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Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin
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Available online at www.sciencedirect.com
ScienceDirect Natural Gas Industry B xx (2017) 1e4 www.elsevier.com/locate/ngib
Research Article
Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin* Wu Furong a,*, Yan Yuanyuan b, Yin Chen a a
Geophysical Exploration Company, CNPC Chuanqing Drilling Engineering Co., Ltd., Chengdu, Sichuan 610213, China b School of Science, China University of Geosciences (Beijing), Beijing 100083, China Received 16 July 2016; accepted 25 November 2016
Abstract Zipper hydraulic fracturing in multiple wells with long horizontal sections is a primary solution means to increase the shale gas production rate and efficiency and to reduce the cost in Southern Sichuan Basin. Microseismic based fracturing monitoring can be used for real-time imaging of hydraulic fractures, so it has been widely used to evaluate the fracturing effect of shale gas reservoirs and to direct the optimization and adjustment of fracturing parameters. In China, however, the microseismic fracturing monitoring on fracturing of shale gas reservoirs cannot be used to evaluate the fracturing results until the fracturing operation in the pad wells is completed according to the parameters which are designed prior to the fracturing monitoring. Its evaluation results can merely provide a guidance for the fracturing parameters of the next pad wells instead of the wells in operation. As a result, the real-time effect of microseismic fracturing monitoring is out of work. In view of this, the fractures induced by zipper hydraulic fracturing in multiple shale gas wells with long horizontal sections in the southern Sichuan Basin, was realtime imaged by using the combined technology of radially arranged microseismic surface monitoring and microseismic well monitoring on the basis of real-time positioning method. The fracturing results were assessed and used in real time for the optimization of prepad fluid parameter, perforation and temporary plugging additive releasing time, so as to effectively avoid repeated fracturing and uneven fracturing effects and improve fracturing stimulation effects. This method is applied in two well groups. It is shown that the average shale gas production rate is increased by 2e5 times. Furthermore, microseismic fracturing real-time monitoring plays a vital role in real-time evaluation of fracturing effect and real-time optimization of fracturing parameters, so it can be used as the reference and should be popularized further. © 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: Shale gas; Microseismic; Real-time monitoring; Horizontal well; Zipper fracturing; Well monitoring; Surface monitoring; Production increase; Southern Sichuan Basin
The Lower Silurian Longmaxi Formation in southern Sichuan Basin is rich in shale gas resources [1e3], but shale gas exploration and production is subject to complex topographic conditions (with elevation difference of 400 m), inconvenient transportation, dense population and inefficient technologies. For the promotion of shale gas exploration and * Supported by the National Science and Technology Major Project “Real-time hydraulic fracturing monitoring and integrated geologic-engineering evaluation” (Grant No. 2016ZX05023004). * Corresponding author. E-mail address: [email protected] (Wu FR.). Peer review under responsibility of Sichuan Petroleum Administration.
production, zipper fracturing in several wells with long horizontal sections was introduced from North America [4e6]. The longest horizontal section in this prospect reached 2000 m. In view of little knowledge about fracturing parameters and results because this technique is still in its infancy in China, it is necessary to conduct microseismic monitoring in real-time evaluation and parameter optimization. But microseismic data were usually used for post-fracturing assessment after the operation was fulfilled with pre-fracturing design parameters [7e9]; the results of assessment may only be used for the operation in the next platform well. Few wells were monitored in real time by hydraulically induced microseism.
