Numerical study of hydraulic fracturing in the sectorial well-factory considering well interference and stress shadowing

In the Changqing Oilfield in northwest China, when traditional petroleum exploitation encounters forestry reserves or water source protection areas, sectorial well-factory design is proposed. The most distinct feature of a sectorial well-factory is the deviation of the well from the minimum horizontal principal stress, resulting in hydraulic fracture deflection after the initiation, along with possible well interference (i.e., fracture hit) and fracture coalescence in the oblique wells. Four indexes describing well deflection are then proposed according to fracture morphology. Several fracturing designs, including stage arrangement, fracturing sequences, and fracturing techniques are applied to study the feasibility of the sectorial well-factory design. The results show that the “gradual” or “sparse” stage arrangement, large injection rate, and simultaneous multifracture treatment can help to optimize the fracture morphology and stimulation design. However, the subsequent stress shadowing effect usually adversely affects the fracturing of adjacent wells. With a small initial horizontal stress difference, large injection rate and staggered stage arrangement can achieve ideal stimulation performance. Our results can provide a guidance for optimizing stimulation design in unconventional well-factory while taking into account environmental protection.

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

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 element method(XFEM)is applied to simulate hydraulic fracturing.The results show that both the axial and transverse hydraulic fracturing cannot effectively create vertical HFs.Therefore,a novel construction method of vertical HF based on the stress shadow effect(SSE)is proposed.The stress reversal region and HF orientation caused by the prefabricated hydraulic fracture(PF)are verified in simulation.The sub-vertical HFs are obtained between two PFs,the vertical extension range of which is much larger than that of directional hydraulic fracturing.The new construction method was used to determine the field plan for fixed-length roof cutting.The roof formed a stable suspended structure and deformation of the main PLRR was improved after hydraulic fracturing.

On stress-forecasting strategy of earthquakes from stress buildup,stress shadow and stress transfer(SSS) based on numerical approach

Global Positioning System （GPS） and Interferometric Synthetic Aperture Radar （InSAR）, used for monitoring crust deformation, are found to be very promising in earthquake prediction subject to stress-forecasting. However, it is recognized that unless we can give reasonable explanations of these curious precursory phenomena that continue to be serendipitously observed from time to time, such high technology of GPS or InSAR is difficult to be efficiently used. Therefore, a proper model revealing the relation between earthquake evolution and stress variation, such as the phenomena of stress buildup, stress shadow and stress transfer （SSS）, is crucial to the GPS or InSAR based earthquake prediction. Here we address this question through a numerical approach of earthquake development using an intuitive physical model with a map-like configuration of discontinuous fault system. The simulation provides a physical basis for the principle of stress-forecasting of earthquakes based on SSS and for the application of GPS or InSAR in earthquake prediction. The observed SSS associated phenomena with images of stress distribution during the failure process can be continuously simulated. It is shown that the SSS are better indicators of earthquake precursors than that of seismic foreshocks, suggesting a predictability of earthquakes based on stress-forecasting strategy.

“Giant pressure shadow” structure and ore-finding method of tectonic stress field in the Tongchang Cu-Au polymetallic orefield,Shaanxi,China:Ⅱ.Dynamics of tectonic ore-forming processes and prognosis of concealed ores

The Tongchang orefield is located in the central part of the Mianxian-Lueyang-Yangpingguan area that is celebrated as a 'gold triangle' area,at the juncture of the latitudinal tectonic zone of South Qinling,the Longmenshan Cathysian tectonic zone and the Sichuan-Yunnan longitudinal tectonic zone,where there are distributed Cu-Au polymetallic ore deposits(occurrences) including the Tongchang,Chenjiaba,Qinjiabian,Hongtushi,Yinshangou and Xiakouyi ore deposits(mineralization).Based on the "giant pressure shadow" structure put forward and demonstrated by numerical modeling of the tectonic stress field and the static photoelasticity experiments on the basis of tectonic ore-controlling laws in the orefield,tectonic metallogenesis driven by orefield tectonic stress has been discussed in terms of its geological setting,orefield geomechanics,and tectonic stress field.It is thought that the dynamic evolution model of the tectonic stress field controls the whole process of formation of the polymetallic ore deposits(mineralization) in the orefield,as well as the deformation field.As a result,it controls the emplacement of rockbodies and the transformation of ore-source bodies,and provides both the channel-ways for ore-forming fluids and ore-hosting space.Furthermore,it controls the migration potential field of fluids and,thereafter,its flow direction,rate and volume;the tectonic stress field also controls the energy field and hence controls the position of occurrence of ore deposits and their scale.The method of tectonic stress field has been applied to ore prognosis in the orefield.The rules of magmatic emplacement and metallogenic fluid migrating and concentrating under the control of the structural stress field were expounded,hence providing the theoretical basis for the prognosis of concealed ores.In addition,a number of important target areas have been defined.

