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首页» 过刊浏览» 2023» Vol.8» Issue(1) 87-101     DOI : 10.3969/j.issn.2096-1693.2023.01.006
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液氮循环压裂裂缝起裂与形态特征可视化研究
洪纯阳, 杨睿月, 黄中伟, 温海涛, 夏志浩, 李根生
中国石油大学 (北京 )油气资源与探测国家重点实验室,北京 102249
Visualization of fracture initiation and morphology by cyclic liquid nitrogen fracturing
HONG Chunyang, YANG Ruiyue, HUANG Zhongwei, WEN Haitao, XIA Zhihao, LI Gensheng
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum-Beijing, Beijing 102249, China

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摘要  针对干热岩储层改造存在的起裂压力高、裂缝单一和易诱发地震的瓶颈难题,本文结合循环水力压裂和 液氮压裂技术优势,探索了一种液氮循环压裂开发干热岩的新思路,即通过“注入—停顿”的方式周期性注入 低温液氮,使岩石在交变热应力—流体压力耦合作用下发生疲劳损伤,促进裂缝起裂、转向、分叉进而形成复 杂缝网,提高储层改造体积。目前,液氮压裂技术的研究多集中在液氮单次或循环冷却致裂岩石力学机制和液 氮压裂造缝机理方面,考虑地应力条件下的液氮循环压裂造缝机理方面的研究未见报道。为了验证液氮循环压 裂开发干热岩的可行性,基于自主研发的真三轴液氮循环压裂实验装置,采用可视化材料PMMA(Polymethyl Methacrylate),研究了水平应力差异系数和循环次数的影响规律,揭示了液氮循环压裂裂缝起裂与形态特征,并 与清水循环压裂进行了对比。结果表明:在较低的循环次数和循环压力下,液氮循环压裂相对于清水循环压裂 可显著降低起裂压力 (下降 47.1%~71.7%),在交变热应力—流体压力耦合作用下液氮循环压裂易形成以“热应 力裂缝+主裂缝”为特征的复杂缝网;液氮循环压裂不易受水平应力差异系数控制,在较大水平应力差异系数 下仍能取得较好的造缝效果;液氮循环冷却预处理是液氮循环压裂的关键,增大液氮循环冷却次数可显著降低 裂缝起裂压力并形成复杂缝网,当直接采用高压液氮压裂时,起裂压力甚至会超过清水压裂;总体来看,液氮 循环压裂相对于清水循环压裂能以较低的循环次数和循环注入压力实现较好的造缝效果,有望为干热岩绿色经 济高效开发提供新途径。研究结果可望为液氮循环压裂开发干热岩提供理论依据和实验基础。
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关键词 : 增强型地热系统 (EGS); PMMA;液氮循环压裂;热应力;疲劳损伤
Abstract

Conventional hydraulic fracturing generally produces high breakdown pressure, results only in single major fracture morphology and increases the risk of seismic events during the stimulation of dry hot rock (HDR) reservoirs. Aiming at addressing the above bottlenecks during hydraulic fracturing, a new reservoir stimulation method, known as cyclic liquid nitrogen (LN2) fracturing, based on cyclic soft stimulation (CSS) and LN2 fracturing was explored in this paper. In cyclic LN2 fracturing, low-temperature LN2 was injected in a cyclic manner, i.e. alternating high-injection-rate and low-injection-rate (or stop injection). Hence, formation rocks would be subjected to fatigue damage under the combined action of alternating thermal stress and fluid pressure, which was expected to promote fracture initiation, propagation and bifurcation to form complex fracture networks and improve the stimulated reservoir volume. However, the research on LN2 fracturing was mainly focused on the mechanisms of cyclic or single cooling treatment on rock and the fracturing performance of LN2 fracturing. No works on the cyclic LN2 fracturing performance, especially subjected to in-situ stresses were published as far as we know. To verify the feasibility of developing HDR by cyclic LN2 fracturing, the fracture initiation and morphology of cyclic LN2 fracturing were revealed by using Polymethyl Methacrylate (PMMA) based on the self-developed true-triaxial experiment of cyclic LN2 fracturing. The effects of horizontal stress difference ratio and the number of cycles on cyclic LN2 fracturing performance were studied. Cyclic water fracturing experiments were also conducted as a comparison. The results show that cyclic LN2 fracturing can significantly reduce the breakdown pressure, with 47.1%~71.7% lower than cyclic water fracturing. Under the combined action of alternating thermal stress and fluid pressure, cyclic LN2 fracturing tends to form a complex fracture network characterized by “thermally-induced fractures + major fractures”. The fracture initiation and morphology of cyclic LN2 fracturing are not easily affected by the horizontal stress difference ratio. Complex fracture networks can still be produced by the cyclic LN2 fracturing under a larger horizontal stress difference ratio. Increasing the number of cycles can reduce more breakdown pressure and generate more pronounced complex fracture networks. When high-pressure LN2 was injected into the wellbore, the breakdown pressure was even higher than that of water fracturing, which indicated that cyclic LN2 cooling pretreatment was the key to enhancing the LN2 fracturing performance. In general, cyclic LN2 fracturing can achieve better fracturing performance with a relatively lower number of cycles and cyclic pretreat pressure compared with cyclic water fracturing. Cyclic LN2 fracturing was expected to provide a new way for the green, economic and efficient development of HDR. The results were expected to provide a theoretical and experimental basis for the development of HDR by using cyclic LN2 fracturing.

Key words: Enhanced Geothermal System (EGS); PMMA; cyclic liquid nitrogen fracturing; thermal stress; fatigue damage
收稿日期: 2023-02-28     
PACS:    
基金资助:国家自然科学基金青年项目“干热岩低温循环压裂造缝机理研究” (52004299) 和国家自然科学基金重大科研仪器研制项目“水力喷射径
向水平井综合实验系统” (51827804) 联合资助
通讯作者: yangruiyue@cup.edu.cn
引用本文:   
洪纯阳, 杨睿月, 黄中伟, 温海涛, 夏志浩, 李根生. 液氮循环压裂裂缝起裂与形态特征可视化研究. 石油科学通报, 2023, 01: 87-101 HONG Chunyang, YANG Ruiyue, HUANG Zhongwei, WEN Haitao, XIA Zhihao, LI Gensheng. Visualization of fracture initiation and morphology by cyclic liquid nitrogen fracturing. Petroleum Science Bulletin, 2023, 01: 87-101.
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