Experimental study and application of hydraulic fracturing in underlying coal seam
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摘要:
采空区下伏煤层气资源储量丰富，长期未能有效开发。水力压裂技术是一种提高煤层气采收率的有效手段，上覆煤层的开采与重新压实会直接影响采空区下伏煤层水力裂缝的扩展行为。通过大尺寸(300 mm×300 mm×300 mm)真三轴水力压裂试验，分析了不同加卸载应力扰动程度下煤体的力学与声发射响应特征，提出了表征煤体损伤程度的损伤变量$T$,明晰了损伤与水力裂缝起裂与扩展规律之间的关系。结果表明：采空区下伏煤体垂向应力加载阶段引起的损伤显著大于卸载阶段，垂向加载应力不超过11 MPa时，煤体处于弹性阶段，损伤极少；加载至11～15 MPa，处于屈服阶段，损伤大幅增加；加载至15～18 MPa，处于强化阶段，煤体孔裂隙逐渐被压实。损伤变量$T$可以有效表征煤体内部损伤程度，Tc为煤体未经过加卸应力扰动时的损伤变量。$T = {T_{\mathrm{c}}}$时，煤体内部的损伤程度与未经过加卸载应力扰动的煤体损伤程度相当；$ T>{T}_{{\mathrm{c}}} $时，煤体呈应力损伤态，$T$越大，损伤程度越高；$ T<{T}_{{\mathrm{c}}} $时，煤体呈应力压实态，$T$越小，压实程度越高。煤体应力损伤程度与破裂压力呈负相关，高损伤程度使得煤体更容易破裂，井筒近端容易形成主水力裂缝，有利于开展水力压裂。煤体的压实程度与破裂压力呈正相关，高压实程度使得水平应力差对水力裂缝扩展的影响减弱，井筒近端水力裂缝发育，不易形成主水力裂缝，阻碍水力压裂开展。基于研究成果制定了采空区下伏煤层水力压裂施工方案优化原则并在现场应用，优化后的方案水力压裂造缝能力显著提高。
Abstract:The coalbed methane resource under goaf is rich, but has not been effectively developed for a long time. Hydraulic fracturing technology stands as an effective means to augment coalbed methane recovery. The extraction and re-compaction of overlying coal seam will directly affect the expansion behavior of hydraulic fractures in the underlying coal seam. Though the large-scale (300 mm×300 mm×300 mm) true triaxial hydraulic fracturing experiments, the mechanical and acoustic emission response characteristics of coal were analyzed under different levels of loading and unloading stress perturbations. The damage variableTwas proposed to characterize the degree of coal damage, and the relationship between damage and the initiation and expansion patterns of hydraulic fractures was elucidated. The results revealed that, the damage induced by vertical stress loading in the underlying coal significantly exceeded that in the unloading stage. When the vertical loading stress was below 11 MPa, the coal remained in an elastic stage with minimal damage. Loading between 11~15 MPa corresponded to the yielding stage, witnessing a substantial increase in damage. Loading between 15~18 MPa led to the strengthening stage, the pores and fractures of coal were gradually compacted. Damage variablesTcan effectively characterize the degree of internal damage of coal. WhenT=Tc, the internal damage of coal was comparable to that of a coal that has not disturbed by loading and unloading stress.Tc is the damage variable of coal without loading and unloading stress disturbance. WhenT＞Tc , the coal exhibited a stress-damaged state, with higher values ofTcorresponding to increasing damage levels. Conversely, whenT＜Tc, the coal demonstrated a stress-compacted state, with smaller values ofTindicating higher compaction degrees. The degree of stress damage in coal was negatively correlated with the fracture pressure, a high degree of damage made coal more prone to fracture, and it was favor to form the primary hydraulic fractures near the wellbore, which was conducive to hydraulic fracturing. The degree of compaction of coal was positively correlated with fracture pressure. The high degree of compaction weakened the effect of horizontal stress difference on hydraulic fracture expansion, and the hydraulic fracture near the wellbore was more developed, which hindered the formation of primary hydraulic fractures. Based on the research results, the principle of hydraulic fracturing construction scheme optimization for underlying coal seam in goaf was formulated and applied in the field. The hydraulic fracturing ability of the optimized scheme was significantly improved.
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Keywords:
- goafs /
- underlying coal seam /
- coalbed methane /
- hydraulic fracturing /
- damage variable /
- fracture expansion
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图 1 真三轴应力扰动试验系统
Figure 1. Connection diagram of true triaxial stress disturbance test system
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图 3 八通道声发射信号定位图
Figure 3. Eight-channel acoustic emission signal localization diagram.
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图 4 加卸载阶段垂向位移及声发射能量曲线
Figure 4. Vertical displacement and acoustic emission energy curves during loading and unloading stages.
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图 5 水力压裂过程中的水压及声发射特性
Figure 5. Water pressure and acoustic emission characteristics during hydraulic fracturing process
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图 6 损伤变量与初次破裂压力关系
Figure 6. Relationship between damage variable and initial fracturing pressure
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图 8 水力裂缝扩展形态
注：坐标轴为以压裂管中心为原点，表示试块的长度。a、b分别为水力裂缝的缝长与缝宽，mm。
Figure 8. Morphology of hydraulic fracture propagation.
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图 9 1号水平井第三段压裂施工曲线
Figure 9. Third section fracturing operation curve of No. 1 horizontal well
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图 10 2号水平井第六段压裂施工曲线
Figure 10. Third section fracturing operation curve of No. 2 horizontal well
表 1 相似模拟材料的物理力学参数
Table 1 Physical and mechanical parameters of cement mortar
孔隙率
$\phi$/%渗透率
K/10−3μm2单轴抗压强度
$ \sigma_ {\mathrm{c}} $/MPa弹性模量
$ E $/GPa抗拉强度
$ \sigma _{\mathrm{t}} $/MPa黏聚力
$ c$/MPa内摩擦角
$ \varphi /( ^\circ) $12.79 1.13 6.27 0.72 1.65 2.54 31.29 
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表 2 水力压裂试验方案
Table 2 Hydraulic fracturing test parameters
试块 垂向应力
$ {\sigma _{\mathrm{v}}} $/MPa最大水平应力
$ {\sigma _{\mathrm{H}}} $/MPa最小水平应力
$ {\sigma _{\mathrm{h}}} $/MPa水力压裂 1 8 16 10 是 2 10→12→8 16 10 是 3 10→14→8 16 10 是 4 10→16→8 16 10 是 5 10→18→8 16 10 是
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表 3 水力裂缝参数
Table 3 Hydraulic fracture parameters
试块 缝长a/mm 缝宽b/mm a/b 裂缝形态 1 310.2 196.5 1.58 水平 2 306.7 190.5 1.61 水平 3 301.3 184.2 1.64 水平 4 350.2 233.6 1.50 水平 5 295.0 211.3 1.39 水平
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