水平井压裂储层应力场分布模拟方法

在研究分析水力压裂对储层岩石力学特性参数影响的基础上,提出一种压力储层应力场分布模拟计算方法。通过建立水平井储层原地应力场模型和水力压裂产生人工裂缝诱导应力场模型,并且利用实际的水力压裂测井参数对储层原地应力场和压裂产生裂缝诱导应力场分布进行了模拟计算。模拟计算结果表明,压裂产生人工裂缝会对储层应力场分布造成很大影响;压裂后储层应力主要在裂缝周围得到积累,并且距离裂缝越远,应力值积累越少;压裂生成裂缝长度也会影响储层应力场分布,裂缝越长,裂缝诱导应力场减小越慢。     

高阶煤煤层气直井低产原因分析及增产措施

我国煤层气井普遍产量低、开发效果差,主要原因是增产改造措施与地质条件匹配性差。通过分析沁水盆地南部郑庄区块直井的低产原因,提出针对性增产措施,并分析相关措施的增产机理及地质适应性,优化增产措施施工参数,并开展实践验证。研究和实践结果表明,埋深大的地区,裂缝开启困难,实施重复压裂可使裂缝转向并增加裂缝长度,增产效果较好。为了充分释放应力,实现裂缝偏转,重复压裂前排采时间至少应在1 000 d以上,重复压裂施工应降低支撑剂用量,且细砂应分段加入;碎裂煤-碎粒煤整体发育的煤层,直接压裂时裂缝延伸较短,实施间接顶板压裂可获得高产,压裂层位顶界至煤层顶板间隔距离为0.5~1.5 m,压裂液排量为5.0~5.5 m3/min,射孔段长度为1.5~2.0 m,单位射孔层段压裂液量为200~300 m3/m时增产效果最好;天然裂缝发育区,实施投球压裂实现裂缝转向,可大幅提高产量,该工艺适应于施工压力下降且低于15 MPa、日产水量为2~5 m3的低产井,其增产措施为先实施以细砂为主的小型预压裂封堵原裂缝,然后投球封堵部分原射孔孔眼,双重封堵可大幅提高重复压裂时的施工压力,形成新裂缝。研究成果对高煤阶煤层气井低产原因分析及增产治理具有指导和借鉴作用。      

用流固耦合方法研究油藏压裂后应力应变和孔渗特性变化

油藏压裂后将引起地应力场发生变化，使岩石变形，导致孔隙度和渗透率变化，进而影响产量，为研究这一问题，作者建立了油藏压裂后流－固耦合渗流模型，考虑了以下因素：油藏岩石变形，地应力，孔隙度和渗透率变化，人工裂缝，流体渗流与岩石应变耦合，储藏渗流与裂缝渗流耦合，非达西效应等。较详细地给出了耦合方程及推导过程，控制方程包括的未知变量有压力，饱和度及位移，11个变量，和11个方程，用有限差分方法将流体渗流和岩石应变方程离散成主对角占优的七对角矩阵，可在修改已有三维二相渗流和三维固体力学程序的基础上，采用隐式迭代方法求解，示例分析表明，用此模型可以研究储层应力变变，孔隙度和渗透率随时间和空间变化规律，为开发方案制定，整体压裂设计，压后生产管理等方面提供定量分析技术。     

定面射孔新工艺对水力裂缝扩展影响研究

定面射孔技术是为了提高体积压裂改造效果而提出的一种新的射孔方案。根据岩石力学及流-固耦合理论,建立了定面射孔地层的三维弹塑性流固-耦合地应力模型,采用数值方法进行求解,结合最大拉应力准则,研究了定面射孔水力裂缝的起裂规律。经过大量计算发现：定面射孔条件下,裂缝首先在射孔面内的射孔眼起裂,然后射孔眼相互贯通而形成扇形裂缝面,增大了水力裂缝的波及体积及井筒的连通性;同一平面内两侧孔眼在中间孔眼上产生的附加应力降低了Y方向主应力而增大了X方向和Z方向主应力,导致正断层采用定面射孔时,起裂压力较大,起裂压力随射孔方位角的增大而降低,只有当射孔方位角大于60°时,定面射孔起裂压力低于螺旋射孔起裂压力;对于逆断层,射孔方位角越小,起裂压力越低,且均低于螺旋射孔起裂压力;减小射孔间夹角、增大射孔直径及深度均可降低起裂压力。     

