页岩气藏地层钻井卸载破坏及对井眼稳定性的影响研究

发布时间：2018-02-04 02:52

本文关键词： 页岩卸载井壁稳定性安全钻井　出处：《西南石油大学》2015年硕士论文　论文类型：学位论文

【摘要】：随着国民经济的高速发展和对环境保护需求呼声的日益高涨,我国对天然气的需求量越来越大,对外依存度呈逐年快速攀升趋势。我国的页岩气勘探开发还处于起步阶段,面临的问题还有很多。其中如何安全钻成长井段规则水平井就是页岩气开发过程中必须首先解决的关键工程技术问题之一,而如何保持井壁的稳定性又是其中最为核心的一个环节。页岩气层的井壁稳定性受其岩石力学特性、地应力环境及施工工艺环节等多因素影响和制约。已有研究表明,页岩气层岩石具有较强的脆性,脆性岩石在钻井过程中极易因钻井应力扰动和卸载而诱发应力卸载缝,引起井周地层岩石结构破坏。而目前国内外对此研究还极为不足,因此.解决页岩地层井眼稳定问题,系统深入地研究卸载条件下硬脆性页岩的破坏机理,对科学有效地解决井壁失稳问题,实现页岩气藏安全高效开发,具有重要的工程实际意义。本文依托国家自然科学基金项目(U1262209),以龙马溪组页岩气层岩石为研究对象,室内加、卸载岩石力学实验为基础,结合地层坍塌压力计算,研究了页岩气层钻井卸载对井周地层岩石力学行为的影响及卸载破坏对井眼稳定性的影响,获得以下认识和结论： (1)单、三轴岩石抗压强度实验表明,研究选区的页岩具有较高强度和明显脆性特征,其黏土矿物含量较低,石英等脆性矿物含量较高,单轴压缩破坏呈现典型的脆性劈裂破坏特征。加载条件下,岩样的抗压强度随着围压的增高而增大,塑性有所显现,岩样破坏模式由劈裂破坏逐渐变为单剪破坏。 (2)卸载力学实验说明,页岩岩样在卸载过程中表现出了与加载过程中不同的变形和破坏特征。卸载导致页岩产生的扩容比加载过程更剧烈,岩样显示出了更强的脆性。低围压下岩样对卸载更加敏感,更易产生张性破坏。在卸荷作用下,页岩力学强度降低,剪切强度参数表现为内聚力少许增大,内摩擦角显著降低。随卸载速率的增大,岩样破坏时的剧烈程度先增大后减小。在实验范围内(卸载速率0.2MPa/s至1.OMPa/s),可能存在一个卸载速率使岩样破坏最剧烈,即在钻井过程中随钻速增大,岩石受应力卸载而发生破坏的程度不是单调增大的,可能存在某一钻速对井壁岩石产生最大破坏。 (3)卸载速率、卸载幅度对致密硬脆性岩石破坏特征和强度有明显影响。不同幅度的卸载实验发现,随卸载幅度增大,岩样破坏程度呈先减小后增大趋势。围压卸载程度越大,对岩样的破坏越大,岩样的破坏程度越剧烈。大幅卸载将加剧致密脆性岩石的损伤演化及破坏。对该类地层的安全钻井,应在充分认识其他地质力学环境、结构特征的基础上,优化钻速及钻井液密度,尽量减小钻井卸载引起的井周岩石破坏和失稳。 (4)基于实际工区地质力学参数资料,研究了卸载对井壁地层坍塌压力的影响。结果表明,考虑卸载影响后得到的井壁地层坍塌压力明显增大。运用岩石加载力学特性分析得到的井周岩石稳定性较实际偏大,特别是在高地应力、高地层压力和高欠压值钻井情况下。并基于卸载力学参数分析对比Mohr-Coulomb强度准则和Hoek-Brown强度准则对地层坍塌压力的影响,结合目前页岩气钻井工程实际表现认为,基于Hoek-Brown强度准则考虑卸载影响计算得出的坍塌压力更能准确反映地层的真实情况。
[Abstract]:With the rapid development of national economy and the increasing demand for environmental protection , China ' s demand for natural gas is increasing and its external dependence has been increasing year by year . Based on the national natural science fund project ( U1262209 ) , the influence of the shale gas layer drilling unloading on the rock mechanics behavior and the influence of unloading failure on the borehole stability are studied based on the research object , the indoor addition and unloading rock mechanics experiment of the Longmaxi Formation shale gas layer rock , and the following understandings and conclusions are obtained : ( 1 ) The compressive strength of single - and tri - axial rock shows that the shale in the study area has higher strength and obvious brittle characteristics , the content of clay minerals is low , the content of brittle minerals such as quartz is high , and the uniaxial compression damage presents typical brittle fracture damage characteristics . The compressive strength of rock samples increases with the increase of confining pressure and the plasticity appears , and the rock sample failure mode gradually becomes single shear failure by splitting failure . ( 2 ) The mechanics experiment shows that the shale rock sample shows different deformation and destruction characteristics during unloading . The unloading results in more severe expansion than the loading process . The rock sample shows stronger brittleness . Under the action of unloading , the rock sample is more sensitive to unloading , and the internal friction angle is decreased . ( 3 ) The unloading rate and the unloading amplitude have obvious influence on the damage characteristics and strength of the tight brittle rock . The larger the unloading amplitude , the greater the damage degree of the rock sample , the more severe the damage degree of the rock sample . ( 4 ) Based on the data of the geological mechanics parameters of the actual working area , the influence of unloading on the collapse pressure of the borehole wall is studied . The results show that the rock stability of the well wall obtained after considering the unloading influence is obviously increased . Based on the analysis of the mechanics characteristics of rock loading , the rock stability of the well wall is relatively large , especially in the case of high ground stress , high formation pressure and high under - pressure value drilling . Based on the analysis of the unloading mechanical parameters , the influence of the Coulomb strength criterion and Hoek - Brown strength criterion on the formation collapse pressure is analyzed .

【学位授予单位】：西南石油大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TE21

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