页岩储层应力敏感性定量评价: 思路及应用

应力敏感性定量评价是页岩油气勘探开发中公认的关键工程难题之一, 然而变应力条件下页岩孔缝尺寸及渗透率下降规律等问题至今尚无定论, 亟待深入探索. 本文从Griffith经典弹性力学解出发, 在充分刻画非均质页岩孔隙和微裂缝的基础上, 通过建立横截面为椭圆的柱体管束模型, 推导得到应力作用下岩石渗透率保持水平的计算公式, 基于此给出适宜于非均质页岩油储层的应力敏感性评价思路及覆压渗透率计算公式, 最后在中国西部和中部典型页岩油储层中开展了工程应用. 研究显示: (1) 相同应力状态下, 页岩油储层应力敏感程度由储渗空间初始长短轴比值、杨氏模量及泊松比共同控制; (2) 裂缝型页岩由于发育长短轴比值较高, 其应力敏感程度略高于裂缝发育程度较低的基质型页岩, 且杨氏模量越小, 两类页岩应力敏感程度差异性越大; (3) 在40 MPa的有效应力作用下, 裂缝型和基质型页岩油储层渗透率损耗程度最高值分别不足10%和8%, 证明页岩应力敏感程度总体较低. 应力敏感性对于页岩油原位储量及实际产能的影响程度亟需在工程实际中予以重新审视. 研究成果将为页岩油储量的精确评估和采收率的高效提升提供新的理论与实践依据.      

考虑多重运移机制耦合页岩气藏压裂水平井数值模拟

页岩作为典型的微纳尺度多孔介质,游离气与吸附气共存,传统的达西定律已无法准确描述气体在页岩微纳尺度的运移规律.基于双重介质模型和离散裂缝模型构建页岩气藏分段压裂水平井模型,其中基岩中考虑气体的黏性流、Knudsen 扩散以及气体在基岩孔隙表面的吸附解吸,吸附采用Langmuir等温吸附方程;裂缝中考虑黏性流和Knudsen扩散,在此基础上建立基岩-裂缝双重介质压裂水平井数学模型并采用有限元方法对模型进行求解.结果表明,基岩固有渗透率越小,表面扩散和Knudsen扩散的影响越大,反之则越小;人工裂缝的性质包括条数、开度、半长以及间距,主要影响压裂水平井生产早期,随着人工裂缝参数值的增加,压裂水平井产能增加,累产气量也越大.其次,页岩气藏压裂诱导缝和天然裂缝的发育程度对页岩气藏的产能有很大的影响,水平井周围只有人工裂缝,周围天然裂缝不开启或不发育时,页岩气藏的水平井的产能较低.      

天然气水合物降压开采沉积物压缩效应数值模拟研究

深海天然气水合物降压开采过程中,沉积物的压缩会改变储层的物理力学特性,进而对天然气的开采效果产生显著影响.为揭示沉积物压缩效应下井周围储层物理力学特性演化规律,本文建立了考虑沉积物压缩效应的理论模型,通过COMSOL模拟研究了不同初始固有渗透率、初始水合物饱和度和井底压力条件下的降压开采中生产井周围储层的物理力学特性演化规律以及开采效果.结果表明:受沉积物压缩的影响,水合物分解区的渗透率随着与井筒距离的增加先增加后减少;产气与产水速率由零立即上升至峰值,然后迅速下降,并且考虑沉积物压缩时的产气与产水速率比不考虑时低;在水合物完全分解区,渗透率的大小与有效应力成负相关关系,未分解区渗透率的大小与水合物饱和度成负相关关系;井底压力越小,有效应力越大,生产井周围储层的渗透率下降越明显;初始水合物饱和度对产气与产水的影响存在拐点,饱和度拐点位于0.25与0.35之间,高水合物饱和度并不代表储层开采效果好,产气速率的高低还与储层的渗透率有关,高水合物饱和度储层的渗透率较低,产气速率较低;储层初始固有渗透率较高时显著促进了开采效果,但储层变形量较大增加了储层的不稳定性.     

