Pump-stopping pressure drop model considering transient leak-off of fracture network

By introducing the coupling flow expressions of main fracture-matrix, secondary fracture-matrix and main fracture-secondary fracture into the traditional main fracture material balance equation, the “main fracture-secondary fracture-matrix” leak-off coupling flow model is established. The pressure-dependent fracture width equation and the wellbore injection volume equation are coupled to solve the pressure-rate continuity problem. The simulation and calculation of the bottomhole pressure drop and fracture network closure after the pump stopping in slickwater volumetric fracturing treatment are realized. The research results show that the log-log curve of pump-stopping bottomhole pressure drop derivative presents five characteristic slope segments, reflecting four dominant stages, i.e. inter-fracture crossflow, fracture network leak-off, fracture network closure and residual leak-off, after pump shutdown. At the initial time of pump shutdown for volumetric fracturing treatment of horizontal well, the crossflow between main and secondary fractures is obvious, and then the leak-off becomes dominant. The leak-off of main and secondary fractures shows a non-uniform decreasing trend. Specifically, the leak-off of main fractures is slow, while that of secondary fractures is fast;the fracture network as a whole presents the leak-off law of fast first, then slow, until close to zero. The influence of fracture network conductivity on the shape of pressure decline curve is relatively weaker than that of fracture network size. The fracture network conductivity is positively correlated with leak-off volume and fracture closure. The secondary fracture size is positively correlated with leakoff volume and closure of the secondary fracture, but negatively correlated with closure of the main fracture. Field data validation proves that the proposed model and simulation results can effectively reflect the closure characteristics of the fracture network, and the interpretation results are reliable and can reflect the non-uniform stimulation performance of each fracturing stage of an actual horizontal well.

Impact of elliptical boreholes on in situ stress estimation from leak-off test data

We developed an inversion technique to determine in situ stresses for elliptical boreholes of arbitrary trajectory. In this approach, borehole geometry, drilling-induced fracture information, and other available leak-off test data were used to construct a mathematical model, which was in turn applied to finding the inverse of an overdetermined system of equations.The method has been demonstrated by a case study in the Appalachian Basin, USA. The calculated horizontal stresses are in reasonable agreement with the reported regional stress study of the area, although there are no field measurement data of the studied well for direct calibration. The results also indicate that 2% of axis difference in the elliptical borehole geometry can cause a 5% difference in minimum horizontal stress calculation and a 10% difference in maximum horizontal stress calculation.

Leak-Off Mechanism and Pressure Prediction for Shallow Sediments in Deepwater Drilling

Deepwater sediments are prone to loss circulation in drilling due to a low overburden gradient. How to predict the magnitude of leak-off pressure more accurately is an important issue in the protection of drilling safety and the reduction of drilling cost in deep water. Starting from the mechanical properties of a shallow formation and based on the basic theory of rock-soil mechanics, the stress distribution around a borehole was analyzed. It was found that the rock or soil on a borehole is in the plastic yield state before the effective tensile stress is generated, and the effective tangential and vertical stresses increase as the drilling fluid density increases; thus, tensile failure will not occur on the borehole wall. Based on the results of stress calculation, two mechanisms and leak-off pressure prediction models for shallow sediments in deepwater drilling were put forward, and the calculated values of these models were compared with the measured value of shallow leak-off pressure in actual drilling. The results show that the MHPS(minimum horizontal principle stress) model and the FIF(fracturing in formation) model can predict the lower and upper limits of leak-off pressure. The PLC(permeable lost circulation) model can comprehensively analyze the factors influencing permeable leakage and provide a theoretical basis for leak-off prevention and plugging in deepwater drilling.

Leak-off behavior and water shut-off performance of a polymer/chromium(Cr^(3+)) gel in fractured media

During gel treatments for fractures, the leak-off behavior of gelant has a great effect on the water shut-off performance of gel. Experiments were carried out using a polymer/chromium(Cr^(3+)) gel system to explore the leak-off behavior and the water shut-off performance of gel in fractured media. Results of the gelant leak-off study show that the gelant leak-off from fracture into matrix contributes to the formation of the gelant leak-off layer during the gelant injection. Moreover, because of the gradual formation of the gelant leak-off layer along fracture, the initial leak-off ratio of gelant is relatively high, but it declines and finally levels off with the increase of the injection volume. The polymer concentration of gelant has a great effect on the chromium output in fluids produced from fractures. With the increase of the polymer concentration, the chromium concentration first decreases and then increases, and the leak-off depth of gelant into matrix is gradually reduced. Results of the water shut-off study present that the decrease of the chromium concentration inside the fracture greatly reduces the water shut-off performance after the gel formation. Therefore, because of the relatively high degree of chromium leak-off, enough injection volume of gelant is essential to ensure the sufficient chromium concentration inside the fracture and to further achieve a favorable water shut-off performance. On the premise of gel strength assurance inside the fracture, the water shut-off performance of gel gradually declines with the extension of the distance from the fracture inlet, and different leak-off degrees of gelant along the fracture are responsible for this phenomenon. Therefore, a proper degree of gelant leak-off contributes to enhancing the water shut-off performance of gel for fractures.

