1 CSIRO Petroleum, Australian Resources Research Centre, PO Box 1130, Bentley, WA 6102, Australia
2 CMIT, Clayton South, Melbourne, VIC 3169, Australia
Several corn-based starches have been developed using local resources to study their suitability to use as drilling fluid additive. Experimental results indicate that some of the newly developed starch products have similar or better filtration control properties than the filtration control properties of a widely used modified starch. The novel products have higher purity and thus expected to be better candidates for exploration and exploitation of oil and gas in environment sensitive areas. The lower production cost of the novel starches indicates their potential to reduce the mud as well as the drilling cost. The products were developed using local resources and thus expected to open new opportunities for Australian farmers and industries.
Several technically better and more environment friendly modified starches have been developed for non-food use of corn-based starch using local resources. The finding opens up a new market for Australian farmers.
Fluid Loss; Novel Starches; Environment Friendly Mud Additive
Starch is the second most abundant biomass found in nature, next to cellulose (Herman Katopo et al., 2002). It consists of two major components. Chemically, it contains amylose, a linear polymer with a molecular weight in the range of 100,000 -500,000 and amylopectin, a highly branched polymer with a molecular weight in the range of 1-2 million ( Wing, 1988). Physically, it has both amorphous and crystalline regions. The short branching chains in the amylopectin are the main crystalline component in granular starch. Variation in the amount of amylose and amylopectin in a starch changes the behaviour of the starch. The amylose component of starch controls the gelling behaviour since gelling is the result of re-association of the linear chain molecules. Amylopectin is usually larger in size. The large size and the branched nature of amylopectin reduce the mobility of the polymer and their orientation in an aqueous environment. Figure 1 (a) and (b) shows the structures of the amylose and amylopectin components of a starch molecule. The abundance in hydroxyl groups in the starch molecules impart hydrophilic properties to the polymer and thus its potential to disperse in water.
Figure 1 Building Units of Starch (a) Amylose and (b) Amylopectin
Different types of chemicals and polymers are used by the petroleum industry to design a drilling mud to meet some functional requirement such as appropriate mud rheology, density, fluid loss control property etc (Amanullah et al. 1997). Modified starch is frequently used at the early stage of drilling. It may be mentioned that several tons of pre-gelatinised starch are used by a rig annually. It is considered as a suitable mud additive for drilling wells having a low bottom hole temperature because of its high biodegradability, no environmental impact, easy availability and low cost. However, the modified starch manufactured by conventional gelatinisation has lower purity due to the use of a solvent during the manufacturing process. Bearing this in mind, several novel starch products have been developed using local resources by gelatinisation using a reactive extrusion technique. This paper describes the novel starch products and their performance as fluid loss additives with respect to a widely used modified starch and demonstrates their suitability to use as fluid loss additives for environmentally sensitive areas.
Over the past decades, starch has been used as the raw material for developing polymeric additives to control the fluid loss properties of drilling mud. Now-a-days, numerous modified starch derivatives have been developed for oilfield applications and some of them have been commercialized. The modification is carried out to produce a range of starches with higher tolerance to thermal, mixing and pH effects and generate easy swelling characteristic in the presence of water. However, most of these modified starches used in oilfields are produced by gelatinization in the presence of a solvent (wet method). This process of gelatinization has lower efficiency and produces a large amount of waste-water as a by-product. Furthermore, it is hard to graft two groups using this process, especially when one is hydrophilic and another is hydrophobic, in one reaction because they need different solvents. Reactive extrusion is a technology developed to carry out a chemical reaction using an extruder. Figure 2 shows a schematic diagram of a Reactive Extruder. The technology has been developed in the last two decades and has been widely used to modify conventional polymers. The advantages of the technology are flexibility, efficiency, no requirement of solvent, lower production cost and ability to carry out multi-reactions at one step.
1 Base Stand
2 Starch Outlet
3 Extruder Barrel
5 Chemical Feeder
6 Starch Feeder
9 Chemical Container
Figure 2 Schematic Diagram of Reactive Extruder
Five different starches namely DF-01, DF-02, DF-03, DF-04 and PS Multi were prepared by gelatinisation using extrusion technology mentioned above. The products have different amylose and amylopectin ratios. They were prepared by putting starch and water in the ratio of 85/15 into an extruder (see Figure 2). The extrusion was carried out at 80 bar extrusion pressure and 140 oC temperature. The approximate resident time during the extrusion process was about 3 minutes. The moisture content of the extruded starch was in the range of 12-13%. For modified starch PSMulti, chemical was introduced into the extrusion chamber before the extrusion operation. PS Multi is hydroxypropyl-modified version of the high amylose DF-04. DF-01 is a waxy corn starch which practically has no amylose in its composition. DF-02 is a low amylose starch and DF-03 has 50% amylose content. After extrusion, the starch was shredded to 2-3 mm sizes. The shredded starch samples were dried at 105 oC for 24 hours and then grinded to obtain <212 micron size fraction to use as fluid loss additive.
Several fresh water-based bentonite muds were designed using 2 gm newly developed starch products. A base bentonite mud and also a bentonite mud having 2 gm of a widely used modified starch were also prepared to compare the fluid loss property control potential of the new starch products. The basic mud components (water and bentonite) were mixed for 20 minutes using a high speed mixture. At the end of mixing of the base mud, starch was added slowly to the agitated mud to avoid any scope of lump formation within the mud system. The pH of the muds was adjusted to 9.8 to 10 by adding and mixing a suitable amount of NaOH to the mud. The starch containing mud contains about 6% bentonite by weight and 0.5% starch by weight.
