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Effects of shear fracture on in-depth profile modification of weak gels

Effects of shear fracture on in-depth profile modification of weak gels 2007 VolA No.1 Petroleum Science Li Xianjie , Song Xinwang", Yue Xiang'an', Hou Jirui', Fang Lichuri' and Zhang Huazhen' (1. Center for Enhanced Oil Recovery, China University ofPetroleum, Beijing 102249, China) (2. Research Institute of Geological Science, Shengli Oilfield Company Ltd., Sinopec, Dongying, Shandong 257015, China) Abstract: Two sand packs were filled with fine glass beads and quartz sand respectively. The characteristics of crosslinked polymer flowing through the sand packs as well as the influence of shear fracture of porous media on the in­ depth profile modification of the weak gel generated from the crosslinked polymer were investigated, The results indicated that under the dynamic condition crosslinking reaction happened in both sand packs, and the weak gels in these two cases became small gel particles after water flooding. The differences were: the dynamic gelation time in the quartz sand pack was longer than that in the glass bead pack. Residual resistance factor (F RR) caused by the weak gel in the quartz sand pack was smaller than that in the glass bead pack. The weak gel became gel particles after being scoured by subsequent flood water. A weak gel with uniform apparent viscosity and sealing characteristics was generated in every part ofthe glass bead pack, which could not only move deeply into the sand pack but also seal the high capacity channels again when it reached the deep part. The weak gel performed in-depth profile modification in the glass bead pack, while in the quartz sand pack, the weak gel was concentrated with 100 em from the entrance of the sand pack. When propelled by the subsequent flood water, the weak gel could move towards the deep part of the sand pack but then became tiny gel particles and could not effectively seal the high capacity channels there. The in-depth profile modification of the weak gel was very weak in the quartz sand pack. It was the shear fracture of porous media that mainly affected the properties and weakened the in-depth profile modification of the weak gel. Key words: Weak gel, shear fracture, in-depth profile modification, glass bead pack, quartz sand pack modification of weak gel, which could be misleading in the field application of weak gel. 1. Introduction In order to overcome the shortcomings of the above­ The weak gel is the reaction product of crosslinked mentioned simulated porous media and study the polymer with organic reagents, with a low polymer influence of shear fracture in porous media, a sand pack, concentration and weak three-dimensional network 200 em long and filled with fine glass beads or quartz structure, and is intermolecular cross linking sand was used. The pores in the fine glass bead pack predominantly and intramolecular crosslinking in the were regular and had good communication and definite second place (Mack and Smith, 1994; Wang, et al., axial tensile force because of the regular shape of fine 2000). Such a gel is considered to have the action of in­ glass beads, while the pores in quartz sand pack were depth profile modification in reservoirs, however, this highly irregular and there was a larger difference in pore action is inevitably affected by the shear fracture in radius and throat radius and strong shear fracture reservoirs. because quartz sand is irregular, fragmented. Therefore, The agitator/rheometer or natural/artificial core is the main difference between these two packs is the often used to study the effect of shear fracture on in­ different shear fracture exerting on the weak gel. depth profile modification of weak gel. But the gel has Comparing the results in the fine glass bead pack with evident wholeness in agitator or rheometer (Li, et al., the results in the quartz sand pack is conducive to an in­ 2000b; Lin, et al., 2004; Robb and Smeulder, 1997), depth study on the effects of shear fracture in a reservoir differing from its micro-distribution in porous media, so on the in-depth profile modification of the weak gel. the inaccuracy is inevitable if the agitator or rheometer is used to study the gel. The natural or artificial core (Li, 2. Experimental et al., 2000a; Tian, et al.,1997; Weng and Wei, 1998; 2.1 Materials Xu, et al., 2001) is similar in pore structure with the Commercial fine glass beads (100-150 mesh) and reservoir, but it is difficult to distinguish the effects of quartz sand (60-100 mesh) were used to fill sand packs. different formation factors, moreover, these simulated Polymer MO-4000 (1,500 mg/L), crosslinker A (1,000 media (including some sand packs (Chen, et al., 1998) ) mg/L) and crosslinker B (100 mg/L) were all provided are so short (within 50 em long in general) that they have many limitations, such as strong end face effect, by Geological Institute of Shengli Oilfield. Simulated especially when used for studying the in-depth profile formation water had a salinity of 19,334 mg/L. 56 2007 Petroleum Science 2.2 Apparatus 3. Results and discussion The experimental setup consists of the following 3.1 Dynamic gelling property main components: sand pack, thermostatic system, 3.1.1 Gelling performance power system, pressure-acquisition system, automatic The curves of the viscosity versus injection time for a fluid-receiving system, intermediate tank. The sand weak gel in the glass bead pack and quartz sand pack are pack was 200 em in height and 2.5 em in diameter. Its shown in Fig. 1. The variation in viscosity of the weak gel inner surface was coated with steel sand (about 100 with injection time was shown in two stages. At the initial mesh) to prevent the fluid from slipping over the wall