基于压裂液自发吸入模型的页岩气压裂液滤失定量预测

非常规油气资源开发常使用水力压裂技术提高单井产量,然而压裂现场数据表明,大量压裂液会滞留在地层中导致压裂液的返排率较低,将对储层和地下水环境造成一定影响。为进一步研究压裂液滤失机理并准确预测压裂液滤失量,首先建立了由毛细管力作用引起的压裂液自发吸入模型,然后对压裂液滤失量进行定量计算,并在现场进行了验证。最后,引入无量纲吸入率参数分析了控制压裂液滤失的关键因素。研究结果表明:(1)页岩气水平井压裂施工中大约50%~95%的压裂液通过基质吸入,压裂液吸入量仅与吸入率参数、裂缝面积和吸收时间有关;(2)当润湿相黏度与非润湿相黏度之比超过阈值时,吸入率参数主要由岩石的孔径分布参数决定,流体黏度的影响非常有限;(3)当孔径分布参数介于0.5~0.7时,吸入率参数达到相对较高的值,即更多的压裂液被吸收到地层中;(4)当润湿相黏度与非润湿相黏度之比大于10,孔径分布参数小于0.8时,优化后的自吸模型具有较大的适用性;(5)由毛细管力机理引起的压裂液滤失量与实际储层观察到的滤失量相近,可认为页岩储层有足够的存储滤失液的能力,不会影响地下水层中的饮用水。结论认为,建立的压裂液自发吸入模型,能够准确预测页岩气水平井压裂液滤失量,为非常规油气的生产及环境保护提供了技术支撑。

青海共和盆地干热岩注采大尺度物理模拟实验

基于自主研制的真三轴多物理场原位注采大型物理模拟实验系统,开展了青海共和干热岩多井长期注采物理模拟实验。通过多井连通性实验获得了岩样内部天然裂缝系统的空间分布特征以及各裂缝与井筒的连通情况,在此基础上选择注入井和生产井,开展了一注两采和一注一采实验,系统分析了生产井的开采流量、开采温度、采热速率和流体采收率随持续注采的变化规律。结果表明:在热冲击、注入压力联合作用下,裂缝导流能力增强,生产井开采温度表现为下降趋势,且流量越大下降越快;当裂缝局部闭合区域逐渐激活,产生新的换热面积,开采温度升高或下降速率降低;采热速率主要由开采流量、注入和采出流体的温度差控制,当滤失通道导流能力增强,生产井流体采收率快速下降;优势通道和换热流体滤失对采热性能的影响机制有所区别,前者限制换热面积,后者影响采出流体流量,二者都是影响干热岩长期高效开发的重要因素。

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。

The effect of natural fracture on the fluid leak-off in hydraulic fracturing treatment

Fluid leak-off phenomenon plays a critical role in hydraulic fracturing operation.This phenomenon can be very impressive in successful operation of hydraulic fracturing.This operation is very complex in fractured reservoirs due to the reaction between induced fracture and natural fractures.In this study with the cohesive element method,the effect of presence of natural fracture on the magnitude of hydraulic fracturing fluid leak-off is investigated.First of all,cohesive element and extended finite element method methods are described.The fluid flow inside hydraulic fracture and the affecting parameters on leak-off of this fluid on adjacent environment are analyzed.Then,effects of natural fracture on hydraulic fracturing direction such as deviation,leak-off and the mutual influences(which includes the changes of stress regime around the natural fracture)and also changes in pore pressure are processed.The results indicate that presence of natural fracture will cause reduction in aperture of hydraulic fracture.This decrease will lead to extension of fluid lag and eventually delaying of leak-off phenomenon.However,this effect is negligible against the positive impact due to shear and normal displacement on increasing leak-off.