页岩气藏的双重介质-离散裂缝模型

考虑页岩气藏开发中渗流的多尺度效应,提出了一个基于裂缝-孔隙双重介质的离散裂缝模型．在该模型中,基质、天然裂缝和人工压裂裂缝采用各自控制方程独立计算,不同介质之间通过流量交换相互关联．为分析模型可靠性,分别和基于渗透率粗化及压裂裂缝导流能力无穷大的模型对比．数值算例显示,伴随着网格细分,该模型与精确渗透率粗化模型具有相同计算精度,两者收敛速度均较快,但该模型易推广到多相流动问题,而等压模型对产量将有所高估．研究了地质参数和工艺参数对气井产量的影响规律．计算结果表明天然裂缝渗透率及基质孔隙扩散系数对产气速率有着重要影响,产气速率伴随着人工压裂裂缝导流能力、长度以及数目的增加而增加,但是增加幅度会逐步趋缓．     

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.      

页岩气藏压裂水平井试井分析

页岩气藏资源丰富,开发潜力巨大,已成为目前研究的热点.与常规气藏相比,页岩气藏运移机制复杂,流动模式呈非线性,有必要考虑页岩气的吸附解吸,天然微裂缝的应力敏感性,人工裂缝内的非达西流等非线性因素对压裂水平井压力响应的影响. 基于双重介质和离散裂缝混合模型,分别采用Langmuir等温吸附方程描述吸附解吸,渗透率指数模型描述应力敏感,Forchheimer方程描述非达西效应,建立页岩气藏压裂水平井数值试井模型. 运用伽辽金有限元法对模型进行求解.根据试井特征曲线,划分流动阶段,着重分析非线性因素对压力响应的影响.结果表明:页岩气藏压裂水平井存在压裂裂缝线性流、压裂裂缝径向流、地层线性流、系统径向流及封闭边界影响5 种流动阶段.吸附解吸的影响发生窜流之后,Langmuir吸附体积增大,拟压力导数曲线凹槽更加明显,系统径向流出现时间与压力波传播到边界时间均延迟;天然裂缝系统的应力敏感性主要影响试井曲线的晚期段,拟压力和拟压力导数曲线均表现为上翘,应力敏感效应越强,上翘幅度越大;高速非达西效应对早期段影响较大,非达西效应越强,拟压力降幅度越大,试井曲线上翘.与解析解的对比以及矿场实例验证了模型的正确性与适用性.      

多簇压裂干扰应力变化规律及对裂纹扩展的影响

为研究水平井分段多簇压裂缝间的干扰应力及其对裂纹扩展的影响,在现有二维未考虑地应力的单裂缝干扰应力解析解的基础上,利用双平面复变函数保角变换得到了包含地应力项的三维干扰应力解析解。基于扩展有限元法建立三维多裂缝扩展力学模型,利用Python脚本二次开发平台实现了三维多裂缝水力压裂参数化建模,通过解析解与数值计算对比分析,得到如下结论。裂纹两侧裂纹面法向和走向干扰正应力分别为压应力和拉应力,均呈纺锤形,法向干扰应力影响范围大约为走向干扰应力的5倍;裂纹尖端裂纹面法向和走向干扰正应力分别为拉应力和压应力;裂纹尖端两侧存在干扰剪应力;考虑初始地应力对干扰应力解析解进行修正后的干扰应力值均变小;多簇压裂中裂缝间的干扰应力叠加,簇间距越小,叠加效果越强;多簇压裂的干扰应力使裂缝间裂纹面法向压应力增大,走向压应力减小,导致裂纹扩展注水压力升高,裂缝张开的宽度降低,不利于单裂缝的扩展;干扰应力使裂缝间应力差降低,甚至局部最小地应力方向发生改变,有利于形成复杂缝网。     