金湖凹陷天96断块阜二段储层裂缝多参数定量预测

基于应变能、表面能理论, 采用格里菲斯岩石破裂准则, 建立了金湖凹陷天96断块造缝期古地应力、现今地应力与裂缝参数之间的关系; 基于构造分析确定了造缝时期古地应力的类型和方向, 用声速法和水力压裂法确定了现今地应力的方向和大小, 将古今地应力场的数值模拟结果导入裂缝参数计算模型中, 模拟计算了天96断块阜二段裂缝的密度、开度、孔渗参数, 并结合古地应力场分析以及岩心裂缝统计数据对裂缝预测结果进行验证; 最后, 综合分析了影响天96断块裂缝发育的因素。      

页岩气藏缝网压裂裂缝间距优化研究

页岩气藏一般具有低孔隙、低渗透等特征,对其实施缝网压裂是高效开发页岩气的最佳途径。采用位移不连续法建立线弹性二维均质地层诱导应力场分布数学模型,通过水平应力差异系数对顺序压裂和交替压裂的裂缝间距进行优化研究。结果表明,水平应力差异系数受到裂缝净压力、裂缝缝长、原地应力场等因素的影响;裂缝净压力越大、缝长越长,水平应力差异系数越小;随着与裂缝距离的增加,水平应力差异系数呈现先减小后增加的趋势,因此,存在后续裂缝形成复杂网络的最佳裂缝间距;顺序压裂裂缝间距不宜过大,且后续压裂裂缝间距应适当减小;交替压裂裂缝间距最优时,缝间水平应力差异系数最小,对中间裂缝形成缝网最有利。     

基于PFC2D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。     

地应力对破裂压力和水力裂缝影响的试验研究

为研究水力压裂时地应力对破裂压力和水力裂缝的影响规律,采用水力压裂试验系统,以相似材料作为研究对象,进行了水力压裂物理模拟试验,并借助煤岩断面三维扫描系统、位移计提取了水力裂缝信息。试验结果表明,随着主应力差的增大,破裂压力逐渐降低,破裂时间也逐渐缩短;随着主应力差的增大,破裂面的表面积逐渐增加,破裂面变得逐渐粗糙;围压相近时水力裂缝易出现转向、分叉,压裂液的动力效应越明显;水力裂缝张开度随着主应力差的增大而逐渐减小。研究结果可为水力压裂试验的进一步研究、裂缝网络系统的建立提供参考。     

川南泸州地区页岩气甜点地质工程一体化关键要素耦合关系及攻关方向

页岩气甜点地质工程一体化关键要素分析与评价是页岩气高效勘探开发的必要工作.从甜点优选、钻完井工程、压裂工艺等多方面对川南泸州地区开展研究,结合地质工程一体化研究思路,系统分析了工程要素与地质要素的耦合关系.结果表明：泸州地区地质条件复杂,水平最大主应力方向为NWW-SEE向,由西向东裂缝发育程度减少;天然裂缝方向与最大主应力方向和井轨迹有效匹配提高压裂改造体积,井轨迹方位与地应力夹角大于60°、与裂缝主方向夹角大于20°时,水平井压裂效果越好;通过加密分簇、提高排量、暂堵转向、提高加砂强度等技术优化,可保证深层页岩改造体积及缝网有效性.综上,利用工程与地质双因素耦合分析,开展深层井轨迹与甜点预测关系、深层裂缝体积改造与地应力关系、深层钻-完井工程与岩石力学关系技术攻关,能够有效推动地质工程一体化的动态运行,提高单井最大可采量和区块最大动用量.     