stage, the viscosity increased slightly with injection time. surface. Fourteen piezometer orifices were arranged At the second stage, i.e. after the weak gel has been along the sand pack in order to measure the variation in injected for 15 and 16.5 h for the glass bead pack and pressure. Two sand packs were used in this paper, one quartz sand pack respectively, the viscosity increased was filled with fine glass beads while the other was sharply. This indicated that during injection the polymer packed with quartz sand. M0-4000 was cross-linked with crosslinkers to form a weak gel with high viscosity in both packs. Thus the shear 2.3 Experimental procedures fracture caused by porous media is not the predominant 1) The sand packs were filled with fine glass beads factor influencing gelling performance. and quartz sand respectively, and then evacuated and saturated with simulated formation water. Finally the porosities of these two sand packs were calculated and their values were 37.5% and 47.3% for the glass bead Y' 25 pack and quartz sand pack respectively. 0- E 20 --<>- Glass bead pack 2) The sand packs were placed in a thermostatic i> --f:r- Quartz sand pack oven at 80°C for 16 h. 'C;; 15 3) After the sand packs were stabilized, their initial cr- ';; 10 water permeabilities were measured. Their values were 6.1 and 3.6 unr', respectively, " 5 8: 4) A weak gel of IPV was injected into the sand packs. PV indicates the total pore volume of the sand pack. -5 o 5 If) 15 20 25 5) After waiting for gelling for 24 h, flood water was Injection time. h subsequently injected at a constant rate of 3 mid until the equilibrium intake pressure was reached. Then the Fig. 1 Curves of apparent viscosity of crosslinked polymer permeability and residual resistance factor (F RR) of sand with injection time packs could be calculated. 3.1.2 Dynamic gelation time 2.4 Data processing The gelation time, or the period of time until the gel Darcy's law is the relationship that explains fluid no longer pumpable, is defined as the time at which flow in porous media and its equation can be written as: crosslinking reaction begins under flowing condition. There are many methods for determining the gelation (1) time, for example, testing the jump point of the crosslinked polymer viscosity or judging the turning From Eq. (1), we have point of elastic modulus and viscous modulus of the polymer. In this paper, the dynamic gelation time was (2) determined by means of testing the jump point of apparent viscosity. Fig. 1 demonstrates that the dynamic where Q is the injection flow rate, cmvs; L is the length gelation time of the weak gel is 15.0 and 16.5 h for the of sand pack, ern; A is the cross-section area of sand glass bead pack and quartz sand pack, respectively. The pack, crrr'; fA, is the viscosity of fluid, ml'a-s; k is the gelation time of the weal gel in the quartz sand pack .initial water permeability of sand pack, urrr'; Sp is the was longer than that in the glass bead pack, which pressure drawdown of sand pack. accords with Zhu's findings (Zhu, et al., 2002). VolA No.1 57 Under flowing condition, the weak gel viscosity was The fine glass beads are well-rounded, smooth­ affected by crosslinking reaction and shear damage surface, regular spheres. The glass bead pack had caused by porous media. Compared with shear action, definite axial shear action and very weak fracture action, adsorptive action was so weak that it was negligible. therefore the separation among crosslinked polymer The crosslinking reaction could reinforce the three­ components and the break-up of polymer molecular dimensional network structure of the weak gel and chains were weaker, and a high-viscosity weak gel, therefore increase the viscosity of the crosslinked which had strong shut off capacity, was generated in the polymer; while the shear fracture could destroy the pores within the glass bead pack. So the residual network structure, thereby decreasing the viscosity of resistance factor in the glass bead pack was larger and the crosslinked polymer. The shear action, decided by the subsequent flood water moved forward evenly, and the shape and surface feature of the particles in the more water was needed to break through the gel. quartz sand pack was stronger than that in the glass In comparison with the fine glass beads, the quartz bead pack, so it decreased the viscosity even more when sand was poorly-rounded, irregular and fragmented. The the same gel formula was used in both sand packs, and quartz sand pack had irregular and variable-radius pores therefore the dynamic gelation time of the crosslinked and strong shear fracture action, which increased flow polymer in the quartz sand pack is longer than that in resistance and reduced the flow rate of macromolecular the glass bead pack. polymer but could not reduce the flowability of micromolecular crosslinker. Therefore the crosslinker 3.2 Sealing characteristics was gradually separated from the polymer, which resulted in the variation in the proportion of crosslinker 3.2.1 Residual resistance factor to polymer, thereby impeding the formation of the high­ The relationship between residual resistance factor viscosity weak gel. On the other hand, some polymer F RR and intake volume of subsequent water flood in could move to the deep part of the sand pack (deep part both packs are shown in Fig. 2. of a reservoir) but it was broken by the shear fracture of Fig. 2 shows that the residual resistance factor in quartz sand, so the high-viscosity gel could not be both packs increased sharply at the initial stage of generated although the concentration of crosslinker was subsequent water flood and then tapered off and high here. Furthermore, the weak gel generated near the steadied gradually with the increase in subsequent entrance