and to provide for-mation protection in payzone applications. KELZAN XC, XCD, and XANVIS are natural choices for off-shore drilling since Kelco’s product specifications are based on viscosity development in ASTM sea-water. Seawater base fluids represent the largest group in this category of salt water systems.

Seawater. KELZAN XC and KELZAN XCD are generally used as the primary viscosifier, with a concentration ranging from 1.0 to 2.0 lb/bbl. Since most offshore locations include large sections of ‘plastic’ or ‘gumbo’ type shales, commercial bentonite is seldom used. In some cases, where drill-water is available, bentonite can be pre-hydrated and added to the system as a supplementary viscosifier. A typical seawater system can be formulated with the following materials:

Table 2

Typical Seawater System Formulation

Additive Concentration

(lb/bbl) Function

KELZAN XC or XCD 1.0 - 1.5 Primary Viscosifier Pre-hydrated bentonite (optional) 5.0 - 7.0 Secondary Viscosifier Polyanionic Cellulose, L.V. 1.0 - 2.0 Filtrate Control Polyacrylamides, etc* 1.0 - 1.5 Shale Inhibition KCl (optional) 10.0 - 25.0 Additional Inhibition Derivatized Starch (optional) 2.0 - 4.0 Filtrate Control Low Molecular Weight Acrylates and Lignosulfonates 1.5 - 4.0 Deflocculants NaOH, KOH 0.25 - 0.50 pH Control (8.0 - 9.0) Barite As needed Weight Material Glutaraldehyde 500-1000 ppm Biocide

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*Various additives, including glycols, polymeric blends, resins, and salts have been developed for shale inhibition. While most are compatible with xanthan and other anionic polymers, pilot testing is highly recommended if cationic inhibitors are used. For optimum stability, xanthan should be pre-hydrated in the seawater/salt makeup fluid prior to adding cationic shale inhibitors. Brines (NaCl, KCl, CaCl2, NaBr). This category includes systems used as drill-in fluids in horizontal drilling and workover completion operations where maximum formation protection is required. Systems are typically formulated as bentonite-free fluids with a minimum number of additives, which can be broken or dissolved using oxidizers and/or acids to provide maximum return permeability. XANVIS® and XANVIS L are the preferred viscosifiers for these applications. Key to obtaining maximum viscosity development and minimizing formation damage due to microgels, is polymer mixing and handling proce-dures. In general, biopolymer hydration depends on: • Mixing time and shear • pH • Salinity • Temperature

Monovalent Salts. XANVIS and XANVIS L can be added directly to sodium or potassium base brines up to saturation without special mixing procedures to obtain full viscosity development. For optimum temperature stability in these bentonite free systems, pH should be maintained in the 8.0 - 9.0 range to minimize acid hydrolysis of the polymer. NaOH, KOH, and MgO can all be used for pH control. Typical drill-in fluid formulations are provided in Table 3. If temperature exceeds 200ºF, use of an antioxidant will help minimize thermal degradation of the bio-polymer. Thermal stability to 300ºF+ is possible with saturated sodium chloride systems formulated with KELZAN® XC, KELZAN XCD or XANVIS (Figure 1). Even higher temperatures (to >350ºF) can be tolerated using formates.

Table 3 Drill-in Fluid Formulations using Monovalent Salts

Additive Concentration (lb/bbl) Function

XANVIS 1.5 - 2.5 Primary Viscosifier Salt (NaCl, KCl, NaBr) Up to saturated Inhibition, Density Sodium Sulfite 0.20 - 0.3 Antioxidant Derivatized Starch (optional) 2.0 - 3.0 Filtration Control MgO 0.5 - 1.0 pH Control in CaCl2 NaOH, KOH 0.3 - 0.5 pH Control in K or Na systems Calcium Carbonate (optional) As needed. Bridging Agent Glutaraldehyde 500 - 1000 ppm Biocide

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1 10 100 1000 Shear Rate, sec-1

1

10

100

1000

Visc

osity

, cP

300 o F 80oF

Figure 1. XANVIS®, 2.2 lb/bbl in Saturated NaCl Brine + 1.0 lb/bbl MgO; Viscosity Profile at 80o and 300oF

Divalent Salts. Systems used in this category are the calcium chloride and calcium bromide brines. Because xanthan will not readily hydrate in saturated, divalent salt brines, special mixing procedures are required. For optimum viscosity development, less than saturated brine is recommended. For example, begin with a 10.5 to 11.0 lb/gal calcium chloride brine, and include the following procedures:

• Use XANVIS L or KELZAN® XC • Use a mechanical shear device • Add polymer through a hopper for good

dispersion • Adjust pH with magnesium oxide

(1.0 ±±±± lb/bbl) while shearing

Due to the relatively harsh environment, hydration time will be longer when viscosifying calcium base brines as opposed to sodium or potassium base systems. Once the 11.0 lb/gal brine is viscosified, additional salt can be added.

