Biodegradable polymer drilling mud prepared from guinea corn

Journal of Brewing and Distilling Vol. 3(1), pp. 6-14, February 1 Available online at http://www.academicjournals.org/jbd DOI: 1.5897/JBD11.18 ISSN 141-197 1 Academic Journals Full Length Research Paper Biodegradable polymer drilling mud prepared from guinea corn C. O. Chike Onyegbula*, O. Ogbobe, S. C. Nwanonenyi Department of Polymer and Textile Engineering School of Engineering and Engineering Technology, Federal University of Technology Owerri, Imo State Nigeria. Accepted 4 February, 1 A biodegradable polymer drilling mud was prepared using Guinea corn starch pre- gelatinized in the absence of a solvent. The filtration and rheological properties of the mud were studied at 5 to C temperature range and.1 to.5 g/ml concentration of starch using filter loss and viscomertric methods, respectively. Experimental results showed that the new mud has better filtration control behavior and thermal stability at all the temperatures than widely used mud prepared with hydroxy propyl- modified starch. The mud displayed thermal degradation at C. The values of flow index were found to be less than 1., showing non Newtonian and Pseudoplastic flow behavior of the mud. Shear stress and yield stress increased with increase in concentration. Viscosity decreased with increasing shear rate, showing shear thinning behavior of the mud. The polymer mud obeyed Henri Darcy and API models for static filtration as well as Power law and Herschel Bulkley models for fluid rheology. The new mud is purer and more suitable for drilling operation in environmentally sensitive areas than the widely used mud. Key words: Biodegradable polymer, drilling mud, filtration, rheology, starch. INTRODUCTION Biodegradable Polymer muds are muds produced from Polymers containing properties that make them susceptible to biological or Microorganisms attack (Alderman et al., 1988; Amanullah et al., 4; Ching et al., 1993; Lenz et al., 1993). Several biodegradable polymers like starch, xanthan gum, guar gum, and cellulose are presently used in preparing muds (Amanullah et al., 4; American Petroleum Institute, 1969; Chilingarian and Varabutre, ; Sorbie, 1994). Starch, specifically, is a naturally occurring polymer of anhydroglucose units having -1, 4 linkages (Amanullah et al., 4). It is the second most abundant biomass found in nature, next to cellulose (Katopo et al., ), and consist of two major weight components. Chemically, *Corresponding author. E-mail: kateoluchi11@yahoo.com. Tel: +34835454139. it contains amylose linear polymer with molecular weight in the range 1, to 5, and amylopectin highly branched with a molecular weight in the range of 1 to million (Wing, 1988). Physically, it has both amorphous and crystalline regions. The short branching chains in the amylopectin are main crystalline component in granular starch. The ratios of amylose and amylopectin vary depending on the origin and nature of the starch, and this variation changes the behavour of the starch. The amylose component of starch controls the gelling behavior since gelling is the result of re-association of the linear chain molecules (Amanullah et al., 4). Amylopectin is usually large in size. The large size and the branched nature of amylopectin reduce the mobility of the polymer and its orientation in a aqueous environment (Amanullah et al., 4). Agricultural crops like Guinea Corn are the major source of starch (Amanullah et al., 4; Katopo et al., ; Stevens, ; Wing, 1988). Starch in its raw form is not water soluble; but simply floats around as starch

Chike Onyegbula et al. 7 particles. Therefore, to prepare an effective polymer mud with starch, it is necessary to rupture the protective shell of amylopectin to release the inner amylose, and this process is called pre gelatinization (Amanullah et al., 4; Ebewele, ). The starch then becomes hydrophilic, nonionic, and soluble in both saturated salt and fresh water. Over the past decades, starch used for preparing drilling muds has been pre- gelatinized through modification with chemicals. Different types of chemicals are used to modify starch used for preparing drilling mud to meet functional requirements such as appropriate mud rheology, density, fluid loss control property, and thermal stability. In recent times, numerous modified starches have been produced for oil field applications, and this is carried out by gelatinization in the presence of a solvent (wet method). This process of gelatinization has lower efficiency and lower purity and produces a large amount of wastewater