基于ABAQUS平台的水力裂缝扩展有限元模拟研究

随着扩展有限元理论的深入研究,利用扩展有限元方法模拟水力压裂具有了一定的可操作性。相比于常规有限元方法,XFEM方法具有计算结果精度高和计算量小的优点。但是,如何模拟射孔孔眼、如何模拟流体与岩石相互作用以及分析水力裂缝的扩展规律仍然是难题。以研究水力压裂裂缝扩展规律为目的,建立了岩石多孔介质应力平衡方程、流体渗流连续性方程和边界条件。通过有限元离散化方法对耦合方程矩阵进行处理。通过富集函数定义初始裂缝（射孔孔眼）,选择最大主应力及损伤变量D分别作为裂缝起裂和扩展判定准则,利用水平集方法模拟水力裂缝扩展过程。数值模拟结果显示：增加射孔方位角、压裂液排量和减小水平地应力差,起裂压力上升;黏度对起裂压力无明显影响。增加射孔方位角、压裂液排量、黏度和减小水平地应力差值有助于裂缝宽度的增加。增加水平地应力差值、压裂液排量和减小射孔方位角以及压裂液黏度有助于裂缝长度增加,反之亦然。基于ABAQUS的水力裂缝扩展有限元法可对不同井型和诸多储层物性参数及压裂施工参数进行分析,且裂缝形态逼真,裂缝面凹凸程度清晰,结果准确。此研究可作为一种简便有效研究水力压裂裂缝扩展规律的方法为油田水力压裂设计与施工提供参考与依据。     

水-力耦合作用下三趾马红土围岩变形特征研究

以地下水位线以下的石楼隧道典型三趾马红土围岩段为例,通过现场监测对三趾马红土围岩的体积含水量、孔隙水压力、围岩应力（土压力）、拱顶沉降与水平收敛进行了分析。在此基础上,通过原位大剪试验获得了可靠的围岩抗剪强度参数,并建立了隧道三维有限元数值模型,分别对考虑水-力耦合效应、不考虑水-力耦合效应的三趾马红土围岩变形规律进行了探讨,分析了孔隙水压力随着隧道开挖的变化和三趾马红土围岩位移场、应力场受水-力耦合效应的影响程度,并提出了围岩破坏变形机制。结果表明:（1）实测拱顶下沉大于围岩水平变形,围岩应力可分为增长期（ < 20d）、调整期（20~60d）、稳定期（>60d）3个阶段,且整体应力水平较高,下台阶含水量大于上台阶,孔隙水压力经历了由负变正的过程。（2）现场剪切试验所测围岩的黏聚力为64.0kPa,内摩擦角为27.7°。（3）数值分析表明,隧道开挖后孔隙水压力场变化十分明显,这是由地下水流速场的改变引起的,水力坡降在衬砌面附近最为明显,渗透动水压力导致土体产生一定的渗透变形;考虑水-力耦合后围岩剪应力、最大剪应变、拱顶沉降、水平收敛、底板隆起均较大。（4）受开挖及支护的影响,地下水产生渗流并依次经过拱顶、边墙,最终汇集于隧底;受开挖、地下水渗流的影响,围岩节理裂隙进一步扩张,成为地下水良好的运移通道;围岩的有效应力随着孔隙水压力的减小而增大,围岩的力学强度在土体趋于饱和状态时骤降,反过来,高有效应力、低围岩强度以及贯通性节理裂隙三者共同改变着地下水渗流场的状态。（5）为保障围岩整体稳定性,建议及时排出隧道底部积水并施做仰拱。     

应力腐蚀和压力溶解作用下双重孔隙岩体中 THMM耦合过程的有限元分析

在使用Yasuhara等建立的裂隙开度的应力腐蚀和压力溶解模型的基础上,将溶质浓度场引入笔者已开发的双重孔隙-裂隙介质热-水-应力耦合的二维有限元分析程序中,针对一个假设的位于非饱和岩体中且有核素泄漏的高放废物地质处置库,拟定2种计算工况：（1）裂隙开度随应力腐蚀和压力溶解而变化（基岩的孔隙率亦是应力的函数）;（2）裂隙开度和基岩的孔隙率均为常数,进行热-水-应力-迁移耦合的数值模拟,考察了岩体中的温度、裂隙开度的闭合速率、闭合量、孔（裂）隙水压力、地下水流速、核素浓度和应力的变化、分布等情况。结果主要显示：应力腐蚀引起的闭合速率要高于压力溶解引起的闭合速率6个数量级,且两种因素产生的闭合速率随时间先增加后减小,并趋于稳定;当考虑应力腐蚀和压力溶解时,近场的负裂隙水压力上升很高;工况1中裂隙开度和孔隙率减小,使得相应的渗透系数降低,故该工况的裂隙和孔隙中核素浓度较工况2为高;由于不计入负的孔（裂）隙水压力对应力平衡的影响,2种工况的岩体中的应力量值及分布基本相同。     

