砂岩预制裂缝定向压裂起裂与扩展规律研究.pdf
万方数据 万方数据 太原理工大学博士研究生学位论文 I 砂岩预制裂缝定向压裂起裂与扩展规律研究 摘 要 在煤炭开采过程中,如果煤层上覆岩层存在坚硬难垮顶板,在工作面 后方极易形成大面积的悬顶,悬顶一旦垮落,产生强冲击载荷,引起飓 风,造成重大的矿难事故。对于无煤柱开采的沿空留巷、沿空掘巷,其巷 旁支护侧采空区也容易形成悬顶,导致巷旁支护载荷增大,使留巷变形 大,难以维护。对于煤柱护巷,采空侧悬顶使煤柱留设宽度增加,回采率 严重下降。因此需要用人工方法强制使坚硬顶板随工作面的推进及时断裂 垮落。目前,煤矿顶板定向水力压裂技术的方法是在钻孔内沿径向平行于 岩层层面机械割缝,从而在有限范围内产生水平定向裂缝,使坚硬顶板分 层,降低整体强度,减小大面积来压的冲击灾害。其缺点是顶板不能在巷 道、采场的指定位置实现充分冒落,多年来应用受到一定限制。由于煤矿 回采的特殊性,要求顶板能够按照指定位置和一定的方向断裂,而现阶段 的顶板压裂难以实现该目的,因此提出垂直岩层层面预制裂缝的定向压裂 技术。针对该技术方案中影响最终压裂效果的影响因素,如预制裂缝方位 角、水平应力差和注液速率等对压裂裂缝的起裂和扩展规律的影响展开研 究。研究内容和研究结果如下 (1)采用真三轴压裂实验装置,研究了不同预制裂缝方位角对压裂 裂缝形态、起裂压力以及扩展规律的影响。实验结果表明随着预制裂缝 方位角的增大(水平应力差为 2MPa) ,起裂压力呈线性递增;起裂位置全 部在预制裂缝尖端,裂缝最终扩展并形成基本对称单一的平直或转向裂 缝,裂缝的弯曲程度随着预制裂缝方位角的变化规律为607545 3015090,通过声发射测试获得剪切破坏比例随预制裂缝方 位角的变化规律与弯曲程度随预制裂缝方位角变化规律一致。随着预制裂 万方数据 太原理工大学博士研究生学位论文 II 缝方位角的增加,达到相同偏转角时,裂缝延伸长度逐渐减小,在 45达 到最小值; 当预制裂缝方位角大于 45,随着预制裂缝方位角的增加, 与 45相比,达到相同的偏转角,裂缝延伸长度先增大后减小。 (2)采用真三轴压裂实验装置,研究了不同水平应力差对定向压裂 裂缝的起裂和扩展规律的影响。实验结果表明随着水平应力差的增加, 能够提供起裂位置的预制裂缝方位角区间逐渐缩小。水平应力差为小于等 于 2MPa 时,沿预制裂缝尖端起裂;水平应力差为 3MPa,即接近岩石抗 拉强度 3.2MPa 时,预制裂缝方位角在 075,沿预制裂缝尖端起裂,在 90,沿钻孔孔壁面最大主应力方向起裂;水平应力差为 4MPa 时,预制 裂缝方位角在 060,沿预制裂缝尖端起裂,在 75和 90,在沿钻孔 孔壁面沿最大水平应力方向起裂。随着水平应力差的增大,裂缝起裂压力 逐渐减小。压裂后形成基本对称单一的平直或转向裂缝,相同预制裂缝方 位角时,随着水平应力差的增加,扩展裂缝由转向裂缝逐渐演变成平直裂 缝,裂缝向最大水平应力方向偏转的越快,达到相同偏转角时,裂缝延伸 长度越短。 (3)采用真三轴压裂实验装置,研究了不同注液速率对定向压裂起 裂压力、裂缝形态以及裂缝扩展控制作用。结果表明随着注液速率的增 大,起裂压力值变化幅度较小,压裂裂缝的扩展更倾向于向预制裂缝方向 延伸;达到相同偏转角时,压裂裂缝延伸长随着注液速率的增加而增加。 (4)初步建立了基于黏聚型裂纹的水力压裂模型,并采用包含裂隙 流水压的零厚度黏聚型界面单元模拟裂纹的起裂和扩展。通过数值模拟结 果与物理实验结果比较,证明该模型可以较好地反映岩石类准脆性材料水 力压裂裂纹的起裂与扩展。 (5)通过本文的研究证明在垂直层面预制裂缝定向压裂技术的主要 影响因素中,在地应力一定的情况下,通过改变预制裂缝方位角和注液速 万方数据 太原理工大学博士研究生学位论文 III 率,可以实现裂缝首先在一定长度范围内沿预制裂缝的方向开裂,随后尽 管裂缝受应力场控制转向沿最大主应力方向扩展,通过控制相邻钻孔的间 距,使相邻钻孔的偏转后的裂缝在允许偏移的范围内相互搭接,从而实现 顶板沿巷道壁一定范围内破断,实现定向切顶的目的。 关键词煤矿开采,坚硬顶板,钻孔轴向预制裂缝,定向压裂,岩层控制 万方数据 太原理工大学博士研究生学位论文 IV 万方数据 太原理工大学博士研究生学位论文 V STUDY ON ORIENTED FRACTURING AND PROPAGATION IN SANDSTONE WITH PREFABRICATED CRACKS ABSTRACT In the process of coal mining, if the coal seam is covered with hard and stable roof,it is easy to a large area of the hanging arch at the rear of the working face. Once the hanging arch collapses, the hurricane and strong impact load are tending to be triggered, which will cause a serious mine accident. For the gob-side entry retaining and gob-side entry driving of non-pillar mining, the roadway side of the support side of the mined-out area is also easy to a hanging roof, leading the roadside support load to increase, so that the roadway des, which is difficult to maintain. For the coal pillar entry retaining, the hanging roof of goaf increases the width of coal pillars, a serious reduction in mining rate. Therefore, there is a need for artificial s to force the roof timely collapse with the advance of the working surface. At present, the of oriented hydraulic fracturing of coal mine is in the borehole along the drilling radial direction parallel to the rock layer for mechanical cutting, resulting