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Mechanical behaviors of cement systems in different conditions
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Available online at www.sciencedirect.com
ScienceDirect Natural Gas Industry B xx (2017) 1e5 www.elsevier.com/locate/ngib
Research Article
Mechanical behaviors of cement systems in different conditions Zheng Youzhi a,b,*, Xu Bingqing c, Pu Junhong d, Mu Naiqu e, Wang Bin a,b, Li Ming f a Engineering Technology Research Institute, PetroChina Southwest Oil & Gas Field Company, Guanghan, Sichuan 618300, China Engineering Technology Research Institute, National Energy High Sulfur Exploitation R&D Center, Guanghan, Sichuan 618300, China c Engineering Division, PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China d Northeast Sichuan Division, PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China e Sichuan Changning Natural Gas Development Ltd., PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China f School of Material Science and Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China b
Received 4 November 2016; accepted 25 January 2017
Abstract At present, the mechanical testing on the ductility of cement sheath is faced with multiple technical difficulties. In this paper, numerical simulation and laboratory evaluation were adopted to compare the mechanical performance of typical elastic, flexible, ductile and neat cement systems at home and abroad by different loading rates, value range of stressestrain curve, confining pressure and temperature. It is shown that when the loading rate is lower, the stressestrain curve of set cement is not smooth, but distorted, and the stress cannot be responded in time; that when the loading rate is higher, the stressestrain curve of set cement is smooth, and the yield stage is remarkable, indicating that higher loading rate can reflect the yield strain behavior of set cement more truly; that the recommended Young's modulus range of set cement should be changed based on the actual downhole conditions; that temperature has more effect on yield stress and ultimate strain, but less effect on elastic modulus; that confining pressure has more effect on ultimate stress and ultimate strain, but less effect on elastic modulus; and that when the confining pressure is lower and temperature is higher, the yield stage of set cement is more remarkable. It is concluded that these mechanical behaviors of cement system in different conditions provide a technical support for understanding the mechanical essence of downhole cement sheath and exploring the mechanical integrity of cement sheath and even that of the whole wellbore. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Well cementing; Set cement; Mechanical testing on ductility of cement sheath; Loading rate; Stressestrain; Confining pressure; Temperature
Some high-temperature deep wells in the Kela 2 gasfield of the Tarim Basin and Luojiazhai and Anyue gasfields of the Sichuan Basin have various abnormal pressures in annulus, impairing their high and stable production. Among the sustained casing pressure issues in sulfur-containing gas wells caused by cement sheath quality, the corrosion of acid media to cement sheath is just a secondary factor, while mechanical damage of cement sheath during engineering operation is the
* Corresponding author. Engineering Technology Research Institute, PetroChina Southwest Oil & Gas Field Company, Guanghan, Sichuan 618300, China. E-mail address: [email protected] (Zheng Y.). Peer review under responsibility of Sichuan Petroleum Administration.