近距离上保护层开采瓦斯运移规律数值分析

采动裂隙是瓦斯运移的通道,搞清瓦斯运移规律是瓦斯治理的前提。在考虑岩石动态破坏过程和含瓦斯煤岩渗流-应力-损伤耦合的基础上,结合平煤五矿实际地质条件和开采工艺,建立了数值计算模型,应用RFPA-Gas程序模拟了近距离上保护层采动顶底板岩层变形破坏、裂隙演化规律与瓦斯运移规律。模拟结果较好地再现了保护层开采过程中煤岩层应力变化、顶底板损伤及裂隙演化过程,得到了上覆岩层移动的“上三带”（冒落带、裂隙带和弯曲下沉带）和底板变形的“下两带”（底板变形破坏带和弹塑性变形带）。得到了被保护层瓦斯流量分布、瓦斯压力分布和透气系数的变化规律,卸压煤层瓦斯透气性增大了2 500倍,得到了煤壁下方压缩区和膨胀区之间的张剪瓦斯渗流通道,并将保护层底板压缩区和膨胀区的瓦斯渗流特征提炼出来：压缩区对应的是渗流减速减量区、膨胀区由卸压膨胀陡变区和卸压膨胀平稳区组成,分别对应着渗流急剧增速增量区和渗流平稳增量区。指出卸压膨胀陡变区是瓦斯突出危险区,为近距离保护层开采瓦斯治理指明了方向。实践表明,瓦斯治理效果显著。     

Analysis of refracturing in horizontal wells: Insights from the poroelastic displacement discontinuity method

In this paper, a fully coupled 2‐dimensional poroelastic displacement discontinuity method is used to investigate the refracturing process in horizontal wells. One of the objectives of refracturing is to access new reserves by adding new hydraulic fractures in zones that were bypassed in the initial fracturing attempt. Pore pressure depletion in the vicinity of old fractures directly affects the state of stress and eventually the propagation of newly created hydraulic fractures. Thus, a poroelastic analysis is required to identify guidelines for the refracturing process, in particular to understand the extension of the pore pressure depletion, and eventually, the orientation of new as well as old fractures. We propose a fully coupled approach to model the whole process of child fracture propagation in a depleted area between 2 parent fractures in the same wellbore. This approach omits the need of using multistep workflow that is regularly used to model the process. The maximum tensile stress criterion (σ criterion) is used for hydraulic fracture propagation. The proposed method is verified using available analytical solutions for total stress and pore pressure loading modes on a line fracture in drained and undrained conditions. Then, test cases of multifractured horizontal wells are studied to calculate the time evolution of the stress and pore pressure fields around old fractures and to understand the effect of these fields on the propagation path of newly created fractures. Finally, the effect of the pore pressure depletion on the propagation path of the newly created fractures in the bypassed area of the same wellbore is studied. The results show that the depleted areas around old fractures are highly affected by the extent and severity of the stress redistribution and pore pressure depletion. It is observed that a successful creation of new fractures may only happen in certain time frames. The results of this study provide new insights on the behavior of newly created fractures in depleted zones. They also clarify the relationship between stress change and pore pressure depletion in horizontal wells.     

Simulating research on pressure distribution of floor pore water based on fluid-solid coupling

In terms of the problem that deep coal resources are under the threats of high water pressure, it is necessary to further study the outburst mechanism of working face floor karst water from the view of studying varying patterns of rock mechanical parameters and confined water pressure distribution of floor rock under the mining influence in the process of mining. Through the theoretical research of fluid-solid coupling and rock seepage experiment, the changing rule of the rock porosity and the Young’s modulus is analyzed, the changing law of the floor displacement is obtained by establishing the floor mechanic model, and the relation between elastic energy density and rock mass unit strain is established. Based on the numerical model of fluid-solid coupling analysis by the Comsol Multiphysics software, the changing law of floor displacement corresponds to the theoretical solution, the different variation laws of rock mass of different depths under the actions of ground pressure are acquired by simulation calculation, the distribution characteristics of pore water pressure are discussed, and the elastic energy density of the rock mass under the action of imbalance water pressure before and after mining is comparatively analyzed. Studies show that the distribution of pore water pressure is horizontally different before and after the stress peak line, and it mainly has four distribution patterns vertically. When mining above the confined water, the spatiotemporal position when and where the upper and lower boundaries of the rock mass in the lower water-resisting layer release energy at the same time is critical to the occurrence of water inrush.     

