New development of hydraulic fracturing technique for in-situ stress measurement at great depth of mines

In-situ stress measurement using the hydraulic fracturing technique was made at Wanfu Coal Mine in Shandong Province, China. To solve problems caused by great measuring depth and extra thick overburden soil layers in the mine, a series of improved techniques were developed for the traditional hydraulic fracturing technique and equipment to increase their pressure-enduring ability and to ensure safe and flexible removal of the sealing packers with other experimental apparatus. Successful in-situ stress measurement at 37 points within 7 boreholes, which were mostly over 1000 m deep, was completed. Through the measurement, detailed information of in-situ stress state has been provided for mining design of the mine. The improved hydraulic fracturing technique and equipment also provide reliable tools for in-situ stress measurement at great depth of other mines.     

基于均匀设计法的水压致裂模拟试验优化研究

水压致裂技术作为一种绝对应力测量方法广泛地运用在原地应力测量领域,在其测量过程中影响岩石破裂值的流体力学影响因素主要是压裂液的注液速率、黏度和密度等,因此应开展流体力学影响因素的室内模拟试验,定量分析各因素对于原地应力测量准确度的影响。由于因素数量较多且该试验对于岩芯试块为破坏性试验,因此试验设计较为复杂且次数较多,全面试验或正交试验方案将严重影响试验实际操作和试验效果。本文在理论分析流体力学因素对于裂缝扩展和破裂压力影响基础上,提出一种基于混合水平均匀设计方法的水压致裂模拟试验优化方案,根据压裂液的注液速率、黏度和密度等影响因素及其参数值（水平数）构建均匀试验设计方案,利用DPS数据处理软件获取最优混合水平的均匀设计表,然后根据最优均匀设计表选择不同流体力学参数的典型点进行试验。最后将试验结果与部分全面试验结果进行对比分析,证明优化试验的岩石破裂值均处于全面试验测量区间内,印证了不同流体力学因素对于岩石破裂值的影响效应。本文中优化算法试验方案通过较少的试验次数简化了试验流程,试验次数仅为正交试验的四分之一,因此显著提高了水压致裂多影响因素模拟试验效率,对于后续建立不同流体力学影响因素的破裂压力修正公式与补偿模型提供了一种快速有效的试验方法,对原地应力进一步精确测算具有一定的积极意义。     

龙马溪组页岩可压性实验评价

针对目标区块龙马溪组露头制作岩样,分别进行矿物组分测量、岩石力学测定及水力压裂实验,以验证目标区块页岩的可压性。测得储层岩样平均脆性指数为56.65,水平应力差异系数为0.11,可知该层在进行水力压裂时能形成复杂的裂缝网络。通过岩样水力压裂破坏实验,发现岩样水力压裂后裂缝条数较多,平均为38条,裂缝网络较复杂。最后验证了该层页岩良好的可压性,在水力压裂施工时可以形成复杂缝网。     

页岩暂堵转向压裂水力裂缝扩展物模试验研究

暂堵转向压裂是提高页岩储层重复压裂缝网改造效果的重要手段，然而该压裂工艺对页岩水力裂缝扩展规律的影响机理尚不明确。因此，基于室内真三轴压裂物模试验，研究页岩暂堵转向压裂裂缝起裂与扩展规律，分析地应力及注入速率等因素对水力裂缝延伸行为的影响，并通过油藏改造面积(SRA)定量表征暂堵转向压裂缝网改造效果。结果表明，暂堵剂可有效封堵初始裂缝，憋压促使水力裂缝发生缝内转向或激活原生天然裂缝系统，提高页岩缝网复杂程度；根据最终水力裂缝形态展，可划分为4种基本类型，包括台阶状缝、激活天然弱面的横切缝、简单多裂缝以及复杂多裂缝网络；不同地应力差异条件下暂堵机理及裂缝延伸规律不同，当水平应力差小于12 MPa时，暂堵剂通过封堵近井筒初始张开的原生层理或天然裂缝，诱导产生与天然裂缝斜交的二次裂缝，当应力差高于15 MPa时，近井筒天然弱面难以激活，暂堵剂通过封堵垂直井筒的初始横切裂缝，诱发形成平行多横切裂缝；增大注液排量可以提高多裂缝形成几率及裂缝沟通面积。     