as a by- product. In addition, it is difficult to graft two groups using this process, especially, when one is hydrophilic and the other is hydrophobic in one reaction because they need different solvents. Amanullah and Long (4) reported on superior corn based starch for oil field application (Amanullah et al., 4). Furthermore, a number of studies on polymers and their use in water based drilling fluids have been carried out (Alderman et al., 1988; Gray and Darly, 198; Kok and Alikaya, 3; 5). The use of polymers like guar gum, carboxymethyl cellulose ( CMC), and hydroxy propyl starch as filtration control agents or as drilling fluids and the effects of temperature on the behavior of these polymers have been discussed (Gray and Darly, 198).This study concluded that the filtration parameters soptivity and diffusivity are dependent on temperature. Sorptivity is a measure of the resistance against the fluid flowing through the filter cake, while diffusivity is a measure of the rate of flow of fluid (Outmans, 1963). Rheological evaluation of polymer as drilling fluids has also been reported (Kok and Alikaya, 3). Also, the effect of polymers on the rheological properties of KC1/ polymer- type drilling fluids has been studied (Kok and Alikaya, 5). The study focused on the effect of addition of polymers on consistency index, flow behavior index, and shear stress. The authors observed that consistency index increased as polymer concentration increased, supplying more shear stress values. Consistency index is a measure of the overall thickness of a mud, while flow index is a measure of the degree of flow behavior of a fluid (Alderman et al., 1988; Mewis et al., 1989). The resistance of the fluid to the applied rate of shear or force is called the shear stress, which in oil field terms is analogous to the pump pressure (Chilingarian and Varabutre, ; Kok and Alikaya, 5; Okafor, 199) The work reported in this paper deals with preparation of biodegradable polymer and water based mud using Guinea Corn starch pre gelatinized by extrusion technique in the absence of solvent. The filtration behavior and rheological properties of such mud were compared to a widely used water- based mud prepared with hydroxy propyl modified starch. Theory The concept involving sorptivity and diffusivity and their computations enable the mathematical description of the filtration behavior of drilling muds. The built up filter cake determines the resistance against the fluid flowing through the filter cake, which thereafter determines the volume of fluid loss in a particular filtration time. Filtration models are usually visualized by means of curves, which are plots either of fluid loss versus square root of the time or of filtration rate versus time. On the other hand, the concepts of shear stress and shear rate and their measurement enable the mathematical description of the flow of drilling muds. The amount of force terms applied to a fluid determines the shear rate, which in oil field terms is determined by the flow rate of the fluid through a particular geometrical configuration. Flow models are plots either of flow pressure versus flow rate or of shear stress versus shear rate. Four different models are described here and used for the characterization of the muds studied. API and Henri Darcy models are for filtration properties, while power law and Herschel Bulkley models are for rheological properties of the muds. API Model The API model is a static filtration model that states that for any static filtration, the total fluid loss is directly proportional to the square root of time 3. The model expresses the relation as: V = St ½ (1) where V is total fluid loss volume or filtrate volume or filter loss, S is sorptivity of fluid, t is time of filtration. Henri Darcy Model The Henri Darcy model relates filtration rate to time in an exponential manner 14 The model is expressed as: (R) = exp Dt where R is filtration rate, is initial volume of fluid and cake, is final volume of filtrate, D is diffusivity of fluid, t ()

