页岩气井强化封堵全油基钻井液体系——以长宁—威远国家级页岩气示范区威远区块为例

Plugging-enhanced whole oil-based drilling fluid system for shale gas wells:A cased study of the Weiyuan Block in the Changning–Weiyuan National Shale Gas Demonstration Area

为了破解四川盆地长宁—威远国家级页岩气示范区威远区块钻井过程中井壁失稳引起的复杂事故频发难题,在分析测定该区下志留统龙马溪组页岩的矿物组成、孔缝构造、表面润湿性和岩样吸液率的基础上,分析了井壁失稳的机理,通过对基础配方的深度优化,以及多种类、多级配复合封堵剂的优选,形成了一套密度介于1.80~2.40 g/cm3、抗温150℃以上的强化封堵全油基钻井液体系。室内性能评价结果表明:①该钻井液体系具有良好的高温高压流变性、沉降稳定性和电稳定性;②该钻井液体系抗污染能力较强,在含页岩岩屑20%、含水10%时的综合性能依然保持稳定;③对龙马溪组页岩岩屑的一次回收率和二次回收率分别为100.00%和99.98%;④添加封堵剂的全油基钻井液能有效地封堵400 mD渗滤砂盘、快速地阻缓流体压力在页岩内部的传递,具有更好的井壁稳定效果。结论认为,该钻井液体系能够满足该区页岩气钻井施工的技术需求,为降低复杂事故发生率、提高页岩气资源开发效率提供了技术思路和解决路径。

Complicated accidents are frequently caused by borehole instability in the drilling process of the Weiyuan Block of Changning–Weiyuan National Shale Gas Demonstration Area in the Sichuan Basin.In order to solve this problem,this paper analyzed and determined the mineral composition,pore and fracture structure,surface wettability and liquid absorption rate of the shale samples of Lower Silurian Longmaxi Formation in this block.Then,the mechanism of borehole instability was studied.Finally,the basic formula was optimized and a variety of multistage compound plugging agents were selected.Thus,a set of plugging-enhanced whole oil-based drilling fluid system was formed,with its density in a range of 1.80–2.40 g/cm3 and temperature resistance over 150℃.And the following laboratory performance evaluation results are obtained.First,this newly developed drilling fluid system is advantageous with good rheology,settlement stability and electrical stability at high temperature and high pressure.Second,its contamination resistance is stronger,and its composite performance still remains stable at a shale cuttings content of 20%and a water content of 10%.Third,the first and second cuttings recovery of Longmaxi shale are 100.00%and 99.98%,respectively.Fourth,the whole oil-based drilling fluid with plugging agent can effectively block the 400 mD filtration sand disc and quickly relieve the transmission of fluid pressure inside the shale,so as to provide a better borehole stability.In conclusion,this plugging-enhanced whole oil-based drilling fluid system can meet the drilling technical requirements of shale gas wells in the Weiyuan Block and provides technical ideas and solutions for reducing the occurrence rate of complicated accidents and improving the development efficiency of shale gas resources.