拖市天然裂缝型低渗透油田压裂工艺及机理研究

采用岩石力学试验,可以求取拖市油田产层、隔层的物性参数,为压裂设计掌握基础资料。在生产中,用Kaiser效应测量对应地层的地应力大小,计算地应力剖面,是为了给压裂设计提供基础数据。同时,还要计算受地应力影响的近井摩阻、压裂液效率、裂缝闭合压力。计算结果与实验结果表明,拖市油田的水力裂缝在近井地带发生偏转,在这种情况下,裂缝的动态缝宽较小,水力压裂应以低砂比、长时间注入为原则,以便降低砂堵风险。     

Guiding-controlling technology of coal seam hydraulic fracturing fractures extension

Aiming at the uncontrollable problem of extension direction of coal seam hydraulic fracturing, this study analyzed the course of fractures variation around the boreholes in process of hydraulic fracturing, and carried out the numerical simulations to investigate the effect of artificial predetermined fractures on stress distribution around fractured holes. The simulation results show that partial coal mass occurs relatively strong shear failure and forms weak surfaces, and then fractures extended along the desired direction while predetermined fractures changed stress distribution. Directional fracturing makes the fractures link up and the pressure on coal mass is relieved within fractured regions. Combining deep hole controlling blasting with hydraulic fracturing was proposed to realize the extension guiding-controlling technology of coal seam fractures. Industrial experiments prove that this technology can avoid local stress concentration and dramatically widen the pressure relief scope of deep hole controlling blasting. The permeability of fractured coal seam increased significantly, and gas extraction was greatly improved. Besides, regional pressure relief and permeability increase was achieved in this study.     

沥青混凝土心墙土石坝水力劈裂计算分析

分析了土石坝水力劈裂原理,针对新疆吐鲁番二塘沟沥青混凝土心墙土石坝,选取河床部位最大断面为典型断面,采用有限元法对其进行水力劈裂计算分析,得出:该土石坝心墙任一高程处的中主应力都小于竖向应力,且中主应力和竖向应力都大于水压力。因此,该坝的心墙不会发生水力劈裂。     

Fixed-length roof cutting with vertical hydraulic fracture based on the stress shadow effect: A case studyOA

Pre-driven longwall retracement roadway(PLRR) is commonly used in large mine shaft. The support crushing disasters occur frequently during the retracement, and roof management is necessary. Taking the 31107 panel as research background, the roof breaking structure of PLRR is analyzed. It is concluded that the roof cutting with vertical hydraulic fracture(HF) at a specified position, that is, fixed-length roof cutting, can reduce support load and keep immediate roof intact. The extended finite el...     

Measurement and study of the distributing law of in-situ stresses in rock mass at great depth

To solve the technical cruxes of the conventional system in deep rock mass, an automatic testing system for hydraulic fracturing that includes a single tube for hydraulic loop, a pressure-relief valve, central-tubeless packers, and a multichannel real-time data acquisition system was used for in-situ stresses measurement at great depths （over 1000 m） in a coalfield in Juye of Northern China. The values and orientations of horizontal principal stresses were determined by the new system. The virgin stress field and its distributing law were decided by the linear regression from the logged 37 points in seven boreholes. Besides, the typical boreholes arranged in both the adjacent zone and far away zone of the faults were analyzed, respectively. The results show that a stress concentration phenomenon and a deflection in the orientation of the maximal horizontal stress exist in the adjacent zone of the faults, which further provides theoretical basis for design and optimization of mining.     

Phenomenon of methane driven caused by hydraulic fracturing in methane-bearing coal seams

The methane concentration of the return current will always be enhanced to a certain degree when hydraulic fracturing with bedding drilling is implemented to a gassy coal seam in an underground coal mine. The methane in coal seam is driven out by hydraulic fracturing. Thus, the phenomenon is named as methane driven effect of hydraulic fracturing. After deep-hole hydraulic fracturing at the tunneling face of the gassy coal seam, the coal methane content exhibits a ‘‘low-high-low" distribution along excavation direction in the following advancing process, verifying the existence of methane driven caused by hydraulic fracturing in methane-bearing coal seam. Hydraulic fracturing causes the change of pore-water and methane pressure in surrounding coal. The uneven distribution of the pore pressure forms a pore pressure gradient. The free methane migrates from the position of high pore(methane) pressure to the position of low pore(methane) pressure. The methane pressure gradient is the fundamental driving force for methane-driven coal seam hydraulic fracturing. The uneven hydraulic crack propagation and the effect of time(as some processes need time to complete and are not completed instantaneously) will result in uneven methane driven. Therefore, an even hydraulic fracturing technique should be used to avoid the negative effects of methane driven; on the other hand, by taking fully advantage of methane driven, two technologies are presented.     

