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Research in particle coating and agglomeration at West Virginia University
Powder Technology 117 Ž2001. 139–148 www.elsevier.comrlocaterpowtec
Research in particle coating and agglomeration at West Virginia University R. Turton a,) , A. Bhatia a , H. Hakim a , G. Subramanian a , Lewis Norman b a
Department of Chemical Engineering, CEMR, West Virginia UniÕersity, P.O. Box 6102, Morgantown, WV 26506-6102, USA b Halliburton Energy SerÕices, 2600 South 2nd St., Duncan, OK 73536, USA Received 14 July 2000; received in revised form 13 October 2000; accepted 15 December 2000
Abstract Over the last three years, work in the Particle Coating Laboratory at West Virginia University has focused on three main areas. The first area concerns the reversible agglomeration of cement to produce a granular product Ž2–10 mm. that can be transported easily and can be broken down and hydrated to form a cement slurry with properties identical to virgin cement. This agglomeration process uses a binding agent consisting of calcium chloride ŽCC. and tartaric acid ŽTA. dissolved in methanol that can be considered an inert solvent. By adjusting the proportions of the cement set accelerating agent ŽCC. and the retarding agent ŽTA. a granular cement product can be formed that gives a cement slurry with essentially the same characteristics as that obtained from virgin cement. The resulting concrete also has the same compressive strength, obtained in a standard 3-day test, as virgin cement. The second research area concerns the formation of encapsulated brittle particles of ammonium persulfate ŽAP. that are used as viscosity breaking agents for fracturing fluids. In order to obtain a coat that under goes brittle fracture when subjected to a compressive load, a coating of a cross-linked acrylate polymer containing up to 80 wt.% of fine Ž- 15 mm. silica was used. By varying the coating level of acrylate, the release of the ammonium persulfate using a standard leach test can be reduced to acceptably low levels Ž- 3%.. By changing the fraction of silica in the coat, the release of the ammonium persulfate when the particles are subjected to a known compressive stress Ž13.8 MPa. can be increased to approximately 70%. The particles formed by this process comprise of agglomerates of between 10 and 20 individually coated particles. When subjected to an applied load, these agglomerates fracture and the coating on the individual particles is sheared away thus releasing AP. These particles can be used as viscosity breaking agents in drilling well fracturing operations. The third project consists of the video imaging of particle movement in a semicircular fluidized bed typically used in coating operations. The particles of interest are 8-mm-diameter tablets. The technique used to capture particle velocity data utilizes two CCD cameras that are synchronized to capture images that are between 1 and 5 ms apart. The mapping of particle velocity within the spray region in the draft tube insert under a variety of conditions is currently underway. Preliminary data is presented and discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Agglomeration; Coating; Fluidized beds; Video imaging
1. Production of dispersible cement agglomerates 1.1. Introduction During the process of drilling a well, cement slurry is pumped into the annular region formed between the borehole and the steel casing in order to stop the motion of fluids between the different geological formations, to pre) Corresponding author. Tel.: q1-304-293-2111; fax: q1-304-2934139. E-mail address: [email protected] ŽR. Turton..
vent contamination of the product and leakage of the product to the surrounding formations, and to provide a support for the casing, w1x. The process by which the cement slurry is prepared usually involves the transportation of the cement and other additives, either separately or as a premixed package, to the site for hydration, mixing, and pumping. Many drilling sites are located in remote areas. The transportation and transfer of very fine Ž15–40 mm. and cohesive drilling cement powders can be problematic due to the tendency of such solids to pack and also to give excessive dust formation after extended periods of transportation. For this rea-
0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 3 2 5 - 4
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son, research was conducted with the production of a granular cement product in mind. By producing a granular cement product, many of the transfer problems associated with the fine cement powder can be eliminated. In addition, the equipment used to transfer and meter granular solids is generally more reliable and less expensive than the equivalent equipment for fine cohesive solids. The formation of concrete from cement is a very complex process due to the myriad compounds present in cement. The most important characteristics of cement in the concrete forming process are the rate of hardening, the rate of total extent of heat evolved during hydration, and the resistance of the hardened cement, w2x. The four main constituents of cement Žtricalcium silicate—3CaOP SiO 2 , dicalcium silicate—2CaO P SiO 2 , tricalcium aluminate— 3CaO P Al 2 O 3 , tetracalcium aluminoferrite— 4CaO P Al 2 O 3 P Fe 2 O 3 . all contribute differently to the process. Initial set and early strength of concrete are attributed to the hydration of tricalcium silicate, while dicalcium silicate is responsible for longer-term strength. A large amount of heat is liberated during the hydration of tricalcium aluminate, which also contributes slightly to early strength. However, tricalcium aluminate also decreases the concrete’s resistance to soils and water containing sulfides. Finally, tetracalcium aluminoferrite decreases the clinkering temperature and can be used to modifyrcontrol the manufacturing process. Not only are the hydration reactions very complex, they are also very sensitive to a wide variety of additives, for example the addition of a 0.01 wt.% sugar Žfructose. can retard significantly the setting of cement and addition of quantities on the order of 0.05–0.10 wt.% can completely prevent the cement from setting. It is indeed true to say that the addition of any small amount of material that can be used as a binding agent for the production of cement granules will change the properties and setting characteristics of the cement to which it is added. Therefore, a second aspect of this work was to find a binder capable of agglomerating cement particles that upon subsequent size reduction would yield cement slurry with essentially identical properties to slurry formed from untreatedrunagglomerated cement. 1.2. Experimental methods The experimental plan followed during this work included the identification and optimization of binder–solvent pairs for the agglomeration of cement particles. In order to test the properties of the cement slurry and cement-set formed by the subsequent grinding and hydration of these agglomerates, a series of tests was performed and the results were compared to similar tests performed on virgin Žunagglomerated. cement powder. 1.2.1. Binder-solÕent pairs Preliminary work was carried out to determine an experimental plan for formulating acceptable binderrsolvent
pairs to be used in the agglomeration work. Many organic liquids do not react with cement and would be suitable solvents. However, in order to obtain a homogeneous, well-dispersed mixture of binder and cement, prior to the evaporation of solvent, it is desirable that the binder should be at least sparingly soluble in the solvent. Coupled with cost considerations the solvent used for the majority of this work was methanol, details of work with other solvents are given by Hakim, w3x. An important criterion for the binder is that it should be soluble in both the solvent Žmethanol. and water. Again there is a wide class of materials that meets this criterion, however, the search of all possible binders was narrowed to those that are currently used as cement additives. Agglomerate strength is usually proportional to the amount of binder added. In order to obtain granules with reasonable strength, binders were chosen based on their agglomerating properties and the relative amounts that could be added to cement without irreversible effects on the setting time. Many binders were investigated, w3x, but the best results were obtained with a mixture of calcium chloride ŽCC. and tartaric acid ŽTA.. Initially, calcium chloride was used exclusively. However, the resulting cement slurries formed with these agglomerates set very quickly. The reason for using the two binding agents is that calcium chloride has a tendency to accelerate the cement set and tartaric acid has the opposite effect. By combining these two binding agents it was found that a neutral binder could be formed. The results for the characterization tests for cement slurries are presented in Section 1.2.2. 1.2.2. Method of granulation The batch granulation of cement was carried out in a mixing bowl using a planetary mixer equipped with a wire–whisk mixing blade that was rotated at approximately 120 rpm. 800 g of dry cement powder were placed in the bowl and between 180 and 200 ml of methanol solution containing the dissolved binders was added to the cement at a rate of 10 mlrmin. The mixture was continually stirred during the addition of the methanol and for a period after the solution had been added. Complete granulation took between 18 and 25 min and the granules produced were approximately spherical and ranged in size from 2 to 10 mm. Depending upon the required application, there are certain chemical and physical characteristics that have to be attained by drilling ŽPortland. cements, w4x. In this work three tests were used to characterize the cement slurry and resulting concrete. 1.2.3. CompressiÕe strength tests Since the strength of a concrete test sample depends on the time after slurry formation at which the test is carried out, as well as the conditions employed in the test, it was imperative to adopt a procedure that was both reliable and
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reproducible. The compressive strength of the set cement was measured by determining the force required to crush a 50 mm Ž2 in.. cube using an unconfined load. The procedure used in this work essentially followed an accepted ASTM procedure, w5x. In the case of the granulated cement, the granules Ž2–10 mm. were first placed in a ball mill and the size was reduced back to that of the original cement Ž15–40 mm.. This test consisted of preparing cement–water slurry of composition 72.5 wt.% cement and 27.5 wt.% water. The slurry was poured into tight fitting bronze cube molds ŽModel a HM-294, Gilson, Wormington, OH.. The sides of the molds were sprayed with a thin coat of release agent ŽWD-40, WD-40 Company, San Diego, CA. and the top and bottom retaining plates were coated with a thin layer of grease ŽLubriplate, Fiske Brothers Refining, Newark, NJ.. After the molds were filled and the top plate secured, they were immersed in a circulating water bath held at a temperature of 388C Ž1008F. and cured for 3 days. After this period, the 50-mm cubes of cured cement were removed and placed in a press between a fixed metal block and a spherically seated platen. A force was then applied to the sample and the load at which the specimen failed was taken as the compressive strength of the concrete. 1.2.4. Tests to determine the Õiscosity of cement–water slurries The second test consisted of evaluating the viscosity of the cement–water slurry as a function of time. Under conditions where cement setting is eliminated, the cement appears to behave as a Bingham Plastic. However, under normal conditions the characteristics of the slurry change with time as the hydration reactions take place. The rheological behavior of cement slurries is therefore very complex. As a first approximation, we can model these fluids with a time dependent Bingham Plastic Model:
t s m0 e b1 t
dÕ dx
q t 0eb2 t
Ž 1.
