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1 SPE Application of Amphoteric Cellulose Ethers in Drilling Fluids Brent Warren/Q'Max Solutions Inc., Peter van der Horst/Akzo Nobel Chemicals bv, Wayne Stewart/Drilling Specialties Company, LLC Copyright 2003, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE International Symposium on ilfield Chemistry held in Houston, Texas, U.S.A., 5 8 February This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.. Box , Richardson, TX , U.S.A., fax Abstract Anionically charged cellulosic ethers have been used for many years in water-based drilling fluids. While the anionic charge imparts a benefit in the filter-cake properties of bentonite containing drilling fluids, this same charge deflocculates (thins) the viscosifying ability of this same bentonite. As a result, additional bentonite is often required in the drilling fluid to maintain an adequate rheological profile. Also, it is well known that cations are viscosity enhancers to bentonite drilling fluids by virtue of the flocculation effect of the cation. This has been used for many years in the potassium chloride (and similar salts) based drilling fluids. Amphoteric Cellulose Ethers represent a new class of drilling fluid additives which combine an anionic and cationic charge onto the same cellulose backbone. The result is a drilling fluid additive having both desirable filter-cake enhancement and partial flocculation properties. Discussion will include the effects of degree of substitution for the anionic and cationic species, as well as the influence of molecular weight on the properties of the Amphoteric Cellulose Ethers. Introduction Gel chemical drilling fluids are among the simplest of fluids used in the drilling of oil and gas wells. The basic components of these fluids are fresh water, bentonite, caustic soda, a polyanionic cellulose (PAC) as well as dispersants/thinners as required. This fluid has a number of inherently desirable properties including: simplicity of design ease of use in the field adequate rheological and filtration control properties simple environmental disposal requirements low cost A large majority of the wells drilled in Western Canada use gel-chemical drilling fluids to drill the main hole sections, including the potential hydrocarbon bearing zones. The clay and the polymers in these drilling fluid systems are responsible for the desired rheology profile (viscosity enhancement) and filtration control. As we aspire to keep the overall cost of drilling as low as possible, and to improve fluid properties, a new polymer additive for the gel-chemical mud system has been developed which made it possible to use less clay and less polymer while achieving an improved rheological profile. This water-soluble polymer is an Amphoteric Cellulose Ether (ACE), a cellulose backbone incorporating both anionic groups and cationic substitution groups. The overall performance of the drilling fluid, having a lower solids and polymer content, is obtained by a subtle interaction between the bentonite clay and the amphoteric polymer, giving a partially flocculated polymer-clay drilling fluid system. This paper deals with the chemistry of the Amphoteric Cellulose Ether and the interactions of new amphoteric species with clay (as compared to regular polyanionic cellulose (PAC)). The physico-chemical characteristics of the amphoteric cellulose ether, which can be altered to modify the degree of flocculation of the clay in the water based drilling fluid system, is also discussed. Field examples will show the ability of ACE systems to lower usage of bentonite and polymer, and will show the reduced costs using these fluids. Comparison of CMC/PAC and ACE The new additive is an Amphoteric Cellulose Ether having anionic carboxymethyl groups as well as cationic quaternary ammonium groups attached to the cellulose backbone. The physical production of Amphoteric Cellulose Ether is very similar to those of the other cellulose ethers. The cellulose is suspended in a diluent to which sodium hydroxide is added to form the reactive and amorphous alkali cellulose. Sodium monochloroacetic acid (NaMCA) is then added which reacts with the alkali cellulose at increased temperature. This is similar to making either CMC (carboxymethyl cellulose) or PAC (polyanionic cellulose), depending on the substitution level of the carboxymethyl groups. To prepare the amphoteric cellulose ether, an additional alkylation reaction is performed with glycidyl trimethylammonium chloride (GTAC), which then forms the cationic quaternary ammonium functional groups of the product. As byproducts of the reaction with

