用水力压裂法获取煤储层地应力参数

地应力可反映煤储层的应力状况,它是认识和评价煤储层的重要参数之一。从水力压裂法获取煤储层地应力的原理和方法入手,通过对3组典型实测数据的分析,着重探讨了用时间平方根法和双对数曲线法在分析地应力时如何准确识别闭合压力和提高地应力测试精度,对煤层气工作者具有很好的借鉴价值。     

The use of the fracture reopening pressure in hydraulic fracturing stress measurements

Summary Practitioners of the hydraulic fracture stress measurement technique commonly estimate the hydraulic fracture tensile strength as the difference between the initial formation breakdown pressure and the fracture reopening pressure. This method, first suggested by Bredehoeft et al. (1976), was developed with the assumption that the stress state in the cracked formation is identical to the stress state in the uncracked formation. In this paper, a relationship for the fracture reopening pressure based on Linear Elastic Fracture Mechanics is developed that takes the hydraulically induced fracture into account. Situations in which the Bredehoeft technique can be appropriately applied are presented.     

水压致裂地应力测试方法在云南大理-丽江铁路隧道工程中的应用

利用水压致裂地应力测试方法对实际工程区进行了地应力测量,确定了工程区隧道围岩现今的地壳应力状态,即原地应力的大小和方向.根据地应力分布特征,分析了隧道开挖过程中发生岩爆等地质灾害的可能性.地应力测量数据的分析结果可作为隧道衬砌的设计、断面的选择及轴线方位的确定的科学依据.     

岩石非均质程度对水压致裂地应力测试方法影响的分析与讨论

准确揭示原位地应力状态,对地下工程开挖支护设计和长期稳定性分析等具有十分重要的意义。利用水压致裂技术开展了纱岭金矿主竖井地应力测试工作,获得了20个测段地应力状态;室内进行了主竖井测试孔岩芯的岩石力学试验,包括巴西劈裂试验、单轴压缩试验及声发射监测试验,获得了岩石空间非均质度和强度分布特征,并分析了岩石非均质度与水压致裂测试结果的关系。结果表明：主应力大小随测量深度近似呈线性增大,测试孔的最大水平主应力值为20.78~45.20 MPa,最小水平主应力值为14.94~35.33 MPa,平均最大水平主应力方向为 NW65°;测试孔岩芯各层位非均质度不同,变辉长岩非均质度系数为0.1~0.3,且岩石不同强度条件下声发射信号数量变化不显著,岩石离散度较小,花岗岩非均质度系数最高,可达1.0,以加载后期强相破裂产生的声发射信号为主;岩石非均质度影响水压致裂裂纹的扩展方向,扩展方向和最大水平主应力方向的夹角$\varphi $影响着最大、最小水平主应力的测量结果,且对最小水平主应力的影响尤为显著。分析水压致裂测量结果与岩石性质之间的关系,对精确探测非均质地层的地应力场分布规律具有一定的指导作用。     

基于FDEM-Flow研究地应力对水力压裂的影响

用提出的FDEM-Flow(考虑流固耦合的离散元-有限元耦合方法)方法作为工具,研究了地应力对水力压裂的影响。通过一个注水圆孔的算例,研究不同地应力状态对压裂裂隙的走向和形态的影响。研究结果表明,起裂压力的大小和压裂裂隙的方位均与地应力有密切关系。在竖向地应力v?保持不变的情况下,且侧压力系数?1时,随着?的增大,起裂压力和失稳压力均减小;?1时且取值较小时,主要产生水平向的裂隙,并有斜向裂隙产生;?1时且取值较大时,裂隙严格按照最大主压应力的方向扩展,不再出现斜向裂隙;?1时,主要产生竖向的裂隙;?=1时,水平地应力和竖向地应力相等,裂隙的扩展不存在优势方向。不同侧压力系数条件下,裂隙的扩展方向与最大主应力的方向一致,水力压裂裂隙的起裂和扩展主要由最大主拉应力控制,裂隙在拉应力集中的区域起裂。这些结果与已有的试验及理论认识是相符的,进一步验证了FDEM-Flow方法用于模拟水力压裂问题的有效性。     

