The issue of drilling depleted zones is increasing in importance as more wells are drilled in mature fields. These zones are typically produced or producing reservoirs overlaid and interbedded with shale layers. Pressure overbalances have been reported as high as 13000 psi but are more typically of the order of a few thousand psi. Wellbore stability problems associated with drilling in these zones can be linked with drilling-induced and preexisting fractures. We describe an approach that links a fracture-fluid-flow model with fluid rheology over a wide range of flow rates and flow behavior in a fracture generation apparatus. The understanding gained is used to develop guidelines for minimising losses into fractures. A numerical fracture simulation scheme with Perkins- Kern-Nordgren (PKN) geometry and flexible rheology of the invading fluid predicts fluid volume lost as a function of time. The drilling environment - differential pressure, fracture gradient, pore pressure and rock properties - can be varied. The effect of fluid rheology on fluid loss rate is demonstrated under various combinations of the parameters relevant to depleted zone drilling. Drilling fluid rheology was investigated in shear flow over the shear rate range 0.001 – 1000 s-1, and in transient flow. Most fluids exhibited shear-thinning and thixotropic behavior that could not be described in terms of PV and yield point (YP) alone. Constitutive rheological models were used to describe the data for input to the simulation model. A wide range in transient behavior was found, and it forms the basis of an experimental test to rank and select fluids to minimize losses in fractures. The fracture generation apparatus enables a fracture to be initiated in a rock core, closed and then re-opened. Weevaluated a suite of water-based and oil-based fluids and lost circulation materials, some of which show unexpected increases in the reopening pressure.
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