in horizontally oriented cracks within a limited range in order to delaminate hard roofs, decrease the overall strength, and reduce the disasters of weighting over great extent. The disadvantage is that the roof cannot be fully caved in the designated place of the roadway and stope, limiting its wide application. Due to the particularity of coal mining,it is required that the roof can be fractured according to the designated location and along a certain direction. The roof fracturing at present stage is difficult to achieve this goal. For this reason, the oriented fracturing technology of the vertical layer prefabricated cracks is proposed. In view of the influence factors of the final fracturing effect in this technical scheme, the influence factors of the final fracturing effect in this technical scheme such as prefabricated fracture azimuth angle, horizontal stress 万方数据 太原理工大学博士研究生学位论文 VI difference and injection rate on the crack initiation and extension law of the fracturing crack are studied in this paper. The study contents and results are as follows 1 Using the true triaxial fracturing experimental device, the effects of the azimuth angle of the prefabricated crack on the fracturing morphology, the crack initiation pressure, and the crack propagation law of the fracturing are studied. Experimental results show with the increase of the azimuth angle of the prefabricated fracture the horizontal stress difference is 2MPa, the initiation pressure is linearly increasing. All the initiation position is at the tip of the prefabricated crack, ing a single symmetrical straight or turning crack, with the degree of bending varying with the azimuth of the prefabricated fracture 60 75 45 30 15 0 90 . Through acoustic emission, it is found that the variation law of the shear failure ratio with the azimuth angle of prefabricated fractures test is consistent with the change of crack diversion degree with azimuth angle of prefabricated fractures. As the azimuth angle increases, the crack extension length decreases gradually, reaching a minimum value at 45. When the azimuth angle of the prefabricated crack is greater than 45, with the increase of the azimuth angle of the prefabricated crack, the length of the crack increases firstly and then decreases, compared with the case of 45. 2 Using the true triaxial fracturing experimental device, the effect of horizontal stress difference on the location of crack initiation, crack morphology, initiation pressure and crack propagation is studied. Experimental results show with the increase of the horizontal stress difference, the azimuth angle interval that can provide the initiation position is gradually reduced. When the horizontal stress difference is less than or equal to 2MPa, the crack propagate along the prefabricated crack tip; when the horizontal stress difference is 3MPa, that is, close to the rock tensile strength of 