水平井段内不同丰度天然气水合物固相颗粒的运移规律

在采用水平井对不同丰度天然气水合物(以下简称水合物)层进行钻进的过程中,含水合物的固相岩屑容易导致水平段固相颗粒沉积或黏附聚并,从而造成携岩不畅。为此,依据水平井多相流条件下的岩屑受力与运移规律及颗粒运移理论,建立了考虑水合物凝聚力条件下水平段岩屑滚动(正常钻进)和跃移(停泵沉砂)时钻井液临界返速模型并进行了数值模拟,分析了正常钻进和停泵条件下钻屑起动的影响因素和运移规律。结果表明:(1)起动临界流速随水合物丰度的增大而降低,该值在考虑水合物凝聚力条件下比不考虑该条件下要高,且水合物丰度越高水合物凝聚力的影响越明显;(2)水合物丰度小于85%时,起动临界流速随钻屑颗粒粒径的增大而增大,水合物丰度超过85%后,起动临界流速随钻屑颗粒粒径的增大而减小;(3)起动临界流速随钻井液密度、黏度的增大而减小;(4)相同条件下跃移所需临界返速约为滚动条件的1.28倍。结论认为,正常钻进条件下应考虑滚动模型,而停泵沉砂后再循环应考虑跃移模型的工艺思路,该成果对于优化水合物钻井施工参数以及降低钻井安全风险均具有重要意义。

Migration laws of natural gas hydrate solid particles with different abundance in horizontal wells

When a horizontal well is drilled through the natural gas hydrate (NGH) strata with different abundance, the NGH bearing solid debris tends to lead to the deposit and adhesion of solid particles in the lateral sections and consequently cuttings carrying is not smooth. In this paper, the critical return rate model of drilling fluid in the case of cutting rolling (normal drilling) and saltation (pump off for sand settling) under NGH adhesion in the laterals was established according to the force and migration laws of cuttings under the condition of multiphase flow in a horizontal well and the particle migration theory. Then, numerical simulation was conducted and the influential factors and migration laws of cuttings starting in the case of normal drilling and pump off were analyzed. And the following results were obtained. First, the critical starting flow rate decreases with the rise of NGH abundance and it is higher when the NGH adhesion is taken into account. Besides, the higher the NGH abundance, the greater the effect of NGH adhesion. Second, the critical starting flow rate increaseswith the rise of drilling cuttings particle size when the NGH abundance is less than 85%; and it decreases with the rise of drilling cuttings particle size when the NGH abundance is more than 85%. Third, the critical starting flow rate decreases with the rise of drilling fluid density and viscosity. And fourth, under the same conditions, the critical return rate for saltation is about 1.28 times that for rolling. It is suggested that the rolling model should be adopted for the normal drilling while the saltation model for the recycle after pump-off sand settling. The research results are of great significance to the optimization of NGH drilling parameters and the reduction of drilling safety risks.