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1.3. Results and discussion The results of the compressive strength tests for virgin cement are shown in Fig. 1. The average compressive strength of all the 2 in. cube samples tested was 78.7 kN Ž17,700 lbf .. When calcium chloride is added to the cement through the agglomeration process, the setting rate of cement is increased considerably and the compressive strength measured at 3 days is very much higher than for virgin cement, Fig. 1. However, by adding small amounts of a setting retarder, such as tartaric acid, this value can be reduced. Fig. 1 shows the effect of adding between 0.25 and 0.45 wt.% tartaric acid along with 1.5 wt.% calcium chloride during the agglomeration process . The choice of 1.5 wt.% CC was arrived at by trial and error and represents a compromise between granule strength and the ability to granulate the cement. Experiments using amounts of CC in the range of 0.5 to 5 wt.% were carried out. For tests using 0.5 and 1.0 wt.% of CC, it was found that a significant portion of the cement remained ungranulated. For 1.5 wt.% CC and greater, all the cement could be granulated but the granule strength increased with increasing CC Žbinder. content. The purpose was to make granules that could withstand abrasion due to transportation but would not be so hard as to be difficult to regrind at the point of use. For this reason, the lowest level of CC Ž1.5 wt.%. that would effectively granulate the cement was chosen. From Fig. 1, it is clear that as the amount of tartaric acid increases the compressive strength Žmeasured after 3 days of cure. decreases. It appears that by adding between 0.275 and 0.325 wt.% TA with 1.5 wt.% CC during the agglomeration process the compressive strength of the cement made from such granules has approximately the same value as that of virgin cement. It should be noted that these additives do not affect the ultimate strength of the concrete that is obtained after a period of approximately 20 days. However, the strength–time profile may be affected greatly and this is very important in well drilling operations.
By evaluating the viscosity of cement–water slurries at different times in a Couette viscometer ŽFann Instrument Company, Houston, TX. the parameters m 0 , t 0 , b 1 , and b 2 can be evaluated for granulated and ungranulated cement samples and the results compared. 1.2.5. Slurry settling tests This test follows a standard API test w6x. The cement slurry described above is mixed at given conditions for given periods of time so that it has a known and prescribed mixing history. It is then poured into a 250-ml graduated cylinder and sealed to prevent evaporation. The slurry is allowed to stand for 2 h and then the supernatant water is decanted and measured. The maximum allowable supernatant water is 3.5 ml.
Fig. 1. The effect of adding between 0.25 and 0.45 wt.% tartaric acid along with 1.5 wt.% calcium chloride during the agglomeration process on the compressive strength of concrete.
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The results of the viscosity tests for virgin cement and granulated cement using 1.5 wt.% CC and varying amounts of TA are shown in Fig. 2a and b. The results for ungranulated Žvirgin. cement are also shown on these figures. It can be seen that the values of m 0 and t 0 for the granulated cement at a TA level of between 0.275 and 0.325 wt.% are similar to those for the virgin cement. Results for the time constants, b 1 and b 2 in Eq. Ž1. show similar agreement although there is considerable scatter in the data. Average values of b 1 s y0.0023 miny1 and b 2 s 0.0108 miny1 were obtained for virgin cement. This means that at constant rate of shear over a period of 2 h, the first term on the right hand side of Eq. Ž1. decreases by approximately 25%, while the yield stress term increases by 365%. For all the samples, both virgin and agglomerated cement, the slurry settling test indicated that the amount of supernatant water was below the 3.5 ml maximum allowed by the API test method. Indeed, there was no detectable supernatant water found for the agglomerated samples. 1.4. Conclusions From the results given above, it appears that cement granules can be formed using a mixture of accelerating ŽCC. and retarding agents ŽTA.. Moreover, these granules
Fig. 2. The results of the viscosity tests for virgin cement and granulated cement using 1.5 wt.% CC and varying amounts of TA, Ža. Results for mo , Žb. Results for to .
appear to give similar slurry and concrete characteristics as those produced from virgin cement. Although size reduction of the agglomerates is necessary prior to use in drilling-well cement applications, this does not pose a large problem since the granules are quite brittle and are easily broken and pulverized by standard mechanical impactors. This novel process is at present under patent review w7x.
2. Production of coated ammonium persulfate particles to facilitate brittle failure of encapsulated viscosity breaking agents 2.1. Introduction It is common practice in the oil and gas production industry to rupture subterranean formations in order to increase permeability and conductivity. This fracturing process is facilitated by pumping a fracturing fluid into a well and applying a high pressure in order to fracture the surrounding rock, thus stimulating the flow of the petroleum product. These fracturing fluids contain a variety of materials. One of the most important of these materials is AproppantB which is essentially nearly spherical sand. The purpose of the proppant is to bridge the gap formed in the formation and to keep it open when the pressure on the fracturing fluid is reduced, thus increasing the permeability of the fracture to hydrocarbon flow. In order to stop the proppant from settling out at the bottom of the well, prior to making a 908 turn and flowing into the fractured formation, it is necessary to control the viscosity of the fracturing fluid. Indeed, a common fracturing fluid is Guar Gum ŽGalactomannan polymer., a complex carbohydrate derived from the seed of specially grown bean plants. This natural polymer has a molecular weight of approximately 1 million and a very high viscosity. The proppant and other solid additives are essentially held in suspension in this gelatinous fluid throughout the fracturing process. However, after fracturing has taken place the viscosity of the fluid must be reduced significantly in order to initiate the flow of hydrocarbons and to allow the proppant to settle out and keep the fractured formation open. This viscosityreducing step is referred to as Aviscosity breakingB or simply Abreaking.B Typical breakers are strong organic oxidizing agents; for example perchlorate, persulfate, and percarbonate compounds are routinely used as viscosity breakers. In this work ammonium persulfate ŽAP., was chosen as the model breaker compound. There are several triggering mechanisms by which the breakers can be activated, w8x. Examples include, direct injection of a breaker chemical into the fracturing fluid, time-release coatings on breakers within the fluid, and semi-permeable membrane coatings on breakers within the fluid. All these methods have certain drawbacks w8x. The purpose of the current work was to examine a new method
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for triggering the release of a viscosity breaker in a fracturing fluid. The idea of this research was to produce an encapsulated breaker that would be activated by the partial closure of the formation. The coated breaker would be added to the fracturing fluid, and after fracturing had taken place and the pressure reduced, the coated breaker particles would be subject to a compressive load due to the gradual closing of the fracture. By making the coating brittle, the application of the compressive load will fracture the outer coat and allow the breaker ŽAP. to release into the fluid and thus reduce the fluid viscosity and aid removing the fluid from the fracture. 2.2. Experimental methods 2.2.1. Coating formulation In order to evaluate potential coating materials a series of screening tests were performed and the details of these tests are given elsewhere, w9x. In developing the coating formulation three important factors were considered. First, the rate at which the AP leached from the uncrushed encapsulated particle should be very low. This is clearly necessary to avoid premature breaking of the fracturing fluid. The second consideration is that the release of the AP from the crushed particle should be both rapid and extensive. Ideally, all of the AP should be released instantaneously into the fracturing fluid upon application of the compressive load. Third, the size of the encapsulated breaker particles should be relatively small so as to pass easily into the fractured formation along with the proppant. A typical size range for the proppant is 1–2 mm and this was chosen to be the largest acceptable size range for the encapsulated breakers. Another consideration regarding the size of the encapsulated breaker is the distribution of these particles within the fracturing fluid. For a given dosage of AP, it is better to have many small particles distributed within the fluid as compared to a few larger ones. Both polyurethane and partially hydrolyzed acrylate coatings were investigated. Preliminary work suggested that the acrylics were more amenable to this application and the focus of this work was on these coatings. The addition of inert silica to the coating matrix has been found to alter the permeability characteristics of the coat formed by cross-linking vinyl acrylic latex polymers, w10x. The addition of fine silica Ž1–15 mm. powder to the coating mixture was also investigated in this work. 2.2.2. Coating process The coating work was carried out in a fluidized bed apparatus equipped with a draft tube insert. A schematic diagram of the equipment is shown in Fig. 3. The uncoated AP particles varied in size from approximately 400 to 800 mm and typical coating thickness was approximately 50– 100 mm. However, due to the tackiness of the coating a certain amount of agglomeration of the partially coated AP
Fig. 3. Fluidized bed apparatus equipped with a draft tube insert used to coat AP particles.