ShadowDB:基于SQL引擎的全链路压力测试分流系统

全链路压力测试作为一种新兴的软件测试技术,直接在生产系统中做压力测试,旨在准确评估线上环境的性能。基于影子库的数据分流技术能够保证全链路压力测试过程中生产数据不受测试污染。基于SQL引擎技术设计并实现了一套完整的针对全链路压力测试的开源数据分流系统ShadowDB,其基本思想是通过SQL解析、SQL路由对用户请求正确分流。ShadowDB目前支持6种关系型数据库系统中所有SQL语句的数据分流;提出了2种数据分流算法:基于列的影子算法和基于Hint的影子算法;实现了所有JDBC的接口,允许线上应用程序无需修改代码即可使用ShadowDB;ShadowDB嵌入应用程序中,无需网络转发请求,对请求效率影响很小,保证了全链路压力测试结果的可靠性。使用2个通用的性能测试工具进行了大量的实验,结果表明ShadowDB的性能远远高于对比方案的。

多级水力压裂应力阴影效应的数值分析

多级水力压裂作为储层改造的主要技术手段,广泛用于提高致密油气藏的气体产量。裂缝在扩展过程中的应力阴影效应对于有效连通储层的天然裂缝并形成有利于油、气体流动的复杂裂缝网络至关重要。本研究采用了自适应的有限元-离散元方法,通过改进网格自动细化和识别多个裂缝扩展模拟含天然裂缝储层的多级水力压裂。数值模型中考虑了水力裂缝、天然裂缝和微观孔隙结构的相互作用,整合了非线性的Carter滤失模型来描述多级压裂过程中压裂液的滤失效应。引入了理想平行板流的支撑剂运输方程,并采用了达西定律来分析裂缝网络中的流体渗透效应。通过比较无裂缝均质模型和天然裂缝模型的裂缝网络和流体流动,评估了天然裂缝对多级压裂裂缝行为和气体产量的影响。研究为确定和优化致密气藏中压裂簇间距提供了一种新的方法。

不同射孔角度对页岩压裂效果的影响

水力压裂是非常规油气开采的关键技术。在进行水力压裂的过程中,水力裂缝和天然裂隙相互作用形成复杂缝网是非常规油气高效开采的前提,而射孔方案设计对缝网的复杂程度有着重要影响。为了更深入研究压裂缝网的演化机理,模拟真实节理裂隙岩体在水力压裂中的裂缝扩展过程,从而科学、合理地优化射孔方案,本文采用全局嵌入cohesive单元的方法,利用Python进行二次开发创建DFN离散裂隙网络,建立了考虑离散裂隙网络的岩体天然裂隙模型,同时设置相同条件下不考虑DFN离散裂隙网络的模型进行对比,研究不同角度射孔方案对压裂效果的影响。通过MATLAB编程提取加载过程中的声发射定位图和声发射能量数据,对裂纹破裂机理和扩展过程进行了进一步分析。结果表明:天然裂缝的活化受射孔间产生的应力阴影、射孔与天然裂缝的角度影响;查明页岩的天然裂缝分布规律,设法提高射孔与天然裂缝的角度,使水力裂缝更容易与天然裂缝沟通并沿着天然裂缝扩展,从而形成复杂缝网。

裂缝性页岩气储层三维多簇水力裂缝竞争扩展数值模拟

页岩气水平井多簇水力裂缝延伸效率是影响压裂效果的关键,现场监测发现大量射孔簇无法进液、多簇非均匀起裂扩展问题突出,深入认识三维空间中天然裂缝对多簇水力裂缝竞争扩展的影响规律具有重要意义。为此,基于离散格子方法建立三维全耦合随机裂缝性页岩储层三簇压裂数值模型,精细刻画多簇水力裂缝竞争起裂、扩展以及天然裂缝相互作用的全过程演化规律,研究地应力差、簇间距、压裂液黏度及排量等参数影响下多簇水力裂缝扩展行为,提出多簇延伸效率的施工对策。结果表明:水力裂缝与天然裂缝相遇时表现为穿透、转向、分叉等单一或多种复合模式;天然裂缝网络会加剧多簇裂缝竞争扩展时的应力阴影效应,中间受到抑制的射孔簇容易沿天然裂缝转向或者分叉并与其他射孔簇发生融合,降低整体改造面积;地应力差越小、簇间距越小,缝间应力干扰越明显,裂缝延伸规模越小;增大压裂液黏度或排量,中间簇水力裂缝受应力干扰程度减弱,延伸距离更远,建议现场在前置液阶段适当提高压裂液黏度、在主压裂阶段增大排量,以提高多簇水力裂缝的延伸效率。研究成果可为页岩气储层射孔分段以及压裂参数优化设计提供指导。

