With exploration advancement in Sichuan Basin,one problem that water-based drilling fluid with high density is contam-inated by CO2 is increasingly prominent in some sour gas blocks or fields(e.g.eastern basin,as well as Penglai,Gaoshiti,and Moxi fields),which causes the fluid's rheology out of control to further increase downhole accidents and complexities,such as pipe sticking and lost circulation.Thus,a system of high-density drilling fluid with resistance to CO2 contamination has been developed by analyz-ing contamination mechanisms and selecting appropriate anti-pollution agents.Results show that(i)slight contamination refers that clay particles transit from coarse to fine dispersion,resulting in enhanced hydration and dispersion,while severe one implies CO32-and HCO3-compress double layers,making the fluid's colloid stability get worse and the system performance take a sharp turn for the worse;and(ii)with anti saline soil,compounding inhibition,and expanding solid phase content,this developed system exhibits stable performance and strong anti-pollution under the contamination of CO2,CO32-and HCO3-with high concentration.Its application to Ga-oXX well in Sichuan Basin demonstrates stable performance consistently when CO2 contamination at the bottomhole,with the average ROP enhanced by 5.6%to 23.8%and the drilling-fluid volume reduced by more than 64%as compared with adjacent wells in the same block.
CO2 contaminationHigh densityWater-based drilling fluidContamination mechanismSichuan Basin
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