水力压裂的支撑剂输送分析

为了研究水力压裂中支撑剂输送过程,本文建立了支撑剂二维输送方程,以及含砂液的二维运动方程。这两个方程考虑了支撑剂沉降,并且采用了含砂液的流态指数和稠度系数与支撑剂体积浓度的函数关系。这两个方程与水力压裂三维裂缝模型中描述裂缝宽度与裂缝面上压力关系的积分方程是耦合的。裂缝延伸判据采用应力强度因子法则。应用有限元方法,模拟了水力压裂中的支撑剂输送过程。计算结果反映了支撑剂输送过程中的输送和沉降。     

中高渗油藏压裂渗流特征的数值模拟研究

为研究中高渗油藏分段压裂井开采机理和渗流规律,通过分段压裂物理模型的岩样筛选、模型制作和封装、模型抽真空及有效驱动的物理模拟评价等方法研究,进一步结合油藏压裂渗流规律。在相同驱替压差下,分段压裂井的压力梯度值要比普通井的压力梯度值高,且随着压裂裂缝半缝长的增加,压力梯度值也增加;当井段长度一定时,储层渗透率越低,分段压裂井的最佳分段数越多;当储层渗透率一定时,井段长度越长,压裂的最佳段数也越多。最佳裂缝半缝长反而呈现减小的趋势;对于油藏压裂井开采来说,对产量最为敏感的是压裂段数,其次是裂缝半缝长,而裂缝导流能力最为不敏感。     

基于压裂砂堵的水击特性研究及预测应用

针对超深井水力压裂发生砂堵时水击效应导致压力大幅振荡升高的问题。综合考虑多相流、支撑剂属性、井筒摩阻、施工压力大以及砂堵形成机理的特点,建立压裂砂堵水击效应预测模型,并采用有限差分法进行数值求解。将某高压深井压裂施工时砂堵的现场数据与预测模型计算结果进行对比分析,结果表明最大误差不大于2.92%,验证了模型的可靠性。针对压裂施工参数的影响开展了对比分析,结果表明：随着排量增大,水击压力波动随之增大,但排量对波速影响小;随着砂比提高,两相流密度增加,动能增大,压力波动小幅度增加,但波速随着砂比增高的规律为先降低再增加;随着支撑剂密度提高,水击压力波动增大而水击波速变化则由支撑剂密度和弹性模量共同决定。     

水力压裂过程的扩展有限元数值模拟方法

建立了基于扩展有限元法的水力压裂数值模拟方法,使水力裂缝独立于网格存在,无需预设裂缝扩展方位。在扩展有限元计算框架下,将裂缝面处理为求解域内边界,将缝内水压力转化为相关单元等效节点力;运用考虑缝内水压力作用的相互积分法来数值求解缝尖应力强度因子;采用最大能量释放率准则确定裂缝是否继续扩展及扩展方位;最终编制了计算机程序。利用该方法数值模拟了单条水力裂缝在恒定水压力作用下作非平面扩展,所得结果分别与室内试验和解析模型相对比。结果表明,数值结果与室内试验和解析解吻合较好,缝尖应力强度因子最大相对误差不高于0.45%,验证了该方法的可行性和准确性。     

基于给定参数的水力裂缝与天然裂缝相互作用结果的预测准则

水力裂缝（HF）和天然裂缝（NF）之间的相互作用在压裂裂缝性储层中扮演着重要角色,并可能导致复杂的裂缝网络。该文基于断裂力学理论提出了一个基于给定参数的显式表达式准则。在进行数值模拟、实验或现场施工之前,根据输入参数,应用本准则预测HF遇到NF后的穿透、打开或滑移行为,以及评估已有的水力压裂试验结果。根据准则可知,在高地应力差和高交汇角下,HF容易穿透NF;在低地应力差和低交汇角下,HF容易打开NF;在适当的应力差和交汇角度以及摩擦系数条件下,NF会在HF的作用下发生剪切滑移。高的岩石断裂韧性和短的HF长度（水平井筒到NF的距离）也会导致HF打开NF。相比其他隐式表达式或通过计算程序给出的准则,该文提出的准则只依赖于外部输入参数,具有显式表达方式,方便压裂实验和工程应用。     

大斜度井压裂工艺研究与应用

水力压裂工艺为低渗透油气藏的开发作出极大的贡献,随着油气田开发的需要,大斜度井的比例会不断增加,井斜度大于30°的井在进行压裂改造中施工成功率较低。研究揭示出在斜井中由于裂缝转向,产生多裂缝及近井摩阻损失等原因会产生不同于直井的更复杂的破裂结构。     