3.2MPa, and the prefabricated fracture oriented in the 0 75 , the crack propagates along the prefabricated crack tip crack; when the azimuth angle is 90 , the crack propagates in the drilling wall 万方数据 太原理工大学博士研究生学位论文 VII along the maximum principal stress direction. For the horizontal stress difference being 4MPa, when the azimuth angle is 0 60 , crack propagates along the prefabricated crack tip; when the azimuth angle is 75 and 90 , the borehole wall is cracked in the direction of maximum principal stress. With the increase of horizontal stress difference, the initiation pressure gradually decreases. After fracturing, a single substantially symmetrical straight or turning crack is ed. For the same prefabricated fracture azimuth angle, with the increase of horizontal stress difference, the transition crack gradually evolves into a straight crack, the fracture diverts to the maximum horizontal stress direction faster, and the crack extension length is shorter when the same deflection angle is reached. 3 Using the true triaxial fracturing experimental device, the effects of different injection rate on initiation pressure, fracture morphology and crack propagation law of rock oriented fracturing were studied. Experimental results show with the increase of the injection rate, the variation range of the initiation pressure value is smaller and the fracturing fracture tends to extend in the direction of the prefabricated crack. When the same deflection angle is reached, extension length of the fracturing fracture increases with the increase of the injection rate. 4 The hydraulic fracturing model based on cohesive fracture is established and used to simulate the crack initiation and propagation using a zero-thickness, cohesive interface element containing fissure water pressure. The variation law of the initiation pressure for the azimuth angle being 0 is simulated. The results of numerical simulation are compared with physical experiment results. It is found that the model can well reflect the initiation and propagation of hydraulic fracturing cracks in rocks. 5 The study of this paper proved that among the main influencing factors of vertical level oriented fracturing with prefabricated cracks, subject to certain 万方数据 太原理工大学博士研究生学位论文 VIII ground stress, the crack can propagate in the direction of prefabricated crack firstly within a certain length by changing the azimuth of prefabricated crack and injection rate, and then controlled by the stress field to expand further along the direction of the maximum principal stress. By controlling the spacing between adjacent drilling holes, the deflected cracks in the adjacent drilling holes can be overlapped with each other within the allowable range so as to achieve the purpose of roof broke along the roadway and roof cutting. KEY WORDS Coal mining, Hard roof, Drilling axial prefabricated cracks, Oriented fracturing, Strata