水合物地层低温钻井液对井底岩石表层强度影响

由于天然气水合物仅在高压低温条件下稳定存在,为了保持水合物稳定,在钻井过程中宜采用低温钻井液,而在低温条件下钻井液能否对井底岩石表层起到软化作用,对于提高机械钻速具有重要意义。在分析钻井液对井底岩石表层软化作用机制的基础上,较系统地建立低温钻井液软化井底岩石表层定量评价方法,以清水为对比浆液,通过试验将6种水合物地层模拟钻井液(分别含有质量分数为0.1%的表面活性剂和有机盐处理剂)在低温条件下对薄片砂岩试样的软化效果进行对比。结果表明:相比于两种有机盐处理剂,含表面活性剂的4种模拟钻井液在低温条件下对岩样软化效果更好,有利于提高岩石破碎效率和机械钻速;4种含表面活性剂钻井液中,含十二烷基硫酸钠(SDS)钻井液对岩样的软化效果最好。

Influence of low temperature drilling fluid on strength of downhole rock surface

Nature gas hydrate is stable only under conditions of high pressure and low temperature. During the process of drilling low temperature, drilling fluid should be used to maintain the hydrate stability. At low temperature whether drilling fluid can play a role in softening the surface of downhole rock is very important for improving drilling speed. This paper analyzes the softening mechanism of drilling fluid for the surface of downhole rock and established the corresponding relationship functions and evaluation method, which is able to provide theoretical and technical support for improving drilling fluid performance and drilling efficiency during drilling in low temperature formations, such as gas hydrate bearing sediments. On the basis, the softening effect of drilling fluid for thin sandstone samples through comparative experiments of six simulation drilling fluids (with the amount of 0.1% wt of common surfactants and organic salts) for drilling in gas hydrate bearing sediments with pure water at low temperature was studied. Experimental results show that the experimental drilling fluids with surfactants at low temperature have good softening effect for used rock samples and are conducive to reduce the strength of downhole rock surface, increase rock breaking efficiency and drilling speed. The drilling fluid with sodium dodecyl sulfate (SDS) has the best softening effect among the four drilling fluids with surfactants.