2 2 BRENT WARREN, PETER VAN DER HRST, WAYNE STEWART SPE MCA and GTAC, sodium chloride and the hydrolyses products of MCA and GTAC are formed which can be removed by extraction of the product with a washing liquid (usually a water / alcohol mixture). 1 A schematic presentation of the chemical structure of amphoteric cellulose ether is shown in Figure 1. As is well known for CMC, PAC and other cellulose ethers, the characteristics of these materials can be varied by having different average numbers of substituents per glucose unit and different molecular weights. 2 Commonly in the drilling of wells, either a low-viscosity or high-viscosity PAC is used in a gel-chemical mud. In the same manner, the product parameters can also be varied on the amphoteric cellulose ether as follows: Average number of sodium carboxymethyl groups per glucose unit (DS CM ) Average number of quaternary ammonium groups per glucose unit (DS QN ) Average molecular weight of the polymer (MW, and MW distribution) Depending upon the molecular weight of the cellulose, the cationic component might result in bridging flocculation, as seen in potassium salt-based muds (i.e. KCl potassium chloride). Cl - + N H Na Na H H H H Na H H H H H Na H + - N Cl Figure 1: Simplified Depiction of the Amphoteric Cellulose Ether showing the Anionic and Cationic Attached Groups. Interaction of ACE s with Bentonite Table 1 shows the rheological values of wide variety of Amphoteric Cellulose Ethers in a simple bentonitic fresh water slurry. The ACE s varied in DS CMC substitution, DS QN substitution and molecular weight. In addition, the comparison is shown against commercially available regular and low viscosity PAC s. The general data trend indicates that in order to achieve fluids with high Yield Points, gel strengths and Brookfield viscosities, an ACE with a moderate degree of quaternary substitution and a lower amount of CMC substitution on the cellulosic ether backbone will provide the best rheological mud system. Figure 2 indicates that a Brookfield viscosity of 50,000 to 110,000 cps (and with a low API fluid loss) is achievable in those molecules with a DS CMC of 0.7 to 0.9 and a DS QN of 0.1 to 0.2. The Brookfield viscosity is a good measure of the suspension characteristics and low shear rate viscosity of the drilling fluid. The effect of molecular weight on the Brookfield rheology is quite variable with the influence of the DS CMC and DS QN playing an important role. As seen in Figure 3 and in Table 1, it is obvious that a low molecular weight ACE or PAC will have a lower Brookfield viscosity. For the higher molecular weight ACE s, both lower and higher Brookfield values were achieved. Although the Plastic Viscosity and Yield Point of the regular grade PAC are among the highest of the fluids tested, the Brookfield viscosity of the regular is less that of many of the ACE s. ACE polymers with a DS QN of greater than 0.2 have proven ineffective in a number of areas. Firstly, these compounds tend to have poor solubility characteristics in water unless subjected to extremely high levels of shearing. More importantly, the API fluid loss values are greater than the ACE s having a DS QN of less than 0.2. It can therefore readily be seen that the CMC substitution portion of the ACE molecule is responsible for filtration control, yet has a negative impact on fluid rheology. Better rheology and poorer fluid losses are typical with the DS QN substitution, as long as the DS QN susbstitution is not greater than 0.2. The aim of an ACE is therefore to combine the best of the DS CMC and DS QN portions of the molecule to flocculate the bentonite as much as possible but without losing the filtration control characteristics. The new ACE additive has been developed in such a way that a balance is obtained in the dispersing and flocculating behavior of the bentonite clay in order to obtain the desired rheology of a gel-chemical system.

3 SPE APPLICATIN F AMPHTERIC CELLULSE ETHERS IN DRILLING FLUIDS 3 This is achieved with the DS CMC of Amphoteric Cellulose Ether in the range of and the DS QN in the range of Therefore, at high molecular weights of polymer, only a few cationic groups are required to obtain the desired amount of weak flocculation. Sample M. Weight DS CMC DS QN PV(mPa.s) YP (Pa) 0/10 Gels (Pa) Brookfield Visc. (cp) API-FL (ml) A Low /5 19, B Low /6 17, C Low / D Medium /11 35, E Medium /7 25, F Medium /21 76, G medium-high /29 104, H High /8 10, I High /33 42, Blank (no ACE/PAC) /3.5 2, Commercial HV-PAC High /11 31, Commercial LV-PAC Low /1.5 13, Table 1: Summary of ACE molecular description, and properties of a mixture of 20 kg/m 3 of unpeptized bentonite and 3 kg/m 3 of Sample at an adjusted ph of 9.5. Note that 4% Rev Dust was added prior to taking API fluid loss. Brookfield viscosity run at 0.3 rpm. Variation of Brookfield Viscosity with Degree of Substitution Degree of Substituion DS-CMC DS-QN Brookfield viscosity (cps at 0.3 rpm) Figure 2: Variation of Brookfield Viscosity with ACE degree of substitution. DS CMC and DS QN refer to the amount of substitution of anionic-cmc and cationic-qn on the cellulose backbone.

