Depleted Zone Drilling: Reducing Mud Losses Into Fractures
Authors: Adachi J. , Bailey L. , Houwen O.H. , Meeten G.H. , Way P.W. , Growcock F.B. , Schlemmer R.P. 
Published: December 2007

Abstract
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 pre-existing 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. We evaluated a suite of water-based and oil-based fluids and lost circulation materials, some of which show unexpected increases in the reopening pressure.

Introduction
The issue of drilling depleted zones is increasing in importance as more fields mature. These zones are typically produced or producing reservoirs overlain and interbedded with shale layers. Pressure overbalances have been reported as high as 13000 psi in the Gulf of Mexico1 but more typically are on the order of a few thousand psi.
Drilling problems in these zones can be broadly categorised into three main areas:
Wellbore StabilityThe presence of normally pressured shales means a higher mud weight is required to prevent collapse even when drilling in the depleted zone.The drilling profile with regard to bedding must be considered both with regard to the overlying shales and weakening of the reservoir rock itself that can result from the depletion process.There can be an issue with mechanical sticking from creeping shales if mud weights are not maintained high enough. Proper levels of inhibition need to be maintained to prevent chemical swelling of shales.If salt structures are being drilled, high salt concentrations are needed to prevent dissolution; therefore it is difficult to lower the mud weight for the depleted zone.
Lost Circulation into Pre-Existing and Drilling-Induced fracturesA high mud weight can result in fracturing of rock already weakened by the depletion process.The loss of fluids into fractures is costly and may lead to well control problems.There could be loss of productivity with blocked fractures.Fractures can increase overall wellbore stability problems