http://dx.doi.org/10.1016/j.ngib.2017.07.010 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: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
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Wu FR. et al. / Natural Gas Industry B xx (2017) 1e4
On the other hand, microseismic monitoring meets challenges in multi-well zipper fracturing. Microseismic events may be acquired at the land surface or in a borehole [10e12]. A borehole microseismic survey has high signal-to-noise ratio. But due to the limitations of recording geometry and azimuth, there are no observation wells with appropriate spacing for monitoring 3e4 wells in zipper fracturing. Thus a well not included in synchronous fracturing is used as the observation well (equipped with geophones) and consequently cannot be monitored by borehole survey. In a borehole survey, vertical positioning is more accurate than lateral positioning. A surface microseismic survey is not constrained by azimuth or observation wells but tends to be affected by topographic conditions, especially in a mountainous region with a vertical detection distance of 3500 m. Thus a surface observation suffers from intense signal attenuation, weak energy recorded and intractable velocity modeling. In a surface survey, lateral positioning is more accurate than vertical positioning [13e16]. In this paper, we use a radially-oriented surface survey combined with borehole survey to image hydraulic fractures and optimize fracturing parameters in real time. 1. Microseismic data acquisition and processing 1.1. Data acquisition In the radially-oriented surface microseismic survey, 8e20 survey lines with 1000e3000 groups of geophones were laid around the wellhead at the center; group interval is 20 m. The borehole survey was equipped with 40-level 3-component geophones placed close to the sections to be fractured; geophone spacing is 15 m (Fig. 1). 1.2. Data processing GeoMonitor, a self-developed system for microseismic monitoring, was used for real-time positioning. (1) Build an interval velocity model. The initial velocity model was built with sonic log derived velocity and seismic interpreted horizons and then corrected by joint surfaceeborehole positioning
of perforation signals. The velocity model was finally accepted when the error of perforation positioning was less than the predefined value. (2) Conduct preprocessing. This included static correction and noise reduction for surface data and noise reduction, vector rotation, and automatic microseism picking for borehole data [17e19]. (3) Conduct real-time positioning [20]. A joint positioning method through automatic searching across 3D grids was employed to position microseismic events in real time. The delay time of real-time positioning was less than 10 s (4) Display microseismic events in 3D space. The positioned microseismic events in the 3D space were delivered to a fracturing engineer for fracturing evaluation and parameter optimization [21]. 2. Real-time evaluation and parameter optimization After real-time positioning, microseismic events projected in the 3D space may be observed by top view or side view to examine hydraulic fractures. The density of events represents the intensity of rock failure and the intensity increases with density. The extension of events denotes the extension of fractures; the lengths in parallel with and perpendicular to the direction of extension are equal to the length and width of the fractured zone, respectively. The height of events in vertical direction is equal to the height of the fractured zone. The volume swept by microseismic events indicates stimulated reservoir volume. Thus fracturing evaluation and parameter optimization in real time could be realized through microseismic events analysis. 2.1. Pad fluids Hydraulic fracturing, especially fracture length [22,23], relies greatly on the selection of pad fluid. Initially 20 m3 acidizing fluid was injected in Well A (the 4th section) was injected with acidizing fluid of 20 m3, directly followed by linear gel. Microseismic events, as shown in Fig. 2-a, manifest inefficient fracturing operation. Hence the pad fluid parameters for the 5th section were adjusted: 20 m3 acidizing fluid followed by 100 m3 slick water, then by linear
Fig. 1. Schematic surface and borehole microseismic survey layouts. Please cite this article in press as: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
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Wu FR. et al. / Natural Gas Industry B xx (2017) 1e4
Fig. 2. Microseismic monitoring results for different pad fluids.
gel. The outcome is shown in Fig. 2-b. More events occurred in the 5th section and mainly extended toward southwest. The 5th section was evidently more fractured than the 4th section. 2.2. Perforation schemes Perforation scheme has a great impact on hydraulic fracture extension [24]. Whether or not the scheme is a good choice may be judged by examining microseismic events. As shown in Fig. 3-a&b, the points of microseismic events, which were generated by fracturing the 4th section in well B, spread over the 4th and 5th sections to form a complex network. The interval to be perforated in the 5th section was shifted by 125 m before fracturing to avoid duplication of fracturing at the 5th section and save cost. The results are shown in Fig. 3-c&d. New microseismic events did not coincide with the preceding events; instead new fractures occurred forward in the perforated interval ahead. 2.3. Temporary blocking agents Longmaxi shale in southern Sichuan Basin is rich in microfaults and natural cracks, which dominate the growth of hydraulic fractures. A large amount of fracturing fluid may flow into microfaults, resulting in heavy filtrate loss and abnormally high surface pump pressure. Therefore, it is hard