模拟水压致裂的另一种二维FDEM-flow方法

可考虑岩石本身渗透性的FDEM-flow方法充分利用原FEMDEM中节理单元和三角形单元独特的拓扑连接关系,将节理单元作为流体流动的天然通道,基于立方定律来表征流体在岩石本身及裂隙中的流动,而岩石本身渗透性则通过标定合适节理单元张开度来表征。用一个含解析解自由面稳态渗流算例初步验证了本文渗流算法及标定岩石本身渗透率方法的正确性。最后计算了一个水力压裂算例,验证了该FDEM-flow方法可同时考虑岩石本身的渗透性和裂隙的渗透性,用简单的纯裂隙渗流完成了对复杂问题的处理。研究结果表明:修改后的FDEM-flow方法能够很好地再现压裂过程中流体压力的分布及裂隙的扩展,可简洁地求解水力压裂这一复杂力学的问题,适用范围更广。为模拟页岩气开采、干热岩地热获取中的水力压裂问题提供新的求解工具。     

水力压裂法地应力测量中的几个问题

回顾了应用水力压裂法测量地应力的基本原理。简要地分析了对水力压裂法地应力测量结果的可靠性影响较大的几个参数。最后,对水力压裂法在深部地应力测量中的应用及其在深钻孔地应力测量方面的发展前景也做了讨论,指出了今后深部地应力测量研究努力的方向。     

Interpretation of in situ stress measurements in anisotropic rocks with the doorstopper method

Summary A correct interpretation of in situ stress measurements by means of the doorstopper technique in cross anisotropic rocks requires that 10 stress concentration factors at the bottom of the borehole be known instead of the 3 which are needed for isotropic rocks. In order to analyse the data obtained in a site investigation in a gneiss of the Alpine region, these factors were determined by a three-dimensional high-order finite-element model. The results are presented and compared with those obtained by means of first approximation methods. The feasibility of carrying out measurements in anisotropic rock by means of only 2 boreholes is discussed and it is shown that in practice 3 properly oriented boreholes are needed in this case too.     

煤层水力压裂应力与裂隙演化的细观规律

水力压裂作为煤层强化增透技术的一种,其应力演化特征及裂隙形态与扩展范围的判断尤为重要。采用离散元数值方法,以导向压裂为背景,建立水力压裂流固耦合模型;通过应力路径、裂纹热点图等手段,探究水力压裂过程中压裂排量、泊松比、天然裂隙密度对应力演化和裂隙演化的影响及其细观规律。结果表明:不同压裂排量下的应力演化方向及最终应力路径曲线形状有着明显的不同,低排量下裂隙附近的应力比值逐渐增大,而在高排量下先增大后减小;煤层泊松比越大,平均压裂半径越低,但对起裂时间及裂隙的扩展形态影响不明显;天然裂隙的发育情况对水力裂隙的扩展起着关键性作用,高裂隙发育煤层水力裂隙扩展的方向性无法预测,应力演化方向会出现反转现象;压裂过程中不同区域的应力演化特征能够反映出裂隙的扩展状态,现场可通过监测压裂区域附近应力变化,判断水力压裂缝网的扩展范围。      

Effect of plastic deformation on hydraulic fracturing with extended element method

The propagation of hydraulic fracture in elastic rocks has widely been investigated. In the paper, we shall focus on numerical modeling of hydraulic fracturing in a class of porous rocks exhibiting plastic deformation. The plastic strain of porous rocks is described by a non-associated plastic model based on Drucker–Prager criterion. The plastic deformation is coupled with fluid pressure evolution described by the lubrication theory. An extended finite element method is used for modeling the propagation of fracture. The fracture propagation criterion is based on the J-integral. The proposed numerical model is validated by comparisons with numerical and analytical results. The influence of plastic deformation on fracture propagation process is investigated.

