The issue of drilling depleted zones is increasing inimportance as more wells are drilled in mature fields. Thesezones are typically produced or producing reservoirs overlaidand interbedded with shale layers. Pressure overbalances havebeen reported as high as 13000 psi but are more typically ofthe order of a few thousand psi.Wellbore stability problems associated with drilling inthese zones can be linked with drilling-induced and preexistingfractures. We describe an approach that links afracture-fluid-flow model with fluid rheology over a widerange of flow rates and flow behavior in a fracture generationapparatus. The understanding gained is used to developguidelines for minimising losses into fractures.A numerical fracture simulation scheme with Perkins-Kern-Nordgren (PKN) geometry and flexible rheology of theinvading fluid predicts fluid volume lost as a function of time.The drilling environment - differential pressure, fracturegradient, pore pressure and rock properties - can be varied.The effect of fluid rheology on fluid loss rate is demonstratedunder various combinations of the parameters relevant todepleted zone drilling.Drilling fluid rheology was investigated in shear flow overthe shear rate range 0.001 – 1000 s-1, and in transient flow.Most fluids exhibited shear-thinning and thixotropic behaviorthat could not be described in terms of PV and yield point(YP) alone. Constitutive rheological models were used todescribe the data for input to the simulation model. A widerange in transient behavior was found, and it forms the basisof an experimental test to rank and select fluids to minimizelosses in fractures.The fracture generation apparatus enables a fracture to beinitiated in a rock core, closed and then re-opened. Weevaluated a suite of water-based and oil-based fluids and lostcirculation materials, some of which show unexpectedincreases in the reopening pressure.
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