major factor causing gas channeling through cement sheath [1e6]. In fact, the sealing of cement sheath under varying wellbore pressure over a long period (for example, before and after pressure test, continuous drilling, shut-in and production) is involved in the development process of each oil and gas well. In this circumstance, the concept of ductile cement is advanced. Keeping the cement sheath in good ductility can give the cement sheath better elastic transformation behavior under the extrusion of casing and the compression of formation, and thus the micro-annulus won't occur at the contact of cement sheath under all kinds of mechanical effects, prolonging the long term sealing capacity of cement sheath, which is of great significance to the wellbore safety and the
http://dx.doi.org/10.1016/j.ngib.2017.07.022 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Zheng Y. et al. / Natural Gas Industry B xx (2017) 1e5
prolonging of oil and gas well life cycle. Ductile cement system is a mainstream direction of cementing development in the future, but elasticity property evaluation method and standard of ductile cement haven't established yet in the world, and the mechanical essence of cement sheath downhole hasn't been understood objectively and realistically, so it is necessary to investigate the mechanical behaviors of oil well cement under different conditions [7e11]. Since there are some technical problems in the mechanical performance test of cement sheath under downhole conditions, numerical simulation and lab evaluation were adopted in this study to find out the mechanical performance of typical elastic, flexible, ductile and neat cement systems (cement system in use in oil and gas fields in the SichuaneChongqing region) under different loading speeds, different value ranges of stressestrain curve, and different confining pressures and temperatures, to provide an important reference for recognizing the mechanical essence of cement sheath downhole and provide a technical support for research on the integrity of the cement sheath and even the whole wellbore [12e16]. 1. Mechanical performance of set cement under different conditions Cement slurry was prepared and set cement was cured according to the oil well cement test method stipulated in GB 19139-2012. The mechanical performance of neat cement, foreign elastic cement, domestic ductile cement and flexible self-stressing cement were tested. Cement slurry test devices used in the experiment included a Waring mixer, an atmospheric thickener, an HTHP filtration instrument and a double-reactor HTHP thickener made in China. The instrument used in the set cement triaxial stress test was GCTS-RTR-1000 rock mechanics system imported from the U.S.A. 1.1. Effect of different loading rates on the mechanical performance of set cement Mechanical loading rate is one of the major factors affecting cement sheath in operations after cementation. In lab testing, keeping the loading rate in the triaxial stress test device consistent with the actual loading rate of cement sheath
under some working conditions can reflect the actual state of cement sheath more truly. The loading rate under pressure test condition calculated by the analysis software for formation casing cement sheath programmed by the Engineering Research Institute of PetroChina Southwest Oilfield Company (Software Registration No. 0910640) was 1.6 kN/min. The stressestrain curves of foreign elastic cement (Fig. 1-a), domestic ductile cement (Fig. 1-b) and flexible self-stressing cement (Fig. 1-c) at the loading rates of 0.5 kN/min, 1.6 kN/min and 5 kN/min (curing conditions: 119 C 20.7 MPa 7 d, density: 2.10 g/cm3) were tested. It can be seen from Figs. 1e3 directly that under different loading rates, the stressestrain curves of domestic ductile cement almost coincide (Fig. 3), indicating that loading rate has little effect on the mechanical performance of this cement system; that under lower loading rates, the stressestrain curves of the three kinds of set cement distort to some extent and are not smooth, indicating no in-time response to the stress; and that under higher loading rates, the stressestrain curves of the three kinds of set cement are smooth and have a distinctive yield stage, indicating that higher loading rates can reflect the yield strain of set cement more truly. 1.2. Effect of different stressestrain value ranges on Young's modulus of set cement As the value range of stressestrain curve varies, the Young's modulus of set cement varies. How to regulate the value range of stressestrain curve to get the true and unique Young's modulus is a key issue in evaluating the elastic performance of set cement. The value ranges of Young's modulus of neat cement (curing condition: 119 C 20.7 MPa 7 d, density: 1.90 g/cm3) and foreign elastic cement were compared through experiment, and the value ranges of neat cement Young's modulus were taken as the reference standard. Fig. 2 shows the stressestrain curves of three parallel samples of foreign elastic cement, and Fig. 3 show the stressestrain curves of two parallel samples of neat cement. It can be seen from Figs. 2 and 3 that all samples have short elastic sections. With the increase of pressure, the plastic deformation accelerates. Stressestrain curves of all samples show an obvious creeping feature. Table 1 shows the elastic
Fig. 1. Effect of loading rates on cement sheath mechanical performance. Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Zheng Y. et al. / Natural Gas Industry B xx (2017) 1e5
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Fig. 2. Tested stressestrain curves of foreign elastic cement.