of the quartz sand pack became tiny gel intake volume, so there was a peak value. The particles and lost its ability to seal the high capacity subsequent intake volumes were 0.5 PV and 0.3 PV channels when they moved further in the sand pack. For respectively in these two packs when the residual these reasons, F RR in the quartz pack was small and the resistance factors reached the peak value, so the flood crossflow of subsequent flood water was strong. water broke through easier in the quartz sand pack. On the other hand, the residual resistance factor in the 3.2.2 Residual resistance factor distribution quartz sand pack was far smaller than that in the glass The sand pack was separated into 15 parts by 14 bead pack, indicating that the shut off capacity of the piezometer orifices. In order to study the shut off weak gel in the quartz sand pack was very weak. capacity of the weak gel in different parts of the sand - . - . - Glass bead pack pack, the residual resistance factor of each part was I Quartz sand pack calculated according to the pressure difference of two :: .... ( 0 adjacent piezometer orifices. The distribution of tl " .... ,,S residual resistance factor along the sand pack was ..... ,,, u 300 I " '" drawn on the coordinates with the x-axis being the ..... en ....... ,-. '0;; length from the entrance of the sand pack to the middle .... " I of the part (L) while the y-axis being the residual "i :s ,'" "0 I'; resistance factor (F RR) of the part. The curves are shown '0;; 0::: " in Figs. 3 and 4. Distribution of F RR in the glass bead pack is shown 0.0 0.3 0.6 0.9 1.2 1.5 in Fig. 3. It is found that F in the pack fluctuated and R R Pore volume injected maintained an average value of about 30 with increasing intake volume of subsequent water flood, so F had a RR Fig. 2 Relationship between F and intake volume of R R relatively even distribution along the glass bead pack, subsequent water flood 58 2007 Petroleum Science indicating that a weak gel with uniform apparent sand pack decreased in varying degrees with increasing viscosity and sealing characteristics was generated in intake volume of subsequent water flood, showing that every part of the sand pack. the weak gel which was generated and sealed the high capacity channels under static condition was able to l'lO move to the deep part of the sand pack; while F RR in 3~ (. -.- 0 5PV other parts of the sand pack did not increase and was even close to zero, indicating that the moving weak gel " 60 could not effectively seal the high capacity channel there again. The pores in the glass bead pack, regular and well '" 20 ~ ,.,. !i-":""--- ~./ interconnected, had only some axial shear action and .§ ,. I ..../.. .~ ••• -1i1 scarcely destroyed the three-dimensional network of the c:::: I._._A o .. I weak gel. Moreover, the weak surface resistance of fine o 50 100 150 200 glass bead allowed the weak gel to slide on the surface, I., em thus increasing the gel's mobility. Therefore, the weak Fig. 3 Distribution of F in the glass bead pack R R gel could move forward and perform in-depth profile modification in the glass bead pack. In contrast to the glass bead pack, the quartz sand -.-O.5PV pack, with irregular shape, variable pore radius, rough ... c • -·-0.7PV surface and a great difference in the radius of throats, had !l 200 ~ - ..-0.9PV strong shear fracture. When the weak gel moved along " • ;; v; the sand pack, this action could destroy the three­ 11~ .. .;; ~ dimensional network of the weak gel and tum the gel into ~\.-~~~~~\ increasingly tiny gel particles with further movement. -€l ... 'if. '.~'.'_'l:_~._ . .:.-!!!!!'i.--1 The rheological property (tested by using a HAAKE 0:: " 0 50 100 150 RS600 rheometer, shown in Fig. 5) of specimens sampled I., ern at the exit of the packs showed that the apparent viscosities of these specimens were lower than that of the Fig.4 Distribution of F in the quartz sand pack RR polymer solution with the same concentration as the crosslinked polymer system and just a little larger than Distribution of F RR in the quartz sand pack is shown the apparent viscosity of simulated formation water, in Fig. 4. It can be found that F converged within 100 RR indicating that the network structure of the weak gel was em from the entrance and was very small in other parts destroyed and the ability to seal the high capacity of the pack. This result indicated that the high-viscosity channels was lost. The gel particles could move deeply weak gel was generated just in the 100 em section from into the sand pack, even reach the exit (equivalent to the the entrance, which made the subsequent flood water to bottom of the producing well in oil field), propelled by flow back to the high capacity channels and decreased the subsequent flood water, and the performance of in­ its swept volume. Moreover, the weak gel near the depth profile modification of the weak gel was entrance consumed most of the displacement energy weakened. and decreased its utilization efficiency. Comparing the results in these two sand packs, the performance of in-depth profile modification of the 3.3 Mobility weak gel was weakened mainly by shear fracture of Fig. 3 illustrates that F RR in the parts near the porous media. entrance (about half of the pack from the entrance) had a trend of decreasing with increasing intake volume of 3.4 Microshape subsequent water flood, especially when the intake A ZEISS optical microscope made in Germany was volume was more than 0.4 PV, while that near the exit used to observe the microshape of the packing materials increased, which demonstrated that the weak gel in the and the gels sampled in the middle of the sand packs glass bead pack could not only move deeply into the respectively, in porous media in order to study and sand pack but also