4 4 BRENT WARREN, PETER VAN DER HRST, WAYNE STEWART SPE Variation of Brookfield Viscosity (cps) with ACE Molecular Weight Brookfield Viscosity (cps at 0.3 rpm) low Molecular Weight medium medium-high high A B C D E F G H I blank com PAC com PAC Sample ID high low Figure 3: Variation of Brookfield Viscosity with molecular weight. E-SEM Structures of ACE s with Bentonite In order to visualize the differences in dispersion and flocculation behavior of the ACE polymer compared to CMC/PAC and PHPA, the wet clay structure containing one of these polymers has been studied using E-SEM. In contrast to SEM 3, E(nvironmental)-SEM is a technique which can be applied to substrates in a wet environment, a wet filtercake in this paper. Figures 4 through 7 show a series of E-SEM photomicrographs of the interaction of bentonite with different polymers. The change from the base bentonite alone (Figure 4) to a mixture of bentonite and ACE (Figure 5) is characterized by an expansion of the interstitial spaces of the bentonite network and the bridging of the ACE polymers along the edges of the bentonite platelets. It is postulated that the expansion of this bentonite clay network by ACE increases the surface area of the clay, thereby increasing viscosity by a greater envelope of water entrapped in the structure. Figures 6 and 7 show the association of bentonite with a polyanionic cellulose (PAC) and a partially-hydrolysed polyacrlyamide (PHPA) based bentonite extender. Polymer association in the PAC figure is similar to that of the ACE, as could be expected. However, the bentonite platelets are more closely associated than in the bentonite and ACE formulation, indicative of less exposed surface area. The PHPA based extender is an interesting figure as the polymer molecule is much longer and is intimately associated with the bentonite clay edges. This is not surprising in that the extender molecule is of an extremely high molecular weight and of a higher charge density than most PAC s or ACE s. The differences in exposed surface area shown in Figures 4 to 7 confirm our theory that the differences in rheology and fluid loss performance are caused by the differences in the degree of dispersion/flocculation of the bentonite.

5 SPE APPLICATIN F AMPHTERIC CELLULSE ETHERS IN DRILLING FLUIDS 5 Figure 4: Base bentonite fluid with no PAC or ACE added. Figure 5: Base bentonite fluid with amphoteric cellulose ether (ACE) added. Bright white strands films of water containing the ACE polymer strands.

6 6 BRENT WARREN, PETER VAN DER HRST, WAYNE STEWART SPE Figure 6: Base bentonite fluid with polyanionic (cellulose) PAC added. Bright white strands are films of water containing the PAC polymer in solution. Figure 7: Base bentonite fluid with partially hydrolyzed polyacrylamide (PHPA) bentonite extender added. Bright white strands solubilized PHPA polymer contained within a water film.

7 Standard Formulations and Drilling Fluid Properties Formulations Table 2 shows the drilling fluid properties obtained for the new amphoteric cellulose compared to a standard high MW, high viscosity PAC material. As seen in the data, good rheological and fluid loss properties are achievable with the new amphoteric cellulose ether at concentrations of 30 kg/m3 of bentonite and 0.5 kg/m3 of amphoteric polymer. The ACE system has a flatter rheological profile than PAC based fluids, and therefore lower Power Law Index ( n value) and improved pressure drops. A standard gel-chemical formulation requires additional bentonite and PAC polymer to match the fluid properties of the amphoteric cellulose ether based mud. Based upon the amphoteric cellulose ether formulation (0.5 kg/m3 of polymer and 30 kg/m3 of bentonite) and the PAC formulation (1.0 kg/m3 of PAC and 45 kg/m3 of bentonite), the estimated selling prices of the two systems in the field are as follows: amphoteric cellulose ether mud - $14.30/m 3 CAN PAC gel-chemical