Fig. 3. Tested stressestrain curves of neat cement. Table 1 Elastic section data of foreign elastic set cement samples. Samples
Elastic section/MPa
Value range of elastic section
Sample 01 Sample 02 Sample 03 The recommended
21e40 21e32 21e54
24e45% 27e42% 32e80% 32e42%
sections and value ranges of three foreign elastic cement samples. Table 2 shows the elastic sections and value ranges of two neat cement samples. It can be seen from Table 1 that with more elastic material, the foreign elastic cement samples have large errors from preparation, so their value ranges of elastic sections differ fairly widely. The recommended value range of elastic section should cover all the tested samples, and therefore is taken to be between 32% and 42%. It can be seen from Table 2 that with simple components, the two neat cement samples are basically the same in their value ranges of elastic sections. Therefore, the value range of elastic section for neat cement is recommended between 35% and 50%. To ensure the repeatability and accuracy of Young's modulus data, it is necessary to keep the value ranges taken Table 2 Elastic section data of 2 neat cement samples. Samples
Elastic section/MPa
Value range of elastic section
Sample 01 Sample 02 The recommended
21e30 21e30
35e50% 36e51% 35e50%
uniform. In the recommendation of value ranges, a reference should be chosen, and the neat cement, with simple component, is the most referential standard sample. Moreover, set cement has different mechanical performances under different downhole conditions, so the recommended value range of Young's modulus should vary according to the actual downhole conditions. 1.3. Effects of temperature and confining pressure on the mechanical performance of set cement The downhole conditions (temperature and pressure) which the cement sheath of an oil well is exposed to change with different engineering operations, like ongoing drilling, pressure test, and stimulation. It is of great significance in evaluating the mechanical integrity of cement sheath to find out the variation patterns of cement sheath mechanical performance under varying temperatures and confining pressures. The mechanical performance of foreign elastic cement (Fig. 4a), flexible self-stressing cement (Fig. 1-b) and neat cement (Fig. 1-c) were tested at 90 C, 119 C, 130 C and at confining pressure of 20.7 MPa, and 10 MPa, 20.7 MPa, 30 MPa and 119 C respectively. Fig. 4 shows the stressestrain curves of the three kinds of cement under different temperatures. It can be seen from Fig. 4 that temperature has a stronger effect on yield stress and ultimate strain, but little effect on elastic modulus. Moreover, under higher temperatures, the yield stage of set cement is more obvious. Fig. 5 shows the stressestrain curves of the three kinds of cement under different confining pressures. It can be seen that
Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Fig. 4. Stressestrain curves of the three kinds of cement under different temperatures.
Fig. 5. Stressestrain of the three kinds of cement under different confining pressures.
confining pressure has a stronger effect on ultimate stress and ultimate strain, but a smaller effect on elastic modulus. Moreover, under lower confining pressures, the yield stage of set cement is more obvious. 2. Conclusions and suggestions In consideration of the issues in mechanical property test of set cement, the mechanical behaviors of cement systems used in oil and gas fields in the SichuaneChongqing region have been explored, and the following conclusions and suggestions have been reached. 1) Under lower loading rates, the stressestrain curve of set cement is distorted and not smooth, indicating that under lower loading rates, set cement can't respond to stress in time. Under higher loading rates, the stressestrain curve of set cement is smooth with an apparent yield stage, indicating that higher loading rates can reflect the yield strain of set cement truly. 2) The recommended value ranges of Young's modulus of oil well set cement should vary with the downhole conditions. 3) Temperature has a stronger effect on yield stress and ultimate strain, but a smaller effect on elastic modulus; confining pressure has a stronger effect on ultimate stress and ultimate strain, but a smaller effect on elastic modulus; under lower confining pressures and higher temperatures, the yield stage of set cement is more obvious.