that it could seal the high capacity verify the above-mentioned laboratory findings. When channels again when it reached the deep part. taking photomicrograph, quartz sand or fine glass beads Fig. 4 shows that F near the entrance of the quartz RR VolA No.1 Effects of Shear Fracture on In-depth Profile Modification of Weak Gels 59 of these samples were spread on the rnicroslide, which model. Photomicrographs are shown in Fig. 6. Fig. 6a is increased the radius of intergranular pores, greater than the photomicrograph of a specimen sampled from the the pore radius between sand particles in closely packed glass bead pack and Fig. 6b from the quartz sand pack. ~ Weak gel [0000 --0- Ungcllcd system --&- Sampled when subsequent injection rate is () PV --f- Sampled when subsequent injection rare is 0.5PV -b- Sampled when subsequent injection rate is 1PV --+- Simulated formation water ~------ 10 100 Shear rate, s' Fig. 5 The rheological curves of specimens sampled at the exit of quartz sand pack (a) Sampled from glass bead pack (b) Sampled from quartz sand pack Fig.6 Photomicrographs of weak gel (lOX 10) Fig. 6 shows that the weak gel became gel particles 4. Conclusions after being scoured by flood water in both sand packs, but there were some differences. The equivalent radius 1) Crosslinking reaction of polymer with crossliker of gel particles is far larger than those of pores and occurred in both sand packs under dynamic condition, throats in the glass bead pack, so the gel particles could but the gelation time in the quartz sand pack was seal the pores and throats; while the equivalent radius of slightly longer than that in the glass bead pack. The gel particles in the quartz sand pack was equal to or weak gel became gel particles after being scoured by smaller than the pore radius in the sand pack, moreover, the subsequent flood water in both sand packs. the low strength gel particles could pass through the 2) A weak gel with uniform apparent viscosity and pores and throats, propelled by the subsequent flood sealing characteristics was generated in the glass bead water, but could not seal the flow channels effectively. pack, and FRR in the glass bead pack was larger than that 60 2007 Petroleum Science gels improve oil recovery efficiency. SPEIDOE, 27780 in the quartz sand pack. The weak gel could not only Robb I. D. and Smeulder 1. B. A. F. (1997) The rheological move towards the deep part of the sand pack but also seal properties of weak gels of poly (vinyl alcohol) and sodium the high capacity channels again when it reached the borate. Polymer, 38(9), 2165-2169 deep part, showing that the gel could perform in-depth Tian G. L., Ju Y., Sun G. Y., et al. (1997) Research on shear profile modification in the glass bead pack. behavior and percolation rules of crosslinked polymer. Oil & 3) In the quartz sand pack, the weak gel was localized Gas Recovery Technology, 4(4), 19-24 (in Chinese) at the first half of the pack next to the entrance and still Wang P. M., Luo 1. H., Li Y. X., et al. (2000) The study of the could move deeply into the sand pack propelled by the characteristics of weak gel in the core test. Oil Drilling & subsequent flood water, but it became tiny gel particles and Production Technology, 22(5), 48-50 (in Chinese) could not effectively seal the high capacity channels when Weng R. and Wei L. (1998) The effect of shear rate and shear it reached the deep part. The in-depth profile modification time on jelling behavior of weak gel formed with solution of of the weak gel was very weak in the quartz sand pack. a low polymer concentration. Petroleum Exploration and Development, 25(5), 65-67 (in Chinese) 4) It was the shear fracture of porous media that mainly Xu G. L., Lin M. Q., Li M. Y., et al. (2001) Plugging performance affected the properties and weakened the performance of of linked polymer solutions. Journal of the University of in-depth profile modification of the weak gel. Petroleum, China, 25(4), 85-87 (in Chinese) Zhu H. 1., Liu Y. Z., Fan Z. H., et al. (2002) The impact of Acknowledgement dynamic process on the gelation mechanism of a HPAM­ This work was supported by the National Key phenol-aldehyde system. Petroleum Exploration and Technologies R&D Program in the 10th Five-year Plan Development, 29(6), 84-86 (in Chinese) of China (No. 2003BA613A-07-05) About the first author References Li Xianjie was born in 1979 and Chen T. L., Zhang L. H. and Zhou 1. M. (1998) Study of the received a M.S degree in oil and gas colloidal dispersion gel and its characteristics of rheology and development engineering from China seepage. Oilfield Chemistry, 15(3),265-268,277 (in Chinese) University of Petroleum (Beijing) in Li L. X., Liu Y. Z., Han M., et at. (2000a) The effect of flow shearing 2005. Now he is a Ph.D student of the on weak gel formation from aqueous polymer/crosslinker gelling oil and gas development engineering, solutions. Oilfield Chemistry, 17(2),148-151 (in Chinese) China University of Petroleum (Beijing), Li M. Y., Lin M. Q. and Zheng X. Y. (2000b) Linked polymer with his research interests in enhanced solution as in-depth permeability control agent: Laboratory study oil recovery and oil chemistry. E-mail: and field test. Oilfield Chemistry, 17(2), 144-147 (in Chinese) upclxjie@126.com Lin M. Q., Luo X. H., Dong Z. X., et at. (2004) The influence of shear action on plugging performance of linked polymer solution. Acta Petro lei Sinica (Petroleum Processing Section), (Received June 5. 2006) 20(5),48-52 (in Chinese) (Edited by Sun Yanhua) Mack J. C. and Smith J. E. (1994) In-depth colloidal dispersion http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

Effects of shear fracture on in-depth profile modification of weak gels

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Publisher
Springer Journals