particles occurs. This agglomerationrcoating process was found to be very beneficial to the overall goals of the research and these results are discussed in the next section. 2.2.3. Leach tests A standard test was developed, in order to quantify the rate at which AP is transferred from the uncrushed encapsulated particle. A fixed weight of encapsulated particles Ž0.11 g. was immersed in 50 cm3 of distilled water and stirred continuously. Small samples of liquid were removed at different points in time, and the quantity of AP in the liquid was calculated using a standardized iodometric titration technique, see Bhatia w9x. The release profile for the AP from the uncrushed particles could then be constructed. 2.2.4. CompressiÕe tests In order to simulate the compressive load, to which the encapsulated particles would be subjected, a test device was constructed. The test method adopted in this work is adapted from API RP 56, which is used for testing the crush resistance of fracture sand Žproppant., w11x. The test method essentially consists of mixing a known weight of encapsulated breaker particles with a known amount of fracture sand or proppant. Typical loadings of proppant to breaker are from 50:1 to 100:1. For all the tests considered here 0.11 g of encapsulated breaker were mixed with 11 g of proppant and loaded into the test cell. This test mixture was placed in a test cylinder Ždiameter of 7.62 cm. and a loose fitting piston placed on top of the mixture. The piston was slowly rotated in order to flatten the test mixture and obtain a sample with a uniform depth. The test cell and piston were then placed in a Carver Press ŽCarver, Wabash, IN. and a load of 62,720 N Ž14,100 lbf . was applied. This load corresponds to an average applied stress of 13.8 MPa Ž2000 psi.. The load was applied uniformly over a period of 1 min and then the load was held constant at this value for a period of 2 min. After the applied load
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had been removed, the sample of sand and crushed and uncrushed breaker particles was added to 50 cm3 of distilled water and stirred. Samples of the liquid were removed and the amount of AP in the liquid was assessed by the iodometric titration procedure mentioned above. 2.3. Results and discussion The effect of the level of coating on encapsulated breaker particles with respect to the amount of AP released under leach and compressive load conditions after 10 min is shown in Fig. 4. The high rate of AP release at the low coating level Ž3.5%. is indicative of non-uniform coverage since both the leach and compression tests show the same level of release. As the coating thickness increases the leach level drops faster than the crushed level. At 15% coating, the leach rate Žapprox 3%. is acceptable but the crushed release is only 40%. In order to increase the crushed release level, without increasing the leach level, a series of experiments was performed in which fine silica was added to the coating mixture. The effect of the silica addition is shown in Fig. 5, where the AP released is plotted as a function of the percentage of silica contained in coatings with two levels of acrylate. In all the results, the level of acrylate coating is reported based on the weight of the AP particles while the weight of silica is based on the weight of the acrylate, thus: wt.% acrylates wt.% silica s
wt. of acrylate wt. of acrylateq wt. of AP wt. of silica
wt. of silica q wt. of acrylate
Ž 2. Ž 3.
From Fig. 5 it appears that there is a significant difference in the AP released from the leach test compared to the compression test and that this difference is influenced by the presence of silica in the coat. This difference increases as the silica content of the coating is increased from 0% to
Fig. 4. The effect of the level of coating on encapsulated breaker particles with respect to the amount of AP released under leach and compressive load conditions after 10 min.
Fig. 5. The effect of the silica addition on the AP released as a function of the percentage of silica contained in coatings at two levels of acrylate coating.
about 60%. Beyond about 60 wt.% silica, there appears to be a decrease in the amount of AP released during the compression test and there is also a significant increase in the amount of AP released during the basic leach test. From these results, acceptable leach and compression results are obtained at acrylate coating levels of 25 wt.% and silica levels of about 60 wt.%. These results indicate that the leach rate is approximately 3.5 wt.% in 10 min and the corresponding compression rate is 66 wt.%. The data shown in Fig. 5 are all taken after 10 min, the release histories for the 25 wt.% acrylater60 wt.% silica runs are shown in Fig. 6. It is clear from Fig. 6, that for the crushed sample, the vast majority of the AP is released within the first few minutes. The structure of the coated particles was also addressed in this work. Due to the stickiness of the coating used in the current research, the particles formed during the fluidized bed coating process consisted of well defined agglomerates of between 10 and 20 individually coated particles that had subsequently adhered to each other and been over coated with the same coating solution. Images of
Fig. 6. The release histories for the 25 wt.% acrylate and 60 wt.% silica runs.
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Fig. 7. A video image of a ruptured ammonium persulfate coated particle agglomerate.
the cross-sections of these agglomerates suggest that when they are subjected to a compressive load the coating around individual particles ruptures and tears. Two video images of ruptured agglomerates are shown in Fig. 7, and individual AP particles are shown exposed. 2.4. Conclusions This work has shown that a coating using a combination of cross-linked acrylic polymers and a high percentage of inert filler particles, such as silica, can be used to encapsulate viscosity breakings agents such as ammonium persulfate. These encapsulated breakers posses low leach rates when immersed in water and therefore posses the desired low water-permeability characteristic required of viscosity breaking agents. Moreover, by controlling the amount of fine silica added to the coating solution, the coating produced exhibits a high release of AP upon crushing. This novel process is at present under patent review w12x.
r particle velocity profiles in 3. Evaluation of voidager fluidized bed coating equipment with draft tubes 3.1. Introduction During the coating of particles in fluidized beds, particles are usually made to spout with the aid of a draft tube. By using the draft tube, the formation of a regular and essentially stable spout is possible. This leads to the regular circulatory movement of particles within the bed. Particles are coated by introducing an atomized spray of coating solution into the bottom of the bed. Particles are repeatedly coated during successive passes through the spray zone and a coherent continuous coat is gradually built up on the particles. For a batch of coated particles, the uniformity of the distribution of coating on the population of particles comprising the bed can be attributed to the distribution of the number of passages that a particle takes
through the spray zone and the distribution of coating material that each particle receives during a single passage through the spray, w13x. This variation in coating mass distribution depends on a variety of operating parameters, and also on the size of the particles, w14x. Moreover, it has been shown for both small particles Ž1 mm., Cheng and Turton w15x and large particles Ž10 mm., Shelukar et al. w16x, that the major contribution to the uniformity of the batch is the distribution of material that each particle receives as it passes through the spray zone. This distribution is a strong function of the sheltering effect that particles have on each other. Therefore, the measurement of the velocity and voidage profiles within the spray zone of the fluidized bed are key variables to know if the uniformity of coating during batch coating operations is to be predicted, w17x. Various techniques can be used to measure the movement of particles within a fluidized bed. During the past several years, work in our group has concentrated on analyzing video imaging data obtained from experiments performed in a 22.5 cm Ž9 in.. diameter semicircular bed fluidized coating apparatus. Voidage and velocity measurements in the draft tuberspray region of this bed using approximately 1 mm diameter spherical glass beads have been reported previously, w18x. Current work is focusing on the recording and analysis of data from this same equipment using 8-mm tablets. Due to the non-spherical shape of these particles, previous techniques to analyze particle velocity and voidage, which were developed for spherical particles, could not be employed. 3.2. Experimental methods The experimental setup is shown in Fig. 8, this semicircular fluidized bed coating apparatus is described elsewhere w18x. Images of the particles are taken with a variable shutter speed CCD camera and these images are stored via a frame grabbing board. When evaluating the
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Fig. 8. Semicircular fluidized bed coating apparatus used to evaluate voidage and velocity profiles in the draft tube region.