After adjustment of the pH of the mud to the desired level, API filtration test was run for all the mud systems. Three tests were run for each mud system. The results presented in Figures 3 and 4 show the average of the three tests. The thermal stability of the novel starch products were assessed by carrying a standard 16 hour hot rolling test at 150, 175 and 200 oC. The muds underwent serious thermal degradation at 200 oC and hence excluded from the results. The filtration properties of the samples recovered from hot roll cells were measured using standard API procedure after a short period of mixing of the mud using a blender.
The effect of different fluid loss additives on the API spurt and filtrate volumes of fresh water-based bentonite muds measured at room temperature and after thermal treatment at elevated temperature are shown in Figures 3 and 4.
Figure 3 API Fluid Losses of the Mud Systems
Figure 4 API Spurt Losses of the Mud Systems
Figure 3 clearly indicates that the presence of most of the newly developed starches in the bentonite mud reduced the API filtration of the bentonite mud measured at room temperature (25 oC) by about 35 to 40%. However, the presence of chemically modified PSmulti shows no improvement in the filtration behaviour of the bentonite mud. Comparison of the API fluid loss behaviour of the newly developed starch containing muds with that of a modified starch containing mud (MS) indicates that most of the newly developed starch products have similar/better behavior with respect to the modified starch containing mud. API spurt loss behavior (see Figure 4) of the newly developed starch containing muds measured at room temperature (25 oC) also shows that most of the newly developed starches have spurt loss characteristics very similar to or better than the spurt loss characteristic of modified starch containing mud. Some of the novel starch products even show a 50% drops in the spurt loss characteristics of the bentonite mud (see Figure 4) with respect to the spurt loss value given by the modified starch containing mud. The lower spurt loss characteristics of the mud indicate lesser scope of spurt related formation damage. However, static spurt loss measured at room temperature may not be a good indicator of formation damage potential of a mud as thermal degradation of the starch at elevated temperature may change the scenario. It may be mentioned that control of spurt and filtrate losses of drilling mud are thought to be the two key elements in controlling mud related formation damage.
Analyses of the filtration behaviour of the muds after thermal treatment at 150 oC and 175 oC indicate that all the muds underwent thermal degradation at 175 oC hot rolling temperature (see Figures 3 and 4). This indicates that the functional behavior of the starches will cease to exist at elevated temperature due to serious thermal degradation of the starch products of the mud. The fluid loss behavior of the muds after thermal treatment at 150 oC indicates that most of the muds possess good thermal stability at 150 oC hot rolling temperature. Even the PSmulti starch containing mud which showed no improvement of bentonite mud filtration at room temperature showed excellent thermal stability at 150 oC. This is reflected by the no change in the spurt and fluid loss characteristics of the mud before and after thermal treatment at 150 oC. Comparison of the fluid loss characteristics of the novel starch containing muds with respect to the fluid loss characteristics of the modified starch containing mud treated at 150 oC temperature indicates that some of the novel starch products have better thermal stability than the widely used modified starch (see Figure 3). API spurt losses of the muds measured after thermal conditioning at 150 oC (see Figure 4) show there is some improvement of the spurt loss characteristic of some starch containing muds. This is due to effective swelling of the starches at this temperature without thermal degradation of the starches. The thermal stability of the starches at 150 oC indicates that the novel starch products could be used in subsurface condition having down hole temperature as high as 150 oC.
The newly developed starches were prepared using no solvent for gelatinisation. Hence, there is no scope of accumulation of any chemicals within the starches. It highlights their higher purity and thus more acceptability for environmentally sensitive areas. The technique used produces no waste-water as a by-product and thus is an environment friendly method of production. The process eliminates the risk of waterway pollution as no waste-water is produced. Due to ease of operation, high efficiency, requirement of no solvent, the newly developed starches have lower production cost and thus expected to reduce the mud as well as drilling cost.
The use of local agricultural resource indicates the opening of new markets for non-food use of corn crops and thus expected to provide higher economic benefit to Australian farmers, development of new industries and generation of new jobs leading to the improvement of the social and economic condition of the public.
- The spurt and API fluid loss values of the novel starch containing muds indicate that some of these starch products have similar or better fluid loss control properties compared to a currently used modified starch.
- The newly developed starch products indicate that muds designed using these starches will prevent the thermal degradation of the drilling mud up to a bottom hole temperature of 150 oC by preventing the hydrolytic and oxidative degradation of the starch products at this temperature.
- The current research finding highlights the opening of new markets for non-food use of Australian grown corn starches and thus expected to provide higher economic benefit to Australian farmers.
- The finding of this study provides new opportunities to Australian farmers and industries and also opportunities for employment as a result of diversified use of agricultural resources.
- The current research finding indicates the potential of development of technically superior and environmentally friendly products by systematic manipulation of starch composition and chemical modification.
Md. Amanullah, J.R. Marsden and H.F. Shaw, (1997). An Experimental Study of the Swelling Behaviour of Mudrocks in the Presence of Drilling Mud Systems, Canadian Journal of Petroleum Tech., Volume 36, No.3, March, pp. 45-50.