8 J. Brew. Distilling is time. Power law model Power law is a two parameter model that relates shear stress to shear rate in a nonlinear manner (Alderman et al., 1988; Okafor, 199). The model does not consider an excess yield stress and states the relation as: = k n (3) where k, and n are consistency and flow index respectively, is the shear stress, and is the shear rate. Taking the logarithm of the equation, the following term is obtained: Log = log k + n log (4) Thus, n = slope and k = intercept. Herschel-Bulkley Model Herschel Bulkley is a three-parameter model that describes the behavior of yield-pseudoplatics fluids (Mewis et al., 1989) = o + k n (5) where is the shear stress, o is the yield stress, k is the consistency index, is the shear rate, and n is the flow index. MATERIALS AND METHODS Guinea Corm was obtained from Tapshang Local farm at Kano in Nigeria. Bentonite and hydroxyl propyl starch were obtained from Global Oil Company Nigeria Limited. Sample preparation was carried out in three stages: extraction of starch from Guinea Corm grains, extrusion of Guinea Corm starch in the absence of a solvent, and preparation of biodegradable polymer mud and widely used drilling mud using the extruded Guinea Corm starch and the chemically modified starch (hydroxyl propyl starch), respectively. All preparations were made using double-distilled water. The mud was prepared using.1 g/ml concentration and.1-.5 g/ml concentration range of the starch in fresh water to study the filtration and rheological properties respectively. Extraction of starch Starch was extracted from Guinea Corm grains. The grains were softened by soaking them overnight in distilled water. The softened grains were removed from the water and ground to a moderately fine consistency. The mash was sieved through a cloth bag into a sufficient volume of distilled water. The whole extract was allowed to stand for an hour, and then the water was pressed out to obtain the starch, which was dried at a low temperature for 4 h. Extrusion of starch The starch was extruded to obtain a novel mud constituent. The new mud constituent, tagged, was prepared by gelatinization using extrusion technique in the absence of a solvent. The process was carried out by mixing starch and water in the ratio of 85: 15 and placing it in an extruder. The extrusion was carried out at 8 1 5 Pa extrusion pressure and 1 C temperature. The approximate resident time during the extrusion process was about 3mins. The moisture content of the extruded starch was in the range 1 to 13%. After extrusion, the extruded starch was shredded to to 3 mm size. The shredded sample was dried at 15 C for 4 h and then ground to obtain fraction of particle size (< 45 µm) to be used for mud preparation. Preparation of muds Several biodegradable polymer muds that are water-based were prepared using first, 3 g of the new starch product () in 3 ml of water and then, 4, 6, 8, and 1 g of the same new product each in ml of water. Also, water-based muds of the same concentration of a widely used modified starch (hydroxy propyl starch), tagged in this study, were prepared to compare the filtration and rheological properties to those of the newly produced muds. The base component of the muds (water) was mixed with bentonite for min using a high-speed mixer. At the end of the mixing, the components, or were added slowly to the agitated mud to avoid any lump formation within the mud system. ph of the muds was adjusted by adding and mixing a suitable amount of sodium hydroxide (NaOH) to the muds. Filter loss method The filtration properties of the new water-based mud () prepared with Guinea Corn starch pre-gelatinized by extrusion technique in the absence of a solvent and the widely used mud () prepared with chemically modified starch, hydroxy propyl starch, containing.1 g/ml concentration of the respective starch, were determined at 5 to C using filter loss method. All sample preparations and balance calibration were carried out following API standard procedures. The tests were run for the mud systems using a standard filter press unit under constant pressure of 1 psi. after assembling the accessories of the filter press, 3 ml of the newly developed biodegradable polymer mud () containing.1 g/ml concentration of the extruded starch was used to run the filtration test for various time intervals (in minutes) at room temperature, 5 C, and the filtrate volume (fluid loss) was collected and measured. The thermal stability of the mud was assessed by carrying out a standard hot turning test at temperature range of 15 to C. Filtration test was run for the mud recovered from the hot test after mixing mud at each time interval. The result of the average of three tests is reported, and the average error observed in the reproducibility (repeatability) is not more than 5%. The same procedure was repeated for the widely used mud () containing hydroxy propyl starch. Viscometric method The rheological properties of the new polymer mud () and the widely used mud () were also determined at.1 to.5 g/ml of each starch concentration using the viscometric method.