NOTE: For overall fluid stability, the maximum brine density recommended is 11.2 - 11.3 lb/gal.

Since the crystallization point of 11.4 - 11.6 lb/gal calcium chloride brines is relatively high (27 - 44oF), gelation of a xanthan viscosified brine in this density range can occur, especially, if temperatures drop below 85oF. If this occurs, heating and applying additional shear will result in a more homogenous fluid. For this reason pilot testing with the proposed brine is highly recommended since composition and impurities will vary from one supplier to another. A fluid formulation, using 1.5 lb/bbl XANVIS in an 11.0 lb/gal calcium chloride brine weighted to 12.0 lb/gal, with calcium carbonate is provided in Table 4. Data illustrates properties before and after hot rolling at 150oF. Magnesium oxide improves temperature stability of calcium chloride viscosified brines, when used in the >275oF ranges (Figure 2). As density of the calcium chloride brine increases, improved thermal stability is obtained. This feature is a result of the increase in transition temperature of xanthan gum under high salinity conditions, Figure 3. Do not adjust the pH of calcium chloride brines with sodium or potassium hydroxide.

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Table 4

XANVIS® (1.5 lb/bbl) in 11.0 lb/gal CaCl2 Brine Weighted to 12.0 lb/gal with 90 lb/bbl Calcium Carbonate, Before and After Aging@150oF

Before Aging After Aging

Temp., oF

Fann 35, f1.0

600 rpm

300 rpm

PV, cP

YP, lb/100 ft2

Gels, 10sec/10min.

Brookfield LV

cP@0.12 sec-1

cP@0.06 sec-1

75oF

85

57

28

29

8/11

75oF

75

49

26

23

6.5/9

10,000

15,600

1 10 100 1000 Shear Rate, sec-1

1

10

100

1000

Visc

osity

, cP

WITH MgO@280°F

INITIAL, 85°F

280°F, w/o MgO

Figure 2. Effect of MgO on Thermal Stability of 2.0 lb/bbl XANVIS in 11.0 lb/gal CaCl2 + Na2SO3

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100 150 200 250 300 250 200 150 100 Temperature, °F

0

25

50

75

100

125

150

Visc

osity

, cP

at 1

00 s

ec-1

10 lb/gal

11.3+ lb/gal

11 lb/gal

Figure 3. Improved Thermal Stability with Increased Brine Density 2.25 lb/bbl XANVIS® in CaCl2 Brines +1.0 lb/bbl MgO; Fann Model 50C@100 sec-1

Table 5 Typical Rheological Properties of XANVIS in Various Makeup Fluids, 78oF

Viscosity, cP

PV YP “n” “K” 5.1 sec-1 0.0636 sec-1

Seawater

1.5 lb/bbl 4.5 17.5 0.2 32.0 890 28,000

2.0 lb/bbl 7.0 24.0 0.19 49.0 1,300 59,000

Saturated NaCl

1.5 lb/bbl 9 19 0.26 28.0 840 33,000

2.0 lb/bbl 11 27 0.24 43.0 1,280 62,000

CaCl2, 11.0 ppg

1.5 lb/bbl 16 19 0.42 13.0 500 9,000

2.0 lb/bbl 20 26 0.41 18.5 700 19,000

“n” and “K” between 3 and 300 rpm; “K” in dynes - secn/cm2 or Poise

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Drill-in fluid design is built around the concept of minimizing formation damage, therefore, keeping fluid/filtrate invasion to a minimum is a key objective. To accomplish this, several practices can be implemented: • Minimize fluid invasion by increasing LSRV