mud - - $20.60/m 3 CAN The laboratory work has proven that the new amphoteric cellulose ether based system has met the desired requirements by showing improved rheological properties and similar filtration control to gel-chemical muds. A significant price savings is also anticipated as the amphoteric cellulose system is estimated to be approximately 30% less costly than a PAC based gel-chemical drilling fluid. In addition, significant savings potential exists in trucking costs due to lesser amounts of additives being used. Application in Sea Water Muds Laboratory work has demonstrated that a seawater slurry, based on prehydrated ACE and bentonite, gives improved carrying capacity and fluid loss control, as compared to a straight prehydrated bentonite slurry. Data in Table 3 shows the fluid loss is about half that of the untreated bentonite and the three rpm reading is 2-3 times higher, providing improved hole cleaning with lower solids content. A simple ACE/bentonite seawater slurry should make a good top-hole fluid for offshore drilling. Effect of Contaminants Simulation of drilling through anhydrite produced similar results for both gel-chemical and amphoteric polymer mud systems. Gypsum or calcium nitrate added to the systems in small concentrations initially gave rise to higher fluid losses and slight decreases in rheology (Table 4). Higher concentrations of calcium increased both the rheology and fluid loss, with the greater increases in rheology seen with a gel chemical system, presumably due to the higher initial bentonite concentration. Soda ash was effective in both fluid types in preventing the flocculation effects due to the calcium. Formation Impairment Considerations The amphoteric cellulose ether system should perform no worse than a gel-chemical drilling fluid system in terms of damaging a hydrocarbon-bearing zone. In practical applications, the amphoteric cellulose fluid may be slightly better in some applications due to the lower concentration of bentonitic solids contained within the system. As per best recommended practices, core flow work and return permeability studies will provide the best answer as to the amount of damage caused by a gel-chemical or an amphoteric cellulose ether drilling fluid. Table 5 provides comparative results for a gel-chemical and amphoteric polymer based fluids. In this particular study, both fluids were essentially non-damaging with the amphoteric polymer fluids return permeability being slightly higher than the gel-chemical mud. TEST MATERIAL TEST MATERIAL CNC. BENTNITE* PV YP 10 sec. GEL 10 min. GEL API-FL (g/l) CNC. (g/l) (mpa.s) (Pa) (Pa) (Pa) (ml/30 min.) Base Fluid Hi Vis PAC Amphoteric Cellulose * - unpeptized bentonite Table 2: Laboratory Comparison of Rheological and Fluid Loss Properties of Amphoteric Cellulose Ether vs. Standard PAC Material. ther Ingredients in all Muds were: Fresh Water, Caustic to ph of 10 and 4% Rev Dust as Drilled Solids. Sample # 600 rpm 300 rpm 6 rpm 3 rpm PV (mpa.s) YP (Pa) API-FL A B After hot roll of 80 o C 16 hours A B Table 3: Rheology and Filtration Control properties of Bentonite and ACE in a Seawater slurry. Sample A is 45 kg/m3 prehydrated bentonite, ph 10 with 45 kg/m3 sea salt added. Sample B is Sample A, plus 2.8 kg/m3 of ACE (added to the prehydrated bentonite). SYSTEM [Ca ++ ] [Na 2 C 3 ] PV YP 10 sec GEL 10 min GEL API-FL (mg/l) (g/l) (mpa.s) (Pa) (Pa) (Pa) (ml/30 min) Gel-chemical

