【作者】 李小龙；

【导师】 曲占庆；

【作者基本信息】 中国石油大学(华东) ， 油气田开发工程， 2018， 博士

【摘要】 径向井辅助压裂技术作为一种增产手段目前在国内一些油田进行了初步探索并取得了良好效果。通过水力喷射钻孔技术在直井筒上形成径向井,可以引导裂缝走向、延伸裂缝穿透距离,降低地层破裂压力,扩大泄油面积。然而,目前国内外对于径向井辅助压裂裂缝形态及扩展规律研究较少,现场应用缺乏理论支撑。明确径向井辅助水力压裂裂缝扩展规律对合理优化径向井参数及压裂方案设计具有重要意义。鉴于此,本文基于数值模拟及物模实验,重点研究了径向井辅助压裂的裂缝起裂机理及其扩展规律,并进一步地探索了适用于径向井辅助压裂改造效果的评价方法。通过柱坐标系建立井筒及径向井眼的应力分布模型,利用最大主应力(最大拉应力)准则预测裂缝的起裂位置,并进行实例分析计算,得到近井带的最大主应力分布情况,结合最大主应力变化曲线的绘制分析、明确不同参数对裂缝起裂的影响规律;同时利用大型真三维物模实验验证径向井对裂缝具备定向起裂作用。结合理论分析及实验结果可知,最优起裂位置位于径向井处。借助ABAQUS有限元软件考虑地应力、孔隙压力等诸多参数模拟应力在井周的分布。ABAQUS中引入Mises及Max Principle应力准则分别对应塑性破坏及拉伸破坏。Mises应力于水平最小主应力方向取得极大值,因此水平最小主应力方向最易发生塑性破坏,应避免于该处布置径向井;Max Principle应力均于水平最大主应力方向取得极大值,因此水平最大主应力方向最易发生拉伸破坏,利于压裂裂缝起裂。应用有限元方法研究不同径向井参数对地应力的影响规律,利用最大主应力(最大拉应力)准则判断裂缝起裂位置并预测裂缝形态,并通过大型真三维物模实验验证数模的准确性。数模及实验结果表明,在径向井内水力加压会改变原始地应力分布并形成诱导应力场,场内的诱导应力在一定范围内可有效改变原始地应力的大小及方向,实现了压裂裂缝的定向扩展。诱导应力场内存在垂直于径向井轴线方向的拉应力,该拉应力是压裂裂缝定向扩展的根本原因。基于地层流-固耦合方程,建立扩展有限元(XFEM)模型,利用最大能量释放率准则判断裂缝扩展,量化分析了不同参数对裂缝形态的影响规律,并对影响因素进行了灰色关联分析,最后通过大型真三维物模实验在一定程度上验证了数模结果的正确性。结果表明,径向井辅助压裂技术对裂缝的引导能力可达40m,水平地应力差和径向井垂向井密是影响径向井对压裂裂缝引导能力的最主要因素,随水平应力差的增加,虽然起裂压力降低,但明显降低径向井对裂缝的引导能力;提高垂向井密可有效提高径向井对裂缝的引导能力同时降低起裂压力,当垂向井密小于1井/m时引导效果较差。相邻径向井间存在干扰应力场,场内应力数值较高,因此干扰应力场内最早发生裂缝的起裂并具备进一步扩展的能力。远井端各径向井仅受诱导应力场的影响,形成各自沿径向井扩展的主裂缝,且主裂缝扩展规律与单径向井辅助压裂裂缝扩展规律相同。主裂缝增大泄油面积,次级裂缝降低近井带油流阻力,两者结合构造的复杂多裂缝形态可有效提高产能。通过利用裂缝在均质储层中的波及面积并叠加模拟多裂缝的波及面积,以最大波及面积为评价标准优化径向井的布孔方式,为多径向井辅助压裂提供最优布孔方案的依据。研究结果表明,多径向井在平面均匀布孔可得到最大波及面积、获得最大产能。结合研究分析的多重因素,最终优化的布孔方案为四径向井、四裂缝形态,且预期裂缝与水平最大主应力方向呈45°夹角,并对称分布,钻孔时需预留裂缝偏转余量,具体余量数值需综合考虑地应力差、岩石参数等。通过产能数值模拟研究,验证了“最大波及面积”理论的正确性,同时验证了最优布孔方案的正确性。径向井压裂辅助蒸汽吞吐技术对于开采低渗稠油油藏非常有效,在优化布孔方案下3年累产油量约为常规压裂辅助蒸汽吞吐的2.65倍。相比冷采,热采中径向井辅助压裂技术的优势更大。更多还原

【Abstract】 The technology combined radial well and hydraulic fracturing has been applied in various oil fields and achieved effective output.The advantages of radial well fracturing in economy and stimulation effect are obvious when compared to horizontal well fracturing and conventional perforation fracturing.Radial well hydraulic fracturing technology can effectively improve the utilization of the reservoir area,and more applicable in production and development of thin reservoir.However,the fracture morphology and propagation law of radial well fracturing are still remain unclear,which limits the development and application of radial well fracturing.The column coordinates establishment of borehole and borehole radial stress distribution model,the tensile fracture criterion to predict crack initiation,and model was imported for programming examples of calculation and analysis,obtained under different conditions of the near wellbore with the maximum principal stress distribution drawing maximum principal stress curve,the analysis of influence of parameters on the crack.Through theoretical analysis and experiment,the conclusion: whether external conditions how to change the optimal location of the crack is still in the radial well.Based on fluid-solid coupling equation,factors that different parameters of radial well on crustal stress were solved nonlinearly by using ABAQUS.Initial fracturing position and propagation morphology were identified according to maximum tension-stress criterion.The conclusions stated that in-situ stress field distribution was changed during the radial well fracturing and the induced stress field along the radial well is established at the same time.The stage of fracture propagation is simulated by extended finite element method(XFEM)in ABAQUS.Initial fracturing position and propagation morphology were identified according to maximum tension-stress criterion and maximum energy release rate fracture criterion.Results obtained from 3D physical experiment in the laboratory were used to verify the accuracy of simulation to a certain degree.The propagation of major fracture is orientated by the induced stress in a certain area.The distance of orientated fracture is able to 40 meters.The guiding function of fracture orientated propagation is evaluated by included angle η between the axis of radial well and equivalent fracture.The smaller the value of η is,the better effect of fracture orientated propagation is.The increase of horizontal stress difference and decrease of vertical density of radial wells are unfavorable to decrease the value of η.The failure pressure grows up with the azimuth angle of radial well increasing.Also the degree of different parameters influence on value of η,failure pressure and maximum fracture width is ranked by grey correlation method.Fracture morphology under three cases(single radial well,vertical multi-radial wells and horizontal multi-radial wells)were identified.The effect of fracture orientated propagation could be strengthen greatly by the design of vertical multi-radial wells.The minor fracture is generated in the interference stress field and connect the two nearby radial wells.With the combination of major fracture and minor fracture,the oil drainage area is improved and the oil flow resistance near the wellbore is decreased greatly.At last the perforation scheme of radial wells is optimized according to the results of simulation.The results of research can be referred to prediction of fracture propagation and design of radial well completion or fracturing.The technology of radial well fracturing combined with steam soak is suitable for tight heavy oil formation.The thermal deliverability of radial well is researched by simulation based on the results above.The results show that multi-cracks morphology is performed under the multi radial wells which increase the drainage area.The thermal deliverability in 3 years of radial well fracturing is 2.65 times larger than that of regular fracturing.Also the parameters of steam soak are optimized.The conclusion is significant and important for the field.更多还原