particle velocities using the 1-mm-diameter spherical glass beads, the shutter speed of the CCD camera is set at between 1 and 2 ms. This setting produced images that were bright particle streaks. The length and direction of the streaks could be used with the camera shutter speed to evaluate the particle velocity projected in the vertical and horizontal directions. In addition, since the particles were essentially the same size, the width of the streaks could be used to determine which particles were out-of-focus. Only in-focus streaks were used for evaluation of particle velocity. By repeatedly taking data at different points in the flow field, the velocity profiles within the draft tube region were constructed, w19x. For the evaluation of voidage measurements, the camera shutter speed was set to 0.1 ms and the resulting video fields gave clear images of the particles in the field-of-view ŽFOV.. Filtering software was used to look at the dark-to-light transitions at the edges of the particle in order to distinguish between in- and out-of-focus particles. The voidage was obtained by knowing the number of particles within a volume element of the bed close to the front wall, equal to the FOV Ž1 cm = 1 cm. multiplied by the depth-of-field ŽDOF s 0.5 cm.. When evaluating the velocity and voidage profiles using large, tablet-shaped particles, the technique for evaluating particle velocity described above for spherical particles no longer applies. The main reason for this is that as the particles move upwards they rotate and the streaks that are formed vary in width and are very indistinct. Therefore, it is impossible to distinguish between in-focus and out-offocus particles. In order to evaluate particle velocity accurately, successive particle images must be captured at times in the region of 1–5 ms apart. This is not possible with a standard CCD camera operating on the RS-170 standard framing rate of 33.33 ms Ž30 Hz.. One method to do this would be to employ a high-speed camera. The
method adopted in this work was to use two synchronized CCD cameras to capture images of the same area of the bed at times of between 1 and 5 ms apart. The timing events are controlled by customized software and the variable time delay can be set by the user. The set-up for the cameras is shown in Fig. 9. It is important that the cameras be arranged in a steep isosceles triangle in order to minimize parallax effects. The greater the angle at the apex, u , the greater is the difference between the images from the two cameras. Once the cameras are set-up, the customized software is used to calibrate the two images from the cameras in order to minimize these errors. When the bed is running, a AgrabB event is initiated that causes both cameras to obtain images at the pre-selected time lag. The center of each particle is then located using a computer-generated crosshair and the horizontal and vertical distances between the successive images is computed and recorded. These data can then be converted into magnitude and direction using the appropriate time lag. The technique for obtaining voidage measurements for the tablets is similar that used for the spherical particles. However, the particle identification algorithm is somewhat different but the idea is still to count the number of particles in a known volume of bed close to the front wall. 3.3. Results and discussion Evaluation of velocity and voidage maps within the draft tube region of the bed is currently underway. Representative velocity distributions at a point 4 cm from the centerline of the bed and at the centerline of the bed and at
Fig. 9. Two camera set-up for measuring tablet velocity.
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Fig. 10. Velocity distributions for tablets in the semicircular fluidized bed shown in Fig. 8 with a gap height of 2.5 cm Ža. radial locations 4 cm from axis and vertical locations 6.5 cm from distributor Žb. radial locations 0 cm from axis and vertical locations 6.5 cm from distributor.
a vertical distance of 6.5 cm from the distributor plate are shown in Figs. 10 and 11. For each location, between 50 and 70 particle velocities were obtained and were used to evaluate the local velocity. For all the data, the cameras were focused on a region approximately 2 mm inside the
front surface of the bed in order to minimize the wall effect of the flat glass partition. In Fig. 10, the gap between the bottom of the draft tube and the distributor plate is 2.5 cm, while in Fig. 11 the gap is 7.5 cm. For these runs, all other operating parameters were the same.
Fig. 11. Velocity distributions for tablets in the semicircular fluidized bed shown in Fig. 8 with a gap height of 7.5 cm Ža. radial locations 4 cm from axis and vertical locations 6.5 cm from distributor Žb. radial locations 0 cm from axis and vertical locations 6.5 cm from distributor.
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For example, the volumetric flowrate of air was 61.4 STD litrs and the bed loading was 3.65 kg. These data show that the direction of particle movement is closer to the vertical the closer the location is to the centerline. This result is consistent with the work of Saadevandi w19x. It is also consistent with visual observations that indicate that at the bottom of the bed particles flow horizontally towards the center of the draft tube from the annular region and then turn upwards as they become entrained by the upward moving central air stream. The particle velocity data varies considerably with location. However, at the top of the draft tube insert the particle velocity is more uniform and shows that the velocity increases with an increase in the height of the gap between the bottom of the draft tube and the distributor plate. Work is ongoing in mapping both the voidage and velocity distributions in the draft tube region of the fluidized bed illustrated in Fig. 8. The experimental program includes varying the following parameters: gap height between the draft tube and the distributor, fluidizing air velocity, distributor plate design, and adding a flow guide at the bottom of the draft tube. 3.4. Conclusions Video imaging of large non-spherical particles Žtablets. using a two-camera system was shown to be a viable technique for estimating particle velocity. Particle velocity data was presented in the form of distributions of magnitude and direction for two locations in a semicircular fluidized bed. This technique is currently being used to map the voidage and particle velocity distributions within the draft tube region of a fluidized bed coating apparatus. Nomenclature b 1 , b 2 constants used in Eq. Ž1. Žminy1 . t time Žmin. dÕrd x shear rate Žsy1 .
Greek Letters shear stress ŽPa. t yield stress ŽPa. t0 m0 plastic viscosity ŽPa s.
Acknowledgements Funding for the agglomeration and coating research by Halliburton Energy Services and funding for the tablet velocity and voidage work by Merck and Company, are gratefully acknowledged.