8 8 BRENT WARREN, PETER VAN DER HRST, WAYNE STEWART SPE Amphoteric Cellulose Ether Table 4: Comparison of the Effects of Calcium on Gel-Chemical and Amphoteric Cellulose Ether Systems. Gel-chemical formulation was 45 kg/m 3 Natural gel, 1.0 kg/m 3 HiVis PAC, ph 10 with Caustic and 4% Rev Dust; amphoteric Cellulose Ether formulation was 30 kg/m 3 Natural gel, 0.5 kg/m 3 Amphoteric Cellulose, ph 10 with Caustic and 4% Rev Dust. SYSTEM INITIAL PERMEABILITY MUD LEAKFF REGAIN PERMEABILITY REGAIN DRAWDWN (md) (cc/240 minutes) (md [%]) (kpa) Gel-chemical * 0.86 [71] [79] [81] [88] 1379 Amphoteric Cellulose * 13.1 [67] 55 Ether 17.8 [91] [100] 1241 * - system did not achieve seal on leak-off test Table 5: Return Permeability studies on Gething sandstone using gel-chemical and Amphoteric Cellulose Ether Mud Systems. Test conditions included: temperature of 82 o C, net overburden pressure of kpag, mud overbalance pressure of 1600 kpag in a gas reservoir. Field Trial Results The new amphoteric cellulose ether system has been proven to be very effective in the field. To date, over 3000 wells throughout Alberta have used this new system to lower drilling fluid costs, while maintaining good rheological and filtration control properties of the muds. Example from W4 Area of Alberta Wells in this area are typically drilled with a gel-chemical fluid and are for the most part relatively trouble free. Wells in the study varied from 1350 to 1471 m in total depth with mudup occurring between 1150 and 1290 m. Table 6 shows the comparison of costs for wells in the study drilled with a gelchemical and Amphoteric Cellulose Ether based systems. The results clearly show that the cost for the drilling fluid (when mudded-up) and the usage of bentonite and polymer (either PAC or Amphoteric Cellulose) were less for the Amphoteric Cellulose system. Interval costs for the amphoteric cellulose system were 42% less than the corresponding gel-chemical fluid costs. The average usage and costs shown in Table 6 are also highlighted in Figure 8. Table 7 and Figure 9 show the rheological and filtration control property averages for the gel-chemical and amphoteric polymer systems. Similar to the results described earlier in the laboratory section of the paper, the rheological properties of the Amphoteric Cellulose Ether based system were markedly improved over the gel-chemical based system. The Amphoteric system had a higher Yield Point, higher and less progressive gel strengths and a significantly lower consistency index value ( n value) than the gel-chemical average. In addition, the fluid loss control results for both systems were similar. WELL LSD SYSTEM TTAL DEPTH MAIN HLE MUD BENTNITE PAC AMPHTERIC CELLULSE [mud-up depth] (m) CSTS ($) (sacks) (sacks) (sacks) W4 Gel-chemical 1429 [1290] W4 Gel-chemical 1425 [1200] W4 Gel-chemical 1471 [1150] W4 Gel-chemical 1351 [1150] Averages 1419 [1200] W4 Amphoteric Cellulose 1407 [1230] W4 Amphoteric Cellulose 1422 [1225] W4 Amphoteric Cellulose 1415 [1269] Averages 1415 [1240] Table 6: Costs and Volume Usages in Main Hole (Mudded-up interval only) for Gel-chemical and Amphoteric Cellulose Ether Mud Systems. All Wells Were Drilled within a Nine Month Time Frame and with a 200 mm Bit in the Main Hole Section.

9 SPE APPLICATIN F AMPHTERIC CELLULSE ETHERS IN DRILLING FLUIDS W4 WELL CMPARISN sxs PAC or Amphot. Cell. * 10 sxs gel $ material 4600 sxs material $ interval main hole mud Gel Chem - 4 WELLS Amphot. Cell. - 3 WELLS 3000 Figure 8: Average Main Hole Material Usage and Interval Costs for Gel-chemical and Amphoteric Cellulose Wells in Central Alberta. Material Usage are in bars and Costs are Depicted by the Line. Note that sxs of PAC or Amphoteric Cellulose Material are Multiplied 10 fold. SYSTEM DENSITY FUNNEL VIS P. VISCSITY YIELD PINT 10 sec GEL 10 min GEL n value API-FL (kg/m 3 ) (seconds) (mpa.s) (Pa) (Pa) (Pa) (dimensionless) (ml/30 min) Gel-chemical Amphoteric Cellulose Table 7: Average Rheological and Fluid Loss Control Values for the W4 Wells. All Values are Derived from the Main Hole Mudded-up Sections and Reflect Values while Actively Drilling TD where Viscosity is Normally Increased for Logging) W4 RHELGICAL CMPARISN Gel-chem Amphoteric Cell PV (mpa.s) YP (Pa) init. gel (Pa) 10 min. gel (Pa) n * 10 Amphoteric Cell. Gel-chem Figure 9: Average Rheological Data for Gel-chemical vs. Amphoteric Cellulose Ether Mud System Comparison.