低渗油层压裂水平井两相流研究

依据压裂水平井不同流动区域的流动规律, 将压裂水平井的渗流分为裂缝中的高速非达西流 动区、裂缝控制影响的椭圆渗流区、远离裂缝的基质非达西渗流区, 考虑启动压力梯度的影 响, 对压裂水平井两相渗流进行了分析, 得到了低渗透油层压裂水平井的产量公式. 研究结 果表明, 裂缝的导流能力越大, 压裂水平井的产量越高. 但随着开采时间的增加, 其产量递减幅度越 大; 压裂裂缝长度越小, 压裂水平井的初始产量越高. 但随着生产时间的推移, 压裂裂缝的 长度越大, 产量的递减幅度越小; 中间裂缝长, 两翼裂缝短的情况下, 压裂水平井的产量最 高.     

多簇压裂干扰应力变化规律及对裂纹扩展的影响

为研究水平井分段多簇压裂缝间的干扰应力及其对裂纹扩展的影响,在现有二维未考虑地应力的单裂缝干扰应力解析解的基础上,利用双平面复变函数保角变换得到了包含地应力项的三维干扰应力解析解。基于扩展有限元法建立三维多裂缝扩展力学模型,利用Python脚本二次开发平台实现了三维多裂缝水力压裂参数化建模,通过解析解与数值计算对比分析,得到如下结论。裂纹两侧裂纹面法向和走向干扰正应力分别为压应力和拉应力,均呈纺锤形,法向干扰应力影响范围大约为走向干扰应力的5倍;裂纹尖端裂纹面法向和走向干扰正应力分别为拉应力和压应力;裂纹尖端两侧存在干扰剪应力;考虑初始地应力对干扰应力解析解进行修正后的干扰应力值均变小;多簇压裂中裂缝间的干扰应力叠加,簇间距越小,叠加效果越强;多簇压裂的干扰应力使裂缝间裂纹面法向压应力增大,走向压应力减小,导致裂纹扩展注水压力升高,裂缝张开的宽度降低,不利于单裂缝的扩展;干扰应力使裂缝间应力差降低,甚至局部最小地应力方向发生改变,有利于形成复杂缝网。     

航空涡轮盘多轴蠕变-疲劳寿命预测及对比研究

针对几种经典和新发展的蠕变-疲劳寿命模型开展综述介绍,并建立预测航空涡轮盘在循环热-机蠕变-疲劳载荷谱下蠕变-疲劳行为的数值流程,对某型航空涡轮盘的蠕变-疲劳损伤和寿命进行预测和对比．结果表明：等效应变法与临界平面法得出的疲劳损伤差距较小,等效应变法由于数值计算简单,工程适用性更强．寿命-时间分数(TF)法由于无法考虑应力松弛效应,给出了最为保守的蠕变损伤预测,其对盘体应力三轴度引起的损伤不敏感;延性耗竭法(DE)法仅以蠕变应变率作为损伤因素,虽考虑多轴蠕变因子的影响,但是给出的蠕变损伤过小;修正应变能密度耗竭(MSEDE)法综合考虑蠕变应变与应力松弛,并且考虑多轴蠕变因子与弹性跟随效应的影响,结合疲劳损伤模型可以给出合理的蠕变、疲劳损伤比例,其预测结果更加合理．     

Experimental Study of Fracture Permeability for Stimulated Reservoir Volume (SRV) in Shale Formation

