高矿化度盐水钻井完井液损害气层防治方法

Preventing measure of high-salinity drilling & completion fluids damaging dry-gas formation

盐水钻井液完井液对防止地层中盐层溶解、粘土水化膨胀分散而导致井壁不稳定和储层产生水敏损害具有很好的作用。室内试验结果表明,高矿化度盐水钻井完井液对气层的渗透率产生严重损害,损害率在90％以上;无固相的高矿化度盐水压井液对气层渗透率的损害率达80％左右。通过研究其损害机理,损害的主要原因是高矿化度盐水钻井液完井液中的盐组分在干气返排过程中的结晶堵塞问题。提出了防治高矿化度盐水钻井完井液和射孔压井液损害气层的技术对策,即,调整井身结构,用套管将盐膏层封隔,将钻井液矿化度降低到气层的临界矿化度再钻开储层,能有效地防止钻井液的盐结晶损害;现场施工中,可以在射孔压井液前打入一段盐结晶溶解液作为前置液,投产返排时,天然气驱替前置液,前置液驱替进入储层的射孔液,这样在射孔液还没有析出盐结晶前,就被驱出了气层,从而消除了高密度盐水射孔液盐结晶损害气层问题,达到保护气层的目的。

High-salinity drilling & completion fluids have good effects on preventing hole instability and formation water-sensitivity damage caused by salt-gypsum formation dissolving and clay minerals hydrating swelling and dispersing in pay-zone. Laboratory experiments have shown that high-salinity drilling & completion fluids can seriously damage dry-gas formation. The permeability damage rates of core and dry-gas formation are as high as 90% and 80% respectively caused by the fluids. Thus, damage mechanisms of the fluids have been investigated and adequate preventing to the damage coun-termeasures have been given in this paper. The countermeasures include: adjusting the casing programme , plugging the salt-gypsum formation with casings between salt-gypsum formation and dry-gas formation, after adjusting salinity of drilling fluid to the critical salinity of dry-gas, then drill reservoir to prevent salt crystallization of drilling fluid; injecting salt crystallizing dissolution as preflush in perforation interval, displacing the preflush with natural gas to displace perforation fluid on producting. Thereby, the dry-gas formation damage comes from the salt crystallization of high density salt water perforation fluid can been prevented because the perforation fluid has been displaced from dry-gas formation before it separate out salt crystallization, and the purpose of protecting reservoir can been a-chieved.