Copyright
Copyright © 2007 by China University of Petroleum
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
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1995-8226
DOI
10.1007/BF03186574
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Abstract

2007 VolA No.1 Petroleum Science Li Xianjie , Song Xinwang", Yue Xiang'an', Hou Jirui', Fang Lichuri' and Zhang Huazhen' (1. Center for Enhanced Oil Recovery, China University ofPetroleum, Beijing 102249, China) (2. Research Institute of Geological Science, Shengli Oilfield Company Ltd., Sinopec, Dongying, Shandong 257015, China) Abstract: Two sand packs were filled with fine glass beads and quartz sand respectively. The characteristics of crosslinked polymer flowing through the sand packs as well as the influence of shear fracture of porous media on the in­ depth profile modification of the weak gel generated from the crosslinked polymer were investigated, The results indicated that under the dynamic condition crosslinking reaction happened in both sand packs, and the weak gels in these two cases became small gel particles after water flooding. The differences were: the dynamic gelation time in the quartz sand pack was longer than that in the glass bead pack. Residual resistance factor (F RR) caused by the weak gel in the quartz sand pack was smaller than that in the glass bead pack. The weak gel became gel particles after being scoured by subsequent flood water. A weak gel with uniform apparent viscosity and sealing characteristics was generated in every part ofthe glass bead pack, which could not only move deeply into the sand pack but also seal the high capacity channels again when it reached the deep part. The weak gel performed in-depth profile modification in the glass bead pack, while in the quartz sand pack, the weak gel was concentrated with 100 em from the entrance of the sand pack. When propelled by the subsequent flood water, the weak gel could move towards the deep part of the sand pack but then became tiny gel particles and could not effectively seal the high capacity channels there. The in-depth profile modification of the weak gel was very weak in the quartz sand pack. It was the shear fracture of porous media that mainly affected the properties and weakened the in-depth profile modification of the weak gel. Key words: Weak gel, shear fracture, in-depth profile modification, glass bead pack, quartz sand pack modification of weak gel, which could be misleading in the field application of weak gel. 1. Introduction In order to overcome the shortcomings of the above­ The weak gel is the reaction product of crosslinked mentioned simulated porous media and study the polymer with organic reagents, with a low polymer influence of shear fracture in porous media, a sand pack, concentration and weak three-dimensional network 200 em long and filled with fine glass beads or quartz structure, and is intermolecular cross linking sand was used. The pores in the fine glass bead pack predominantly and intramolecular crosslinking in the were regular and had good communication and definite second place (Mack and Smith, 1994; Wang, et al., axial tensile force because of the regular shape of fine 2000). Such a gel is considered to have the action of in­ glass beads, while the pores in quartz sand pack were depth profile modification in reservoirs, however, this highly irregular and there was a larger difference in pore action is inevitably affected by the shear fracture in radius and throat radius and strong shear fracture reservoirs. because quartz sand is irregular, fragmented. Therefore, The agitator/rheometer or natural/artificial core is the main difference between these two packs is the often used to study the effect of shear fracture on in­ different shear fracture exerting on the weak gel. depth profile modification of weak gel. But the gel has Comparing the results in the fine glass bead pack with evident wholeness in agitator or rheometer (Li, et al., the results in the quartz sand pack is conducive to an in­ 2000b; Lin, et al., 2004; Robb and Smeulder, 1997), depth study on the effects of shear fracture in a reservoir differing from its micro-distribution in porous media, so on the in-depth profile modification of the weak gel. the inaccuracy is inevitable if the agitator or rheometer is used to study the gel. The natural or artificial core (Li, 2. Experimental et al., 2000a; Tian, et al.,1997; Weng and Wei, 1998; 2.1 Materials Xu, et al., 2001) is similar in pore structure with the Commercial fine glass beads (100-150 mesh) and reservoir, but it is difficult to distinguish the effects of quartz sand (60-100 mesh) were used to fill sand packs. different formation factors, moreover, these simulated Polymer MO-4000 (1,500 mg/L), crosslinker A (1,000 media (including some sand packs (Chen, et al., 1998) ) mg/L) and crosslinker B (100 mg/L) were all provided are so short (within 50 em long in general) that they have many limitations, such as strong end face effect, by Geological Institute of Shengli Oilfield. Simulated especially when used for studying the in-depth profile formation water had a salinity of 19,334 mg/L. 56 2007 Petroleum Science 2.2 Apparatus 3. Results and discussion The experimental setup consists of the following 3.1 Dynamic gelling property main components: sand pack, thermostatic system, 3.1.1 Gelling performance power system, pressure-acquisition system, automatic The curves of the viscosity versus injection time for a fluid-receiving system, intermediate tank. The sand weak gel in the glass bead pack and quartz sand pack are pack was 200 em in height and 2.5 em in diameter. Its shown in Fig. 1. The variation in viscosity of the weak gel inner surface was coated with steel sand (about 100 with injection time was shown in two