水力压裂过程中射孔冲蚀效应的数值模拟研究

针对水力压裂过程中射孔冲蚀效应使得射孔簇处射孔数目难以确定的问题,使用FrackOptima全三维水力压裂软件和前期研究建立的射孔冲蚀模型建立算例。通过边界元进行网格离散,使用时间步长算法进行流固耦合数值模拟,定量研究射孔冲蚀效应对裂缝最终形态、压裂液流量分布、射孔摩阻、射孔直径、流量系数、井底压力这6个压裂结果参数的影响规律。研究结果表明:传统非限流压裂过程中射孔摩阻的值和射孔冲蚀导致的摩阻瞬时动态变化较小,因此可以忽略射孔冲蚀效应;而在使用同时多簇限流压裂方法开发非常规油气的过程中,射孔摩阻的值较大且射孔冲蚀导致的摩阻瞬时动态变化较明显,所以必须考虑射孔冲蚀效应。在算例参数条件下,选取8孔/簇可以获得较合适的射孔摩阻,从而较好地平衡应力阴影效应,获得较均匀的多条裂缝发育;同时,确保井底压力在实际压裂安全范围内。

水力压裂分段射孔簇多裂缝空间偏转模拟研究

深部致密油气储层水力压裂工程形成复杂缝网形态是影响油气采收率的关键因素,需要准确评估和优化压裂裂缝扩展行为。水平井多射孔簇分段压裂涉及储层和孔隙−裂隙内流体之间的热扩散、流体流动与岩体基质变形,热扩散效应和多物理场耦合作用是深部致密岩体压裂的典型特征;同时,压裂缝网扩展与裂缝间的扰动作用有关,压裂工艺中的射孔簇间距、起裂顺序等造成平行裂缝发生不同程度的非稳定扩展。理解多物理场耦合、裂缝间扰动等内外因素的影响机制,对有效评估压裂缝网具有重要意义。综合考虑深部储层的热−流−固耦合效应,研究水力压裂缝网三维扩展之间的应力阴影效应和多裂缝扰动偏转行为。研究建立水平井分段压裂的工程尺度三维数值模型,利用典型工况计算分析了压裂裂缝三维扩展的热扩散效应影响、不同射孔簇间距以及不同压裂方案(顺序、同步、交替压裂)下裂缝网络的扩展扰动行为。结果表明:深部致密油气储层压裂裂缝扩展引起的应力扰动区域在多裂缝中存在叠加、覆盖行为,形成应力阴影效应、造成裂缝空间偏转;水平井多射孔簇间距的减小,将增大应力阴影区,加剧裂缝间相互干扰;相比多射孔簇顺序压裂,同步压裂将增大应力阴影区,交替压裂可减小应力阴影区,交替压裂成为缓解压裂缝网三维扩展扰动、优化空间缝网形态的有效方案;深部致密油气储层岩体裂缝内的压裂液与岩体基质进行热交换,各压裂方案下的裂缝扩展面积、体积均有提升,表明热效应对裂缝扩展有促进作用,成为影响压裂裂缝扩展的重要因素。

基于PHF-LSM模型节理对应力阴影效应的影响

采用PHF-LSM(permeability-based hydraulic fracture-level set method)水力压裂计算模型,建立了含节理分布的岩石材料模型,将其计算结果与裂缝几何特征和诱发应力理论解对比验证模型正确性,在此基础上分析节理对水力裂缝应力阴影效应的影响.计算结果表明:基于弥散裂缝模型的PHF模型应使用等效开裂区域最外侧结点位移差作为裂缝等效开度;节理的存在会影响裂缝注水过程中水压力变化,同时会引起裂缝等效开度的增大;较大的节理入射角度与距注水点较小的节理高度会产生更大诱发应力,同时会引起最小主应力方向的改变,导致更强应力阴影效应,裂缝偏转加强.

