基于分步有限元法的多级压裂过程中套管应力敏感性分析

目前中国四川地区页岩气开发大多采用大排量分段压裂工艺技术,部分页岩气井在施工过程中出现套管变形的现象,导致后续施工改造无法顺利进行,严重影响了页岩气井的正常生产。因此准确了解压裂过程中套管应力的变化规律具有重要意义。区别于传统模型,基于分步有限元方法,考虑了钻井、完井、压裂整个施工过程,构建了页岩各向异性下套管-水泥环-地层组合体有限元分析模型,模拟多级压裂过程中组合体应力以及温度场随时间的变化特征。分析了两种模型下注液温度、套管内压、地应力变化、地层孔隙压力变化、水泥环弹性模量以及地层弹性模量等因素对套管受力状态的影响。研究结果表明:(1)压裂过程中,采用传统模型会低估套管应力的增大程度;(2)大排量压裂施工过程中,井筒内温度震荡变化,注液温降导致套管应力明显升高;(3)地应力分均匀性以及孔隙压力的增大会导致套管应力有所增加;(4)页岩各向异性对套管应力的影响较小,其中地层性质的下降会导致套管损坏的几率增加。因此在今后的页岩气压裂改造过程中,有必要采用分步有限元模型,综合考虑这些影响因素,合理优化相关的压裂作业参数,从而确保后续压裂完井作业的正常进行。

Stress sensitivity analysis of casing in multi - stage fracturing based on step - by - step finite element method

At present, most of the shale gas development in Sichuan area of China adopts the large displacement fracturing technology, some of the shale gas wells in the process of casing deformation phenomenon, resulting in follow-up construction can not be carried out smoothly, seriously affecting the shale gas wells normal production. Therefore, it is very important to understand the variation law of casing stress in fracturing process. Based on the step-by-step finite element method, the whole construction process of drilling, completion and fracturing is considered in this paper. A finite element analysis model of casing-cement ring-stratum assemblage under shale anisotropy is established. The results show that the stress and temperature field in the fracturing process are different with the time. The effects of injection temperature, casing pressure, ground stress, pore pressure, elastic modulus of cement ring and elastic modulus of formation on the stress state of casing are analyzed. The results show that: 1) during the fracturing process, the traditional model will underestimate the degree of casing stress increase; 2) During the process of large displacement fracturing, the temperature fluctuation changes in the wellbore, 3) The stress uniformity and the increase of pore pressure lead to the increase of casing stress. 4) The effect of shale anisotropy on casing stress is small, in which the decrease of formation property leads to the increase of casing damage probability. Therefore, in the future shale gas fracturing process, it is necessary to adopt the step-by-step finite element model to comprehensively consider these influencing factors and optimize the relevant fracturing operation parameters in order to ensure the normal operation of the subsequent fracturing completion.