3
to generate a complex fracture network and enlarge stimulated reservoir volume. When microfaults are detected by microseismic monitoring and hydraulic fractures mainly run parallel to the faults instead of extending toward other directions, temporary blocking agents should be injected into the well without hesitation; thus hydraulic fractures may be compelled to occur relatively far away from the faulted zone. Fig. 4-a shows the top view of microseismic events hydraulically induced before temporary blocking agent injection. The events occurred unevenly at both sides of the horizontal wellbore track. A limited number of events existed perpendicular to the wellbore track in a limited southwestern area smaller than that at the other side (on the northeast). A large number of events at the other side (on the northeast) of the wellbore track indicate a complex fracture network generated in more fractured formations. The linear distribution of microseismic events, marked by a black arrow, was detected in real time later to extend in northesouth direction; the magnitude (indicated by the size of point) is 8 times larger than preceding values. No additional events were located after that. This implies there is a microfault in northesouth trend. Consequently temporary blocking agents were injected into the well promptly. As a result, additional hydraulic fractures were compelled to occur in a previously unbroken zone marked by a solid circle in spite of some events still existing around the fault (pointed by a black arrow in Fig. 4-b). There were no microseismic events detected in a zone, marked by a dashed circle and a capital B, close to the fault. The fault-guided fracturing was inhibited by temporary blocking agents and more area was then fractured. 3. Post-frac evaluation Stimulated reservoir volume was estimated after multi-well zipper fracturing. For the well groups A and B with joint surfaceeborehole microseismic monitoring in real time for parameter optimization, the stimulated reservoir volume was estimated to be 1.8 108 m3 and 2.5 108 m3, respectively, while the volume for well group C without real-time optimization was estimated to be 0.8 108 m3. The average daily yields for A and B were tested to be 8.2 104 m3 and
Fig. 3. Microseismic monitoring for different perforation schemes. Please cite this article in press as: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
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Wu FR. et al. / Natural Gas Industry B xx (2017) 1e4
Fig. 4. Microseismic monitoring results before and after temporary blocking agents injection.
15.3 104 m3, respectively, much higher than that of 3.8 104 m3 for C. The benefit of real-time microseismic monitoring was confirmed by later production tests. 4. Conclusions Hydraulic fractures generated by multi-well zipper fracturing were imaged and positioned in real time by joint monitoring of a radially-oriented surface microseismic survey and borehole survey; this made it possible to optimize fracturing parameters in real time and to enlarge stimulated reservoir volume. The benefit of real-time microseismic monitoring was confirmed by later production tests. Real-time microseismic monitoring serves as an eye of fracturing engineers keep an eye on the outcome of hydraulic fracturing; hence it is suggested that conducting real-time microseismic monitoring be conducted for in all the shale gas producers wells to be fractured. Here we only discussed In this paper, only the effects on stimulated reservoir volume and post-frac tested yield were discussed. Next we will the impact on the actual production of shale gas producers producing wells and microseismic monitoring results will be investigated. References [1] Liu Zhenwu, Sa Liming, Yang Xiao, Li Xiangyang. Needs of geophysical technologies for shale gas exploration. Oil Geophys Prospect 2011;46(5):810e8. [2] Li Zhirong, Deng Xiaojiang, Yang Xiao, Wu Furong, Liu Dingjin, Zhang Hong, et al. New progress in seismic exploration of shale gas reservoirs in the southern Sichuan Basin. Nat Gas Ind 2011;31(4):40e3. [3] Wu Mengjie, Zhong Guangfa, Li Yalin, Yang Xiao. Seismic-logging sequence analysis of Longmaxi shale gas reservoirs in the Sichuan Basin. Nat Gas Ind 2013;33(5):51e5. [4] Qian Bin, Zhang Juncheng, Zhu Juhui, Fang Zeben, Kou Shuangfeng, Chen Rui. Application of zipper fracturing of horizontal cluster wells in the Changning shale gas pilot zone, Sichuan Basin. Nat Gas Ind 2015;35(1):81e4. [5] Qian Bin, Zhu Juhui, Li Jianzhong, Li Guoqing, Xiang Lanying. Field application of abrasive jet multi-stage fracturing with coiled tubing annular frac BHA. Nat Gas Ind 2011;31(5):67e9.