变形介质流固耦合渗流的数值模型及其应用

在地下流体的开采过程中,随着地下流体的不断采出,必然造成孔隙流体压力的逐渐降低,由此导致储层岩石骨架的有效应力增大,地层产生变形或压实,甚至造成地表沉陷.针对饱和土固结问题,利用可变形多孔介质中流体渗流的流固耦合有限元数值模型,对该问题进行了数值模拟.同时,对油井开采过程进行了模拟.      

热-水-应力-迁移耦合条件下双重孔隙-裂隙介质的抗剪强度及有限元分析

考虑裂隙的连通率、间距、孔隙基质和裂隙材料在表征单元（REV）中的体积分数,并假定双重孔隙-裂隙介质的等效内摩擦角保持常数,而等效黏聚力是固有黏聚力、等效塑性应变、基质吸力、溶质浓度及温度的函数,提出了一种在热-水-应力-迁移耦合条件下确定表征单元内任一平面上等效的黏聚力及内摩擦角的方法。针对一个假定的位于非饱和双重孔隙-裂隙岩体中的高放废物地质处置模型进行了数值模拟及分析。结果表明,基质吸力对于等效黏聚力的增强作用大于等效塑性应变和溶质浓度的减弱作用,使得等效黏聚力得到了提高,故减少了围岩中的塑性区;由此岩体应力、孔（裂）隙水压力及流速、孔（裂）隙溶质浓度的分布及量值也发生相应的改变。     

Experimental Researches on Hydro-Mechanical Properties of Altered Rock Under Confining Pressures

Altered rock, as the abutment materials of Xiaowan Hydropower Station in China, is a kind of geological defective rock mass. It is loosely structured and its strength is low, with some development of pores and cavities. Research on the hydro-mechanical coupling of the altered rock are of important significance to hydropower projects. In this study, the advanced fully automatic triaxial fluid flow-rheological test servo system is employed to study the hydro-mechanical coupling characteristics of the altered rock, and the water pressures and confining pressures in the laboratory tests are set to simulate the conditions of excavation and impoundment of Xiaowan Hydropower Station. Based on the test results, the stress–strain laws of the rock specimens under the effect of complete hydro-mechanical coupling, as well as the lateral strain and volumetric strain characteristics, are studied. The fluid flow laws of the rock specimens and the effects of the confining pressures on the fluid flow are analyzed. The fluid flow failure characteristic under the effect of the complete hydro-mechanical coupling is discussed. The research achievements show that with the change of the stress states, the permeability of the rock also changes, and the permeability evolution shows the phase characteristic during the process of stress and strain. The impacts of the confining pressures on the strength and deformation and permeability of the altered rock are obvious. The failure behaviours of the rock specimens under the effect of coupling relates to the confining pressures, including two kinds of splitting failure and shear failure. The fluid flow failure characteristic of the rock specimens depend upon the initiation, growth and coalesce of micro-cracks, heterogeneity, confining pressures and properties of the rock.     

Onset of Hydraulic Fracture Initiation Monitored by Acoustic Emission and Volumetric Deformation Measurements

In this paper, the results of laboratory studies of fracture initiation, early propagation and breakdown are reported. Three experiments were conducted on a low permeability sandstone block, loaded in a polyaxial test frame, to representative effective in situ stress conditions. The blocks were instrumented with acoustic emission (AE) and volumetric deformation sensors. In two experiments, fluids of different viscosity were injected into the wellbore, fluid injection was interrupted soon after the breakdown pressure had been reached. This allowed us to investigate hydraulic fracture initiation. In the third test, fracture initiation criteria were applied to stop hydraulic fracture propagation significantly earlier, prior to breakdown, and as it propagated a short distance from the wellbore. The analysis of AE results shows an increase in AE activity and a change in the AE spatial correlation, during the fracture initiation. This early stage of fracturing correlates strongly with the onset of rock volumetric deformation, and is confirmed by the analysis of ultrasonic transmission monitoring. The rock microstructure, after the test, was investigated by analysis of scanning electron microscope images. These indicated the development of leak-off zone near the wellbore and a dry hydraulic fracture at the farther distance from the wellbore.