References w1x G.C. Howard, J.B. Clark, Factors to be Considered in Obtaining Proper Cementing of Casing, Drill and Prod. Prac. API, Dallas, 1948, pp. 257–272. w2x F.M. Lea, The Chemistry of Cement and Concrete, 3rd edn., Chemical Publishing, NY, 1971. w3x H. Hakim, The Agglomeration of Cement to Facilitate Transportation, MS Thesis, Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506, 1999. w4x D.K. Smith, Cementing, 2nd edn., Monograph vol. 4, Society of Petroleum Engineers, NY, 1990. w5x Standard Test Method for Compressive Strength of Hydraulic Cement Mortars Žusing 2-in or 50-mm Cube Specimens., C1090-90, 13th edn., Vol. 2, American Society for Testing of Materials, Philadelphia, PA, 1993. w6x Specifications for Materials and Testing of Well Cements, Determination of Free Water Content of Slurry, API Specification 10, 5th edn., American Petroleum Institute, Dallas, TX, 1990. w7x Agglomeration of Hydraulic Cement Powder, L. Norman, R. Turton, and H. Hakim, US Patent Application HES 2000 IP-000054, July, 2000. w8x J.W. Ely, Fracturing Fluids and Additives, Recent Advances in Hydraulic Fracturing, SPE Monograph Series vol. 12, Society of Petroleum Engineers, Richardson, TX, 1989, pp. 131–146. w9x A. Bhatia, Encapsulation of Particles using Brittle Coatings for Subterranean Applications, MS Thesis, Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506, 1999. w10x L.R. Norman, S.B. Laramay, Encapsulated Breakers and Method for use in Treating Subterranean Applications, US Patent No. 5,373,901, 1994. w11x Procedures for Testing Frac Sand, Section 8, and Recommended Frac Sand Crush Resistance Test, American Petroleum Institute RP 56, Dallas, 1994. w12x Encapsulated Breakers And Method For Use In Treating Subterranean Formations, L.R. Norman, R. Turton, and A.L. Bhatia, US Patent Application HES 2000 IP-344U1, July, 2000. w13x U. Mann, Analysis of spouted-bed coating and granulation. 1. Batch operation, Ind. Eng. Chem. Proc. Des. Dev. 22 Ž1983. 288–292. w14x K. Sudsakorn, R. Turton, Non-uniformity of coating on a size distribution of particles in a fluidized bed coater, Powder Technol. 110 Ž2000. 37–43. w15x X.X. Cheng, R. Turton, The prediction of variability occurring in fluidized bed coating equipment: Part 1. The measurement of particle circulation rates in a bottom spray fluidized bed Coater, Pharm. Dev. Technol. 5 Ž3. Ž2000. 311–322. w16x S. Shelukar, J. Ho, J. Zega, E. Roland, N. Yeh, D. Quiram, A. Nole, A. Katdare, S. Reynolds, Quantification of factors affecting wurster coating process, Proceedings of the PTF, AIChE Annual Meeting, Miami, Nov. 15–20 vol. II, 1998, pp. 406–411. w17x X.X. Cheng, R. Turton, The prediction of variability occurring in fluidized bed coating equipment Part 2: the role of non-uniform particle coverage as particles pass through the spray zone, Pharm. Dev. Technol. 5 Ž3. Ž2000. 323–332. w18x B.A. Saadevandi, R. Turton, The application of computer-based imaging to the measurements of particle and voidage profiles in a fluidized bed, Powder Technol. 98 Ž1998. 183–189. w19x B.A. Saadevandi, The Use of Imaging Techniques to Study the Hydrodynamics of Particle Motion in a Fluidized Bed Coating Device in the Region of Liquid Spray, PhD Dissertation, West Virginia University, Morgantown, WV, 1996.
Research in particle coating and agglomeration at West Virginia University R. Turton a,) , A. Bhatia a , H. Hakim a , G. Subramanian a , Lewis Norman b a
Department of Chemical Engineering, CEMR, West Virginia UniÕersity, P.O. Box 6102, Morgantown, WV 26506-6102, USA b Halliburton Energy SerÕices, 2600 South 2nd St., Duncan, OK 73536, USA Received 14 July 2000; received in revised form 13 October 2000; accepted 15 December 2000
Abstract Over the last three years, work in the Particle Coating Laboratory at West Virginia University has focused on three main areas. The first area concerns the reversible agglomeration of cement to produce a granular product Ž2–10 mm. that can be transported easily and can be broken down and hydrated to form a cement slurry with properties identical to virgin cement. This agglomeration process uses a binding agent consisting of calcium chloride ŽCC. and tartaric acid ŽTA. dissolved in methanol that can be considered an inert solvent. By adjusting the proportions of the cement set accelerating agent ŽCC. and the retarding agent ŽTA. a granular cement product can be formed that gives a cement slurry with essentially the same characteristics as that obtained from virgin cement. The resulting concrete also has the same compressive strength, obtained in a standard 3-day test, as virgin cement. The second research area concerns the formation of encapsulated brittle particles of ammonium persulfate ŽAP. that are used as viscosity breaking agents for fracturing fluids. In order to obtain a coat that under goes brittle fracture when subjected to a compressive load, a coating of a cross-linked acrylate polymer containing up to 80 wt.% of fine Ž- 15 mm. silica was used. By varying the coating level of acrylate, the release of the ammonium persulfate using a standard leach test can be reduced to acceptably low levels Ž- 3%.. By changing the fraction of silica in the coat, the release of the ammonium persulfate when the particles are subjected to a known compressive stress Ž13.8 MPa. can be increased to approximately 70%. The particles formed by this process comprise of agglomerates of between 10 and 20 individually coated particles. When subjected to an applied load, these agglomerates fracture and the coating on the individual particles is sheared away thus releasing AP. These particles can be used as viscosity breaking agents in drilling well fracturing operations. The third project consists of the video imaging of particle movement in a semicircular fluidized bed typically used in coating operations. The particles of interest are 8-mm-diameter tablets. The technique used to capture particle velocity data utilizes two CCD cameras that are synchronized to capture images that are between 1 and 5 ms apart. The mapping of particle velocity within the spray region in the draft tube insert under a variety of conditions is currently underway. Preliminary data is presented and discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Agglomeration; Coating; Fluidized beds; Video imaging
1. Production of dispersible cement agglomerates 1.1. Introduction During the process of drilling a well, cement slurry is pumped into the annular region formed between the borehole and the steel casing in order to stop the motion of fluids between the different geological formations, to pre) Corresponding author. Tel.: q1-304-293-2111; fax: q1-304-2934139. E-mail address: [email protected] ŽR. Turton..
vent contamination of the product and leakage of the product to the surrounding formations, and to provide a support for the casing, w1x. The process by which the cement slurry is prepared usually involves the transportation of the cement and other additives, either separately or as a premixed package, to the site for hydration, mixing, and pumping. Many drilling sites are located in remote areas. The transportation and transfer of very fine Ž15–40 mm. and cohesive drilling cement powders can be problematic due to the tendency of such solids to pack and also to give excessive dust formation after extended periods of transportation. For this rea-
0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 3 2 5 - 4
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son, research was conducted with the production of a granular cement product in mind. By producing a granular cement product, many of the transfer problems associated with the fine cement powder can be eliminated. In addition, the equipment used to transfer and meter granular solids is generally more reliable and less expensive than the equivalent equipment for fine cohesive solids. The formation of concrete from cement is a very complex process due to the myriad compounds present in cement. The most important characteristics of cement in the concrete forming process are the rate of hardening, the rate of total extent of heat evolved during hydration, and the resistance of the hardened cement, w2x. The four main constituents of cement Žtricalcium silicate—3CaOP SiO 2 , dicalcium silicate—2CaO P SiO 2 , tricalcium aluminate— 3CaO P Al 2 O 3 , tetracalcium aluminoferrite— 4CaO P Al 2 O 3 P Fe 2 O 3 . all contribute differently to the process. Initial set and early strength of concrete are attributed to the hydration of tricalcium silicate, while dicalcium silicate is responsible for longer-term strength. A large amount of heat is liberated during the hydration of tricalcium aluminate, which also contributes slightly to early strength. However, tricalcium aluminate also decreases the concrete’s resistance to soils and water containing sulfides. Finally, tetracalcium aluminoferrite decreases the clinkering temperature and can be used to modifyrcontrol the manufacturing process. Not only are the hydration reactions very complex, they are also very sensitive to a wide variety of additives, for example the addition of a 0.01 wt.% sugar Žfructose. can retard significantly the setting of cement and addition of quantities on the order of 0.05–0.10 wt.% can completely prevent the cement from setting. It is indeed true to say that the addition of any small amount of material that can be used as a binding agent for the production of cement granules will change the properties and setting characteristics of the cement to which it is added. Therefore, a second aspect of this work was to find a binder capable of agglomerating cement particles that upon subsequent size reduction would yield cement slurry with essentially identical properties to slurry formed from untreatedrunagglomerated cement. 1.2. Experimental methods The experimental plan followed during this work included the identification and optimization of binder–solvent pairs for the agglomeration of cement particles. In order to test the properties of the cement slurry and cement-set formed by the subsequent grinding and hydration of these agglomerates, a series of tests was performed and the results were compared to similar tests performed on virgin Žunagglomerated. cement powder. 1.2.1. Binder-solÕent pairs Preliminary work was carried out to determine an experimental plan for formulating acceptable binderrsolvent