“Stimulated reservoir volume”(SRV) makes shale gas production economic through new completion techniques including horizontal wells and multiple hydraulic fractures. However, the mechanism behind these treatments that provide sufficient permeability is not well understood. The effects of different stimulation treatments need to be further explored. To understand the effects of fracture surface roughness, fracture registration, confining pressure, proppant type and distribution mode, fiber and acidizing treatment on fracture permeability, a series of laboratory permeability experiments were performed on fractured cores from shale formation of Shengli Oilfield. The results of this study demonstrate that sedimentary bedding of shale has important influence on matrix permeability. At 35 MPa confining pressure, the permeability of aligned fracture (unpropped and without fracture offset) can increase about 1–3 orders of magnitude over shale matrix. The permeability of displaced fracture can increase about 1–2 orders of magnitude over the aligned fracture. The permeability of fracture propped with proppant can increase about 2–4 orders of magnitude over unpropped fracture. The greater the fracture surface roughness, the higher the permeability. The increasing degree of displaced fracture permeability is not proportional to the amount of fracture offset. In the microfracture of shale, the effect of ceramic proppant is still better than that of quartz sand, and the permeability of a centralized fairway distribution of proppant is about 1.2 times better than an even monolayer distribution of proppant. Under high pressure, proppant is easy to cause the break of fracture faces of brittle shale, and increase local fracture permeability to some extent. However, quartz sand are more easily broken to embed and block microcracks just made, which results in fracture permeability lower than that of ceramic proppant. At the same time, the argillation phenomenon is easy to happen on propped fracture faces of shale, which is one of the main factors that leads to a substantial decline in fracture permeability. The permeability of displaced fracture propped with proppant is greater than that of aligned fracture propped with proppant. Because of added fiber presence, the permeability of microfractures presented in SRV is greatly reduced. The pressure dependence of aligned fractures in shale obeys Walsh’s theory, but the pressure dependence of propped and displaced fractures in shale obeys Walsh’s law over a limited range of pressures. Deviations reflect proppant seating, proppant embedding and breaking. For shale formation with the high carbonate content, acidizing treatment should be carefully implemented. Experimental results may provide more valuable information for effective design of hydraulic fracturing in shale reservoir.     

Investigation on nonlinear multi-scale effects of unsteady flow in hydraulic fractured horizontal shale gas wells

A unified mathematical model is established to simulate the nonlinear unsteady percolation of shale gas with the consideration of the nonlinear multi-scale effects such as slippage, diffusion, and desorption. The continuous inhomogeneous models of equivalent porosity and permeability are proposed for the whole shale gas reservoir including the hydraulic fracture, the micro-fracture, and the matrix regions. The corresponding semi-analytical method is developed by transforming the nonlinear partial differential governing equation into the integral equation and the numerical discretization. The non-linear multi-scale effects of slippage and diffusion and the pressure dependent effect of desorption on the shale gas production are investigated.     

Laboratory Investigation of LNAPL Migration in Double-Porosity Soil Under Fractured Condition Using Digital Image Analysis

Gas production from shale gas reservoirs plays a significant role in satisfying increasing energy demands. Compared with conventional sandstone and carbonate reservoirs, shale gas reservoirs are characterized by extremely low porosity, ultra-low permeability and high clay content. Slip flow, diffusion, adsorption and desorption are the primary gas transport processes in shale matrix, while Darcy flow is restricted to fractures. Understanding methane diffusion and adsorption, and gas flow and equilibrium in the low-permeability matrix of shale is crucial for shale formation evaluation and for predicting gas production. Modeling of diffusion in low-permeability shale rocks requires use of the Dusty gas model (DGM) rather than Fick’s law. The DGM is incorporated in the TOUGH2 module EOS7C-ECBM, a modified version of EOS7C that simulates multicomponent gas mixture transport in porous media. Also included in EOS7C-ECBM is the extended Langmuir model for adsorption and desorption of gases. In this study, a column shale model was constructed to simulate methane diffusion and adsorption through shale rocks. The process of binary \(\hbox {CH}_{4}{-}\hbox {N}_{2}\) diffusion and adsorption was analyzed. A sensitivity study was performed to investigate the effects of pressure, temperature and permeability on diffusion and adsorption in shale rocks. The results show that methane gas diffusion and adsorption in shale is a slow process of dynamic equilibrium, which can be illustrated by the slope of a curve in \(\hbox {CH}_{4}\) mass variation. The amount of adsorption increases with the pressure increase at the low pressure, and the mass change by gas diffusion will decrease due to the decrease in the compressibility factor of the gas. With the elevated temperature, the gas molecules move faster and then the greater gas diffusion rates make the process duration shorter. The gas diffusion rate decreases with the permeability decrease, and there is a limit of gas diffusion if the permeability is less than \(1.0\,\times \,10^{-15}\, \hbox { m}^{2}\). The results can provide insights for a better understanding of methane diffusion and adsorption in the shale rocks so as to optimize gas production performance of shale gas reservoirs.     