of the system • Particle bridging on the wellbore and/or

near-wellbore region • Filtrate/fluid chemistry to suppress

hydration of in-situ clays When using filter cake building materials and bridging agents, such as calcium carbonate, selection should be made based on the type of completion and whether or not pre-packed screens or slotted liners will be used. Proper sizing is important to avoid plugging-off completion equipment. In general, pref-erence should be made toward an easily removable cake to allow maximum flow from the exposed pay-zone. Acid washes or oxidizers can be applied to enhance removal of bridging materials and to break the viscosity contributed by xanthan. XANVIS® formulated fluids can be engineered to provide a high degree of flexibility in terms of minimizing fluid invasion. In most cases, developing LSRV in the 25,000+cP range (as measured at 0.3 rpm, Brookfield LV series viscometer, No. 2 cylindrical spindle) at 100 - 120ºF can assist in controlling fluid invasion.

Formation permeability will dictate optimum value(s) for minimizing depth of invasion. In addition, a com-bination of sized particulates, including drilled solids from the payzone, can assist in the bridging of pores in the near-wellbore region. Key is to minimize fines migration or forming of a deep, internal filter cake. In so doing, cleanup operations will be held to a mini-mum, and permeability impairment will be localized to the near-wellbore region. In the salt inhibited systems, KELZAN® XC, XCD or XANVIS are the viscosifiers of choice. Any need to increase viscosity, suspension properties and cuttings carrying capacity should be accomplished by adding these biopolymers, as opposed to cellulosics, attapulgite or bentonite. This will help retain the shear thinning features of the system by not adversely affecting high shear rate viscosity. In saturated salt systems, aeration/foaming can be a problem, especially if hopper discharges are not submerged, or there is an excessive use of mud-guns. A pre-hydration tank is recommended so that maintenance treatments can be added to a separate tank, where foaming can be controlled, then the pre-mix can be incorporated into the active system. When initially viscosifying a brine with KELZAN XC, KELZAN XCD or XANVIS , adding a defoamer prior to the biopolymer has provided positive results in the field. Table 5 provides typical rheological properties for XANVIS in various salt environments.

Technical Bulletin

KELZAN ®, XANVIS ®, and BIOZAN ® are registered trademarks of CP Kelco U.S., Inc. and may be registered or applied for in other countries. © 2001 CP Kelco U.S., Inc.

The information contained herein is, to our best knowledge, true and accurate, but all recommendations or suggestions are made without guarantee, since we can neither anticipate nor control the different conditions under which this information and our products are used. It is our policy, to assist our customers and to help in

the solution of particular problems, which may arise in connection with application of our products. Rev. 09/05

Table 6 Diagnostic Chart for KELZAN® or XANVIS® Formulated Systems

PROBLEM SYMPTOMS POTENTIAL CAUSE / TREATMENT

Viscosity loss Reduction in yield point, 3 rpm reading or Brookfield LSRV.

1. Biodegradation if fresh water system, and low pH - add biocide; pre-treat makeup water with 3% bleach (1 gal per 50± bbl). Maintain 9.0+pH.

2. High temperature (>200ºF) with a low MBT, fresh water system. Add 02 scavenger; control pH at 8.5 - 9.5; add BIOZAN® or change over to a K or NaCl system (2 - 3% salt).

3. Cement contamination. If pH increased above 10.5 with soluble calcium. Add SAPP/ Bicarb to reduce Ca below 200 ppm, then add KELZAN, XANVIS or BIOZAN.

4. Certain cationic additives, i.e. quaternary amines, corrosion/scale inhibitors may be incompatible with xanthan; can cause precipitation of the polymer. Pre-hydrate biopolymer, add 3% salt, then add cationic additive - pilot test.

High viscosity Progressive gels; high PV

1. Solids contamination. Check MBT, total LDS, solids control equipment. Dilute, evaluate mechanical equipment, add deflocculant. Control LDS ≤5% by volume.

2. Cross-linking. Can occur in xanthan solutions, e.g. gravel packing, if soluble iron is present; add citric acid (0.25 - 0.5 lb/bbl) to chelate iron.

Increase in filtrate High API or HTHP fluid loss, drop in viscosity.

Bacteria problem if starch in use; oxidation due to high temperature. Add biocide, oxygen scavenger, change out filtrate control additive.

No viscosity development

Low YP, 3 rpm and Brookfield LSRV

Insufficient mixing time; high salinity; cold water temperature. Increase shear, buffer pH with NaOH, KOH, MgO to 8.0 - 9.0 dependent on makeup fluid (in calcium brines use MgO).