4) It is recommended to carry out further mechanical property test of oil well cement sheath under actual downhole conditions, to set up an evaluation standard for the elastic mechanical performance of set cement, provide a strong technical support for the toughness improvement of cement sheath, and lay a foundation for the solution to sustained annulus pressure, a problem of world class. References [1] Zheng Youzhi, She Chaoyi, Yao Kunquan, Ma Faming, Wang Xin, Tang Geng, et al. Corrosion mechanism analysis of cement sheath interfaces of sour gas wells in Sichuan and Chongqing areas. Nat Gas Ind 2011;31(12):85e9. [2] Yan Siming, Dai Zhenzhen, Pei Guibin, Tie Qiang, Wu Zongguo. An analysis of CO2 gas corrosion on the set cement of gas wells. Nat Gas Ind 2010;30(9):55e9. [3] Guo Xiaoyang, Gu Tao, Li Zaoyuan, Cheng Xiaowei, Zheng Youzhi. Corrosion mechanism of hydrogen sulfide on well cement under water wet environments. Nat Gas Ind 2015;35(10):93e8. [4] Lu Yafeng, Zheng Youzhi, She Chaoyi, Tang Geng, Liu Xiangkang, Guo Jianhua, et al. Analysis of cement-sheath mechanical integrity based on the experiment data of cement paste. Nat Gas Ind 2013;33(5):77e81. [5] Zheng Youzhi, Liu wei, Yu Caijun, Ma Faming, Tang Geng, Xie Minghua, et al. Application of rotating-casing cementing technology in the well LG-A of the LG gas field. Nat Gas Ind 2010;30(4):74e6. [6] Li Ming, Yang Yujia, Zhang Guanhua, Zheng Youzhi, Cheng Xiaowei, Guo Xiaoyang. Impact of a biological tackifier in a drilling fluid system on the performance and structure of a cement slurry. Nat Gas Ind 2014;34(9):93e8. [7] Wang Yi, Chen Dajun, Yu Zhiyong, Qi Zhigang. Experimental assessment of the mechanical performance of the set cement for horizontal wells. Nat Gas Ind 2012;32(10):63e6.
Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Zheng Y. et al. / Natural Gas Industry B xx (2017) 1e5 [8] Fang Zhiyi, Wu Shirong, Yao Kunquan, Li Zaoyuan, Guo Xiaoyang, Shi Qing. Application of three-stage cementing slurry system in deep and ultradeep wells. Nat Gas Ind 2008;28(7):58e9. [9] Li Zaoyuan, Guo Xiaoyang, Yang Yuanguang. Plasticity improving of set cement and in-house evaluation method. Nat Gas Ind 2004;24(2):55e8. [10] Li Zaoyuan, Guo Xiaoyang, Han Lin, Luo Pingya. Deformation behavior of oil-well set cement under confining pressure. Nat Gas Ind 2007;27(9):62e4. [11] Li Ming, Yang Yujia, Jin Jianzhou, Yu Yongjin, Guo Xiaoyang. Reinforced mechanical performance and mechanism of well-cementing stones by brucite fiber. Nat Gas Ind 2015;35(6):82e6. [12] Li Ming, Qi Fengzhong, Jin Jianzhou, Yu Yongjin. Development of light weight cement slurry technology for CBM gas wells. Nat Gas Ind 2011;31(7):63e6.
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[13] Bu Yuhuan, Wang Ruihe, Mu Haipeng. Compressive strength model for fiber cement. Nat Gas Ind 2007;27(9):56e8. [14] Chen Dajun, Liao Gang, Li Fude, Cao Zhi. Research and application of plastic cement slurry system of deep well in Sichuan. Nat Gas Ind 2001;21(3):36e8. [15] Gu Jun, Gao Xingyuan, Liu Hong. Two-interface isolation system of oil/ gas well cementing and its damage model. Nat Gas Ind 2006;26(7):74e6. [16] Zhang Junya, Xia Boru, Cao Yongbin, Li Hongqian, Zhao Shengying. Handling of Ø177.8 mm liner sticking occurred when cementing in Rabaa-1 well. Oil Drill Prod Technol 2010;32(4):33e6.