Apparent Permeability and Gas Flow Behavior in Carboniferous Shale from the Qaidam Basin,China: An Experimental Study

Shale can act as an unconventional gas reservoir with low permeability and complex seepage characteristics. Study of the apparent permeability and percolation behavior of shale gas is important in understanding the permeability of shale reservoirs, to evaluate formation damage, to develop gas reservoirs, and to design wells. This study simulated methane percolation at 298.15 K under inlet pressures ranging from 0.2 to 4 MPa and a constant outlet pressure of 0.1 MPa to investigate shale gas percolation behavior and apparent permeability. Five representative shale cores from the Carboniferous Hurleg and Huitoutala formations in the eastern Qaidam Basin, China, were analyzed. Each experiment measured the volume flow rate of methane and the inlet pressure. Pseudopressure approach was used to analyze high-velocity flow in shale samples, and apparent permeability at different pressures was calculated using the traditional method. A nonlinear apparent permeability model that considers diffusion and slippage is established from theory and experimental data fitting, and the shale gas flow characteristics affected by slippage and diffusion are analyzed. The results indicate that the pseudopressure formulation that considers the effect of gas properties on high-velocity flow produces a more accurate linear representation of the experimental data. The apparent gas permeability of shale consists of contributions from Darcy permeability, slippage, and diffusion. The apparent permeability and gas flow behavior in the studied shales strongly depended on pressure. The diffusion contribution increased greatly as pressure decreased from 2 to 0.2 MPa, and the smaller the shale permeability, the greater the relative contribution of diffusion flow. At pressures greater than 2 MPa, slip flow contributes \(\sim \)20% of the total flux, Darcy flow contributes up to 70%, and diffusion makes only a minor contribution. This study provides useful information for future studies of the mechanism of shale gas percolation and the exploration and development of Qaidam Basin shale gas specifically.     

Numerical Simulation of Hydraulic Fracturing Water Effects on Shale Gas Permeability Alteration

Hydraulic fracturing has been recognized as the necessary well completion technique to achieve economic production from shale gas formation. However, following the fracturing, fluid–wall interactions can form a damaged zone nearby the fracture characterized by strong capillarity and osmosis effects. Here, we present a new reservoir multi-phase flow model which includes these mechanisms to predict formation damage in the aftermath of the fracturing during shut-in and production periods. In the model, the shale matrix is treated as a multi-scale porosity medium including interconnected organic, inorganic slit-shaped, and clay porosity fields. Prior to the fracturing, the matrix holds gas in the organic and the inorganic slit-shaped pores, water with dissolved salt in the inorganic slit-shaped pores and the clay pores. During and after fracturing, imbibition causes water invasion into the matrix, and then, the injected water–clay interaction may lead to clay-swelling pressure development due to osmosis. The swelling pressure gives additional stress to slit-shaped pores and cause permeability reduction in the inorganic matrix. We develop a simulator describing a system of three pores, two phases (aqueous and gaseous phases), and three components (\(\hbox {H}_{2}\hbox {O}, \hbox {CH}_{4}\), and salt), including osmosis and clay-swelling effect on the permeability. The simulation of aqueous-phase transport through clay shows that high swelling pressure can occur in clays as function of salt type, salt concentration difference, and clay-membrane efficiency. The new model is used to demonstrate the damage zone characteristics. The simulation of two-phase flow through the shale formation shows that, although fracturing is a rapid process, fluid–wall interactions continue to occur after the fracturing due to imbibition mechanism, which allows water to penetrate into the inorganic pore network and displace the gas in-place near the fracture. This water invasion leads to osmosis effect in the formation, which cause clay swelling and the subsequent permeability reduction. Continuing shale–water interactions during the production period can expand the damage zone further.     