[6] Ren Yong, Qian Bin, Zhang Jian, Zhuo Zhichuan, Qiao Lin. Practice and understanding of industrial fracturing for shale gas of Longmaxi Formation in Changning region. Oil Drill Prod Technol 2015;37(4):96e9. [7] Qiu Jian, Duan Shufa. Application of microseismic monitoring technology to Yang 201-H2 well fracturing. Nat Gas Explor Dev 2013;36(4):49e53. [8] Liu Zhenwu, Sa Liming, Wu Furong, Dong Shitai, Li Yanpeng. Microseismic monitor technology status for unconventional resource E&P and its future development in CNPC. Oil Geophys Prospect 2013;48(5): 843e53. [9] Zhao Boxiong, Wang Zhongren, Liu Rui, Lei Liqun. Review of Microseismic monitoring technology research. Prog Geophys 2014;29(4): 1882e8. [10] Sun Keming, Zhang Shucui, Xin Liwei. Impacts of bedding directions of shale gas reservoirs on hydraulically induced crack propagation. Nat Gas Ind 2016;36(2):45e51. [11] Bian Xiaobing, Jiang Tingxue, Jia Changgui, Wang Haitao, Li Shuangming, Su Yuan, et al. A new post-fracturing evaluation method for shale gas wells based on fracturing curves. Nat Gas Ind 2016; 36(2):60e5. [12] Wang Wanxun, Ye Lianchi, Wang Lizhi, Duan Zhengzhong. Microseismic fracturing mapping and fracturing of open-hole long horizontal Well Su 75-70-6H. Eng Sci 2012;14(4):4e7. [13] Yin Chen, Liu Hong, Li Yalin, Wu Furong, He Guangming, Chen Chunhua. The precision analysis of the microseismic location. Prog Geophys 2013;28(2):800e7. [14] Yin Chen, Wu Furong, Li Yalin, Liu Hong, Chen Bo, Kang Liang, et al. The effect of the velocity calibrated by the perforation on microsesmic location error. Prog Geophys 2013;28(4):1809e16. [15] Yin Chen, Liu Hong, Wu Furong, Li Yalin, Kang Liang, Chen Bo, et al. The effect of the calibrated velocity on the microseismic event location precision. 83th SEG Technical Program Expanded Abstracts 2013, 22e27 September, Houston, Texas, USA. 2013. [16] Yin C, Wu FR, Li YL, Liu H, He GM, Chen CH. The application of the microseismic monitoring in natural fracture detection. 76th EAGE Conference and Exhibition, 16e19 June, Amsterdam, The Netherlands. 2014. [17] Hu Yongquan, Yin Cheng, Pan Shulin, Wu Furong, Li Yalin, Liu Yuhai. A microseismic signal recognition technique based on improved time-varying Skewness and Kurtosis Method. Geophys Prospect Pet 2012;51(6):625e32. [18] Hu Yongquan, Yin Cheng, Pan Shulin, Wu Furong, Li Yalin, Liu Yuhai. Denoising method for microseismic data based on single-channel SVD. J Oil Gas Technol 2013;35(4):64e9. [19] Yin Chen, He Zhenhua, Li Yalin, Wu Furong, Cao Libin, Liu Hong, et al. Research and application of the relative magnitude technique based on microseism. Chin J Geophys 2015;58(6):2210e20. [20] Li Gaoming, Chen Jingyi, Han Mei, Gajraj A, Xing Yang, Wu Furong, et al. Accurate microseismic event location inversion using a gradientbased method. SPE Annual Technical Conference and Exhibition, 8e10 October, San Antonio, Texas, USA. 2012. [21] Lu¨ Shichao, Guo Xiaozhong, Jia Likun. Microseismic monitoring data processing and interpretation of horizontal fracturing wells. Reserv Eval Dev 2013;3(6):37e42. [22] Zhang Jun. A method of evaluating the quantity of ahead fluid in one hydraulic fracture treatment. Well Test 2002;11(3):9e11. [23] Yao Haijing. Ahead fluid volume calculation during fracturing. Pet Geol Oilfield Dev Daqing 2007;26(6):107e9. [24] Li Yongming, Chen Xiyu, Zhao Jinzhou, Shen Feng, Qiao Hongjun. The effects of crack interaction in multi-stage horizontal fracturing. J Southwest Pet Univ Sci Technol Ed 2016;38(1):76e83.
