核磁共振法研究钻井液侵入对甲烷水合物沉积层分解及渗透率的影响

Effect of fluid invasion on methane hydrate dissociation and sediment damage using low-field NMR

海域天然气水合物储层钻井过程中,井筒工作液侵入诱发储层水合物分解易导致储层骨架变形、微粒运移、出砂堵塞等类型的储层损害。为探明水合物储层伤害类型及机制,基于核磁共振技术建立井筒工作液侵入水合物储层的模拟试验方法,探究在不同液相侵入温度下水合物分解特性及储层伤害程度。结果表明:外来液相侵入过程中沉积层水合分解程度差异较大,将沉积层从左到右划分为3个区域(Ⅰ、Ⅱ、Ⅲ),当注热温度为20℃时沉积层水合物仅在近入口端少量分解;热流以40℃注入时,区域Ⅰ平均水合物饱和度由54%降低至24%,区域Ⅱ中水合物饱和度由65%下降至43%,区域Ⅲ中水合物饱和度由67%下降至49%;液相侵入温度为60℃时区域Ⅰ中平均水合物饱和度降至2%,基本已完全分解,区域Ⅱ中水合物饱和度仅剩12%,区域Ⅲ中平均水合物饱和度降为26%;侵入过程中水合物分解前缘呈楔形推进,注入温度升高,分解速率和分解程度明显加剧;随着水合物分解沉积层渗透率增大,但同时沉积层胶结性减弱,微粒运移加剧,堵塞孔喉,造成储层伤害。

In the process of drilling operation in methane hydrate sediments,the invasion of wellbore working fluid can induce the decomposition of gas hydrate in the reservoir,which easily results in reservoir damage,such as reservoir skeleton deformation,particle migration and sand blockage.In order to clarify the damage types and mechanism of methane hydrate damage,a simulation experiment method for the invasion of wellbore working fluid into hydrate reservoir was established based on nuclear magnetic resonance technology,and the hydrate decomposition characteristics and reservoir damage degree at different liquid invasion temperatures were investigated.The results show that the decomposition degree of hydrate varies greatly at different areas in the sand pack during the process of external liquid intrusion.The sediment can be divided into three regions(Ⅰ,ⅡandⅢ)from inlet to outlet sides.When the heat injection temperature is 20℃,the hydrate decomposes near the inlet part.When the heat flow is injected at 40℃,the average hydrate saturation in regionⅠdecreases from 54%to 24%,while the saturation decreases from 65%to 43%in regionⅡ,and this value decreases from 67%to 49%in regionⅢ.When the liquid phase intrusion temperature is 60℃,the average hydrate saturation in regionⅠdecreases to 2%,which has basically been completely decomposed.The hydrate saturation is 12%in regionⅡ,and the average hydrate saturation decreases to 26%in regionⅢ.During the intrusion process,the front edge of hydrate dissociation advances in a wedge shape,and the decomposition rate and degree increase obviously with the increase of injection temperature.As the hydrate decomposes,the permeability of the sedimentary layer increases,but at the same time,the cementation of the sedimentary layer weakens,and the particle migration intensifies,blocking the pore throat and causing reservoir damage.