摘要
随着页岩气资源的不断开发,水平井段的硬脆性页岩井壁失稳问题也越来越突出。为此,通过对水基、油基钻井液浸泡后的页岩岩样直剪试验,并结合单组弱面强度理论,分析了内聚力和内摩擦角对页岩强度的影响规律以及不同钻井液浸泡后的页岩强度弱化规律。对试验数据的分析结果表明:①水基、油基钻井液浸泡后内聚力和内摩擦角均下降,其垂直层理面方向的内摩擦角和内聚力均高于平行层理面方向;②页岩强度随钻井液浸泡时长增加而降低,受水化作用影响层理面导致强度降低,微裂缝表现为更易扩展,页岩容易产生破坏;③内聚力控制页岩强度大小,内摩擦角控制页岩沿弱面破坏的范围;④水基钻井液对页岩影响较为明显,页岩受其浸泡后强度下降较快,并随浸泡时间增加页岩强度持续下降,同时沿弱面破坏的角度限明显增大,油基钻井液能较好地保持岩样强度水平。最后,以四川盆地志留系龙马溪组M气藏为例,通过水平井段坍塌压力的计算,认识到水基钻井液基本不能满足钻井要求,而应采用油基钻井液且优先考虑沿最小水平地应力方向钻进。
With the development of shale gas resources, the borehole wall instability is more and more obvious in hard brittle shale beds of horizontal sections. In this paper, therefore, direct shear tests were performed on shale samples which were soaked in water or oil base drilling fluids, and then analysis was conducted of the effect rules of cohesive force and internal friction angle on shale strength and the weakness rules of shale strength after the shale samples were soaked in different drilling fluids. Based on the analy- sis of experimental data, the following conclusions were reached. Firstly, cohesive force and internal friction angle decrease in both cases of water base and oil base drilling fluids, but those vertical to the beddings are higher than those horizontal to the beddings. Secondly, the strength of shale decreases with the lengthening of soak time because of the effect of hydration on bedding planes, so microfractures tend to propagate and shale failure occurs easily. Thirdly, the shale strength is dominated by cohesive action and the shale failure scope along the weak plane is controlled by internal friction angle. And fourthly, shale is more affected by water base drilling fluids, and its strength decreases faster after being soaked in water base drilling fluids and continues decreasing with the lengthening of soak time. And moreover, angle limit for the failure along the weak planes increases remarkably, so oil base drilling fluids can keep the shale strength level. Finally, a case study was performed on M gas reservoir in Silurian Longmaxi Formation, Sichuan Basin. Based on the calculated collapse pressure of horizontal sections, it is realized that drilling requirements can not be met by water base drilling fluids, so oil base drilling fluids should be adopted and drilling should be preferentially carried out along mini- mum horizontal earth stress.
出处
《天然气工业》
EI
CAS
CSCD
北大核心
2015年第11期71-77,共7页
Natural Gas Industry
基金
国家重点基础研究发展计划(973计划)项目"页岩气水平井钻完井关键基础研究"(编号:2013CB228003)
四川省科技支撑计划项目"页岩气钻完井工程技术研究"(编号:2015SZ0003)
关键词
页岩
直剪试验
水化作用
弱面
岩石强度
坍塌压力
四川盆地
志留纪
水基钻井液
油基钻井液
Shale
Direct shear test
Hydration
Weak plane
Rock strength
Collapse pressure
Sichuan Basin
Silurian
Water basedrilling fluid
Oil base drilling fluid