Please cite this article in press as: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
MODEL
Available online at www.sciencedirect.com
ScienceDirect Natural Gas Industry B xx (2017) 1e4 www.elsevier.com/locate/ngib
Research Article
Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin* Wu Furong a,*, Yan Yuanyuan b, Yin Chen a a
Geophysical Exploration Company, CNPC Chuanqing Drilling Engineering Co., Ltd., Chengdu, Sichuan 610213, China b School of Science, China University of Geosciences (Beijing), Beijing 100083, China Received 16 July 2016; accepted 25 November 2016
Abstract Zipper hydraulic fracturing in multiple wells with long horizontal sections is a primary solution means to increase the shale gas production rate and efficiency and to reduce the cost in Southern Sichuan Basin. Microseismic based fracturing monitoring can be used for real-time imaging of hydraulic fractures, so it has been widely used to evaluate the fracturing effect of shale gas reservoirs and to direct the optimization and adjustment of fracturing parameters. In China, however, the microseismic fracturing monitoring on fracturing of shale gas reservoirs cannot be used to evaluate the fracturing results until the fracturing operation in the pad wells is completed according to the parameters which are designed prior to the fracturing monitoring. Its evaluation results can merely provide a guidance for the fracturing parameters of the next pad wells instead of the wells in operation. As a result, the real-time effect of microseismic fracturing monitoring is out of work. In view of this, the fractures induced by zipper hydraulic fracturing in multiple shale gas wells with long horizontal sections in the southern Sichuan Basin, was realtime imaged by using the combined technology of radially arranged microseismic surface monitoring and microseismic well monitoring on the basis of real-time positioning method. The fracturing results were assessed and used in real time for the optimization of prepad fluid parameter, perforation and temporary plugging additive releasing time, so as to effectively avoid repeated fracturing and uneven fracturing effects and improve fracturing stimulation effects. This method is applied in two well groups. It is shown that the average shale gas production rate is increased by 2e5 times. Furthermore, microseismic fracturing real-time monitoring plays a vital role in real-time evaluation of fracturing effect and real-time optimization of fracturing parameters, so it can be used as the reference and should be popularized further. © 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: Shale gas; Microseismic; Real-time monitoring; Horizontal well; Zipper fracturing; Well monitoring; Surface monitoring; Production increase; Southern Sichuan Basin
The Lower Silurian Longmaxi Formation in southern Sichuan Basin is rich in shale gas resources [1e3], but shale gas exploration and production is subject to complex topographic conditions (with elevation difference of 400 m), inconvenient transportation, dense population and inefficient technologies. For the promotion of shale gas exploration and * Supported by the National Science and Technology Major Project “Real-time hydraulic fracturing monitoring and integrated geologic-engineering evaluation” (Grant No. 2016ZX05023004). * Corresponding author. E-mail address: [email protected] (Wu FR.). Peer review under responsibility of Sichuan Petroleum Administration.
production, zipper fracturing in several wells with long horizontal sections was introduced from North America [4e6]. The longest horizontal section in this prospect reached 2000 m. In view of little knowledge about fracturing parameters and results because this technique is still in its infancy in China, it is necessary to conduct microseismic monitoring in real-time evaluation and parameter optimization. But microseismic data were usually used for post-fracturing assessment after the operation was fulfilled with pre-fracturing design parameters [7e9]; the results of assessment may only be used for the operation in the next platform well. Few wells were monitored in real time by hydraulically induced microseism.