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Available online at www.sciencedirect.com
ScienceDirect Natural Gas Industry B xx (2017) 1e5 www.elsevier.com/locate/ngib
Research Article
Mechanical behaviors of cement systems in different conditions Zheng Youzhi a,b,*, Xu Bingqing c, Pu Junhong d, Mu Naiqu e, Wang Bin a,b, Li Ming f a Engineering Technology Research Institute, PetroChina Southwest Oil & Gas Field Company, Guanghan, Sichuan 618300, China Engineering Technology Research Institute, National Energy High Sulfur Exploitation R&D Center, Guanghan, Sichuan 618300, China c Engineering Division, PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China d Northeast Sichuan Division, PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China e Sichuan Changning Natural Gas Development Ltd., PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China f School of Material Science and Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China b
Received 4 November 2016; accepted 25 January 2017
Abstract At present, the mechanical testing on the ductility of cement sheath is faced with multiple technical difficulties. In this paper, numerical simulation and laboratory evaluation were adopted to compare the mechanical performance of typical elastic, flexible, ductile and neat cement systems at home and abroad by different loading rates, value range of stressestrain curve, confining pressure and temperature. It is shown that when the loading rate is lower, the stressestrain curve of set cement is not smooth, but distorted, and the stress cannot be responded in time; that when the loading rate is higher, the stressestrain curve of set cement is smooth, and the yield stage is remarkable, indicating that higher loading rate can reflect the yield strain behavior of set cement more truly; that the recommended Young's modulus range of set cement should be changed based on the actual downhole conditions; that temperature has more effect on yield stress and ultimate strain, but less effect on elastic modulus; that confining pressure has more effect on ultimate stress and ultimate strain, but less effect on elastic modulus; and that when the confining pressure is lower and temperature is higher, the yield stage of set cement is more remarkable. It is concluded that these mechanical behaviors of cement system in different conditions provide a technical support for understanding the mechanical essence of downhole cement sheath and exploring the mechanical integrity of cement sheath and even that of the whole wellbore. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Well cementing; Set cement; Mechanical testing on ductility of cement sheath; Loading rate; Stressestrain; Confining pressure; Temperature
Some high-temperature deep wells in the Kela 2 gasfield of the Tarim Basin and Luojiazhai and Anyue gasfields of the Sichuan Basin have various abnormal pressures in annulus, impairing their high and stable production. Among the sustained casing pressure issues in sulfur-containing gas wells caused by cement sheath quality, the corrosion of acid media to cement sheath is just a secondary factor, while mechanical damage of cement sheath during engineering operation is the
* Corresponding author. Engineering Technology Research Institute, PetroChina Southwest Oil & Gas Field Company, Guanghan, Sichuan 618300, China. E-mail address: [email protected] (Zheng Y.). Peer review under responsibility of Sichuan Petroleum Administration.
