Dominating Effects Controlling the Extent of Drilling-Induced Formation Damage
Authors: Francis Paul 
Published: December 2007

Abstract
An analysis of a large amount of data generated from reservoir conditions core flood testing for Shell Operating Units has lead to an increased understanding of the dominant effects controlling the extent of drilling-induced formation damage. Results were confirmed by full-scale tests in a drilling simulator.
In nearly every case, for all drilling fluids tested, the dominant form of impairment was an extremely fine layer of residual solids after removal of mud filter cake from the wellbore face. The impact of this permeability barrier is dependent on initial core permeability and the reservoir fluid used in the drawdown phase (oil or gas). These results indicate that the formation damage remaining after drawdown is more dependent on the virgin reservoir properties than the drilling fluid itself.

Introduction
The loss of revenue caused by damaged wells producing below their potential strongly influences the profitability of any venture. A significant effort by the industry is currently being put into avoiding drilling and completion induced damage, especially in horizontal open-hole completions, where bypassing damage by perforation is not usually an option. In order to be able to avoid or remedy damage, it is first necessary to understand the underlying causes. Reservoir conditions core flood tests have been used by Shell Operating Units to assess the effects of various drilling fluids and clean-up chemicals on their reservoirs. The tests have not only focused on return permeability to assess the magnitude of damage, but also, by mineralogical analysis of cores before and after tests, visualisation of the type of damage. This paper contains a summary of what has been seen from a large number of reservoir conditions core flood tests and the conclusions drawn. The results of these small scale reservoir conditions core flood tests have been confirmed by full scale tests in a drilling simulator. Reservoir Conditions Core Flood Tests Reservoir conditions core flood tests comprise flowing drilling fluids against the face of core plugs at wellbore conditions of overbalance followed by drawdown to the reservoir fluid phase (oil or gas). These tests simulate the type of damage that may be observed in open-hole completed horizontal wells. Permeability measurements before and after testing give the magnitude of damage — which can be equated to an equivalent inflow performance. Mineralogical analyses before and after testing permit visualisation of the type of damage caused.
Previous papers have described the important factors for a representative core flood test. These factors have been now been incorporated into a 'Recommended Practice' and are summarised below:
The technique attempts to simulate, as closely as possible, the conditions occurring in the reservoir during drilling-in through the use of:–Real, used field muds sampled from previous drilling campaigns–Fresh, well-preserved core material from the target reservoir, and if possible, original native pore fluids, i.e., both pore water and hydrocarbons–Realistic, long-duration exposure of the core to the drilling fluid under reservoir conditions, with fluid flow past the core face at realistic borehole velocities–Simulated back production of damage by flowing hydrocarbon phase (oil or gas) at realistic drawdown pressure
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