To select bridging agents properly is a critical factor inrndesigning the no-damaging or low-damaging drill-in fluids.rnHistorically, the Abrams's rule has been used for this purpose.rnAccording to this rule, the median particle size of a bridgingrnagent should be equal to or slightly greater than 1/3 of thernmean pore size for a given target formation. However,rnAbrams' rule only addresses the size of particle required torninitiate a bridge. The rule does not give optimum size orrnaddress an ideal packing sequence for minimizing fluidrninvasion and optimizing sealing.rnThis paper elaborates an ideal packing approach to solvernthe sealing problem, with the aim being to minimize formationrndamage by sealing pores with different size, especially thosernlarge pores which usually make dominant contribution tornpermeability and thereby preventing the solids and filtrate ofrndrill-in fluids from invading into formations more effectively,rncompared with the conventionally used techniques.rnA practical software has been developed to optimize thernblending proportion of several bridging agents, so as tornachieve ideal packing effectiveness. It is very convenient forrnuse in the field only by inputting some data of formation, suchrnas the maximum pore size or permeability.rnIt has been confirmed from numerous experimental resultsrnthat the core sample contaminated by the drill-in fluidrndesigned following ideal packing approach can acquire higherrnreturn permeabilities, and have a shallower invading depth, arnlower breakthrough pressure as well as a lower dynamicrnfiltration rate than cores contaminated by the drill-in fluidrndesigned applying the Abrams's rule.rnThe method and its use in selecting the best blendingrnproportion of several bridging agents, focusing on an idealrnpacking sequence for minimizing fluid invasion, are alsorndiscussed in this paper. A carefully designed drill-in fluidrnusing the ideal paking technique (named the IPT fluid) forrnoffshore drilling operations at the Weizhou oilfield located inrnthe west of South China Sea is presented. The near 100%rnreturn permeabilities from the dynamic damage tests usingrnreservoir cores prove the excellent bridging effect can bernprovided by this drill-in fluid.
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