http://dx.doi.org/10.1016/j.ngib.2017.07.010 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: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
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MODEL
Wu FR. et al. / Natural Gas Industry B xx (2017) 1e4
On the other hand, microseismic monitoring meets challenges in multi-well zipper fracturing. Microseismic events may be acquired at the land surface or in a borehole [10e12]. A borehole microseismic survey has high signal-to-noise ratio. But due to the limitations of recording geometry and azimuth, there are no observation wells with appropriate spacing for monitoring 3e4 wells in zipper fracturing. Thus a well not included in synchronous fracturing is used as the observation well (equipped with geophones) and consequently cannot be monitored by borehole survey. In a borehole survey, vertical positioning is more accurate than lateral positioning. A surface microseismic survey is not constrained by azimuth or observation wells but tends to be affected by topographic conditions, especially in a mountainous region with a vertical detection distance of 3500 m. Thus a surface observation suffers from intense signal attenuation, weak energy recorded and intractable velocity modeling. In a surface survey, lateral positioning is more accurate than vertical positioning [13e16]. In this paper, we use a radially-oriented surface survey combined with borehole survey to image hydraulic fractures and optimize fracturing parameters in real time. 1. Microseismic data acquisition and processing 1.1. Data acquisition In the radially-oriented surface microseismic survey, 8e20 survey lines with 1000e3000 groups of geophones were laid around the wellhead at the center; group interval is 20 m. The borehole survey was equipped with 40-level 3-component geophones placed close to the sections to be fractured; geophone spacing is 15 m (Fig. 1). 1.2. Data processing GeoMonitor, a self-developed system for microseismic monitoring, was used for real-time positioning. (1) Build an interval velocity model. The initial velocity model was built with sonic log derived velocity and seismic interpreted horizons and then corrected by joint surfaceeborehole positioning
of perforation signals. The velocity model was finally accepted when the error of perforation positioning was less than the predefined value. (2) Conduct preprocessing. This included static correction and noise reduction for surface data and noise reduction, vector rotation, and automatic microseism picking for borehole data [17e19]. (3) Conduct real-time positioning [20]. A joint positioning method through automatic searching across 3D grids was employed to position microseismic events in real time. The delay time of real-time positioning was less than 10 s (4) Display microseismic events in 3D space. The positioned microseismic events in the 3D space were delivered to a fracturing engineer for fracturing evaluation and parameter optimization [21]. 2. Real-time evaluation and parameter optimization After real-time positioning, microseismic events projected in the 3D space may be observed by top view or side view to examine hydraulic fractures. The density of events represents the intensity of rock failure and the intensity increases with density. The extension of events denotes the extension of fractures; the lengths in parallel with and perpendicular to the direction of extension are equal to the length and width of the fractured zone, respectively. The height of events in vertical direction is equal to the height of the fractured zone. The volume swept by microseismic events indicates stimulated reservoir volume. Thus fracturing evaluation and parameter optimization in real time could be realized through microseismic events analysis. 2.1. Pad fluids Hydraulic fracturing, especially fracture length [22,23], relies greatly on the selection of pad fluid. Initially 20 m3 acidizing fluid was injected in Well A (the 4th section) was injected with acidizing fluid of 20 m3, directly followed by linear gel. Microseismic events, as shown in Fig. 2-a, manifest inefficient fracturing operation. Hence the pad fluid parameters for the 5th section were adjusted: 20 m3 acidizing fluid followed by 100 m3 slick water, then by linear
Fig. 1. Schematic surface and borehole microseismic survey layouts. Please cite this article in press as: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
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MODEL
Wu FR. et al. / Natural Gas Industry B xx (2017) 1e4
Fig. 2. Microseismic monitoring results for different pad fluids.
gel. The outcome is shown in Fig. 2-b. More events occurred in the 5th section and mainly extended toward southwest. The 5th section was evidently more fractured than the 4th section. 2.2. Perforation schemes Perforation scheme has a great impact on hydraulic fracture extension [24]. Whether or not the scheme is a good choice may be judged by examining microseismic events. As shown in Fig. 3-a&b, the points of microseismic events, which were generated by fracturing the 4th section in well B, spread over the 4th and 5th sections to form a complex network. The interval to be perforated in the 5th section was shifted by 125 m before fracturing to avoid duplication of fracturing at the 5th section and save cost. The results are shown in Fig. 3-c&d. New microseismic events did not coincide with the preceding events; instead new fractures occurred forward in the perforated interval ahead. 2.3. Temporary blocking agents Longmaxi shale in southern Sichuan Basin is rich in microfaults and natural cracks, which dominate the growth of hydraulic fractures. A large amount of fracturing fluid may flow into microfaults, resulting in heavy filtrate loss and abnormally high surface pump pressure. Therefore, it is hard