An experimental study of the dynamic features of shadow areas of caustics in response to loading/unloading fracture

Using a plexiglass sample and by means of real-time holographic interferometry and shadow optical method of caustics, the different features of dynamic variation in stress (strain) field, plastic area and nucleation zone (shadow area) when the sample fractures during loading (loading-fracture) and unloading (unloading-fracture) are studied visually. The results show that the strain nuclei (zones with dense fringes) appear first at the tips of prefabricated cracks at low stress, and then the shadow areas of caustics form with the increase of load. These nuclei and shadow areas can become larger, or smaller, when the process of loading, or unloading, goes on. When the stress is kept within a certain range, the shadow areas of caustics can become larger and smaller alternatively with repeated loading and unloading (cyclic loading). However, when loading and unloading at high stress, in particular when the macrofracture is about to appear, the variations of the shadow areas of caustics are irreversible and quite different. The shadow areas of caustics expand rapidly at an increasing speed when loading-fracture appears. In contrast, the shadow areas of caustics expand at a lower speed when unloading-fracture appears; besides, there is a circular shadow in front of the sharp-angle shaped area.

页岩气藏压裂缝网扩展数值模拟

为探究页岩气藏水力压裂复杂裂缝网络的形成机理,开展了缝网扩展的数值模拟研究.考虑应力阴影和天然裂缝作用,建立了井筒和裂缝中流体流动模型,利用位移不连续方法求解应力与位移不连续量,然后构建了压力与裂缝宽度的迭代方程,并采用牛顿迭代法求解.通过比较数值解经典模型解析解,验证了模型和迭代解法的正确性.多簇裂缝同步扩展时裂缝间距越小,压裂液分配到各条裂缝越不均匀,靠近井筒跟部的裂缝的分流量越大,从而裂缝宽度越大;考虑天然裂缝作用时,逼近角越小或者应力各向异性越弱,水力裂缝越容易发生转向扩展,裂缝网络越复杂.

应力阴影效应对水平井压裂的影响

为揭示水平井分段改造中的应力阴影效应及其影响因素,基于水平井分段体积改造理论,借鉴Bunger的应力场分析假设条件,从水平井单一横切缝入手,建立起了考虑应力阴影的应力场叠加数学模型并推导出数学公式,在此基础上得出简化的水平井多段应力阴影效应应力场模型。对应力阴影效应影响因素的分析结果表明:储层地质参数中杨氏模量、水平主应力差对应力阴影效应的影响大,而泊松比对其影响则很小;施工参数中施工排量对应力阴影效应的影响明显,在一定储层条件下,分段及簇间距越小,应力阴影效果对水平井裂缝附近其他裂缝的扩展产生应力阴影影响越大。利用该研究成果对现场实际水平井压裂进行了优化设计,微地震监测结果表明:利用应力阴影效应只压裂6段就实现了以往10段压裂的改造体积,较之以往减少压裂4段,节约成本在30%以上。这对合理利用应力阴影效应进行水平井体积改造裂缝尺寸设置及工艺参数优化具有重要的指导意义。

水平井交替压裂裂缝间距优化及影响因素分析

为使水平井交替压裂在非常规油气储集层中形成大规模高效复杂体积缝网,需对交替压裂裂缝间距进行优化分析。从相邻2条压裂裂缝诱导应力叠加效应对初始水平主应力差的影响出发,建立了裂缝高度不同且缝内净压力也不同的物理模型,提出了交替压裂第一次压裂裂缝和第二次压裂裂缝临界裂缝间距及中间裂缝最佳起裂位置的确定方法。分析发现:前两次压裂裂缝临界裂缝间距随储集层岩石泊松比增大而减小,随裂缝高度和缝内净压力增大而增大;中间裂缝最佳起裂位置距第一次压裂裂缝的距离随储集层岩石泊松比增大而减小,随裂缝高度增大而增大,而缝内净压力对中间裂缝最佳起裂位置的影响较小。通过实例,将改进后的方法与已有方法进行对比分析后发现,改进后的临界裂缝间距和中间裂缝最佳起裂位置较已有方法能更准确地计算出诱导应力场,对非常规油气储集层交替压裂形成高效复杂体积缝网具有指导意义。