control 万方数据 太原理工大学博士研究生学位论文 IX 目 录 摘 要 .................................................................I ABSTRACT .................................................................V 第一章 绪论 ............................................................1 1.1 研究背景及意义 ...................................................1 1.2 水力压裂研究现状 .................................................3 1.2.1 水力压裂理论研究现状.......................................3 1.2.2 水力压裂裂缝形态与扩展规律研究现状.........................7 1.3 定向水力压裂研究现状 ............................................14 1.3.1 定向压裂理论研究现状......................................14 1.3.2 定向水力压裂裂缝扩展规律研究现状..........................16 1.3.3 煤矿坚硬顶板定向压裂研究现状..............................19 1.4 研究现状的评述 ..................................................21 1.5 本文的研究内容 ..................................................23 1.6 本文的研究技术路线 ..............................................24 第二章 预制裂缝方位角对裂缝起裂及扩展规律的影响 ........................25 2.1 实验系统及实验方法 ..............................................25 2.1.1 预制裂缝定向压裂实验系统..................................25 2.1.2 预制裂缝定向压裂实验方案及步骤............................30 2.2 预制裂缝方位角对裂缝形态和起裂压力的影响规律 ....................34 2.2.1 不同预制裂缝方位角压裂过程中声发射的时序特征..............34 2.2.2 预制裂缝方位角对压裂裂缝形态的影响........................37 2.2.3 预制裂缝方位角对起裂压力的影响规律........................42 2.3 预制裂缝方位角对裂缝扩展的影响 ..................................43 2.4 本章小结 ........................................................46 第三章 水平应力差对裂缝起裂及扩展规律的影响 ............................47 3.1 不同应力差定向压裂实验方案 ......................................47 3.2 水平压力差对裂缝形态和起裂压力的影响规律 ........................48 万方数据 太原理工大学博士研究生学位论文 X 3.2.1 应力差对裂缝起裂位置的影响规律 ........................... 48 3.2.2 应力差对压裂裂缝形态的影响规律 ........................... 49 3.2.3 应力差对起裂压力的影响规律 ............................... 58 3.3 水平应力差对裂缝扩展规律的影响.................................. 60 3.4 本章小结........................................................ 64 第四章 注液速率对裂缝起裂及扩展的控制作用 .............................. 65 4.1 不同注液速率定向压裂实验方案.................................... 65 4.2 注液速率对裂缝形态及起裂压力的控制作用.......................... 65 4.2.1 不同注液速率压裂过程中声发射时序特征 ..................... 65 4.2.2 注液速率对裂缝形态的控制作用 ............................. 67 4.2.3 注液速率对起裂压力的控制作用 ............................. 70 4.3 注液速率对压裂裂缝扩展的控制作用................................ 70 4.4 本章小结........................................................ 74 第五章 预制裂缝定向压裂数值模拟 ....................................... 75 5.1 水力压裂黏聚型裂纹模型 ......................................... 75 5.1.1 经典水力压裂模型 ......................................... 75 5.1.2 基于黏聚型裂纹的水力压裂模型 ............................. 80 5.2 基于黏聚型裂纹的水力压裂模型的离散及求解方法.................... 87 5.2.1 水力压裂模型的有限元离散方程............................. 88 5.2.2 水力压裂有限元耦合方程求解............................... 91 5.3 零度预制裂缝定向压裂数值模拟 ................................... 93 5.3.1 计算模型 ................................................. 