3
to generate a complex fracture network and enlarge stimulated reservoir volume. When microfaults are detected by microseismic monitoring and hydraulic fractures mainly run parallel to the faults instead of extending toward other directions, temporary blocking agents should be injected into the well without hesitation; thus hydraulic fractures may be compelled to occur relatively far away from the faulted zone. Fig. 4-a shows the top view of microseismic events hydraulically induced before temporary blocking agent injection. The events occurred unevenly at both sides of the horizontal wellbore track. A limited number of events existed perpendicular to the wellbore track in a limited southwestern area smaller than that at the other side (on the northeast). A large number of events at the other side (on the northeast) of the wellbore track indicate a complex fracture network generated in more fractured formations. The linear distribution of microseismic events, marked by a black arrow, was detected in real time later to extend in northesouth direction; the magnitude (indicated by the size of point) is 8 times larger than preceding values. No additional events were located after that. This implies there is a microfault in northesouth trend. Consequently temporary blocking agents were injected into the well promptly. As a result, additional hydraulic fractures were compelled to occur in a previously unbroken zone marked by a solid circle in spite of some events still existing around the fault (pointed by a black arrow in Fig. 4-b). There were no microseismic events detected in a zone, marked by a dashed circle and a capital B, close to the fault. The fault-guided fracturing was inhibited by temporary blocking agents and more area was then fractured. 3. Post-frac evaluation Stimulated reservoir volume was estimated after multi-well zipper fracturing. For the well groups A and B with joint surfaceeborehole microseismic monitoring in real time for parameter optimization, the stimulated reservoir volume was estimated to be 1.8 108 m3 and 2.5 108 m3, respectively, while the volume for well group C without real-time optimization was estimated to be 0.8 108 m3. The average daily yields for A and B were tested to be 8.2 104 m3 and
Fig. 3. Microseismic monitoring for different perforation schemes. Please cite this article in press as: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010
+ 4
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Wu FR. et al. / Natural Gas Industry B xx (2017) 1e4
Fig. 4. Microseismic monitoring results before and after temporary blocking agents injection.
15.3 104 m3, respectively, much higher than that of 3.8 104 m3 for C. The benefit of real-time microseismic monitoring was confirmed by later production tests. 4. Conclusions Hydraulic fractures generated by multi-well zipper fracturing were imaged and positioned in real time by joint monitoring of a radially-oriented surface microseismic survey and borehole survey; this made it possible to optimize fracturing parameters in real time and to enlarge stimulated reservoir volume. The benefit of real-time microseismic monitoring was confirmed by later production tests. Real-time microseismic monitoring serves as an eye of fracturing engineers keep an eye on the outcome of hydraulic fracturing; hence it is suggested that conducting real-time microseismic monitoring be conducted for in all the shale gas producers wells to be fractured. Here we only discussed In this paper, only the effects on stimulated reservoir volume and post-frac tested yield were discussed. Next we will the impact on the actual production of shale gas producers producing wells and microseismic monitoring results will be investigated. References [1] Liu Zhenwu, Sa Liming, Yang Xiao, Li Xiangyang. Needs of geophysical technologies for shale gas exploration. Oil Geophys Prospect 2011;46(5):810e8. [2] Li Zhirong, Deng Xiaojiang, Yang Xiao, Wu Furong, Liu Dingjin, Zhang Hong, et al. New progress in seismic exploration of shale gas reservoirs in the southern Sichuan Basin. Nat Gas Ind 2011;31(4):40e3. [3] Wu Mengjie, Zhong Guangfa, Li Yalin, Yang Xiao. Seismic-logging sequence analysis of Longmaxi shale gas reservoirs in the Sichuan Basin. Nat Gas Ind 2013;33(5):51e5. [4] Qian Bin, Zhang Juncheng, Zhu Juhui, Fang Zeben, Kou Shuangfeng, Chen Rui. Application of zipper fracturing of horizontal cluster wells in the Changning shale gas pilot zone, Sichuan Basin. Nat Gas Ind 2015;35(1):81e4. [5] Qian Bin, Zhu Juhui, Li Jianzhong, Li Guoqing, Xiang Lanying. Field application of abrasive jet multi-stage fracturing with coiled tubing annular frac BHA. Nat Gas Ind 2011;31(5):67e9.
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Please cite this article in press as: Wu FR, et al., Real-time microseismic monitoring technology for hydraulic fracturing in shale gas reservoirs: A case study from the Southern Sichuan Basin, Natural Gas Industry B (2017), http://dx.doi.org/10.1016/j.ngib.2017.07.010