Fluid loss V (ml) (ml) Fluid loss V (ml) Fluid loss V (ml) (ml) (ml) Fluid loss V (ml) (ml) Fluid loss V (ml) (ml) Fluid loss V,( ml ) Chike Onyegbula et al. 9 Figure 4.1 : Fluid loss versus sqare root of time curves for the Muds at 15 oc 5 15 1 5 3. 5.5 7.1 8.4 9.5 1.5 11.4 1.3 13 13.8 Square root of time, t1/ Square time, t ½ (min), (min) * 5 15 1 5 5 15 1 5 3. 5.5 7.1 8.4 9.5 1.5 11.4 1.3 13 13.8 Square root of time, t1/ (min) Square time, t ½, (min) -1 - * -3-4 5 15 1 5 3 (c) 5 3 5 5 15 1 515 15 5 1 1 155 5 1 5 (d) (c) (c) (d) Figure 1. Plot of fluid loss against square root of time for and samples at 5, 15, (c)18 and (d) C. The experiment was performed following standard procedures. The apparatus used was a Brookfield viscometer. A ml amount of each of the muds with.1 to.5 g/ml concentration range of the respective starches was used for the test. Each volume of the mud to be tested was agitated by remixing it for min. Each mud concentration,.1 to.5 g/ml, was subjected to four different speeds, 6, 1, 3, and 6 rpm, under the shear rate range of.1 to 1.s -1. RESULTS AND DISCUSSION Starches are very effective in the control of filtration and rheological behavior of water-based drilling muds. Different starches pre-gelatinized and modified in different ways have different effects on the muds prepared with them. The aim of this study is to prepare biodegradable and water based mud () using starch pre-gelatinized by extrusion technique and to compare the filtration and rheological properties of such mud to hydroxy propyl-modified starch mud (). The experiments were carried out by using API filter loss and viscometric methods. Throughout the study two different models (API and Henry Darcy) were used for filtration behavior identification, and two other models (Power Law and Herschel-Bulkley) were used for rheological model identification. The results of the experiments showed that fluid loss increased with temperature. It was also observed that both sorptivity and diffusivity increased with increase in temperature. The new mud () gave higher values of sorptivity and lower values of diffusivity than mud at the temperatures. mud has lower fluid loss than mud at all the temperatures. The highest value of fluid loss was obtained at C. The flow index values were found to be less than 1 (n < 1) at all the concentrations for both muds. The flow index values and the consistency index values were observed to increase with increase in starch concentration. The yield stress increased with increase in concentration of starch. It was also observed that the viscosity of the muds decreased with increasing shear rate. Filtration properties Figures 1 to (d) shows the fluid loss versus square root of time curves for the mud s at 5 C, 15 C, (c) 18 C and (d) C. it could be observed from the figures that the fluid loss increased with increase in square root of time. This shows that the results obtained with muds agreed with the API model for static filtration, which states a directly proportional relationship between fluid loss and square root of time (Equation (1)). It was also observed that the fluid loss increased with increase in temperature, and highest fluid was obtained at C. Figure 1 (d) shows the extent of thermal stability of the muds. Since the increase in fluid loss with temperature is as a result of increase in fluid flow, it means that there is a particular high temperature at which the fluid loss continues until there is no more free fluid to flow, then, thermal degradation may set in. it is also observed that mud gave lower fluid loss values at all the temperatures than mud. The lower fluid loss values of mud should result from the starch particles, which may be highly compacted and capable of closing the interstitial openings of the filter cake. Table 1 shows

Rate of filtration (dv/dt) ml/min Rate of filtration (dv/dt)ml/min Rate of filtration (dv/dt)ml/min Rate of filtration (dv/dt) ml/min 1 J. Brew. Distilling Table 1. Computed values of fluid sorptivity (S) for mud and mud containing.1g/ml concentration of the respective starch at different temperatures. Mud sample Fluid sorptivity, S (ml/min) 5 C 15 C 18 C C 5.7 7.93 3. 