Fractured rock mass hydraulic fracturing under hydrodynamic and hydrostatic pressure joint action

According to the stress state of the crack surface, crack rock mass can be divided into complex composite tensile-shear fracture and composite compression-shear fracture from the perspective of fracture mechanics. By studying the hydraulic fracturing effect of groundwater on rock fracture, the tangential friction force equation of hydrodynamic pressure to rock fracture is deduced. The hydraulic fracturing of hydrostatic and hydrodynamic pressure to rock fracture is investigated to derive the equation of critical pressure when the hydraulic fracturing effect occurs in the rock fracture. Then, the crack angle that is most prone to hydraulic fracturing is determined. The relationships between crack direction and both lateral pressure coefficient and friction angle of the fracture surface are analyzed. Results show that considering the joint effect of hydrodynamic and hydrostatic pressure, the critical pressure does not vary with the direction of the crack when the surrounding rock stationary lateral pressure coefficient is equal to 1.0. Under composite tensile-shear fracture, the crack parallel to the direction of the main stress is the most prone to hydraulic fracturing. Under compression-shear fracture, the hydrodynamic pressure resulting in the most dangerous crack angle varies at different lateral pressure coefficients; this pressure decreases when the friction angle of the fracture surface increases. By referring to the subway tunnel collapse case, the impact of fractured rock mass hydraulic fracturing generated by hydrostatic and hydrodynamic pressure joint action is calculated and analyzed.     

水力压裂优化设计方法研究

综合根据施工规模预测压裂效果和根据增产要求设计施工规模的两种设计思想体系,将裂缝三维延伸模拟技术应用于压裂施工设计,提出了从地层条件出发、以最佳的压裂效果为目标的水力压裂优化设计方法,通过不断地自动调整各段压裂液体积、地面加砂浓度、支撑剂类型和粒径等施工参数,设计出最优的施工方案,并在此基础上,采用ＶｉｓｕａｌＢａｓｉｃ 语言研制了一套Ｗｉｎｄｏｗｓ 环境下的三维压裂优化设计软件,在现场应用中取得了令人满意的施工效果。     

Numerical modeling of simultaneous hydraulic fracturing in the mode of multi-well pads

In order to investigate propagation regularity of hydraulic fractures in the mode of multi-well pads, numerical modeling of simultaneous hydraulic fracturing of multiple wells was conducted. The mathematical model was established coupling rock deformation with fluid flow in the fractures and wellbores. And then the model was solved by displacement discontinuity method coupling with implicit level set method. The implicit method was based on fracture tip asymptotical solution and used to determine fracture growth length. Simulation results showed that when multiple wells were fractured simultaneously, adjacent fractures might propagate towards each other, showing an effect of attraction other than repulsion. Fracture spacing and well spacing had significant influence on the propagation path and geometry of multiple fractures. Furthermore, when multiple wells were fractured simultaneously, stress reversal regions had a large area, and stress reversal regions were distributed not only in the area between fractures but also on the outside of them. The area of stress reversal regions was related to fracture spacing and well spacing. Results indicated that multi-well fracturing induced larger area of stress reversal regions than one-well fracturing, which was beneficial to generating complex fracture network in unconventional reservoirs.     

黏土心墙坝水力劈裂发生机理及判别方法

采用基于Biot固结理论的有效应力分析方法,对某黏土心墙土石坝进行了数值计算,分析坝体的有效小主应力、总小主应力及应力水平的分布特点,并在分析黏土心墙坝水力劈裂发生机理的基础上,对坝体心墙发生水力劈裂的可能性进行判断.结果表明:分别采用有效应力法和总应力法判断水力劈裂是否发生,二者结果是一致的;此外,蓄水后心墙上游侧应力水平较大,其可能的剪切破坏会导致心墙产生裂缝,进而引发心墙水力劈裂,因此,判断心墙水力劈裂是否发生时,应考虑应力水平的影响.     