stages. At the initial mesh) to prevent the fluid from slipping over the wall stage, the viscosity increased slightly with injection time. surface. Fourteen piezometer orifices were arranged At the second stage, i.e. after the weak gel has been along the sand pack in order to measure the variation in injected for 15 and 16.5 h for the glass bead pack and pressure. Two sand packs were used in this paper, one quartz sand pack respectively, the viscosity increased was filled with fine glass beads while the other was sharply. This indicated that during injection the polymer packed with quartz sand. M0-4000 was cross-linked with crosslinkers to form a weak gel with high viscosity in both packs. Thus the shear 2.3 Experimental procedures fracture caused by porous media is not the predominant 1) The sand packs were filled with fine glass beads factor influencing gelling performance. and quartz sand respectively, and then evacuated and saturated with simulated formation water. Finally the porosities of these two sand packs were calculated and their values were 37.5% and 47.3% for the glass bead Y' 25 pack and quartz sand pack respectively. 0- E 20 --<>- Glass bead pack 2) The sand packs were placed in a thermostatic i> --f:r- Quartz sand pack oven at 80°C for 16 h. 'C;; 15 3) After the sand packs were stabilized, their initial cr- ';; 10 water permeabilities were measured. Their values were 6.1 and 3.6 unr', respectively, " 5 8: 4) A weak gel of IPV was injected into the sand packs. PV indicates the total pore volume of the sand pack. -5 o 5 If) 15 20 25 5) After waiting for gelling for 24 h, flood water was Injection time. h subsequently injected at a constant rate of 3 mid until the equilibrium intake pressure was reached. Then the Fig. 1 Curves of apparent viscosity of crosslinked polymer permeability and residual resistance factor (F RR) of sand with injection time packs could be calculated. 3.1.2 Dynamic gelation time 2.4 Data processing The gelation time, or the period of time until the gel Darcy's law is the relationship that explains fluid no longer pumpable, is defined as the time at which flow in porous media and its equation can be written as: crosslinking reaction begins under flowing condition. There are many methods for determining the gelation (1) time, for example, testing the jump point of the crosslinked polymer viscosity or judging the turning From Eq. (1), we have point of elastic modulus and viscous modulus of the polymer. In this paper, the dynamic gelation time was (2) determined by means of testing the jump point of apparent viscosity. Fig. 1 demonstrates that the dynamic where Q is the injection flow rate, cmvs; L is the length gelation time of the weak gel is 15.0 and 16.5 h for the of sand pack, ern; A is the cross-section area of sand glass bead pack and quartz sand pack, respectively. The pack, crrr'; fA, is the viscosity of fluid, ml'a-s; k is the gelation time of the weal gel in the quartz sand pack .initial water permeability of sand pack, urrr'; Sp is the was longer than that in the glass bead pack, which pressure drawdown of sand pack. accords with Zhu's findings (Zhu, et al., 2002). VolA No.1 57 Under flowing condition, the weak gel viscosity was The fine glass beads are well-rounded, smooth­ affected by crosslinking reaction and shear damage surface, regular spheres. The glass bead pack had caused by porous media. Compared with shear action, definite axial shear action and very weak fracture action, adsorptive action was so weak that it was negligible. therefore the separation among crosslinked polymer The crosslinking reaction could reinforce the three­ components and the break-up of polymer molecular dimensional network structure of the weak gel and chains were weaker, and a high-viscosity weak gel, therefore increase the viscosity of the crosslinked which had strong shut off capacity, was generated in the polymer; while the shear fracture could destroy the pores within the glass bead pack. So the residual network structure, thereby decreasing the viscosity of resistance factor in the glass bead pack was larger and the crosslinked polymer. The shear action, decided by the subsequent flood water moved forward evenly, and the shape and surface feature of the particles in the more water was needed to break through the gel. quartz sand pack was stronger than that in the glass In comparison with the fine glass beads, the quartz bead pack, so it decreased the viscosity even more when sand was poorly-rounded, irregular and fragmented. The the same gel formula was used in both sand packs, and quartz sand pack had irregular and variable-radius pores therefore the dynamic gelation time of the crosslinked and strong shear fracture action, which increased flow polymer in the quartz sand pack is longer than that in resistance and reduced the flow rate of macromolecular the glass bead pack. polymer but could not reduce the flowability of micromolecular crosslinker. Therefore the crosslinker 3.2 Sealing characteristics was gradually separated from the polymer, which resulted in the variation in the proportion of crosslinker 3.2.1 Residual resistance factor to polymer, thereby impeding the formation of the high­ The relationship between residual resistance factor viscosity weak gel. On the other hand, some polymer F RR and intake volume of subsequent water flood in could move to the deep part of the sand pack (deep part both packs are shown in Fig. 2. of a reservoir) but it was broken by the shear fracture of Fig. 2 shows that the residual resistance factor in quartz sand, so the high-viscosity gel could not be both packs increased sharply at the initial stage of generated although the concentration of crosslinker was subsequent water flood and then tapered off and high here. Furthermore, the weak gel