     

基于水压致裂法的岩石抗拉强度研究

为了揭示水压致裂法和巴西劈裂法测量岩石抗拉强度的关系,开展了理论和现场试验研究。基于经典的水压致裂法理论,推导了不同围压下钻孔破裂压力和抗拉强度。利用断裂力学理论建立了水压致裂法和巴西劈裂法测得抗拉强度的关系。利用预制切槽方法模拟天然裂纹,对水力裂缝的起裂压力进行了研究。结果表明：围压为最大主应力等于3倍最小主应力测得的抗拉强度大于围压为0测得的抗拉强度;水压致裂法和巴西劈裂法测量抗拉强度关系与应力场、裂纹长度、断裂韧度3个变量有关;通过在晋城矿区王台铺矿的预制切槽试验,运用断裂力学建立的抗拉强度计算式更为符合现场实际。研究结果可为坚硬难垮落顶板预制切槽的水力压裂设计提供参考。     

Formation stability after hydraulic fracturing

This paper investigates stress changes resulting from fracturing in a weak formation and estimates the reduced risk of formation failure. The analysis is based on fracture propagation and closure of a plane strain elasto–plastic fracture. It is shown that during fracture propagation the area near the fracture tip undergoes plastic deformation, with the result that the in situ stresses there are significantly reduced from the original compressive state. The stress relief is driven by the reduction of the minimum in situ stress and the consistency condition which requires the stress state to remain on the yield or failure envelope. After fracture closure, due to permanent deformation the stress state does not return to its original state, as in the case of elastic material. The risk of formation failure, which is quantified with the introduction of a yield factor, is significantly reduced after fracturing and closure. The residual width from plastic deformation results in a non-uniform closure stress on proppant with higher values near the tip and lower value near the wellbore which is detrimental to the stability of proppant. The closure stress becomes more uniform with increasing fracture length. Copyright © 1999 John Wiley & Sons, Ltd.     

Three-dimensional analysis of hydraulic fracturing

The formulation, development, implementation and application of a complete three-dimensional hydraulic fracturing simulator, 3DHFRAC, is described. The simulator consists of two principal components: the first represents the solid deformation, crack opening and propagation, using a surface integral scheme, hybridised with finite-elements as needed, and a condition of critical stress-intensity-factor or energy-release-rate to track the motion of the fracture perimeter; the second captures the fluid flow along the open fracture, and into the reservoir. The simulator has been tested extensively by comparison to analytical results, simpler models and laboratory experiments for which good agreement is obtained. It is finally used to study the general growth of 3-D fracture propagation under different fluid and reservoir conditions.     

基于煤储层水力压裂动态渗透率变化的压裂效果评价方法及其应用

压裂施工曲线是反映压裂效果的重要依据,而压裂阶段储层渗透率的动态变化能够更直观地反映造缝效果。借鉴试井渗透率测试原理,建立一种压裂阶段储层动态渗透率定量评价方法,并将该方法应用到准南某区块2口煤层气井水力压裂效果评价中,获得压裂阶段储层动态渗透率曲线;同时采用G函数对压裂效果进一步评价。结果表明:动态渗透率曲线所反映压裂效果与G函数分析和基于排量、井底流压关系的评价结果吻合较好,能够反映储层内裂缝开启、延伸效果;其中,CMG-01井通过实施煤储层与围岩大规模缝网改造,压裂阶段储层渗透率最高达到2.5 μm2,造缝效果良好;而CBM-02井实施煤储层常规水力压裂,储层渗透率保持在1.8 μm2之下,显示出煤储层常规水力压裂与煤储层?围岩大规模缝网改造的差异性。动态渗透率定量评价方法弥补前期压裂改造效果缺乏量化评价的不足,为煤层气/煤系气储层水力压裂工艺的优化提供依据。      

In situ magnetic measurements on igneous rock bodies

A simple method for an in situ measurement of susceptibility and low-field hysteresis on rock bodies like dykes and flows is described. Extensive use of this method indicates that in some dykes and flows, the nature of the magnetic grains varies rapidly across the bodies. Making use of such observations, it seems possible to select stable rock samples for palaeomagnetic work and also understand to some extent the origin of remanent magnetization in different types of rock bodies.     