采油井重复压裂的方式方法及技术参数研究

压裂技术是开发低渗透油藏主要措施之一,但经过水力压裂的油气井,在生产过程中由于各种原因可能导致水力裂缝失效。为此,系统地分析了压裂井失效的原因和重复压裂方式,阐述了重复压裂的选井选层原则和重复压裂参数的选择,研究了重复压裂工艺方法。     

间接压裂技术提高煤层气采收率的探讨

探讨提高煤层气采收率的间接压裂新方法。根据煤体基质岩块与裂缝的渗流物性规律,以及常规水力压裂又很难适应煤层的特殊地质特性,提出在煤层相邻的砂岩部位进行压裂构想。其机理是煤岩的裂缝传导率差,易被压碎产生煤粉且有突出的各项异性、应力集中的特点,煤层中的面割理垂直于煤层,垂向渗透率通常高于水平渗透率,面割理的方向性使煤层与间接压裂诱导水力裂缝自动沟通,形成高传导渗流通道,从而促进煤层气的解吸和渗流。     

定向射孔多缝压裂工艺案例分析

一种新型的压裂工艺在中国长庆油田试验成功,该工艺在层内压裂形成了2条独立裂缝,大幅增加了裂缝与油藏的接触面积,提高了产量。长庆油田开发的超低渗透油藏岩芯分析渗透率一般在0.5 mD以下,且受到储层条件、注采井网、压裂工艺等多重限制,常规压裂工艺改造难以实现该类油藏的有效开发。历经多年研究并结合自身实际,长庆油田于2006年提出了体积压裂的理念,通过在油层内造多缝扩大油藏的泄流体积以提高单井产量。室内物模试验研究射孔对压裂影响时观察到,当射孔方向与最大水平主应力方位存在夹角的情况下裂缝发生转向,即裂缝均先沿射孔孔眼方向起裂,后转向最大主应力方向。在同一油层内上下各射一排孔,两排孔方位与最大主应力方向呈一定夹角,一排偏右,一排偏左。分别对两排孔眼进行压裂,这样由于初始裂缝转向导致两条裂缝在井眼处虚拟相交,但在地层内不会重合,形成近井类似于"X"形4条裂缝,远井形成2条近于平行的裂缝。提出了一种产生多裂缝的新型压裂工艺———定向射孔多裂缝压裂。开展了大量物模试验,证实了思路的可行性,并且长庆油田在A井开展了现场压裂试验。井下微地震裂缝测试表明:形成了"X"形多裂缝;产量数据表明:定向射孔多缝压裂井相比邻井产量提高38.1%。     

基于多分量信号分析的微地震声源定位基础研究

当今世界上对油气田增产以及非常规油气田开发时,应用最广泛最有效的石油工程技术方法是水力压裂。评价水力压裂效果的最成功的方法是微地震监测。国内目前在这方面仍处于起步阶段。通过数值模拟方法,研究不同位置的压裂导致微地震源产生的弹性波场特征,采用相邻井中二分量接收信号进行微地震源反演模拟研究。结合纵波横波的传播时间以及偏振特性等信息,追踪微地震波传播路径,计算声源位置,确定压裂裂缝的前沿,从而达到评估压裂效果的目的。数值模拟结果表明,该方法在微地震监测的声源定位方面有较高的精确度,在微地震监测的反演中有一定的借鉴意义。     

Poro-elasto-plastic modeling of complex hydraulic fracture propagation: simultaneous multi-fracturing and producing well interference