generated near the steadied gradually with the increase in subsequent entrance of the quartz sand pack became tiny gel intake volume, so there was a peak value. The particles and lost its ability to seal the high capacity subsequent intake volumes were 0.5 PV and 0.3 PV channels when they moved further in the sand pack. For respectively in these two packs when the residual these reasons, F RR in the quartz pack was small and the resistance factors reached the peak value, so the flood crossflow of subsequent flood water was strong. water broke through easier in the quartz sand pack. On the other hand, the residual resistance factor in the 3.2.2 Residual resistance factor distribution quartz sand pack was far smaller than that in the glass The sand pack was separated into 15 parts by 14 bead pack, indicating that the shut off capacity of the piezometer orifices. In order to study the shut off weak gel in the quartz sand pack was very weak. capacity of the weak gel in different parts of the sand - . - . - Glass bead pack pack, the residual resistance factor of each part was I Quartz sand pack calculated according to the pressure difference of two :: .... ( 0 adjacent piezometer orifices. The distribution of tl " .... ,,S residual resistance factor along the sand pack was ..... ,,, u 300 I " '" drawn on the coordinates with the x-axis being the ..... en ....... ,-. '0;; length from the entrance of the sand pack to the middle .... " I of the part (L) while the y-axis being the residual "i :s ,'" "0 I'; resistance factor (F RR) of the part. The curves are shown '0;; 0::: " in Figs. 3 and 4. Distribution of F RR in the glass bead pack is shown 0.0 0.3 0.6 0.9 1.2 1.5 in Fig. 3. It is found that F in the pack fluctuated and R R Pore volume injected maintained an average value of about 30 with increasing intake volume of subsequent water flood, so F had a RR Fig. 2 Relationship between F and intake volume of R R relatively even distribution along the glass bead pack, subsequent water flood 58 2007 Petroleum Science indicating that a weak gel with uniform apparent sand pack decreased in varying degrees with increasing viscosity and sealing characteristics was generated in intake volume of subsequent water flood, showing that every part of the sand pack. the weak gel which was generated and sealed the high capacity channels under static condition was able to l'lO move to the deep part of the sand pack; while F RR in 3~ (. -.- 0 5PV other parts of the sand pack did not increase and was even close to zero, indicating that the moving weak gel " 60 could not effectively seal the high capacity channel there again. The pores in the glass bead pack, regular and well '" 20 ~ ,.,. !i-":""--- ~./ interconnected, had only some axial shear action and .§ ,. I ..../.. .~ ••• -1i1 scarcely destroyed the three-dimensional network of the c:::: I._._A o .. I weak gel. Moreover, the weak surface resistance of fine o 50 100 150 200 glass bead allowed the weak gel to slide on the surface, I., em thus increasing the gel's mobility. Therefore, the weak Fig. 3 Distribution of F in the glass bead pack R R gel could move forward and perform in-depth profile modification in the glass bead pack. In contrast to the glass bead pack, the quartz sand -.-O.5PV pack, with irregular shape, variable pore radius, rough ... c • -·-0.7PV surface and a great difference in the radius of throats, had !l 200 ~ - ..-0.9PV strong shear fracture. When the weak gel moved along " • ;; v; the sand pack, this action could destroy the three­ 11~ .. .;; ~ dimensional network of the weak gel and tum the gel into ~\.-~~~~~\ increasingly tiny gel particles with further movement. -€l ... 'if. '.~'.'_'l:_~._ . .:.-!!!!!'i.--1 The rheological property (tested by using a HAAKE 0:: " 0 50 100 150 RS600 rheometer, shown in Fig. 5) of specimens sampled I., ern at the exit of the packs showed that the apparent viscosities of these specimens were lower than that of the Fig.4 Distribution of F in the quartz sand pack RR polymer solution with the same concentration as the crosslinked polymer system and just a little larger than Distribution of F RR in the quartz sand pack is shown the apparent viscosity of simulated formation water, in Fig. 4. It can be found that F converged within 100 RR indicating that the network structure of the weak gel was em from the entrance and was very small in other parts destroyed and the ability to seal the high capacity of the pack. This result indicated that the high-viscosity channels was lost. The gel particles could move deeply weak gel was generated just in the 100 em section from into the sand pack, even reach the exit (equivalent to the the entrance, which made the subsequent flood water to bottom of the producing well in oil field), propelled by flow back to the high capacity channels and decreased the subsequent flood water, and the performance of in­ its swept volume. Moreover, the weak gel near the depth profile modification of the weak gel was entrance consumed most of the displacement energy weakened. and decreased its utilization efficiency. Comparing the results in these two sand packs, the performance of in-depth profile modification of the 3.3 Mobility weak gel was weakened mainly by shear fracture of Fig. 3 illustrates that F RR in the parts near the porous media. entrance (about half of the pack from the entrance) had a trend of decreasing with increasing intake volume of 3.4 Microshape subsequent water flood, especially when the intake A ZEISS optical microscope made in Germany was volume was more than 0.4 PV, while that near the exit used to observe the microshape of the packing materials increased, which demonstrated that the weak gel in the and the gels sampled in the middle of the sand packs glass bead pack could not only move deeply into the