控制致裂岩体应力测试原理

阐明了控制致裂岩体应力测试方法的实质和用控制致裂法测试岩体应力的依据。介绍了各种控制致裂测试钻孔布置原则及原岩应力的计算方法。讨论了控制致裂岩体应力测试中应该注意的问题。     

水力压裂煤储层卸压增透技术的适用性分析

水力压裂煤储层技术在不同矿区应用过程中受不同煤体破坏类型和围压条件的影响,其卸压增透效果差异性较大.为了明确该项技术的井下适用条件,优化其实施工艺,切实提高煤层瓦斯抽采率,通过分析和总结河南省不同矿区实际煤储层的水力压裂试验数据资料发现,水力压裂增透技术对Ⅰ,Ⅱ破坏类型煤体的增透效果比较明显,而在Ⅲ,Ⅳ,Ⅴ破坏类型的松软煤层中适用性则具有一定的局限性,具有压裂范围小、裂缝闭合快、增透效果不明显等特点.所得结论对于选择合理的水力压裂储层并进一步优化完善高效预抽本煤层瓦斯技术具有一定的指导意义.     

井下低频脉动水力压裂技术参数优化及现场应用

对于地层压力高、岩石破裂压力大的储层,水力压裂施工面临井口泵压大、设备要求高的难题。井下低频脉动水力压裂技术通过在井底产生低频脉动,可以显著降低岩石起裂压力和延伸压力,提高压裂增产效果。通过室内实验以及理论计算,对低频脉动水力压裂技术的脉动频率及脉动压力幅值两个关键参数进行了优化。结果表明:当脉动频率为20 Hz时,岩石的破坏最为严重; 脉动频率越高,振幅衰减越快; 岩石的抗压强度与脉动压力幅值成反比,幅值越大,岩石的抗压强度越低。最后,在陕北地区低渗透油田进行了3口井的低频脉动水力压裂现场试验。试验结果表明,低频脉动水力压裂后的试验井与相邻常规压裂井相比,砂堵问题明显减少,且在降低施工压力方面具有显著效果。     

Variable frequency of pulse hydraulic fracturing for improving permeability in coal seam

Variable frequency, a new pattern of pulse hydraulic fracturing, is presented for improving permeability in coal seam. A variable frequency pulse hydraulic fracturing testing system was built, the mould with triaxial loading was developed. Based on the monitor methods of pressure sensor and acoustic emission, the trials of two patterns of pulse hydraulic fracturing of single frequency and variable frequency were carried out, and at last fracturing mechanism was analyzed. The results show that the effect of variable frequency on fracture extension is better than that of single frequency based on the analysis of macroscopic figures and AE. And the shortage of single frequency is somewhat remedied when the frequency is variable. Under variable frequency, the pressure process can be divided into three stages: low frequency band, pressure stability band and high frequency band, and rupture pressure of the sample is smaller than that of the condition of single frequency. Based on the Miner fatigue theory, the effect of different loading sequences on sample rupture is discussed and the results show that it is better to select the sequence of low frequency at first and then high frequency. Our achievements can give a basis for the improvement and optimization of the pulse hydraulic fracturing technology.     

Effect of ground stress on hydraulic fracturing of methane well

Most of the coal reservoirs in China are of low-permeability, so hydraulic fracturing is widely used to improve the permeability in the extraction of gas by ground drilling. The ground stress around the well was analyzed by using theory of elasticity. The pressure when the well fractured is formulated and the effect of ground stress on pressure is discussed. The effect of ground-stress-differences on hydraulic fracturing was analyzed by using the numerical software RFPA2D-Flow in reference to the tectonic stress in Jincheng coal area. The results show that: 1) the position where initial fracture appears is random and fracture branches emerge when the fractures expand if ground stresses in any two directions within a horizontal plane are equal; 2) otherwise, the fractures expand in general along the direction of maximum ground stress and the critical pressure decreases with increasing ground-stress-differences and 3) the preferred well-disposition pattern is diamond shaped. The preferred well spacing is 250 m×300 m. This study can provide a reference for the design of wells.     

煤层气垂直井重复水力压裂综合评价方法研究

系统分析了重复水力压裂选井的主要影响因素,建立了重复水力压裂选井事故树模型;根据多层次模糊数学综合评价方法结合事故树模型,建立了煤层气垂直井重复水力压裂选井评价指标体系.根据评价指标体系,对沁水盆地东南樊庄区块部分产气效果不好的煤层气井能否对二次压裂进行评价.结果表明,该方法能较好地为现场煤层气垂直井重复水力压裂选井提供理论依据,投资风险大大降低.