major factor causing gas channeling through cement sheath [1e6]. In fact, the sealing of cement sheath under varying wellbore pressure over a long period (for example, before and after pressure test, continuous drilling, shut-in and production) is involved in the development process of each oil and gas well. In this circumstance, the concept of ductile cement is advanced. Keeping the cement sheath in good ductility can give the cement sheath better elastic transformation behavior under the extrusion of casing and the compression of formation, and thus the micro-annulus won't occur at the contact of cement sheath under all kinds of mechanical effects, prolonging the long term sealing capacity of cement sheath, which is of great significance to the wellbore safety and the
http://dx.doi.org/10.1016/j.ngib.2017.07.022 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Zheng Y. et al. / Natural Gas Industry B xx (2017) 1e5
prolonging of oil and gas well life cycle. Ductile cement system is a mainstream direction of cementing development in the future, but elasticity property evaluation method and standard of ductile cement haven't established yet in the world, and the mechanical essence of cement sheath downhole hasn't been understood objectively and realistically, so it is necessary to investigate the mechanical behaviors of oil well cement under different conditions [7e11]. Since there are some technical problems in the mechanical performance test of cement sheath under downhole conditions, numerical simulation and lab evaluation were adopted in this study to find out the mechanical performance of typical elastic, flexible, ductile and neat cement systems (cement system in use in oil and gas fields in the SichuaneChongqing region) under different loading speeds, different value ranges of stressestrain curve, and different confining pressures and temperatures, to provide an important reference for recognizing the mechanical essence of cement sheath downhole and provide a technical support for research on the integrity of the cement sheath and even the whole wellbore [12e16]. 1. Mechanical performance of set cement under different conditions Cement slurry was prepared and set cement was cured according to the oil well cement test method stipulated in GB 19139-2012. The mechanical performance of neat cement, foreign elastic cement, domestic ductile cement and flexible self-stressing cement were tested. Cement slurry test devices used in the experiment included a Waring mixer, an atmospheric thickener, an HTHP filtration instrument and a double-reactor HTHP thickener made in China. The instrument used in the set cement triaxial stress test was GCTS-RTR-1000 rock mechanics system imported from the U.S.A. 1.1. Effect of different loading rates on the mechanical performance of set cement Mechanical loading rate is one of the major factors affecting cement sheath in operations after cementation. In lab testing, keeping the loading rate in the triaxial stress test device consistent with the actual loading rate of cement sheath
under some working conditions can reflect the actual state of cement sheath more truly. The loading rate under pressure test condition calculated by the analysis software for formation casing cement sheath programmed by the Engineering Research Institute of PetroChina Southwest Oilfield Company (Software Registration No. 0910640) was 1.6 kN/min. The stressestrain curves of foreign elastic cement (Fig. 1-a), domestic ductile cement (Fig. 1-b) and flexible self-stressing cement (Fig. 1-c) at the loading rates of 0.5 kN/min, 1.6 kN/min and 5 kN/min (curing conditions: 119 C 20.7 MPa 7 d, density: 2.10 g/cm3) were tested. It can be seen from Figs. 1e3 directly that under different loading rates, the stressestrain curves of domestic ductile cement almost coincide (Fig. 3), indicating that loading rate has little effect on the mechanical performance of this cement system; that under lower loading rates, the stressestrain curves of the three kinds of set cement distort to some extent and are not smooth, indicating no in-time response to the stress; and that under higher loading rates, the stressestrain curves of the three kinds of set cement are smooth and have a distinctive yield stage, indicating that higher loading rates can reflect the yield strain of set cement more truly. 1.2. Effect of different stressestrain value ranges on Young's modulus of set cement As the value range of stressestrain curve varies, the Young's modulus of set cement varies. How to regulate the value range of stressestrain curve to get the true and unique Young's modulus is a key issue in evaluating the elastic performance of set cement. The value ranges of Young's modulus of neat cement (curing condition: 119 C 20.7 MPa 7 d, density: 1.90 g/cm3) and foreign elastic cement were compared through experiment, and the value ranges of neat cement Young's modulus were taken as the reference standard. Fig. 2 shows the stressestrain curves of three parallel samples of foreign elastic cement, and Fig. 3 show the stressestrain curves of two parallel samples of neat cement. It can be seen from Figs. 2 and 3 that all samples have short elastic sections. With the increase of pressure, the plastic deformation accelerates. Stressestrain curves of all samples show an obvious creeping feature. Table 1 shows the elastic
Fig. 1. Effect of loading rates on cement sheath mechanical performance. Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Zheng Y. et al. / Natural Gas Industry B xx (2017) 1e5
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Fig. 2. Tested stressestrain curves of foreign elastic cement.