93 5.3.2 模拟结果分析 ............................................. 94 5.4 本章小结........................................................ 99 第六章 结论和展望 .................................................... 101 6.1 结论........................................................... 101 6.2 不足与展望..................................................... 102 参考文献 ............................................................... 103 致 谢 ............................................................... 117 万方数据 太原理工大学博士研究生学位论文 XI 攻读博士学位期间发表的论文 .............................................119 博士学位论文独创性说明 .................................................121 附录 ...................................................................122 万方数据 太原理工大学博士研究生学位论文 XII 万方数据 太原理工大学博士研究生学位论文 1 第一章 绪论 1.1 研究背景及意义 我国地域辽阔,煤层赋存条件复杂,煤层顶板属于坚硬顶板[1]的煤层约占三分之 一,且分布在百分之五十以上的矿区,煤矿坚硬顶板控制是采场矿山压力研究的重要 内容之一。坚硬难垮顶板是指赋存在煤层上方或薄层直接顶上面厚而稳定、坚硬的石 灰岩、砂岩和砂砾岩等岩层,具有厚度大、整体性强、强度大以及节理裂隙不发育等 特点。在煤层开采时,随着工作面推进,其后方容易形成大面积悬顶,悬顶一旦垮落 波及范围大,极易形成飓风和强冲击载荷,造成人员伤亡和设备损坏,而且瓦斯瞬间 涌出,容易诱发重特大事故[1-2]。另一方面,坚硬顶板的存在,对于无煤柱开采的沿空 留巷、沿空掘巷,其巷旁支护侧采空区也容易形成悬顶,导致巷旁支护载荷增大,使 留巷变形大,维护成本高[3-5]。对于高瓦斯矿井,为了满足通风、运输安全等需要,工 作面回采巷道采用“两进两回”或者是“三进两回”的布置方式,为了减少巷道掘进 率,提高经济效益,其中相邻的一条或者两条回采巷道需保留作为相邻的下个工作面 利用,由于坚硬岩层的悬顶使巷道煤柱受采动压力影响很大,难以维护。多需采取补 强、加固等措施;或者加大护巷煤柱宽度,个别已达 50m 宽,使采出率严重下降,资 源损失严重;同时煤柱留设过宽,容易引起冲击地压灾害。可见坚硬顶板威胁着矿井 的安全生产,给回采、巷道支护和资源回收都带来了重大影响。 对待煤矿坚硬顶板的管理,国内外专家早有共识[6],那就是通过人为改变坚硬顶板 的物理力学特性和整层性,变难垮顶板为易冒顶板,在需要的位置能及时充分垮落, 巷旁支护 留巷 悬顶 巷旁支护 留巷 悬顶 巷旁支护 留巷 切缝 巷旁支护 留巷 切缝 a 巷道处于高应力区 b 垂直切缝后巷道顶板处于低应力区 图 1-1 沿空巷道采动应力分布 Fig.1-1 Mining stress distribution of gob-side entry 万方数据 太原理工大学博士研究生学位论文 2 达到消除冲击,减小采场、巷道、煤柱上的采动压力。图1-1为沿空巷道的采动压力分 布,其中图1-1a为悬顶时的支承压力分布,巷道和巷旁支护均处于高应力区,因此维 护特困难。图1-1b为在指定位置垂直切缝后顶板及时垮落,支承压力向煤体转移,巷 道处于低应力区,降低了维护难度和成本。图1-2为宽煤柱下的采动压力分布,其中图 1-2a为悬顶时的支承压力分布,煤柱完全处在高应力集中区,如果留设煤柱宽度不 足,将使整条巷道垮塌,无法维护。如果煤柱留设过宽,煤炭损失严重,而且回采这 种孤岛煤柱十分困难,代价会更大,且安全无保障。图1-2b为指定位置切缝后,顶板 及时垮落,煤柱上只有上覆的岩层自重,其压力大大降低,只用窄的护巷煤柱就可安 悬顶悬顶 护巷煤柱护巷煤柱护巷煤柱护巷煤柱 切缝切缝 巷道巷道巷道巷道 全开采。图1-3为采场的初次或周期来压的压力分布,其中图1-3a为悬顶时支架上承受 着高的支承压力,经常出现压垮支架的事故。图1-3b为预先垂直切缝切断顶板的联 系,则采场支架处于低应力区,开采安全可靠。因此,通过人工切顶,消除坚硬顶板 垮落时冲击,减小采场、巷道、煤柱上的采动压力,这就是国内外专家对坚硬顶板管 理的共识。 悬顶悬顶 切缝切缝 目前,弱化坚硬顶板强度的方法有岩层静动压注水、聚能爆破和水力压裂。岩层 a 煤柱处于高应力区 b 垂直切缝后煤柱处于低应力区 图 1-2 煤柱下的采动应力分布 Fig.1-2 Mining stress distribution of coal pillar a 采场支架处于高应力区 b 垂直切缝后采场支架处于低应力区 图 1-3 采场支架下的采动应力分布 Fig.1-3 Mining stress distribution of stope hydraulic support 万方数据 太原理工大学博士研究生学位论文 3 静动压注水方式使顶板含水量提高,降低岩体强度,减小垮落步距。该方法要求顶板 具有强的吸水性和弱化性,即使这样,也不能保证在指定的位置充分及时冒落。聚能 爆破能够使顶板在指定的位置断裂,取得了一定的成功,但是由于井下爆破会引起钻 孔工程量多、炸药量大等,造成生产成本较高,同时爆破严重污染井下空气。水力压 裂利用高压水形成水楔、扩展裂隙和弱面,以及高压水渗流等产生各种物理化学作 用,从而降低岩石的强度,达到顶板强度弱化、减小冲击灾害的目的。为了保证坚硬 顶板压裂效果,采用定向水力压裂,沿钻孔径向(平行于岩层层面)机械水平切槽, 产生定向裂缝,使坚硬顶板分层,降低整体强度,减小大面积来压的冲击灾害。其缺 点是顶板不能在巷道、采场的指定位置实现充分冒落,多年来应用受到一定限制。由 于煤矿回采的特殊性,要求顶板能够按照指定位置和一定的方向断裂,而现阶段的顶 板压裂难以实现该目的,因此提出垂直岩层层面预制裂缝的定向压裂技术。 垂直岩层层面预制裂缝的定向压裂技术特征是在采掘巷道一侧的顶板上施工垂直 岩层层面的向上钻孔,然后在钻孔内沿轴向预制裂缝,预制裂缝方位角根据地应力差 选取,使压裂裂缝在有限长度内沿着预设的方向扩展,从而使坚硬顶板沿着需要的方 向断裂,大幅度减小顶板大面积来压和沿空巷道变形,减小护巷煤柱尺寸,实现坚硬 难垮