10 Example from W5 Area of Alberta. Table 8 presents the results from another Amphoteric Cellulose Ether study. The area is commonly drilled with gelchemical drilling fluids. Although the drilling in the main hole mud section is usually without major problems, difficulty in logging at total depth is an issue in the area. As a result, gel-chemical drilling fluids often use Xanthan gum polymers to boost the low-end rheology prior to logging to increase the likelihood of trouble-free logging run(s). As evident in Table 8, the Amphoteric Cellulose ether grouping of wells used less bentonite, less polymer and had lower interval costs than the offset gel-chemical wells. Cost savings over the interval averaged 38% for the Amphoteric polymer system. It is worth noting that the average total well depth for the amphoteric cellulose fluid group was 230 m deeper than the gel-chemical group and that the average meters of drilling with mud was also 160 m greater. Some of the savings were a result of the Xanthan polymer usage that averaged 0.7 sacks for the Amphoteric polymer wells and 3.7 sacks for the gel-chemical wells. The wells drilled with amphoteric polymer achieved increased rheology prior to logging by adding small amounts of bentonite and amphoteric cellulose. Figure 10 presents the average results of the wells listed in Table 8. WELL LSD SYSTEM TTAL DEPTH MAIN HLE MUD BENTNITE PAC AMPHTERIC CELL. XANTHAN [mud-up depth] (m) CSTS ($) (sacks) (sacks) (sacks) (sacks) W5 Gel-chemical 1468 [1300] W5 Gel-chemical 1877 [1477] W5 Gel-chemical 1595 [1400] W5 Gel-chemical 1473 [1284] W5 Gel-chemical 1602 [1300] W5 Gel-chemical 1513 [1321] W5 Gel-chemical 1681 [1450] Averages 1601 [1360] W5 Amphoteric cell [1500] W5 Amphoteric cell [1301] W5 Amphoteric cell [1474] Averages 1830 [1425] Table 8: Costs and Volume Usages in Main Hole (Mudded-up interval only) for Gel-chemical and Amphoteric Cellulose Ether Mud Systems. All Wells Were Drilled within a Six Month Time Frame and with a 200 mm Bit in the Main Hole Section W5 AREA - USAGE & CST IN MAIN HLE sxs PAC or Ampho. Cell * 10 sxs gel $ cost main mud sxs material Aver. Well depth = 1601 m Aver. Well depth = 1830 m $ main mud interval cost Gel-chem (7 wells) Amphoteric Cell. (3 wells) 4000 Figure 10: Average Main Hole Material Usage and Interval Costs for Gel-chemical and Amphoteric Cellulose Wells in Northwestern Alberta. Material Usage are in bars and Costs are Depicted by the Line. Note that sxs of PAC or Amphoteric Cellulose Material are Multiplied 10 fold. Conclusions 1. Amphoteric Cellulose Ether polymer chemistry represents a significant breakthrough in the drilling fluids industry. This type of polymer provides an enhancement to both rheological and filtration control properties of fresh water

11 SPE APPLICATIN F AMPHTERIC CELLULSE ETHERS IN DRILLING FLUIDS 11 drilling fluids. 2. The rheological and filtration control properties of fresh water bentonitic muds containing amphoteric cellulose ethers can be varied greatly, depending upon the degrees of substitution of the carboxymethyl (DS CM ) and quaternary (DS QN ) groups and polymer molecular weight. 3. The ideal drilling fluid ACE polymer will have a DS CMC between 0.8 and 1.0, and a DS QN of 0.05 to In addition, the molecular weight of the ACE needs to be of a minimum level to achieve adequate viscosity. 4. The drilling fluid containing the new amphoteric cellulose ether requires less bentonite and less polymer than gelchemical muds to achieve similar filtration control and improved rheological properties. 5. The new amphoteric cellulose ether system costs less than a typical gel-chemical system. Chemical savings with the new system should be ~ 20-50% as compared to offset wells drilled with gel-chemical muds. The examples shown in this paper showed savings of 38-42%. 6. Contaminants such as anhydrite can be treated out in the new amphoteric polymer system in a manner similar to a gel-chemical mud. 7. Core flow return permeability studies suggest that the new amphoteric cellulose system will have a similar or slightly better ability to minimize formation impairment. References 1. Quaternary Nitrogen Containing Amphoteric Water Soluble Polymers and Their Use in Drilling Fluids, International Patent Application Number PCT/EP00/02884, Publication Number W 00/60023, ctober 12, Drilling Fluid Selection, Performance and Quality Control, Kelly Jr., John, J. Petroleum Technol., p 889, Visualization of Fluid-Loss Polymers in Drilling-Mud Filter Cakes, Plank, J.P. and Gossen, F.A., SPEDE, 1991.