Fig. 3. Tested stressestrain curves of neat cement. Table 1 Elastic section data of foreign elastic set cement samples. Samples
Elastic section/MPa
Value range of elastic section
Sample 01 Sample 02 Sample 03 The recommended
21e40 21e32 21e54
24e45% 27e42% 32e80% 32e42%
sections and value ranges of three foreign elastic cement samples. Table 2 shows the elastic sections and value ranges of two neat cement samples. It can be seen from Table 1 that with more elastic material, the foreign elastic cement samples have large errors from preparation, so their value ranges of elastic sections differ fairly widely. The recommended value range of elastic section should cover all the tested samples, and therefore is taken to be between 32% and 42%. It can be seen from Table 2 that with simple components, the two neat cement samples are basically the same in their value ranges of elastic sections. Therefore, the value range of elastic section for neat cement is recommended between 35% and 50%. To ensure the repeatability and accuracy of Young's modulus data, it is necessary to keep the value ranges taken Table 2 Elastic section data of 2 neat cement samples. Samples
Elastic section/MPa
Value range of elastic section
Sample 01 Sample 02 The recommended
21e30 21e30
35e50% 36e51% 35e50%
uniform. In the recommendation of value ranges, a reference should be chosen, and the neat cement, with simple component, is the most referential standard sample. Moreover, set cement has different mechanical performances under different downhole conditions, so the recommended value range of Young's modulus should vary according to the actual downhole conditions. 1.3. Effects of temperature and confining pressure on the mechanical performance of set cement The downhole conditions (temperature and pressure) which the cement sheath of an oil well is exposed to change with different engineering operations, like ongoing drilling, pressure test, and stimulation. It is of great significance in evaluating the mechanical integrity of cement sheath to find out the variation patterns of cement sheath mechanical performance under varying temperatures and confining pressures. The mechanical performance of foreign elastic cement (Fig. 4a), flexible self-stressing cement (Fig. 1-b) and neat cement (Fig. 1-c) were tested at 90 C, 119 C, 130 C and at confining pressure of 20.7 MPa, and 10 MPa, 20.7 MPa, 30 MPa and 119 C respectively. Fig. 4 shows the stressestrain curves of the three kinds of cement under different temperatures. It can be seen from Fig. 4 that temperature has a stronger effect on yield stress and ultimate strain, but little effect on elastic modulus. Moreover, under higher temperatures, the yield stage of set cement is more obvious. Fig. 5 shows the stressestrain curves of the three kinds of cement under different confining pressures. It can be seen that
Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Fig. 4. Stressestrain curves of the three kinds of cement under different temperatures.
Fig. 5. Stressestrain of the three kinds of cement under different confining pressures.
confining pressure has a stronger effect on ultimate stress and ultimate strain, but a smaller effect on elastic modulus. Moreover, under lower confining pressures, the yield stage of set cement is more obvious. 2. Conclusions and suggestions In consideration of the issues in mechanical property test of set cement, the mechanical behaviors of cement systems used in oil and gas fields in the SichuaneChongqing region have been explored, and the following conclusions and suggestions have been reached. 1) Under lower loading rates, the stressestrain curve of set cement is distorted and not smooth, indicating that under lower loading rates, set cement can't respond to stress in time. Under higher loading rates, the stressestrain curve of set cement is smooth with an apparent yield stage, indicating that higher loading rates can reflect the yield strain of set cement truly. 2) The recommended value ranges of Young's modulus of oil well set cement should vary with the downhole conditions. 3) Temperature has a stronger effect on yield stress and ultimate strain, but a smaller effect on elastic modulus; confining pressure has a stronger effect on ultimate stress and ultimate strain, but a smaller effect on elastic modulus; under lower confining pressures and higher temperatures, the yield stage of set cement is more obvious.
