关键词:  盐水侵, 污染机理, 复杂工况, 室内实验, 现场试验

Abstract:  There is compound brine in the Leikoupo formation of the Daxingchang structure. In the drilling process, the brine intrusion may lead to drilling fluid change, wellbore instability, pipe sticking, and lost circulation. Analysis reveals that the brine sample consists of sodium, magnesium, and calcium ions mainly. Experiments have been conducted to investigate the effects of ion concentrations on plastic viscosity, apparent viscosity, and dynamic shear force. Under sodium ion contamination, these three properties first increase and then decrease, with repeated fluctuation in dynamic shear force following the decrease; calcium and magnesium ions result in a rise in all of them, although to different extents. Based on our analysis, the positive charges compress the double electrical layer, causing the dispersion to decrease, so flocs emerge in the drilling fluid and then become particle pellets. However, different Coulomb forces from monovalent and divalent ions result in different degrees of compression and thus different macro effects. The pipe sticking and lost circulation cases in the drilling process of Dashen 001 - X1 and 001 - X4 wells reveal the diversity of the collapsing formations. Combined with field tests and experiments, it shows that the primary harms caused by brine intrusion are differential sticking, contamination-caused water loss increase in mud cake,contamination-caused collapse of upper wellbore, and hydro chop-caused lost circulation. Based on the contamination mechanism, drilling fluid recipe is improved, and magnesium and calcium deionization additives are added. Corresponding well killing methods are applied to Well Dashen 001 X4, which demonstrates the accuracy of theoretical analysis and lays the foundation for future drilling at Daxingchang structure.

Key words:  , combined brine intrusion, pollution mechanism, complex conditions, laboratory experiment, field test