pairs to be used in the agglomeration work. Many organic liquids do not react with cement and would be suitable solvents. However, in order to obtain a homogeneous, well-dispersed mixture of binder and cement, prior to the evaporation of solvent, it is desirable that the binder should be at least sparingly soluble in the solvent. Coupled with cost considerations the solvent used for the majority of this work was methanol, details of work with other solvents are given by Hakim, w3x. An important criterion for the binder is that it should be soluble in both the solvent Žmethanol. and water. Again there is a wide class of materials that meets this criterion, however, the search of all possible binders was narrowed to those that are currently used as cement additives. Agglomerate strength is usually proportional to the amount of binder added. In order to obtain granules with reasonable strength, binders were chosen based on their agglomerating properties and the relative amounts that could be added to cement without irreversible effects on the setting time. Many binders were investigated, w3x, but the best results were obtained with a mixture of calcium chloride ŽCC. and tartaric acid ŽTA.. Initially, calcium chloride was used exclusively. However, the resulting cement slurries formed with these agglomerates set very quickly. The reason for using the two binding agents is that calcium chloride has a tendency to accelerate the cement set and tartaric acid has the opposite effect. By combining these two binding agents it was found that a neutral binder could be formed. The results for the characterization tests for cement slurries are presented in Section 1.2.2. 1.2.2. Method of granulation The batch granulation of cement was carried out in a mixing bowl using a planetary mixer equipped with a wire–whisk mixing blade that was rotated at approximately 120 rpm. 800 g of dry cement powder were placed in the bowl and between 180 and 200 ml of methanol solution containing the dissolved binders was added to the cement at a rate of 10 mlrmin. The mixture was continually stirred during the addition of the methanol and for a period after the solution had been added. Complete granulation took between 18 and 25 min and the granules produced were approximately spherical and ranged in size from 2 to 10 mm. Depending upon the required application, there are certain chemical and physical characteristics that have to be attained by drilling ŽPortland. cements, w4x. In this work three tests were used to characterize the cement slurry and resulting concrete. 1.2.3. CompressiÕe strength tests Since the strength of a concrete test sample depends on the time after slurry formation at which the test is carried out, as well as the conditions employed in the test, it was imperative to adopt a procedure that was both reliable and
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reproducible. The compressive strength of the set cement was measured by determining the force required to crush a 50 mm Ž2 in.. cube using an unconfined load. The procedure used in this work essentially followed an accepted ASTM procedure, w5x. In the case of the granulated cement, the granules Ž2–10 mm. were first placed in a ball mill and the size was reduced back to that of the original cement Ž15–40 mm.. This test consisted of preparing cement–water slurry of composition 72.5 wt.% cement and 27.5 wt.% water. The slurry was poured into tight fitting bronze cube molds ŽModel a HM-294, Gilson, Wormington, OH.. The sides of the molds were sprayed with a thin coat of release agent ŽWD-40, WD-40 Company, San Diego, CA. and the top and bottom retaining plates were coated with a thin layer of grease ŽLubriplate, Fiske Brothers Refining, Newark, NJ.. After the molds were filled and the top plate secured, they were immersed in a circulating water bath held at a temperature of 388C Ž1008F. and cured for 3 days. After this period, the 50-mm cubes of cured cement were removed and placed in a press between a fixed metal block and a spherically seated platen. A force was then applied to the sample and the load at which the specimen failed was taken as the compressive strength of the concrete. 1.2.4. Tests to determine the Õiscosity of cement–water slurries The second test consisted of evaluating the viscosity of the cement–water slurry as a function of time. Under conditions where cement setting is eliminated, the cement appears to behave as a Bingham Plastic. However, under normal conditions the characteristics of the slurry change with time as the hydration reactions take place. The rheological behavior of cement slurries is therefore very complex. As a first approximation, we can model these fluids with a time dependent Bingham Plastic Model:
t s m0 e b1 t
dÕ dx
q t 0eb2 t
Ž 1.
141
1.3. Results and discussion The results of the compressive strength tests for virgin cement are shown in Fig. 1. The average compressive strength of all the 2 in. cube samples tested was 78.7 kN Ž17,700 lbf .. When calcium chloride is added to the cement through the agglomeration process, the setting rate of cement is increased considerably and the compressive strength measured at 3 days is very much higher than for virgin cement, Fig. 1. However, by adding small amounts of a setting retarder, such as tartaric acid, this value can be reduced. Fig. 1 shows the effect of adding between 0.25 and 0.45 wt.% tartaric acid along with 1.5 wt.% calcium chloride during the agglomeration process . The choice of 1.5 wt.% CC was arrived at by trial and error and represents a compromise between granule strength and the ability to granulate the cement. Experiments using amounts of CC in the range of 0.5 to 5 wt.% were carried out. For tests using 0.5 and 1.0 wt.% of CC, it was found that a significant portion of the cement remained ungranulated. For 1.5 wt.% CC and greater, all the cement could be granulated but the granule strength increased with increasing CC Žbinder. content. The purpose was to make granules that could withstand abrasion due to transportation but would not be so hard as to be difficult to regrind at the point of use. For this reason, the lowest level of CC Ž1.5 wt.%. that would effectively granulate the cement was chosen. From Fig. 1, it is clear that as the amount of tartaric acid increases the compressive strength Žmeasured after 3 days of cure. decreases. It appears that by adding between 0.275 and 0.325 wt.% TA with 1.5 wt.% CC during the agglomeration process the compressive strength of the cement made from such granules has approximately the same value as that of virgin cement. It should be noted that these additives do not affect the ultimate strength of the concrete that is obtained after a period of approximately 20 days. However, the strength–time profile may be affected greatly and this is very important in well drilling operations.
By evaluating the viscosity of cement–water slurries at different times in a Couette viscometer ŽFann Instrument Company, Houston, TX. the parameters m 0 , t 0 , b 1 , and b 2 can be evaluated for granulated and ungranulated cement samples and the results compared. 1.2.5. Slurry settling tests This test follows a standard API test w6x. The cement slurry described above is mixed at given conditions for given periods of time so that it has a known and prescribed mixing history. It is then poured into a 250-ml graduated cylinder and sealed to prevent evaporation. The slurry is allowed to stand for 2 h and then the supernatant water is decanted and measured. The maximum allowable supernatant water is 3.5 ml.
Fig. 1. The effect of adding between 0.25 and 0.45 wt.% tartaric acid along with 1.5 wt.% calcium chloride during the agglomeration process on the compressive strength of concrete.
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The results of the viscosity tests for virgin cement and granulated cement using 1.5 wt.% CC and varying amounts of TA are shown in Fig. 2a and b. The results for ungranulated Žvirgin. cement are also shown on these figures. It can be seen that the values of m 0 and t 0 for the granulated cement at a TA level of between 0.275 and 0.325 wt.% are similar to those for the virgin cement. Results for the time constants, b 1 and b 2 in Eq. Ž1. show similar agreement although there is considerable scatter in the data. Average values of b 1 s y0.0023 miny1 and b 2 s 0.0108 miny1 were obtained for virgin cement. This means that at constant rate of shear over a period of 2 h, the first term on the right hand side of Eq. Ž1. decreases by approximately 25%, while the yield stress term increases by 365%. For all the samples, both virgin and agglomerated cement, the slurry settling test indicated that the amount of supernatant water was below the 3.5 ml maximum allowed by the API test method. Indeed, there was no detectable supernatant water found for the agglomerated samples. 1.4. Conclusions From the results given above, it appears that cement granules can be formed using a mixture of accelerating ŽCC. and retarding agents ŽTA.. Moreover, these granules
Fig. 2. The results of the viscosity tests for virgin cement and granulated cement using 1.5 wt.% CC and varying amounts of TA, Ža. Results for mo , Žb. Results for to .
appear to give similar slurry and concrete characteristics as those produced from virgin cement. Although size reduction of the agglomerates is necessary prior to use in drilling-well cement applications, this does not pose a large problem since the granules are quite brittle and are easily broken and pulverized by standard mechanical impactors. This novel process is at present under patent review w7x.