In situ strength measurements on natural upper-mantle minerals

Using in situ strength measurements at pressures up to 10 GPa and at room temperature, 400, 600, and 700°C, we examined rheological properties of olivine, orthopyroxene, and chromian-spinel contained in a mantle-derived xenolith. Mineral strengths were estimated using widths of X-ray diffraction peaks as a function of pressure, temperature, and time. Differential stresses of all minerals increase with increasing pressure, but they decrease with increasing temperature because of elastic strain on compression and stress relaxation during heating. During compression at room temperature, all minerals deform plastically at differential stress of 4–6 GPa. During subsequent heating, thermally induced yielding is observed in olivine at 600°C. Neither orthopyroxene nor spinel shows complete stress relaxation, but both retain some stress even at 700°C. The strength of the minerals decreases in the order of chromian-spinel ≈ orthopyroxene > olivine for these conditions. This order of strength is consistent with the residual pressure of fluid inclusions in mantle xenoliths.     

压裂液黏度对循环水力压裂影响的试验研究

中国石油大学（北京）石油天然气工程学院,北京 102249     

煤层定点水力压裂防冲的机制研究

为实现在掘进或回采前对冲击危险煤层的一次性卸压,达到区域防冲的目的,提出了煤层定点水力压裂技术。为了从理论上说明煤层定点水力压裂技术防治冲击地压的可行性和有效性,首先,通过建立煤层压裂力学模型,从定性和定量两方面分析煤层定点水力压裂防冲的机制,认为煤层压裂通过增阻和降能2方面实现冲击地压的防治;提出了基于压裂效能指数In的压裂防冲效果评价方法以及关键冲击块临界体积的估算公式,为煤层压裂施工参数的选择提供参考;最后,在华丰煤矿1412工作面成功应用该项技术,从现场观测、管路压力、微震事件和应力变化4方面验证了煤层定点水力压裂技术防治冲击地压的效果。     

In situ stress field of eastern Australia

New in situ data based on hydraulic fracturing and overcoring have been compiled for eastern Australia, increasing from 23 to 110 the number of in situ stress analyses available for the area between and including the Bowen and Sydney Basins. The Bowen Basin displays a consistent north‐northeast maximum horizontal stress (σ_H) orientation over some 500 km. Stress orientations in the Sydney Basin are more variable than in the Bowen Basin, with areas of the Sydney Basin exhibiting north‐northeast, northeast, east‐west and bimodal σ_H orientations. Most new data indicate that the overburden stress (σ_V) is the minimum principal stress in both the Bowen and Sydney Basins. The Sydney Basin is relatively seismically active, whereas the Bowen Basin is relatively aseismic. Despite the fact that in situ stress measurements sample the stress field at shallower depth than the seismogenic zone, there is a correlation between the stress measurements and seismicity in the two areas. Mohr‐Coulomb analysis of the propensity for failure in the Sydney Basin suggests 41% of the new in situ stress data are indicative of failure, as opposed to 13% in the Bowen Basin. The multiple pre‐existing structural grains in the Sydney Basin further emphasise the difference between propensity for failure in the two areas. Previous modelling of intraplate stresses due to plate boundary forces has been less successful at predicting stress orientations in eastern than in western and central Australia. Nonetheless, stress orientation in the Bowen Basin is consistent with that predicted by modelling of stresses due to plate boundary forces. Variable stress orientations in the Sydney Basin suggest that more local sources of stress, such as those associated with the continental margin and with local structure, significantly influence stress orientation. The effect of local sources of stress may be relatively pronounced because stresses due to plate boundary forces result in low horizontal stress anisotropy in the Sydney Basin.