respectively, in porous media in order to study and sand pack but also that it could seal the high capacity verify the above-mentioned laboratory findings. When channels again when it reached the deep part. taking photomicrograph, quartz sand or fine glass beads Fig. 4 shows that F near the entrance of the quartz RR VolA No.1 Effects of Shear Fracture on In-depth Profile Modification of Weak Gels 59 of these samples were spread on the rnicroslide, which model. Photomicrographs are shown in Fig. 6. Fig. 6a is increased the radius of intergranular pores, greater than the photomicrograph of a specimen sampled from the the pore radius between sand particles in closely packed glass bead pack and Fig. 6b from the quartz sand pack. ~ Weak gel [0000 --0- Ungcllcd system --&- Sampled when subsequent injection rate is () PV --f- Sampled when subsequent injection rare is 0.5PV -b- Sampled when subsequent injection rate is 1PV --+- Simulated formation water ~------ 10 100 Shear rate, s' Fig. 5 The rheological curves of specimens sampled at the exit of quartz sand pack (a) Sampled from glass bead pack (b) Sampled from quartz sand pack Fig.6 Photomicrographs of weak gel (lOX 10) Fig. 6 shows that the weak gel became gel particles 4. Conclusions after being scoured by flood water in both sand packs, but there were some differences. The equivalent radius 1) Crosslinking reaction of polymer with crossliker of gel particles is far larger than those of pores and occurred in both sand packs under dynamic condition, throats in the glass bead pack, so the gel particles could but the gelation time in the quartz sand pack was seal the pores and throats; while the equivalent radius of slightly longer than that in the glass bead pack. The gel particles in the quartz sand pack was equal to or weak gel became gel particles after being scoured by smaller than the pore radius in the sand pack, moreover, the subsequent flood water in both sand packs. the low strength gel particles could pass through the 2) A weak gel with uniform apparent viscosity and pores and throats, propelled by the subsequent flood sealing characteristics was generated in the glass bead water, but could not seal the flow channels effectively. pack, and FRR in the glass bead pack was larger than that 60 2007 Petroleum Science gels improve oil recovery efficiency. SPEIDOE, 27780 in the quartz sand pack. The weak gel could not only Robb I. D. and Smeulder 1. B. A. F. (1997) The rheological move towards the deep part of the sand pack but also seal properties of weak gels of poly (vinyl alcohol) and sodium the high capacity channels again when it reached the borate. Polymer, 38(9), 2165-2169 deep part, showing that the gel could perform in-depth Tian G. L., Ju Y., Sun G. Y., et al. (1997) Research on shear profile modification in the glass bead pack. behavior and percolation rules of crosslinked polymer. Oil & 3) In the quartz sand pack, the weak gel was localized Gas Recovery Technology, 4(4), 19-24 (in Chinese) at the first half of the pack next to the entrance and still Wang P. M., Luo 1. H., Li Y. X., et al. (2000) The study of the could move deeply into the sand pack propelled by the characteristics of weak gel in the core test. Oil Drilling & subsequent flood water, but it became tiny gel particles and Production Technology, 22(5), 48-50 (in Chinese) could not effectively seal the high capacity channels when Weng R. and Wei L. (1998) The effect of shear rate and shear it reached the deep part. The in-depth profile modification time on jelling behavior of weak gel formed with solution of of the weak gel was very weak in the quartz sand pack. a low polymer concentration. Petroleum Exploration and Development, 25(5), 65-67 (in Chinese) 4) It was the shear fracture of porous media that mainly Xu G. L., Lin M. Q., Li M. Y., et al. (2001) Plugging performance affected the properties and weakened the performance of of linked polymer solutions. Journal of the University of in-depth profile modification of the weak gel. Petroleum, China, 25(4), 85-87 (in Chinese) Zhu H. 1., Liu Y. Z., Fan Z. H., et al. (2002) The impact of Acknowledgement dynamic process on the gelation mechanism of a HPAM­ This work was supported by the National Key phenol-aldehyde system. Petroleum Exploration and Technologies R&D Program in the 10th Five-year Plan Development, 29(6), 84-86 (in Chinese) of China (No. 2003BA613A-07-05) About the first author References Li Xianjie was born in 1979 and Chen T. L., Zhang L. H. and Zhou 1. M. (1998) Study of the received a M.S degree in oil and gas colloidal dispersion gel and its characteristics of rheology and development engineering from China seepage. Oilfield Chemistry, 15(3),265-268,277 (in Chinese) University of Petroleum (Beijing) in Li L. X., Liu Y. Z., Han M., et at. (2000a) The effect of flow shearing 2005. Now he is a Ph.D student of the on weak gel formation from aqueous polymer/crosslinker gelling oil and gas development engineering, solutions. Oilfield Chemistry, 17(2),148-151 (in Chinese) China University of Petroleum (Beijing), Li M. Y., Lin M. Q. and Zheng X. Y. (2000b) Linked polymer with his research interests in enhanced solution as in-depth permeability control agent: Laboratory study oil recovery and oil chemistry. E-mail: and field test. Oilfield Chemistry, 17(2), 144-147 (in Chinese) upclxjie@126.com Lin M. Q., Luo X. H., Dong Z. X., et at. (2004) The influence of shear action on plugging performance of linked polymer solution. Acta Petro lei Sinica (Petroleum Processing Section), (Received June 5. 2006) 20(5),48-52 (in Chinese) (Edited by Sun Yanhua) Mack J. C. and Smith J. E. (1994) In-depth colloidal dispersion

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Published: Apr 14, 2010

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