考虑压驱注水诱发裂缝影响的注水井压力分析

压驱注水井底压力分析对压驱效果评价与储层参数反演具有重要的意义。为了获得压驱注水井井底压力响应和压驱注水产生的动态裂缝参数,从裂缝扩展角度出发,运用渗流力学理论与数值模拟方法,在考虑裂缝扩展、裂缝形态、注入流体滤失等因素影响的基础上,形成了考虑压驱注水诱发裂缝影响的注水井压力分析模型,获得了压驱注水井井底压力响应,分别分析了注入流量、产液量、地层渗透率和井距因素对压力的影响,并将其运用于实际压驱井,以验证模型的准确性。研究发现:压驱注水井流体流动分为5个流动阶段,分别为压驱起裂初期阶段、裂缝扩展阶段、线性流动阶段、过渡流动阶段和边界控制流动阶段;随着注入流量的增大,裂缝扩展阶段越明显,且产液量越大,在压驱注水后期双对数压力分析曲线上翘幅度越小。该研究对压驱注水开发低渗透油藏不稳定渗流机理的研究和认识具有重要的意义,对注水井动态裂缝对水井井底压力的影响研究具有一定意义。

考虑压裂缝网动态滤失的停泵压降模型

通过在传统主裂缝物质平衡方程中引入主裂缝-基质、次裂缝-基质、主裂缝-次裂缝耦合流动表达式,建立了“主裂缝-次裂缝-基质”滤失耦合流动模型,同时将主、次裂缝缝宽的应力敏感方程和井筒注入量方程进行压力-流量连续性耦合求解,实现了滑溜水体积压裂改造停泵后井底压降和缝网闭合的模拟计算。研究表明:停泵井底压降导数双对数曲线呈现5个特征斜率段,反映了停泵后“缝间窜流控制”、“缝网滤失控制”、“缝网闭合控制”和“残余滤失控制”4个主控阶段。水平井体积压裂改造停泵初始时刻,主、次裂缝间的窜流现象明显,随后滤失占据主导,主、次裂缝的滤失呈现出非均匀的递减趋势:主裂缝滤失速度慢,次裂缝滤失速度快,缝网整体呈现先快、后慢,直至接近于零的滤失规律。缝网导流能力对停泵压降曲线形态的影响相对弱于缝网规模;缝网导流能力与滤失量、裂缝闭合程度正相关;次裂缝规模与滤失量、次裂缝闭合程度正相关,但与主裂缝闭合程度负相关。经现场施工数据检验,该模型能够有效地反映缝网的闭合特征,解释结果可靠且能够反映实际水平井各压裂段的非均匀改造效果。

某核电机组阀门引漏管开裂原因

某核电机组阀门引漏管发生开裂,采用宏观观察、化学成分分析、金相检验、维氏硬度测试、扫描电镜和能谱分析等方法对引漏管的开裂原因进行分析。结果表明:引漏管开裂的原因为应力腐蚀,腐蚀介质为氯离子,最后提出了相关建议。

基于FrackOptima的PKN型水力裂缝数值模拟分析

使用FrackOptima全三维水力压裂软件模拟计算了PKN型水力裂缝在存储效应主导条件下和滤失效应主导条件下的扩展过程。该软件首先使用边界元素进行离散化网格划分,然后通过边界元算法进行数值模拟。将数值计算结果和相关参数条件下的PKN模型半解析解进行对比。结果表明,在缝高方向上仅设置5个元素的条件下,数值计算结果就能够和半解析解较好地吻合,最大误差不超过10%。研究结果验证了该软件的精度和效率,为进一步使用该软件进行水力压裂实际方案的优化提供了理论基础。

裂缝性地层H-B流型钻井液漏失流动模型及实验模拟

随着油气勘探开发逐步面向深层、超深层、深水、高温高压高含硫及多压力层系等复杂地层,井漏问题异常严峻,严重迟滞了油气勘探开发进程。因此,开展钻井液漏失诊断研究,揭示钻井液漏失动态行为及其特征,对认识井漏和优化防漏堵漏技术有重要意义。建立了二维平面裂缝H-B流型钻井液漏失流动模型,揭示了钻井液漏失动态行为及其影响因素。研究结果表明,二维平面裂缝的纵横比、裂缝面积、延伸长度、裂缝变形及裂缝面倾角越大,钻井液漏失速率及累积漏失量则越大。钻井液稠度系数及动切力越大,钻井液漏失速率及累积漏失量则越小。利用高温高压钻井液漏失动态评价仪评价了0.5 mm和1mm缝宽的平面裂缝的钻井液漏失行为,与漏失模型模拟结果整体趋势吻合,误差小于25%,表明所建二维平面裂缝钻井液漏失流动模型具有一定的合理性。

沁南地区煤体结构对煤层气开发的影响

为了研究煤体结构对煤层气开发的影响,对比了沁南地区3号煤层中原生结构煤、碎裂煤、碎粒煤、糜棱煤等不同煤体结构的钻井、压裂、排采资料,并对部分煤矿进行了井下观测分析。研究发现煤体结构越破碎,井壁稳定性越差,井径、抽采孔钻屑量越大,同时,储层污染范围与井径大致呈正相关关系;碎粒煤及糜棱煤发育处,井筒周围形成厚层的水泥环,水力压裂初始施工压力将快速上升,导致水泥环破裂,同时压裂液的大量滤失会降低压裂裂缝的延伸范围,甚至导致砂堵等工程问题;在排采过程中,煤层水及煤层气均有携带煤粉的能力,易造成排采通道的堵塞,导致气井产能迅速降低,同时也容易造成裂缝闭合,降低解吸范围,煤体结构越破碎,煤粉产出越多,裂缝闭合越严重。