关于应力触发和应力影概念在地震预报中应用的一些思考

应力触发和应力影概念及其在地震预报中的应用 ,是学术界关注的热点 ,尚有一系列需要研究的问题。例如初始应力场的确定 ,合理流变模型的选取和计算 ,孔隙压力的变化和影响 ,断层强度随时间的变化等。特别在估计应力场变化时 ,人们现今只注意到大地震对应力场造成的变化。一些新的观测事实表明 ,大量的小地震效应 ,慢地震的效应和无震滑动的效应 ,也不容忽视。定量考虑它们的效应 。

基于PFC^(2D)数值模拟的交替压裂中应力阴影效应研究

为了探究应力阴影效应对交替压裂中压裂间距选取的影响,基于优化后的颗粒流离散元流固耦合计算模型,模拟并分析了双初始水力裂缝下因应力阴影效应产生的诱导应力的分布情况,并与理论解析解进行对比,证明了该数值方法的合理性。在此基础上,分析了应力阴影效应在不同各向异性地应力场及初始压裂间距条件下对新水力裂缝的起裂压力及扩展形态的影响,研究结果表明:初始各向异性应力场不改变裂缝周边的应力场,不影响新水力裂缝的起裂压力;随着初始压裂间距的减小,应力阴影效应增强,新水力裂缝的起裂压力逐渐增加。初始水力裂缝间距与初始各向异性应力场共同影响新水力裂缝的扩展形态,随着初始水力裂缝间距或初始水平地应力场差异系数的增大,应力阴影对新水力裂缝的扩展方向的影响逐步减弱;初始水力裂缝对新水力裂缝的扩展有一定的限制作用,在一定程度上不利于形成复杂的裂缝网络。根据以上分析结果,对交替压裂中压裂间距的优化进行了定性的探讨。

准脆性地层中水平井多段水压致裂的扩展有限元法模拟

在实际的水力压裂过程中,裂缝总是沿着垂直于最小地应力的方向扩展,地应力的分布形式和多个压裂段之间的互相影响(应力阴影效应)对于形成复杂的裂缝网络具有重要的影响。本文基于扩展有限单元法(XFEM)模拟页岩等多孔介质在水压作用下裂缝的任意扩展,由于在传统有限元法的基础上引入了扩充自由度和可以描述间断的位移阶跃函数,所以裂缝可以独立于网格扩展,而不需要重新剖分网格。通过引入一维流动假设,求解润滑方程,并考虑流体在裂缝内的流动。同时也考虑裂缝向基质中流动的滤失效应。研究实际施工中不同段间距下裂缝的扩展模式和段间距对裂缝形态的影响,结果表明,压裂段间距过小时中间的裂缝会被屏蔽;此外,裂缝会由于应力阴影效应而发生转向。

行波管栅控电子枪热形变研究

行波管栅控电子枪各组件尤其是阴极和栅网的热形变对行波管电子光学性能有着较大的影响。基于有限元分析方法,利用热力场耦合对栅控电子枪热形变问题进行了研究,结果表明:阳极的总形变量最大,最高处达0.28mm;控制栅网、阴影栅、阴极及部分支撑结构向着阳极方向发生形变,由温升引起的形变不会使控制栅网和阴影栅发生接触或交叉,而阴极形变量较大时,阴极与阴影栅则存在接触的危险,这对电子枪的工作性能有很大的影响,甚至会使阴影栅烧坏。降低电子截获率,适当增加控制栅网和阴影栅的厚度,使用热传导率高、线膨胀系数低的材料,可以减小栅控电子枪各组件的热形变。

基于PHF-LSM模型岩石分段水力压裂应力阴影效应

采用PHF-LSM(permeability-based hydraulic fracture-level set method)水力压裂数值计算模型,模拟并分析了均质与分层岩石材料中分段压裂过程引起的应力阴影效应.通过与单一裂缝诱发应力理论解的对比,验证了PHF-LSM考虑应力阴影效应的可行性.以此模型为基础分析了单一裂缝发展过程中,岩石材料参数与初始应力条件对诱发应力场的影响,以及均质与分层岩石材料的多裂缝起裂过程中不同压裂间距与压裂顺序产生的应力阴影效应.数值计算结果表明:泊松比、抗拉强度与孔隙率的增加会增大裂缝引起的诱发应力与净压力比值的峰值,而弹性模量和水平主应力差的增大会减小该峰值;裂缝间距的扩大可以减弱裂缝间应力阴影效应;两步压裂与顺序压裂的裂缝总面积均大于同时压裂时的面积,但顺序压裂与两步压裂会带来更高的起裂压力;产层厚度与压裂间距的减小会增大应力阴影效应.