With the increasing wide use of hydraulic fractures in the petroleum industry, it is essential to accurately predict the behavior of fractures based on the understanding of fundamental mechanisms governing the process. For effective reservoir exploration and development, hydraulic fracture pattern, geometry and associated dimensions are critical in determining well stimulation efficiency. In shale formations, non-planar, complex hydraulic fractures are often observed, due to the activation of preexisting natural fractures and the interaction between multiple, simultaneously propagating hydraulic fractures. The propagation of turning non-planar fractures due to the interference of nearby producing wells has also been reported. Current numerical simulation of hydraulic fracturing generally assumes planar crack geometry and weak coupling behavior, which severely limits the applicability of these methods in predicting fracture propagation under complex subsurface conditions. In addition, the prevailing approach for hydraulic fracture modeling also relies on linear elastic fracture mechanics (LEFM) by assuming the rock behaves purely elastically until complete failure. LEFM can predict hard rock hydraulic fracturing processes reasonably, but often fails to give accurate predictions of fracture geometry and propagation pressure in ductile and quasi-brittle rocks, such as poorly consolidated/unconsolidated sands and ductile shales, even in the form of simple planar geometry. In this study, we present a fully coupled poro-elasto-plastic model for hydraulic fracture propagation based on the theories of extend finite element, cohesive zone method and Mohr–Coulomb plasticity, which is able to capture complex hydraulic fracture geometry and plastic deformations in reservoir rocks explicitly. To illustrate the capabilities of the model, example simulations are presented including ones involving simultaneously propagating multiple hydraulic fractures and producing well interference. The results indicate that both stress shadow effects and producing well interference can alter hydraulic fracture propagation behavior substantially, and shear failure in ductile reservoir rocks can indeed make a significant difference in fracturing pressure and final fracture geometry.     

A three-dimensional hydraulic fracturing simulator

The propagation of non-planar hydraulic fractures is modelled using a three-dimensional numerical simulator. This paper describes the different components of the model (stress/displacement analysis, fluid-flow analysis, propagation criterion) with an emphasis on the numerical techniques used. A few examples of out-of-plane fracture geometries are provided.     

Geomechanical optimization of hydraulic fracturing in unconventional reservoirs: a semi-analytical approach

Unconventional drilling and completion architecture includes drilling multilateral horizontal wells in the direction of minimum horizontal stress and simultaneous multistage fracturing treatments perpendicular to the wellbore. This drilling and stimulation strategy is utilized in order to raise the connectivity of the reservoir to the wellbore, thereby remedying the low permeability problem, increasing reserve per well, enhancing well productivity, and improving project economics in this type of reservoir. However, in order to have the highest production with the least cost, an optimization technique should be used for the fracturing treatment. According to the fact that aperture, propagation direction, and propagation potential of hydraulic fractures are of paramount importance in optimization of the fracking treatment, in this research paper, these three major factors are studied in detail, the control variables on these three factors are examined, and the effect of each factor is quantified by proposing a complete set of equations. Using the proposed set of equations, one can make a good estimate about the fracture aperture (directly controlling the fracture conductivity), the stress shadow size (directly controlling the fracture path), and the change of stress intensity factor (directly controlling the fracture propagation potential). A geomechanical optimization procedure is then presented for toughness-dominated and viscosity-dominated regimes based on the proposed equations that can be used for estimation of different optimal fracturing patterns. The most efficient fracturing pattern can be determined afterward via considering the cumulative production using a reservoir simulator e.g. ECLIPSE, Schlumberger. This procedure is likely to offer an optimal simultaneous multistage hydraulic fracture treatment without deviation or collapse, with no fracture trapping, with the highest possible propagation potential in the hydrocarbon producing shale layer, and a predicted aperture for proppant type/size decision and conductivity of the fractures.     

An approximate solution for toughness-dominated near-surface hydraulic fractures

Asymptotic solutions for fracture opening, volume and specific surface energy for small and large fracture radii are presented from the literature. By comparison to numerical simulation of static circular fractures subject to constant and uniform internal pressures, it is found that a good approximation for fracture opening, for intermediate fracture radii, is obtained from a power mean (with exponent 1/2) of the small and large fracture radii limiting cases. The power mean equation for fracture opening is used to derive corresponding equations for fracture volume, specific surface energy and mode I and II stress intensities. These are then combined to form an approximate solution to describe the propagation of circular toughness-dominated near-surface hydraulic fractures, suitable for small, large and intermediate fracture radii. The approximate solution is shown to closely approximate results from equivalent numerical simulation.     

The growth of the loosening zone

The problem of fracture of rocks will be considered on the base of the kinetic mechanism. Namely, the method proposed will be based on statistical consideration of the accumulation of microfractures. In this paper, based on this concept, we will model massive fractures in rock mass around the borehole as the result of pumping the viscous fluid through this borehole into rock. The experiments have shown that, as the result of the pumping and depending on the properties of rocks, either a hydraulic fracture or a loosening zone spreads in rocks surrounding the borehole. We propose a simple qualitative model to describe how a loosening zone grows with time and to evaluate the parameters controlling this growth. We expect that the model proposed will help to predict the external parameters and rock properties for which one or another mechanism of hydraulic fracturing occurs (the hydraulic fracture or the loosening zone).     