34.91 11.9 15.3 18.88.15 9 8 7 18 16 14 6 1 5 4 3 1 1 3 5 7 9 11 13 15 17 19 Time (min) 1 8 6 4 1 3 5 7 9 11 13 15 17 19 Time (min) 18 16 14 1 1 8 6 4 1 3 5 7 9 11 13 15 17 19 Time (min) (c) 18 16 14 1 1 8 6 4 1 3 5 7 9 11 13 15 17 19 Time (mins) (min) m Figure. Plot of rate filtration against time for and samples at 5, 15, (c)18 and (d) C. (d) (d) Table. Computed values of fluid diffusivity (D) for mud and mud containing.1g/ml concentration of the respective starch at different temperatures Mud sample Fluid diffusivity, D (ml/min ) 5 C 15 C 18 C C.9.13.17..16.18.4.7 the values of fluid sorptivity (S) obtained as slope from figures 1 to (d) from equation (1). It is seen from the table that values of sorptivity (S) increased with increase in temperature. This could be explained on the basis that an increase in the sorptivity results from an increase of the resistance against the fluid flowing through the filter cake. Thus, the increase in the sorptivity with temperature should result from an increase in the built-up filter cake. Also, it was observed that mud gave higher values of sorptivity at all the temperatures than mud. This is as a result of mud having greater ability than mud to build up filter cake, which increases the resistance against the fluid flowing through the filter cake at the temperatures. Figure to (d) shows filtration rate versus time curves for the muds at 5 C, 15 C, (c) 18 C and (d) C. Inspection of the figures reveals that filtration rate decreased with increase in time in an exponential manner. This implies that the mud filtration behavior agreed with the Henri Darcy model {Equation ()}. It could also be observed that filtration rate increased with increase in temperatures. The increase in filtration rate with temperature resulted from an increase in fluid loss. The values of fluid diffusivity (D) obtained as a slope from Figures to (d) considering Equation () are listed in Table. It is observed from the table that values of

Shear stress (mpa) Shear stress (mpa) Chike Onyegbula et al. 11 18 16 14 1 1 8 6 4.1..5 1 Shear rate (s -1 ).1 g/ml. g/ml.3 g/ml.4 g/ml.5 g/ml 14 1 1 8 6 4.1..5 1 Shear rate (s -1 ).1 g/ml. g/ml.3 g/ml.4 g/ml.5 g/ml Figure 3. Plot of shear stress as a function of shear rate for samples and at different concentrations. diffusivity increase with increase in temperature. The increase in diffusivity with temperature resulted from an increase in the rate of flow of fluid. The values of diffusivity (D) obtained with mud at all the temperatures are higher than that obtained with mud. Rheological properties The pattern of the curves obtained from shear stress and shear rate relationship at the various concentrations showed that the shear stress and shear rate were related in a nonlinear manner (Figures 3 and ). This nonlinear relationship between shear stress and shear rate showed that the muds obeyed the power law model for non-newtonian fluid (Equation (3)). In addition, the nonlinear relationship between shear stress and shear rate showed that the muds are pseudoplastic. According to Alderman et al. (1988), a fluid is pseudoplastic when the consistency curve obtained from shear stress and shear rate relationship passes through the origin and is nonlinear. Table 3 shows the values of flow index, n, and consistency index, k, obtained with the muds at the various starch concentrations. The slope and the intercept of the plots of log shear stress versus log shear rate (Figures 4 and ) are respectively equal to the flow index, n, and consistency index, k (Equation (4)). It is

Log shear stress Log shear stress 1 J. Brew. Distilling Table 3. Rheological parameter from power law equation for mud and mud with different starch concentrations Mud sample Concentration of starch (g/ml) Flow index n Consistency index k.1.48 1.89..6.57.3.69.9.4.75 3.1.5.85 3.37 PS- MUL.1.4 1.7..49..3.61.64.4.71.9.5.8 3.7 3.5 3.5 1.5 1.5-1 -.69 -.3.1 g/ml. g/ml.3 g/ml.4 g/ml.5 g/ml Log shear rate 3.5 3.5 1.5 1.5-1 -.69 -.3.1 g/ml. g/ml.3 g/ml.4 g/ml.5 g/ml Log Shear rate Figure 4. Plot of log of shear stress as a function of log of shear rate for samples and at different concentrations. observed from table III that the flow index, n, is less than1 for each mud at the various concentrations studied. According to Mewis et al. (1989), a fluid for which n value is less than 1 (n < 1) is said to have pseudo plastic flow behavior. Further examination of the table reveals that the flow index n, and the consistency index, k, increased with increase in concentration of starch. This may be due to the fact that as concentration increases, the resistance of the fluid to the applied rate of shear or force, called shear stress, increases, and gives rise to an increase in flow index. Thus, the increase in flow index, n, with starch concentration resulted from the increase in shear stress