压裂液综合滤失系数的计算方法研究

准确认识和计算压裂液综合滤失系数,既是压裂施工设计中的一个关键问题,也是压后分析评估的一个核心内容。通过对压裂液综合滤失系数的四种计算方法,即理论计算方法、压后压力降落分析、瞬时关井压力梯度法和压裂过程压力反演方法综合分析后认为,理论计算方法得到的压裂液综合滤失系数往往小于实际情况,而且地层渗透率越低问题越严重,对此给出了理论计算方法计算的压裂液综合滤失系数的修正关系式,具有重要的借鉴和指导意义。

大庆深层致密气藏高温压裂液的研制与应用

针对深层致密气藏储层埋藏深、岩性复杂且微裂缝发育、压裂施工规模大的特点,研制了深层致密气藏压裂液体系(CHTG),改变了传统压裂液受热、剪切快速降解的特点,研究了该压裂液体系性能.室内评价及现场应用表明:CHTG压裂液在170℃高温环境中性能稳定,在150℃条件下,压裂液抗剪切可达到4.0 h,同时可根据需要控制压裂液的流变性,降低压裂液摩阻,并当排量为7 m3/min时,CHTG压裂液的摩阻是清水的16.7%;采用的降滤剂能显著降低双重介质下的压裂液滤失,压裂后表皮因数为-5.92,2口井压后无阻流量超过100×104m3/d,其增产效果显著.

天然裂缝压裂液滤失模型

在裂缝性储层水力压裂过程中,天然裂缝在水力裂缝的作用下产生剪切滑移或张开,使压裂液的滤失量显著增加,从而增大了施工风险。目前的压裂液滤失模型大多是针对均质储层的,不适用于裂缝性储层。为此,从天然裂缝内压裂液的动态滤失过程出发,描述了压裂液滤失的物理过程;根据压力连续及流体体积守恒原理,建立了天然裂缝压裂液滤失模型;结合模型求解思路和方法,编制了计算程序。研究结果表明,压裂液性质、天然裂缝性质和施工参数等对压裂液的滤失影响较大,重点模拟分析了充填带对滤失量的影响。天然裂缝压裂液滤失模型计算结果表明:当最大缝宽较窄时,存在最小滤失量的临界缝宽;当最大缝宽较大时,累积滤失量与临界缝宽成反比;增大充填物的体积浓度可减小压裂液滤失量;而增大充填带临界厚度会增大滤失量,但存在最佳临界厚度。

渗透性砂岩地层漏失压力预测模型

为了预测渗透性砂岩地层的漏失压力,提高钻井的安全性。根据塑性流体的本构方程和毛细管渗流理论,分析了钻井液(宾汉流体)在井壁泥饼带、地层侵入带和原地层孔隙中的流动特性,分别建立了井壁有无泥饼2种情况下的渗漏压力计算模型。利用建立的数学模型,对崖城13-1-A15井压力衰竭储层段的漏失压力进行了预测,结果表明:井壁上未形成泥饼时,漏失压力当量密度为0.613kg/L;井壁上形成厚度1mm、渗透率小于4×10-5 D的泥饼时,漏失压力当量密度可提高到1.21kg/L以上。崖城13-1-A15井钻井施工中,采用密度1.10kg/L的抗高温Versaclean油基钻井液,没有发生井漏。建立的渗透性砂岩地层漏失压力预测模型,可为渗透性砂岩地层安全钻井提供参考。

天然裂缝滤失计算和控制技术应用

引用天然裂缝滤失系数和基质滤失系数的指数关系式,对塔里木油田裂缝性K气藏小型测试压裂的压降分析,计算出K气藏天然裂缝滤失系数为10^-3～10^-2 m/min^1/2级别;净压力控制在10 MPa以内,即能将裂缝滤失系数控制在基质滤失系数的10倍以内.现场应用表明,使用多级高质量浓度段塞和大段塞量是控制K气藏加砂压裂净压力和天然裂缝滤失的有效措施之一.塔里木油田裂缝性砂岩储层加砂压裂的段塞级数为4-6级,质量浓度60～520 kg/m^3,体积7～15 m3,段塞量增加1倍,平均净加砂量增加1.5倍.研究结果对类似天然裂缝发育储层的裂缝滤失规律研究和加砂压裂优化设计有很好的借鉴意义.