非渗透性低失水水泥浆保护储层的机理研究

固井水泥浆会对储层造成损害已是不争的事实。实验分析和理论研究表明：①高pH值条件下水泥浆中的大量过饱和无机物离子与地层水作用将产生大量结晶物和沉淀物（诸如Ca(OH)₂、CaCO₃、CaSO₄等），它们进入储层必然会缩小裂缝和孔喉，导致储层导流能力下降而损害储层；②钻井液动态污染后，若去掉外滤饼，再经水泥浆污染后岩心的整段渗透率恢复值只有35%～45%，此时仅靠内滤饼作用，油层保护效果不是太好，固相颗粒侵入深度将超过5.5 cm；钻井液动态污染后，若不去掉外滤饼，再经水泥浆污染后岩心的整段渗透率恢复值高达60%～70%，此时内外滤饼共同作用，油气层保护效果较好，固相颗粒侵入深度将只有2.5 cm。这说明外滤饼的存在对水泥浆的固相和液相确有很大的阻渗作用，而以降失水剂RC800为主配出的非渗透性低失水水泥浆会在近井壁地带形成一层高分子薄膜，阻止了水泥浆固相和滤液侵入储层，它相当于恢复了因要提高固井质量而人为破坏了的外滤饼的作用，从而保护了油气储层。

Study on Reservoir-protection Mechanism of Cement Slurry with Impermeability and Low Water Loss (LWL)

It's an undisputable truth the cementing slurry will do damage to the reservoir. The experiments and theoretical studies demonstrate: ① under the condition of high pH value, the large amounts of over saturated inorganic ions in the cement slurry will react with the formation water to produce a lot of crystallisates and sediments like Ca(OH)2, CaCO3, CaSO4 etc., which will decrease the size of fractures and pore throats once entering the reservoirs to reduce the conductivity of the reservoir; ② the core permeability recovery is only 35%～45% if the core re contaminated by the cement slurry after being polluted by dynamic drilling fluids and the external filtrate cakes be removed. Only the internal filtrate cake cannot effectively protect the reservoir, the solids intrusion depth will exceed 5.5cm. If the external filtrate cake is not removed, the permeability recovery will be 60%～70% even the core is contaminated by the slurry, both the external and internal filtrate cakes will help protect the reservoir, the solids intrusion will be only 2.5cm. It can be seen that the presence of external filtrate cake will provide a resistance for solids and fluid permeation. In addition, RC800 dominated impermeable and LWL cement slurry will form a layer of high molecular thin membrane at the near wellbore region to prevent the slurry solids and filtrates from entering the reservoir, so as to compensate the effects of the external filtrate cake which has been destroyed intentionally during cementing for better cementing quality, and thus the oil/gas reservoir is preserved.