Viscosity (Mpa.s) Viscosity (Mpa.s) Chike Onyegbula et al. 13 Table 4. Calculated values of yield stress from Herschel Bulkley model for mud and PS- MUL mud with different starch concentrations. Mud sample Concentration of starch (g/ml) Yield stress o (mpa).1 48.. 18..3 3..4 46..5 69. PS- MUL.1 9.. 7..3 165..4 7..5 38. 5 15 1 5.1..5 1 Shear rate (s -1 ).1 g/ml. g/ml.3 g/ml.4 g/ml.5 g/ml 15 1 5.1..5 1 Shear rate(s -1 ).1 g/ml. g/ml.3 g/ml.4 g/ml.5 g/ml Figure 5. Plot of viscosity as a function of shear rate for samples and at different concentration required to cause the mud to flow. The increase in consistency index, k, with starch concentration resulted from the increase in the overall thickness of the mud. The values of yield stress obtained from Equation (5) are given in Table 4. It is clearly seen from the table that yield stress increases with increase in starch concentration. The increase in yield stress with concentration resulted from an increase in shear stress required to break the gel structure of the mud before flow started. Furthermore, it could be considered that, at rest, the mud s chains were entangled and the gel formation resulted from the polymer network formed by the physical aggregation with region of local order acting as network junctions. Figure 5 shows the plot of shear rate as a function of viscosity for and. It is observed from

14 J. Brew. Distilling the figure that the viscosity of the mud decreased with increasing shear rate; this is known as shear thinning. This resulted from the fact that the entanglement of the mud s chain at low shear rate impeded shear flow and the viscosity was high. The viscosity decreased as shear rate was increased, showing shear thinning behavior of the muds. Conclusion The following conclusions are drawn from the work: (i) Guinea Corn starch pre-gelatinized by extrusion technique in the absence of the solvent is suitable for preparing biogradable polymer mud. (ii) The new polymer mud has better fluid loss control behavior, better filtration rates, higher sorptivity, and lower diffusivity than a widely used mud prepared with chemically modified starch, thus, the new mud can control fluid flow even at stoped circulation during drilling operation. (iii) The thermal stability of the new mud at 18 C implies that the mud is suitable for drilling well bores having bottom hole temperatures as high as 18 C, whereas the functional behavior of the mud will cease at temperatures above C due to serious thermal degradation at these temperatures. (iv) Shear stress, yield stress, and viscosity are dependent on concentration and have higher values showing higher gel strength and flocculation for the new mud than for the widely used mud. (v) The viscosity of the mud decreases with increasing shear rate, therefore, the new mud has shear thinning behavior. (vi) The values of flow index are less than 1., showing that the mud is non-newtonian and has pseudoplastic flow model. (vii) The new polymer mud is therefore more suitable for exploration and exploitation of oil and gas in environmentally sensitive areas due to its high efficiency and purity. REFERENCES Alderman NJ, Babu DR, Hughes TL, Maitland GC (1988). Rheological Properties of water-based drilling muds, in 4 th international Congress on Rheological, Sydney, I: p. I. Amanullah MD, Long Y (4). Superior corn-based starches for oil field application, in New Director for a Diverse Planet: Proceedings for the 4 th International Conference on Corns. Brisbane, Australia, American Petroleum Institute (1969). Principles of Drilling Fluid Control, 1 th ed., I, University of Texas, Continuing Education Petroleum Extension service, Austin. Chilingarian GV, Varabutre P (). Drilling and Drilling Muds: Development in Petroluem Scince, 44. Elsevier, Amsterdam. Ching C, Kaplan D, Thomas E (1993). Biodegradable Polymers and Packaging, Technomic. Lancaster, Penn. Ebewele RO (). Polymer Science and Technology, 15-16, CRC Press, Boca Raton, Fla. Gray GR, Darly HCH (198). Composition and Properties of Oil Well Drilling Fluid, 4 th ed., 19-6, Gulf Publishing Co., Houston. Katopo H, Song Y, Jane JL (). Effect and mechanism of ultrahigh hydrostatic pressure on the structure and properties of starches, carbohydr. Polym., 47(3): 33-44. Kok MV, Alikaya T (3). Rheological evaluation of polymers as drilling fluids, pet. Sci. Technol., 1(1-): 133. Kok MV, Alikaya T (5). Effect of polymers on the rhological properties of KCI/polymer type drilling fluid, Energy Sources, 7: 45. Lenz RW (1993), Biodegradable polymers, in Biopolymers Springer, Heidelberg. I: 1-4. Mewis J, Willaim JF, Trevor AS, Russel WB (1989). Rheology of suspensions containing polymerically stabilized particles, J. Chem. Engineers, Res. Develop. ALCHE, 19: 415. Okafor MN (199). SPE Paper no. 486. Outmans FHD (1963). Mechanics of static and dynamic filtration, Soc. Pet. Eng. J., 63: 1. Sorbie KS (1994). Polymer-Improved Oil Revovery, 7, Blackie, Glasgow. Stevens ES (). Green Plastic: An Introduction to the New Science of Biodegradable Polymers, Princeton University Press, Princeton, N.J. Wing RE (1988). Non-chemically modified corn starch serves as an entrapment agent, In Proceedings of Corn Utilization Conference II, National Corn Growers Association, Australia, November 17-18, pp. 1-4.