2. Production of coated ammonium persulfate particles to facilitate brittle failure of encapsulated viscosity breaking agents 2.1. Introduction It is common practice in the oil and gas production industry to rupture subterranean formations in order to increase permeability and conductivity. This fracturing process is facilitated by pumping a fracturing fluid into a well and applying a high pressure in order to fracture the surrounding rock, thus stimulating the flow of the petroleum product. These fracturing fluids contain a variety of materials. One of the most important of these materials is AproppantB which is essentially nearly spherical sand. The purpose of the proppant is to bridge the gap formed in the formation and to keep it open when the pressure on the fracturing fluid is reduced, thus increasing the permeability of the fracture to hydrocarbon flow. In order to stop the proppant from settling out at the bottom of the well, prior to making a 908 turn and flowing into the fractured formation, it is necessary to control the viscosity of the fracturing fluid. Indeed, a common fracturing fluid is Guar Gum ŽGalactomannan polymer., a complex carbohydrate derived from the seed of specially grown bean plants. This natural polymer has a molecular weight of approximately 1 million and a very high viscosity. The proppant and other solid additives are essentially held in suspension in this gelatinous fluid throughout the fracturing process. However, after fracturing has taken place the viscosity of the fluid must be reduced significantly in order to initiate the flow of hydrocarbons and to allow the proppant to settle out and keep the fractured formation open. This viscosityreducing step is referred to as Aviscosity breakingB or simply Abreaking.B Typical breakers are strong organic oxidizing agents; for example perchlorate, persulfate, and percarbonate compounds are routinely used as viscosity breakers. In this work ammonium persulfate ŽAP., was chosen as the model breaker compound. There are several triggering mechanisms by which the breakers can be activated, w8x. Examples include, direct injection of a breaker chemical into the fracturing fluid, time-release coatings on breakers within the fluid, and semi-permeable membrane coatings on breakers within the fluid. All these methods have certain drawbacks w8x. The purpose of the current work was to examine a new method
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for triggering the release of a viscosity breaker in a fracturing fluid. The idea of this research was to produce an encapsulated breaker that would be activated by the partial closure of the formation. The coated breaker would be added to the fracturing fluid, and after fracturing had taken place and the pressure reduced, the coated breaker particles would be subject to a compressive load due to the gradual closing of the fracture. By making the coating brittle, the application of the compressive load will fracture the outer coat and allow the breaker ŽAP. to release into the fluid and thus reduce the fluid viscosity and aid removing the fluid from the fracture. 2.2. Experimental methods 2.2.1. Coating formulation In order to evaluate potential coating materials a series of screening tests were performed and the details of these tests are given elsewhere, w9x. In developing the coating formulation three important factors were considered. First, the rate at which the AP leached from the uncrushed encapsulated particle should be very low. This is clearly necessary to avoid premature breaking of the fracturing fluid. The second consideration is that the release of the AP from the crushed particle should be both rapid and extensive. Ideally, all of the AP should be released instantaneously into the fracturing fluid upon application of the compressive load. Third, the size of the encapsulated breaker particles should be relatively small so as to pass easily into the fractured formation along with the proppant. A typical size range for the proppant is 1–2 mm and this was chosen to be the largest acceptable size range for the encapsulated breakers. Another consideration regarding the size of the encapsulated breaker is the distribution of these particles within the fracturing fluid. For a given dosage of AP, it is better to have many small particles distributed within the fluid as compared to a few larger ones. Both polyurethane and partially hydrolyzed acrylate coatings were investigated. Preliminary work suggested that the acrylics were more amenable to this application and the focus of this work was on these coatings. The addition of inert silica to the coating matrix has been found to alter the permeability characteristics of the coat formed by cross-linking vinyl acrylic latex polymers, w10x. The addition of fine silica Ž1–15 mm. powder to the coating mixture was also investigated in this work. 2.2.2. Coating process The coating work was carried out in a fluidized bed apparatus equipped with a draft tube insert. A schematic diagram of the equipment is shown in Fig. 3. The uncoated AP particles varied in size from approximately 400 to 800 mm and typical coating thickness was approximately 50– 100 mm. However, due to the tackiness of the coating a certain amount of agglomeration of the partially coated AP
Fig. 3. Fluidized bed apparatus equipped with a draft tube insert used to coat AP particles.
particles occurs. This agglomerationrcoating process was found to be very beneficial to the overall goals of the research and these results are discussed in the next section. 2.2.3. Leach tests A standard test was developed, in order to quantify the rate at which AP is transferred from the uncrushed encapsulated particle. A fixed weight of encapsulated particles Ž0.11 g. was immersed in 50 cm3 of distilled water and stirred continuously. Small samples of liquid were removed at different points in time, and the quantity of AP in the liquid was calculated using a standardized iodometric titration technique, see Bhatia w9x. The release profile for the AP from the uncrushed particles could then be constructed. 2.2.4. CompressiÕe tests In order to simulate the compressive load, to which the encapsulated particles would be subjected, a test device was constructed. The test method adopted in this work is adapted from API RP 56, which is used for testing the crush resistance of fracture sand Žproppant., w11x. The test method essentially consists of mixing a known weight of encapsulated breaker particles with a known amount of fracture sand or proppant. Typical loadings of proppant to breaker are from 50:1 to 100:1. For all the tests considered here 0.11 g of encapsulated breaker were mixed with 11 g of proppant and loaded into the test cell. This test mixture was placed in a test cylinder Ždiameter of 7.62 cm. and a loose fitting piston placed on top of the mixture. The piston was slowly rotated in order to flatten the test mixture and obtain a sample with a uniform depth. The test cell and piston were then placed in a Carver Press ŽCarver, Wabash, IN. and a load of 62,720 N Ž14,100 lbf . was applied. This load corresponds to an average applied stress of 13.8 MPa Ž2000 psi.. The load was applied uniformly over a period of 1 min and then the load was held constant at this value for a period of 2 min. After the applied load
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had been removed, the sample of sand and crushed and uncrushed breaker particles was added to 50 cm3 of distilled water and stirred. Samples of the liquid were removed and the amount of AP in the liquid was assessed by the iodometric titration procedure mentioned above. 2.3. Results and discussion The effect of the level of coating on encapsulated breaker particles with respect to the amount of AP released under leach and compressive load conditions after 10 min is shown in Fig. 4. The high rate of AP release at the low coating level Ž3.5%. is indicative of non-uniform coverage since both the leach and compression tests show the same level of release. As the coating thickness increases the leach level drops faster than the crushed level. At 15% coating, the leach rate Žapprox 3%. is acceptable but the crushed release is only 40%. In order to increase the crushed release level, without increasing the leach level, a series of experiments was performed in which fine silica was added to the coating mixture. The effect of the silica addition is shown in Fig. 5, where the AP released is plotted as a function of the percentage of silica contained in coatings with two levels of acrylate. In all the results, the level of acrylate coating is reported based on the weight of the AP particles while the weight of silica is based on the weight of the acrylate, thus: wt.% acrylates wt.% silica s
wt. of acrylate wt. of acrylateq wt. of AP wt. of silica
wt. of silica q wt. of acrylate
Ž 2. Ž 3.
From Fig. 5 it appears that there is a significant difference in the AP released from the leach test compared to the compression test and that this difference is influenced by the presence of silica in the coat. This difference increases as the silica content of the coating is increased from 0% to
Fig. 4. The effect of the level of coating on encapsulated breaker particles with respect to the amount of AP released under leach and compressive load conditions after 10 min.
Fig. 5. The effect of the silica addition on the AP released as a function of the percentage of silica contained in coatings at two levels of acrylate coating.
about 60%. Beyond about 60 wt.% silica, there appears to be a decrease in the amount of AP released during the compression test and there is also a significant increase in the amount of AP released during the basic leach test. From these results, acceptable leach and compression results are obtained at acrylate coating levels of 25 wt.% and silica levels of about 60 wt.%. These results indicate that the leach rate is approximately 3.5 wt.% in 10 min and the corresponding compression rate is 66 wt.%. The data shown in Fig. 5 are all taken after 10 min, the release histories for the 25 wt.% acrylater60 wt.% silica runs are shown in Fig. 6. It is clear from Fig. 6, that for the crushed sample, the vast majority of the AP is released within the first few minutes. The structure of the coated particles was also addressed in this work. Due to the stickiness of the coating used in the current research, the particles formed during the fluidized bed coating process consisted of well defined agglomerates of between 10 and 20 individually coated particles that had subsequently adhered to each other and been over coated with the same coating solution. Images of
Fig. 6. The release histories for the 25 wt.% acrylate and 60 wt.% silica runs.