4) It is recommended to carry out further mechanical property test of oil well cement sheath under actual downhole conditions, to set up an evaluation standard for the elastic mechanical performance of set cement, provide a strong technical support for the toughness improvement of cement sheath, and lay a foundation for the solution to sustained annulus pressure, a problem of world class. References [1] Zheng Youzhi, She Chaoyi, Yao Kunquan, Ma Faming, Wang Xin, Tang Geng, et al. Corrosion mechanism analysis of cement sheath interfaces of sour gas wells in Sichuan and Chongqing areas. Nat Gas Ind 2011;31(12):85e9. [2] Yan Siming, Dai Zhenzhen, Pei Guibin, Tie Qiang, Wu Zongguo. An analysis of CO2 gas corrosion on the set cement of gas wells. Nat Gas Ind 2010;30(9):55e9. [3] Guo Xiaoyang, Gu Tao, Li Zaoyuan, Cheng Xiaowei, Zheng Youzhi. Corrosion mechanism of hydrogen sulfide on well cement under water wet environments. Nat Gas Ind 2015;35(10):93e8. [4] Lu Yafeng, Zheng Youzhi, She Chaoyi, Tang Geng, Liu Xiangkang, Guo Jianhua, et al. Analysis of cement-sheath mechanical integrity based on the experiment data of cement paste. Nat Gas Ind 2013;33(5):77e81. [5] Zheng Youzhi, Liu wei, Yu Caijun, Ma Faming, Tang Geng, Xie Minghua, et al. Application of rotating-casing cementing technology in the well LG-A of the LG gas field. Nat Gas Ind 2010;30(4):74e6. [6] Li Ming, Yang Yujia, Zhang Guanhua, Zheng Youzhi, Cheng Xiaowei, Guo Xiaoyang. Impact of a biological tackifier in a drilling fluid system on the performance and structure of a cement slurry. Nat Gas Ind 2014;34(9):93e8. [7] Wang Yi, Chen Dajun, Yu Zhiyong, Qi Zhigang. Experimental assessment of the mechanical performance of the set cement for horizontal wells. Nat Gas Ind 2012;32(10):63e6.
Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022
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Zheng Y. et al. / Natural Gas Industry B xx (2017) 1e5 [8] Fang Zhiyi, Wu Shirong, Yao Kunquan, Li Zaoyuan, Guo Xiaoyang, Shi Qing. Application of three-stage cementing slurry system in deep and ultradeep wells. Nat Gas Ind 2008;28(7):58e9. [9] Li Zaoyuan, Guo Xiaoyang, Yang Yuanguang. Plasticity improving of set cement and in-house evaluation method. Nat Gas Ind 2004;24(2):55e8. [10] Li Zaoyuan, Guo Xiaoyang, Han Lin, Luo Pingya. Deformation behavior of oil-well set cement under confining pressure. Nat Gas Ind 2007;27(9):62e4. [11] Li Ming, Yang Yujia, Jin Jianzhou, Yu Yongjin, Guo Xiaoyang. Reinforced mechanical performance and mechanism of well-cementing stones by brucite fiber. Nat Gas Ind 2015;35(6):82e6. [12] Li Ming, Qi Fengzhong, Jin Jianzhou, Yu Yongjin. Development of light weight cement slurry technology for CBM gas wells. Nat Gas Ind 2011;31(7):63e6.
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[13] Bu Yuhuan, Wang Ruihe, Mu Haipeng. Compressive strength model for fiber cement. Nat Gas Ind 2007;27(9):56e8. [14] Chen Dajun, Liao Gang, Li Fude, Cao Zhi. Research and application of plastic cement slurry system of deep well in Sichuan. Nat Gas Ind 2001;21(3):36e8. [15] Gu Jun, Gao Xingyuan, Liu Hong. Two-interface isolation system of oil/ gas well cementing and its damage model. Nat Gas Ind 2006;26(7):74e6. [16] Zhang Junya, Xia Boru, Cao Yongbin, Li Hongqian, Zhao Shengying. Handling of Ø177.8 mm liner sticking occurred when cementing in Rabaa-1 well. Oil Drill Prod Technol 2010;32(4):33e6.
Please cite this article in press as: Zheng Y, et al., Mechanical behaviors of cement systems in different conditions, Natural Gas Industry B (2017), http:// dx.doi.org/10.1016/j.ngib.2017.07.022