页岩基质解吸-扩散-渗流耦合实验及数学模型

采用川南地区龙马溪组页岩样品,设计了页岩基质解吸-扩散-渗流耦合物理模拟实验,揭示了页岩基质气体流动特征以及压力传播规律.推导了页岩气解吸-扩散-渗流耦合数学模型并且利用有限差分法对数学模型进行数值求解,与实验结果相比较表明该数学模型能够很好地描述气体在页岩基质中的流动规律.同时对页岩基质气体流动的影响因素进行了分析,认为页岩基质的渗透率、扩散系数、解吸附常数等因素均能影响页岩基质气体的流量和压力传播规律,在页岩气藏的开发过程中需要考虑这些参数的影响,该数学模型为页岩气井产能计算提供了更准确的计算方法.      

页岩储层体积压裂的地应力变化研究

页岩储层属于致密超低渗透储层,需改造形成复杂缝网才有经济产能.体积压裂是页岩储层增产改造的主要措施,而地应力场特别是水平主应力差值是体积压裂的关键控制因素. 理论研究表明:(1)当初始两向水平主应力差较小时,容易形成缝网,反之不易产生缝网;(2)人工裂缝的形成能够改变地层初始应力场. 因此应在前人研究的基础上优化设计压裂方式,以克服和翻转初始水平主应力差值,产生体积缝网.基于此,建立了页岩气藏水平井体积压裂数值模型,模型中采用多孔介质流固耦合单元模拟页岩基质的行为,采用带有孔压的"cohesive"单元描述水力裂缝的性质,模型对"Texas Two-Step" 压裂方法进行了数值模拟,模拟结果得到了压裂过程中地层应力场的分布及其变化,模拟结果和解析公式计算结果吻合良好.模拟结果表明:(1)裂缝的产生减弱了地层应力场的各向异性;(2 对于低水平应力差页岩储层,采用"Texas Two-Step"压裂方法可以产生缝网. 对于采用"Texas Two-Step"压裂方法无法产生缝网的高应力差页岩储层,提出了三次应力"共振" 和四次应力"共振" 压裂方法并进行了数值模拟,模拟结果得到了压裂过程中页岩储层应力场的分布及其变化,得到了缝网形成的区域,模拟结果表明:(1)对于高应力差页岩储层,采用"Texas Two-Step" 压裂方法无法产生缝网;(2)对于高应力差页岩储层,三次应力"共振" 和四次应力"共振"压裂方法是有效的体积压裂缝网形成的方法.      

压裂页岩气藏多尺度耦合流动数值模拟研究

在碳达峰的国策背景之下,页岩气成为传统能源向绿色清洁低碳能源转型的重要过渡和能源支点.压后页岩气藏流体流动力学成为高效开发页岩气的关键力学问题.文章将小尺度低导流天然裂缝等效升级为连续介质,建立有机质-无机质-天然裂缝三重连续介质模型,同时对大尺度高导流裂缝采用离散裂缝模型刻画,嵌入天然裂缝连续介质中,构建多重连续/离散裂缝模型.综合考虑吸附气的非平衡非线性解吸附和表面扩散,自由气的黏性流和克努森扩散,给出页岩气在多尺度复杂介质中的非线性耦合流动数学模型.提出多尺度扩展有限单元法对离散裂缝进行显式求解,创新性构建三类加强形函数捕捉离散裂缝的局部流场特征,解决了压后页岩海量裂缝及多尺度流动通道的流动模拟难题.文章提出的模型和方法既能准确刻画高导流裂缝对渗流的影响,又克服了海量多尺度离散裂缝导致计算量增大的问题.通过算例展示了压后页岩各连续介质的压力衰减规律,发现裂缝中自由气、有机质中自由气、无机质中吸附气依次滞后的压力(浓度)扩散现象,重点分析了吸附气表面扩散系数、自由气克努森扩散系数、天然裂缝连续介质渗透率和吸附气解吸附速率对页岩气产量的影响.文章重点解决压后页岩多尺度流动通道的表征和...