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Fig. 7. A video image of a ruptured ammonium persulfate coated particle agglomerate.
the cross-sections of these agglomerates suggest that when they are subjected to a compressive load the coating around individual particles ruptures and tears. Two video images of ruptured agglomerates are shown in Fig. 7, and individual AP particles are shown exposed. 2.4. Conclusions This work has shown that a coating using a combination of cross-linked acrylic polymers and a high percentage of inert filler particles, such as silica, can be used to encapsulate viscosity breakings agents such as ammonium persulfate. These encapsulated breakers posses low leach rates when immersed in water and therefore posses the desired low water-permeability characteristic required of viscosity breaking agents. Moreover, by controlling the amount of fine silica added to the coating solution, the coating produced exhibits a high release of AP upon crushing. This novel process is at present under patent review w12x.
r particle velocity profiles in 3. Evaluation of voidager fluidized bed coating equipment with draft tubes 3.1. Introduction During the coating of particles in fluidized beds, particles are usually made to spout with the aid of a draft tube. By using the draft tube, the formation of a regular and essentially stable spout is possible. This leads to the regular circulatory movement of particles within the bed. Particles are coated by introducing an atomized spray of coating solution into the bottom of the bed. Particles are repeatedly coated during successive passes through the spray zone and a coherent continuous coat is gradually built up on the particles. For a batch of coated particles, the uniformity of the distribution of coating on the population of particles comprising the bed can be attributed to the distribution of the number of passages that a particle takes
through the spray zone and the distribution of coating material that each particle receives during a single passage through the spray, w13x. This variation in coating mass distribution depends on a variety of operating parameters, and also on the size of the particles, w14x. Moreover, it has been shown for both small particles Ž1 mm., Cheng and Turton w15x and large particles Ž10 mm., Shelukar et al. w16x, that the major contribution to the uniformity of the batch is the distribution of material that each particle receives as it passes through the spray zone. This distribution is a strong function of the sheltering effect that particles have on each other. Therefore, the measurement of the velocity and voidage profiles within the spray zone of the fluidized bed are key variables to know if the uniformity of coating during batch coating operations is to be predicted, w17x. Various techniques can be used to measure the movement of particles within a fluidized bed. During the past several years, work in our group has concentrated on analyzing video imaging data obtained from experiments performed in a 22.5 cm Ž9 in.. diameter semicircular bed fluidized coating apparatus. Voidage and velocity measurements in the draft tuberspray region of this bed using approximately 1 mm diameter spherical glass beads have been reported previously, w18x. Current work is focusing on the recording and analysis of data from this same equipment using 8-mm tablets. Due to the non-spherical shape of these particles, previous techniques to analyze particle velocity and voidage, which were developed for spherical particles, could not be employed. 3.2. Experimental methods The experimental setup is shown in Fig. 8, this semicircular fluidized bed coating apparatus is described elsewhere w18x. Images of the particles are taken with a variable shutter speed CCD camera and these images are stored via a frame grabbing board. When evaluating the
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Fig. 8. Semicircular fluidized bed coating apparatus used to evaluate voidage and velocity profiles in the draft tube region.
particle velocities using the 1-mm-diameter spherical glass beads, the shutter speed of the CCD camera is set at between 1 and 2 ms. This setting produced images that were bright particle streaks. The length and direction of the streaks could be used with the camera shutter speed to evaluate the particle velocity projected in the vertical and horizontal directions. In addition, since the particles were essentially the same size, the width of the streaks could be used to determine which particles were out-of-focus. Only in-focus streaks were used for evaluation of particle velocity. By repeatedly taking data at different points in the flow field, the velocity profiles within the draft tube region were constructed, w19x. For the evaluation of voidage measurements, the camera shutter speed was set to 0.1 ms and the resulting video fields gave clear images of the particles in the field-of-view ŽFOV.. Filtering software was used to look at the dark-to-light transitions at the edges of the particle in order to distinguish between in- and out-of-focus particles. The voidage was obtained by knowing the number of particles within a volume element of the bed close to the front wall, equal to the FOV Ž1 cm = 1 cm. multiplied by the depth-of-field ŽDOF s 0.5 cm.. When evaluating the velocity and voidage profiles using large, tablet-shaped particles, the technique for evaluating particle velocity described above for spherical particles no longer applies. The main reason for this is that as the particles move upwards they rotate and the streaks that are formed vary in width and are very indistinct. Therefore, it is impossible to distinguish between in-focus and out-offocus particles. In order to evaluate particle velocity accurately, successive particle images must be captured at times in the region of 1–5 ms apart. This is not possible with a standard CCD camera operating on the RS-170 standard framing rate of 33.33 ms Ž30 Hz.. One method to do this would be to employ a high-speed camera. The
method adopted in this work was to use two synchronized CCD cameras to capture images of the same area of the bed at times of between 1 and 5 ms apart. The timing events are controlled by customized software and the variable time delay can be set by the user. The set-up for the cameras is shown in Fig. 9. It is important that the cameras be arranged in a steep isosceles triangle in order to minimize parallax effects. The greater the angle at the apex, u , the greater is the difference between the images from the two cameras. Once the cameras are set-up, the customized software is used to calibrate the two images from the cameras in order to minimize these errors. When the bed is running, a AgrabB event is initiated that causes both cameras to obtain images at the pre-selected time lag. The center of each particle is then located using a computer-generated crosshair and the horizontal and vertical distances between the successive images is computed and recorded. These data can then be converted into magnitude and direction using the appropriate time lag. The technique for obtaining voidage measurements for the tablets is similar that used for the spherical particles. However, the particle identification algorithm is somewhat different but the idea is still to count the number of particles in a known volume of bed close to the front wall. 3.3. Results and discussion Evaluation of velocity and voidage maps within the draft tube region of the bed is currently underway. Representative velocity distributions at a point 4 cm from the centerline of the bed and at the centerline of the bed and at
Fig. 9. Two camera set-up for measuring tablet velocity.
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Fig. 10. Velocity distributions for tablets in the semicircular fluidized bed shown in Fig. 8 with a gap height of 2.5 cm Ža. radial locations 4 cm from axis and vertical locations 6.5 cm from distributor Žb. radial locations 0 cm from axis and vertical locations 6.5 cm from distributor.
a vertical distance of 6.5 cm from the distributor plate are shown in Figs. 10 and 11. For each location, between 50 and 70 particle velocities were obtained and were used to evaluate the local velocity. For all the data, the cameras were focused on a region approximately 2 mm inside the
front surface of the bed in order to minimize the wall effect of the flat glass partition. In Fig. 10, the gap between the bottom of the draft tube and the distributor plate is 2.5 cm, while in Fig. 11 the gap is 7.5 cm. For these runs, all other operating parameters were the same.
Fig. 11. Velocity distributions for tablets in the semicircular fluidized bed shown in Fig. 8 with a gap height of 7.5 cm Ža. radial locations 4 cm from axis and vertical locations 6.5 cm from distributor Žb. radial locations 0 cm from axis and vertical locations 6.5 cm from distributor.
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For example, the volumetric flowrate of air was 61.4 STD litrs and the bed loading was 3.65 kg. These data show that the direction of particle movement is closer to the vertical the closer the location is to the centerline. This result is consistent with the work of Saadevandi w19x. It is also consistent with visual observations that indicate that at the bottom of the bed particles flow horizontally towards the center of the draft tube from the annular region and then turn upwards as they become entrained by the upward moving central air stream. The particle velocity data varies considerably with location. However, at the top of the draft tube insert the particle velocity is more uniform and shows that the velocity increases with an increase in the height of the gap between the bottom of the draft tube and the distributor plate. Work is ongoing in mapping both the voidage and velocity distributions in the draft tube region of the fluidized bed illustrated in Fig. 8. The experimental program includes varying the following parameters: gap height between the draft tube and the distributor, fluidizing air velocity, distributor plate design, and adding a flow guide at the bottom of the draft tube. 3.4. Conclusions Video imaging of large non-spherical particles Žtablets. using a two-camera system was shown to be a viable technique for estimating particle velocity. Particle velocity data was presented in the form of distributions of magnitude and direction for two locations in a semicircular fluidized bed. This technique is currently being used to map the voidage and particle velocity distributions within the draft tube region of a fluidized bed coating apparatus. Nomenclature b 1 , b 2 constants used in Eq. Ž1. Žminy1 . t time Žmin. dÕrd x shear rate Žsy1 .
Greek Letters shear stress ŽPa. t yield stress ŽPa. t0 m0 plastic viscosity ŽPa s.
Acknowledgements Funding for the agglomeration and coating research by Halliburton Energy Services and funding for the tablet velocity and voidage work by Merck and Company, are gratefully acknowledged.
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