The growth of the loosening zone

The problem of fracture of rocks will be considered on the base of the kinetic mechanism. Namely, the method proposed will be based on statistical consideration of the accumulation of microfractures. In this paper, based on this concept, we will model massive fractures in rock mass around the borehole as the result of pumping the viscous fluid through this borehole into rock. The experiments have shown that, as the result of the pumping and depending on the properties of rocks, either a hydraulic fracture or a loosening zone spreads in rocks surrounding the borehole. We propose a simple qualitative model to describe how a loosening zone grows with time and to evaluate the parameters controlling this growth. We expect that the model proposed will help to predict the external parameters and rock properties for which one or another mechanism of hydraulic fracturing occurs (the hydraulic fracture or the loosening zone).     

Meso-structures evolution rules of coal fracture with the computerized tomography scanning method

In order to observe the meso-structure and fracture process of coal interior, the computerized tomography (CT) scanning experimental system was developed. This system includes the nonmetal cylinder shell, loading head, strain test equipment, hydraulic pump and vacuum pump, which can scan coal or rock samples with or without gas under uniaxial and triaxial load. The uniaxial compression experiments of coal were done and CT images were obtained at different stress level. The results show that the raw coal is a kind of heterogeneous material and the internal distribution of density is inhomogeneous. The cracks are uneven and their directions are random. During the loading process the inner structures of coal changed. The density of the coal increases at the beginning of load and decreases with the increment of stress. The fractures of coal occur mainly between the skeleton and matrix and on the weak region of the skeleton. Moreover, CT images from the experiments were used to reconstruct 3D images of coal under different stress states. The deformation and fracture process can be observed obviously. These investigations can provide theoretical foundations for understanding of fracture mechanism of coal.     

Stresses between 3D fractures in infinite and layered elastic solids

Periodically distributed opening mode fractures are often found in layered sedimentary rocks. The stress analysis related to opening mode fractures in layered solids is solved by a new numerical approach combining (3D) fast Fourier transform with the theory of periodic eigenstrain and the conjugate gradient method. Results show that the fracture spacing to layer thickness ratio for embedded opening mode fractures, using a three-dimensional (3D) model, is in good comparison with that of the plane strain case (two-dimensional model). The critical value of the fracture spacing to layer thickness ratio increases for a stiff layer case and decreases for a stiff surrounding solid case. Out-of-plane fracture length is also studied as a parameter in the 3D modeling. Opening mode surface fractures in a layered half-space were also studied. The results show that a critical fracture saturation ratio exists for this case and occurs when the normal surface stress transitions from tensile to compressive. This stress state is shown to be caused by a bending effect in the layer.     

A method for 3-D hydraulic fracturing simulation

We present a method for the simulation of 3-D hydraulic fracturing in fully saturated porous media. The discrete fracture(s) is driven by the fluid pressure. A cohesive fracture model is adopted where the fracture follows the face of the elements around the fracture tip which is closest to the normal direction of the maximum principal stress at the fracture tip. No predetermined fracture path is needed. This requires continuous updating of the mesh around the crack tip to take into account the evolving geometry. The updating of the mesh is obtained by means of an efficient mesh generator based on Delaunay tessellation. The governing equations are written in the framework of porous media mechanics theory and are solved numerically in a fully coupled manner. An examples dealing with a concrete dam is shown.     

Numerical simulation study of shale gas reservoir with stress-dependent fracture conductivity using multiscale discrete fracture network model

The exploitation of shale gas has increasingly become the focus of worldwide energy industry. Due to the existence of natural/hydraulic fractures, most of the shale gas reservoirs exhibit high degree of heterogeneity and complexity which leads to the stress-dependent fracture conductivity of shale gas reservoir. Discrete fracture network (DFN) model is adopted in this research since the conventional continuum model cannot meet the numerical simulation requirements of fractured shale gas reservoir. A series of experiments about the fracture properties stress-dependent have been conducted on some shale core samples, the stress-dependent fracture conductivity correlation is selected and incorporated into the mathematical model to characterize the reduction of fracture conductivity potential with the reservoir pressure drop. The DFN model is applied to a shale gas reservoir with fracture network to study the effect of the stress-dependent fracture conductivity on the shale gas well performance. The results show that the effect of fracture conductivity reduction with pressure drop on the shale gas well performance depends on both the initial fracture conductivity and matrix permeability. The complex interactions between the fracture and matrix permeability should be considered when select the appropriate size of proppants for fracturing.